CSWIP MT Level 2 Textbook (2016).pdf

Magnetic Particle Testing (MT) NDT30M

Training and Examination Services Granta Park, Great Abington Cambridge CB21 6AL United Kingdom Copyright © TWI Ltd

Magnetic Particle Testing – NDT30M Contents Section

Subject

Preliminary pages Standards and Associated Reading COSHH, H&S, Cautions and Warnings Introduction to NDT Methods NDT Certification Schemes 1

The Principles of MPI

1.1

1.8

Introduction Types of magnetisation - diamagnetism and paramagnetism Ferromagnetism and domain theory Permanent magnetism Electromagnetism Magnetic hysteresis Definition of terms Flux leakage

2

Methods of Magnetisation

2.1 2.2

Portable equipment Fixed equipment

3

Detecting Media, UV Light and Other Equipment

3.1 3.2

Inks and powders Visible or fluorescent

4

Application Techniques and Demagnetisation

4.1

4.5

Continuous technique Residual technique Demagnetisation Principle of demagnetisation Methods of demagnetisation

5

Current Waveforms

5.1

5.5

Direct current Alternating current Half-wave rectified (HWR) [or HWRAC] Full-wave rectified current (single phase [FWRAC] Converting between RMS and peak values

6

Assessing Magnetising Force and Amperage

6.1

Portable equipment Alternative standards Fixed equipment Verification of magnetisation Factors affecting MPI sensitivity Assessment and reporting of indications and test procedure

1.2 1.3 1.4 1.5 1.6 1.7

4.2 4.3 4.4

5.2 5.3 5.4

6.2 6.3 6.4 6.5 6.6

NDT30M-60615 Contents

Copyright © TWI Ltd

7

Control and Maintenance Checks

7.1

Detection media Fluorescent ink intensity Overall performance check Viewing efficiency Magnetising units Tank levels Ultraviolet lamp maintenance Ammeters Demagnetiser

7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

Glossary of Terms Product Technology Notes Coursework

NDT30M-60615 Contents


Standards and Associated Reading BS EN ISO 1330-1

Non-destructive Testing – Terminology – Part 1: List of general terms.

BS EN ISO 1330-2

Non-destructive Testing – Terminology – Part 2: Terms common to NDT methods.

BS EN ISO 1330-7

Non-destructive Testing – Terminology – Part 7: Terms used in magnetic particle testing.

BS EN ISO 9934-1

Non-destructive Testing – Magnetic particle testing – Part 1: General Principles.

BS EN ISO 9934-2

Non-destructive Testing – Magnetic particle testing – Part 2: Detection media.

BS EN ISO 9934-3

Non-destructive Testing – Magnetic particle testing – Part 3: Equipment.

ASTM E1444-05

Standard Practice for Magnetic Particle Examination.

BS EN 1290

Non-destructive examination of welds – Magnetic particle examination of welds (SUPERSEDED by BS EN ISO 17638).

BS EN 1291

Non-destructive testing of welds. Magnetic particle testing of welds. Acceptance levels.

BS EN 3059

Non-destructive Testing – Penetrant testing and magnetic particle testing – Viewing conditions.

BS EN 10228-1

Non-destructive testing of steel forgings. Magnetic particle inspection.

BS EN 12062

Non-destructive examination of welds. General rules for metallic materials.

BS 7773

Code of practice for cleaning and preparation of metal surfaces. (SUPERSEDED by BS EN ISO 27831 Parts 1 and 2: Metallic and other inorganic coatings – cleaning and preparation of metal surfaces. Part 1. Ferrous metals and alloys. Part 2. Non-ferrous metals and alloys).

BS 4069

Specification for magnetic flaw detection inks and powders (SUPERSEDED, WITHDRAWN)

BS 5044

Specification for contrast aid paints used in magnetic particle flaw detection. (SUPERSEDED, WITHDRAWN)

BS 6072

Method for magnetic particle flaw detection. (SUPERSEDED by BS EN ISO 9934-1)

BS EN ISO 17638

Non-destructive testing of welds. Magnetic particle testing.

BS EN 1369

Founding – magnetic particle testing.

NDT30M-60615 Prelims


EN 473

Superseded by BS EN ISO 9712.

BS EN ISO 9712

Non-destructive testing – qualification and certification of NDT personnel.

PD 6513

Magnetic particle flaw detection. A guide to the principles and practice of applying magnetic particle flaw detection in accordance with BS 6072. (SUPERSEDED by BS EN ISO 99 34-1)

Associated Reading Magnetic Particle Inspection: A Practical Guide. David Lovejoy Mathematics and formulae in NDT (ISBN 0 903 132 214) published by ‘The British Institute of Non-destructive Testing’, Newton Building, St George’s A venue, Northampton, NN2 6JB, UK. NDT Education and Resource Centre. Magnetic Particle Inspection . http://www.ndted.org/EducationResources/CommunityCollege/MagParticle/cc_mpi_index.htm



COSHH, H&S, Caution and Warnings Relevant to TWI Training & Examination Services Introduction The use of chemicals in NDT is regulated by law under the Control of Substances Hazardous to Health (COSHH) Regulations 2005. These regulations require the School to assess and control the risk of health damage from every kind of substance used in training. Students are also required by the law to co-operate with the School’s risk management efforts and to comply with the control measures adopted. Hazard Data Sheets The School holds Manufacturers Safety Data Sheets for every substance in use. Copies are readily available for students to read before using any product. The Data Sheets contain information on:     

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The trade name of the product; eg Magnaglo, Ardrox, etc. Hazardous ingredients of the products. The effect of those ingredients on people’s health. The hazard category of the substance; eg irritant, harmful, corrosive or toxic, etc. Special precautions for use; eg the correct Personal Protective Equipment (PPE) to wear. Instructions for First Aid. Advice on disposal.

Disposing of chemicals Personnel Protective Equipment (PPE) must be used at all times according to the manufactures instructions and including the wearing of safety equipment and clothing and also recognising that dangerous fumes might be present particularly for solvent contrast paints, inks and cleaners. Safety Conditions 

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Where required carbon filters will be used for the removal of odours, gases and chemical vapours. Similarly ultrafiltration can be used for the treatment of rinse water used during cleaning. Safety precautions when using UV radiation is cover in Section 3.2.2, Ultraviolet Lamp Maintenance - Safety precautions and operating instructions of UV light sources. Electrical Hazards include the following: Electrical shock and burns from contact with live parts  Injury from exposure to arcing or fire from faulty equipment   Explosion caused by electrical apparatus (or static electricity)  Electric shocks can lead to other types of injury such as falling from ladders or scaffolds. It is therefore important that workers know h ow to use electrical equipment and that it should be properly maintained and switched off when cleaning, adjusting or setting up.  As is the case with all items of test equipment and safety equipment, national regulations in the country of operation must be adhered to.



EH40 – Occupational Exposure Limits What is Exposure? Exposure to a substance is uptake into the body. The exposure routes are by:    

Breathing fume, dust, gas or mist. Skin contact. Injection into the skin. Swallowing.

Many thousands of substances are used at work but only about 500 substances have Workplace Exposure Limits (WELs). Until 2005 it had been normal for HSE to publish a new edition of EH40, or at least an amendment, each year. However with increasing use of the website facilities the HSE no longer always publishes a revised hardcopy edition, or amendment. The web based list which became applicable from 1st October 2007 can now be found at http://www.hse.gov.uk/coshh/table1.pdf.



Introduction to Non Destructive Testing Non-destructive testing (NDT) is the ability to examine a material (usually for discontinuities) without degrading it or permanently altering the article being tested as opposed to destructive testing which renders the product virtually useless after testing. Other advantages of NDT over destructive testing are that every item can be examined with no adverse consequences, materials can be examined for conditions internally and at the surface and most importantly parts can be examined whilst in service making a good balance between cost effectiveness and quality control. NDT is used in almost every industry with the majority of applications coming from the aerospace, power generation, automotive, rail, oil and gas, petrochemical and pipeline markets, safety being the main priority of these industries. When properly applied, NDT saves money, time, materials and lives. NDT as it is known today has been developing since around the 1920s with the methods used today taking shape later with vast technological advancements being made during the Second World War. The five principal methods, other than visual inspection, are:     

Penetrant testing. Magnetic particle inspection. Eddy current testing. Ultrasonic testing. Radiography.

In all NDT methods interpretation of results is critical. Much depends on the skill and experience of the technician, although properly formulated test techniques and procedures will improve accuracy and consistency. Visual testing (VT) With sufficient light and access, visual techniques provide simple, rapid methods of testing whilst also being the least expensive. Close Visual Testing (CVT) refers to viewing directly with the eye (with or without magnification) whereas Remote Visual Inspection (RVI) refers to the use of optical devices such as the boroscope and fibrescope. Visual testing begins with the eye; however, the first boroscopes used a hollow tube and a mirror with a small lamp at the end to investigate the bores of rifles and cannon for problems and discontinuities. In the 1950s, the lamps were replaced by glass fibre bundles which were used to transmit the light. These became known as fibrescopes which were also less rigid, increasing the capabilities of testing. With usage expanding, many users began to suffer from eye fatigue which led to the development of video technology. This was first used in the 1970s and relies on electronics to transmit the images rather than fibreoptics. Further enhancements to video technology include pan, tilt and zoom lenses, mounting cameras to platforms and wheels, all allowing more parts to be tested and better images for improved inspection. Video devices also allow recording of inspections to be taken meaning permanent records can be kept. This has a number of advantages such as enabling other inspectors to observe the test as it was performed and allowing further review and evaluation.



Penetrant testing Penetrant testing locates surface-breaking discontinuities by covering the item with a penetrating liquid, which is drawn into the discontinuity by capillary action. After removal of excess penetrant the indication is made visible by application of a developer. Colour contrast or fluorescent systems may be used.

Advantages Applicable to non-ferromagnetics Able to test large parts with a portable kit Batch testing Applicable to small parts with complex geometry Simple, cheap, easy to interpret Sensitivity

Disadvantages Only detects defects open to the surface Careful surface preparation required Not applicable to porous materials Temperature dependant Cannot retest indefinitely Compatibility of chemicals

History of penetrant testing A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later, it became the practice in railway workshops to examine iron and steel components by the oil and whiting method. In this method, heavy oil, commonly available in railway workshops, was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for ferromagnetic iron and steels. A different (though related) method was introduced in the 1940s. The surface under examination was coated with a lacquer and after drying, the sample was caused to vibrate by the tap of a hammer. The vibration causes the brittle lacquer layer to crack generally around surface defects. The brittle lacquer (stress coat) has been used primarily to show the distribution of stresses in a part and not for finding defects. Many of these early developments were carried out by Magnaflux in Chicago, IL, USA in association with Switzer Bros, Cleveland, OH, USA. More effective penetrating oils containing highly visible (usually red) dyes were developed by Magnaflux to enhance flaw detection capability. This method, known as the visible or colour contrast dye penetrant method, is still used quite extensively today. In the 1940s, Magnaflux introduced the Zyglo system of penetrant inspection where fluorescent dyes were added to the liquid penetrant. These dyes would then fluoresce when exposed to ultraviolet light (sometimes referred to as black light) rendering indications from cracks and other surface flaws more readily visible to inspectors. UV lights have become increasingly portable with hand held UV torches now readily available.



Magnetic particle inspection Magnetic particle inspection (MPI) is used to locate surface and slightly sub-surface discontinuities in ferromagnetic materials by introducing a magnetic flux into the material. Advantages Will detect some sub-surface defects Rapid and simple to understand Pre-cleaning not as critical as with dye penetrant inspection (DPI) Will work through thin coatings Cheap rugged equipment Direct test method

Disadvantages Ferromagnetic materials only Requirement to test in two directions Demagnetisation may be required Odd shaped parts difficult to test Not suited to batch testing Can damage the component under test

History of magnetic particle inspection (MPI) The origins of MPI can be traced to the 1860s when cannon barrels were tested for defects by first magnetising the barrel and then running a compass down the length of the barrel. By monitoring the needle of the compass, defects within the barrel could be detected. This form of NDT became much more common post First World War, in the 1920s, when William Hoke discovered that flaws in magnetised materials created distortions in the magnetic field. When a fine ferromagnetic powder was applied to the parts, it was observed that they built up around the defects providing a visible indication. Magnetic particle inspection superseded the oil and chalk method in the 1930s as it proved far more sensitive to surface breaking flaws. Today it is still preferred to the penetrant method on ferromagnetic material and much of the equipment being used then, is very similar to today, with the only advances coming in the form of fluorescent coating to increase the visibility of indications and more portable devices being used. In the early days battery packs and direct current were the norm and it was some years before alternating current proved acceptable. Magnetism The phenomenon called magnetism is said to have been discovered in the ancient Greek city of Magnesia, where naturally occurring magnets were found to attract iron. The use of magnets in navigation goes back to Viking times or maybe earlier, where it was found that rods of magnetised material, when freely suspended, would always point in a north-south direction. The end of the rod which pointed towards the North Pole star became known as the North Pole and consequently the other end became the South Pole. Hans Christian Oersted (1777-1851) discovered the connection between electricity and magnetism, to be followed by Michael Faraday (1791-1867) whose experiments revealed that magnetic and electrical energy could b e interchanged.



Electromagnetic Testing Historical perspective Electromagnetic testing – the interaction of magnetic fields with circulating electrical currents - had its origin in 1831 when Michael Faraday discovered electromagnetic induction. He induced current flow in a secondary coil by switching a battery on and off. D E Hughes performed the first recorded eddy current test in 1879. He was able to distinguish between different metals by noting a change in excitation frequency resulting from effects of test material resistivity and magnetic permeability. Introduction to electromagnetic testing Many electromagnetic induction or eddy current comparators were patented in the period from 1952. Innumerable examples of comparator tests were reported in the literature and in patents. Many provided simple comparator coil into which round bars or other test objects were placed, producing simple changes in amplitudes of test signals, or unbalancing simple bridge circuits. In nearly all cases, particularly where ferromagnetic test materials were involved, no quantitative analyses of test objects dimensions, properties, or discontinuities were possible with such instruments. Often, difficulties were encountered in reproducing test results, for some test circuits were adjusted or balanced to optimise signal differences between a known good test object and a known defective test object, for each group of objects to be tested. Little or no correlation could then be obtained between various types of specimens, each type having been compared to an arbitrarily selected specimen of the same specific type. Developments in electromagnetic induction tests Rapid technological developments in many fields before and during World War 2, (193945) contributed both to the demand for NDT and to the development of advanced test methods. Radar and sonar systems made acceptable the viewing of test data on the screens of cathode-ray tubes or oscilloscopes. Developments in electronic instrumentation and in magnetic sensors used both for degaussing ships and for actuating magnetic mines, brought a resurgence of activity. Friedrich Förster The introduction by Förster of sophisticated, stable, quantitative test equipment and of practical methods for analysis of quantitative test signals on the complex plane were by far the most important factors contributing to the rapid development and acceptance of electromagnetic induction and eddy current testing. Förster is rightly identified as the father of modern eddy current testing. By 1950, he had developed a precise theory for many basic types of eddy current tests, including both absolute and differential or comparator test systems and probe or fork coil systems used with thin sheets and extended surfaces. Continued advances in research and development, advanced electronics and digital equipment have led to eddy currents becoming one of the most versatile of the surface methods of inspection.



Eddy current inspection Eddy current inspection is based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by coils in the test probe and monitored simultaneously by measuring the coils electrical impedance. As it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the material must be an electrical conductor. Advantages Sensitive to surface defects Can detect through several layers Can detect through surface coatings Accurate conductivity measurements Can be automated Little pre-cleaning required Portability

Disadvantages Very susceptible to permeability changes Only on conductive materials Will not detect defects parallel to surface Not suitable for large areas and/or complex geometries Signal interpretation required No permanent record (unless automated)

History of eddy current testing The principles of eddy currents arose in 1831 with Faraday’s discovery of electromagnetic induction; eddy current testing methods have their origins in a period just after the First World War, when materials with a high magnetic permeability were being developed for electrical power transformer cores and motor armatures. Eddy currents are a considerable nuisance in electrical engineering – they dissipate heat and efforts to reduce their effect led to a discovery that they could be used to detect material changes and cracks in magnetic materials. The first eddy current testing devices for NDT were by Hughes in 1879 who used the principles of eddy currents to conduct metallurgical sorting tests and the stray flux tube and bar tester. It was left to Dr. Friedrich Förster in the late 1940s to develop the modern day eddy current testing equipment and formulate the theories which govern their use. Since then, eddy current methods have developed into a wide range of uses and are recognised as being the forerunner of NDT techniques today. From the mid 1980s the microprocessor based eddy current testing instruments were developed which had many advantages for inspectors. Modern electronics have made instruments more user friendly, providing reduced noise levels which made certain test applications very difficult, but also improving methods of signal p resentation and recording capabilities. Microcomputer chips abound, from giving lift-off suppression in simple crack detection to providing signal processing for immediate analysis of condenser tube inspection. As with other testing methods, improvements to the equipment have been made to increase its portability and computer-based systems now allow easy data manipulation and signal processing. Eddy current testing is now a widely used and understood inspection method for flaw detection as well as for thickness and conductivity measurements.



Ultrasonic testing Ultrasonic testing measures the time for high frequency (0.5-50MHz) pulses of ultrasound to travel through the inspection material. If a discontinuity is present, the ultrasound will return to the probe in a time period other than would be expected of a fault free specimen. Advantages Sensitive to cracks at various orientations Portability Safety Able to penetrate thick sections Measures depth and through-wall extent

Disadvantages No permanent record (unless automated) Not easily applied to complex geometries and rough surfaces. Unsuited to coarse grained materials Reliant upon defect orientation

History of ultrasonic testing (UT) In Medieval times craftsmen casting bells for churches were aware that a properly cast bell rang true when struck and that a bell with flaws would give out a false note. This principle was used by wheel-tappers inspecting rolling stock on the railways; they struck wheels with a hammer and listened to the note given out. A loose tyre sounded wrong. The origin of modern ultrasonic testing (UT) is the discovery by the Curie brothers in 1880 that quartz crystals cut in a certain way produce an electric potential when subjected to pressure - the piezo-electric effect, from the Greek piedzein, to press or strike. In 1881 Lippman theorised that the effect might work in reverse and that quartz crystals might change shape if an electric current was applied to them. He found this was so and experimented further. Crystals of quartz vibrate when alternating currents are applied to them. Crystal microphones in a modern stereo rely on this principle. When the Titanic sank in 1912, the Admiralty tried to find a way of locating icebergs by sending out sound waves and listening for an echo. They experimented further with sound to detect submarines during the First World War. Between the wars, marine echo sounding was developed and in the Second World War ASDIC (Anti-Submarine Detection Investigation Committee) was extensively used in the Battle of the Atlantic against the U-boats. In 1929 a Russian physicist, Sokolov, experimented with through transmission techniques of passing vibrations through metals to find flaws; this work was taken up by the Germans. In the 1930s the cathode ray tube was developed and miniaturised in the Second World War to fit small airborne radar sets into aircraft. It made the UT set as we know it possible. Around 1931 Mulhauser obtained a patent for a system using two probes to detect flaws in solids and following this Firestone (1940) and Simons (1945) developed pulsed UT using a pulse-echo technique. In the years after the Second World War researchers in Japan began to experiment on the use of ultrasound for medical diagnostic purposes. Working largely in isolation until the 1950s, the Japanese developed techniques for the detection of gallstones, breast masses and tumours. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation. The first flaw detector was made by Sproule in 1942 while he was working for the Scottish firm Kelvin & Hughes. Similar work was carried out by Firestone in the USA and by German physicists. Sproule went on to develop the shear-wave probe.



Initially UT was limited to testing aircraft, but in the 1950s it was extensively used in the building of power stations in Britain for examining thick steel components safely and cheaply. UT was found to have several advantages over radiography in heavy industrial applications: 

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It did not have health hazard associated with radiography and a UT technician could work next to welders and other employees without endangering them of holding up work. It was efficient in detecting toe cracks in boilers – a major cause of explosions and lack of fusion in boiler tubes. It could find planar defects, like laminations, which were sometimes missed by radiography. A UT check on a thick component took no more time than a similar check on a thin component as opposed to long exposure times in radiography.

Over the next twenty years, improvements focused on accurate detection and sizing of the flaws with limited success, until 1977 when Silk first discovered an accurate measurement and display of the top and bottom edges of a discontinuity with the Time of Flight technique (TOFD). Advances in computing technology have now expanded the use of TOFD as real time analyses of results are now available. It was also during the 1970s that industries focused on reducing the size and weight of Ultrasonic flaw detectors and making them more portable. This was achieved by using semi-conductor technology and during the 1990s microchips were introduced into the devices to allow calibration parameters and signal traces to be stored. LCD display panels and digital technology have also contributed to reducing the size and weight of Ultrasonic flaw detectors. With the development of Ultrasonic Phased Array and increased computing power, the future f or Ultrasonic inspection is very exciting. Radiography Radiography monitors the varying transmission of ionising radiation through a material with the aid of photographic film or fluorescent screens to detect changes in density and thickness. It will locate internal and surface-breaking defects. Advantages Gives a permanent record, the radiograph Detects internal flaws Detects volumetric flaws readily Can be used on most materials Can check for correct assembly Gives a direct image of flaws Fluoroscopy can give real time imaging


Disadvantages Radiation health hazard Can be sensitive to defect orientation and so can miss planar flaws Limited ability to detect fine cracks Access is required to both sides of the object Skilled radiographic interpretation is required Relatively slow method of inspection High capital cost High running cost


History of radiographic testing X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was a Professor at Wϋrzburg University in Germany. Whilst doing some experiments in which he passed an electric current through Crookes tubes, an evacuated glass tube with an anode and a cathode. When a high voltage was applied, the tube produced a fluorescent glow. Roentgen noticed that some nearby photographic plates became fogged. This caused Roentgen to conclude that a new type of ray was being emitted from the tube. He believed that unknown rays were passing from the tube and through the plates. He found that the new ray could pass through most substances casting shadows of solid objects. Roentgen also discovered that the ray could pass through the tissue of humans, but not bones and metal objects. One of Roentgen's first experiments late in 1895 was a film of the hand of his wife. Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896 French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period were working with cathode rays and other scientists were gathering evidence on the theory that the atom could be subdivided. Some of the new research showed that certain types of atoms disintegrate by themselves. It was Becquerel who discovered this phenomenon while investigating the properties of fluorescent minerals. One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with photographic plates. Later when he developed these plates, he discovered that they were fogged (exhibited exposure to light). Becquerel questioned what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light, in addition, he noticed that only the plates that were in the drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium. Becquerel did not pursue his discovery of radioactivity, but others did. While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitchblende contained other radioactive elements. Marie and her husband, French scientist Pierre Curie, started looking for these other elements. In 1898, the Curies discovered another radio-active element in pitchblende and named it polonium in honour of Marie’s native homeland. Later that year, the Curies discovered another radioactive element which they named ‘radium’, or shining element. Both polonium and radium were more radioactive than uranium. Due to her lifelong research in this field, Marie Curie is widely credited with the discovery of gamma radiation and the introduction of the new term: radio-active.



Since these discoveries, many other radioactive elements have been discovered or produced. Radiography in the form of NDT took shape in the early 1920s when Dr. H.H. Lester began testing on different materials. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. During the Second World War industrial radiography grew tremendously as part of the Navy's shipbuilding programme. In 1946, man-made gamma ray sources such as cobalt and iridium became available. These new sources were far stronger than radium and much less expensive. The man-made sources rapidly replaced radium and use of gamma rays grew quickly in industrial radiography. William D Coolidge's name is inseparably linked with the X-ray tube popularly called the Coolidge tube. This invention completely revolutionised the generation of Xrays and remains the model upon which all X-ray tubes for medical applications are patterned. He invented ductile tungsten, the filament material still used in such lamps. He was awarded 83 patents. Although the theories and practices have changed very little, radiographic equipment has developed. These developments include better images through higher quality films and also lighter, more portable equipment. In addition to conventional film radiography Digital Radiographic systems are now wide spread within the NDT industry. The use of Phosphor stimulated imaging plates (PSP) with photomultipliers to capture image signals and Analogue to Digital Converters (ADC) to convert to digital image are used extensively in Computed Radiography (CR). Direct radiography systems (DR) are also used based upon complimentary metal oxide sensor (CMOS) technology and TFT (thin film transistors). These systems have the ability to directly convert light into digital format, additionally they may be coupled with a scintillator which coats CMOS and CCD (charged couple device) sensors, the scintillator converts photon energy to light before the sensor and ADC converts to digital format. Systems which use scintillators in this way are often referred to as indirect systems. Quality issues of any digital system are based upon the effective pixel size and the SNR (signal to noise ratio).The benefits of using Digital systems is the speed of inspection, the absence of chemical processing requirements and wet film, however, the initial equipment costs will be high.



NDT Certification Schemes CSWIP – Certification Scheme for Personnel Managed by TWI Certification Ltd (TWICL), a TWI Group company formed in 1993 to separate TWI’s activities in the field of personnel and company certification thus ensuring continued compliance with international standards for certification bodies and is accredited by UKAS to ISO 17024. TWICL establishes and implements certification schemes, approves training courses and authorises examination bodies and assessors in a large variety of inspection fields, including; non-destructive testing (NDT), welding and plant inspectors, welding supervisors, welding coordination, plastic welders, underwater inspectors, integrity management, general inspection of offshore facilities, cathodic protection, heat treatment. TWI Certification Ltd Granta Park, Great Abington, Cambridge CB21 6AL, United Kingdom Tel: +44 (0) 1223 899000 Fax: +44 (0) 1223 894219 Email: [email protected] Website: www.cswip.com PCN – Personal Certification in Non-destructive testing Managed and marketed by the British Institute of Non-Destructive Testing (BINDT) which owns and operates the PCN Certification Scheme, it offeres a UKAS accreditied certification of competence for NDT and condition monitoring in a variety of product sectors. The British Institute of Non-Destructive Testing Certification Services Division Newton Building St Georges Avenue Northampton NN2 6JB, United Kingdom Tel: +44 (0)1604 893811 Fax: +44 (0)1604 893868 Email: [email protected] Website: http://www.bindt.org/Certification/General_Information Both schemes offer NDT certification conforming to BS EN ISO 9712 – Non- destructive testing - qualification and certification of NDT personnel.



The PCN Scheme A summary of the general requirements for qualification and PCN certification of NDT personnel as described in PCN/GEN Issue 5 Revision R PCN Certification is a scheme which covers the qualification of NDT inspection staff to meet the requirements of European and International Standards. Typically a standard or procedure will call for the Inspector to be certified in accordance with EN ISO 9712. Non destructive testing – qualification and certification of NDT personnel and/or PCN requirements. The PCN Gen Document describes how the PCN system works. The points below cover extracts from this document which are major items, the full document can be viewed on the BINDT website – www.bindt.org/certification/PCN. References PCN documents PSL/4 PSL/8A PSL/30 PSL/31 PSL/42 PSL/44 PSL/49 PSL/51 PSL/57C PSL/67 PSL/70 CP9 CP16 CP17 CP19 CP22 CP25 CP27 PCN/GEN


Examination availability PCN documents – issue status Log of pre-certification experience Use of PCN & UKAS logo Log of pre-certification on-the-job training Vision requirements Examination exemptions for holders of certification other than PCN Acceptable certification for persons supervising PCN candidates gaining experience prior to certification Application for certification, experience gained post examination Supplementary 56 day waiver Request for L2 certificate issue to a L3 holder Requirements for BINDT authorised qualifying bodies Renewal and recertification of PCN Levels 1 and 2 certificates Renewal and recertification of PCN Level 3 certificates Informal access to authorised qualifying bodies by third parties Marking and grading PCN examinations Guidelines for the preparation of NDT procedures and instructions in PCN examinations Code of ethics for PCN certificate holders Appendix ZI – NDT training syllabi


Levels of PCN certification CSWIP certification operates with similar rules and requirements to PCN. The requirements for PCN certification are shown here. For a comprehensive view of CSWIP scheme documents got to www.cswip.com/schemes. Level 1 personnel are qualified to carry out NDT operations according to written instructions under the supervision of appropriately qualified Level 2 or 3 personnel. Within the scope of the competence defined on the certificate, Level 1 personnel may be authorised by the employer to perform the following in accordance with NDT instructions:    

Set up equipment. Carry out the test. Record and classify the results in terms of written criteria. Report the results.

Level 1 personnel have not demonstrated competence in the choice of test method or technique to be used, nor for the assessment, characterisation or interpretation of test results. Level 2 personnel have demonstrated competence to perform and supervise nondestructive testing according to established or recognised procedures. Within the scope of the competence defined on the certificate, Level 2 personnel may be authorised by the employer to:      

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Select the NDT technique for the test method to be used. Define the limitations of application of the testing method. Translate NDT standards and specifications in to NDT instructions. Set up and verify equipment settings. Perform and supervise tests. Interpret and evaluate results according to applicable standards, codes or specifications. Prepare written NDT instructions. Carry out and supervise all Level 1 duties. Provide guidance for personnel at or below Level 2. Organise and report the results of non-destructive tests.

Level 3 personnel are qualified to direct any NDT operation for which they are certificated and may be authorised by the employer to:  

  



Assume full responsibility for a test facility or examination centre and staff. Establish, review for editorial and technical correctness and validate NDT instructions and procedures. Interpret codes, standards, specifications and procedures. Designate the particular test methods, techniques and procedures to be used. Within the scope and limitations of any certification held carry out all Level 1 and 2 duties and; Provide guidance and supervision at all levels.



Level 3 personnel have demonstrated: 





Competence to interpret and evaluate test results in terms of existing codes, standards and specifications. Possession of the required level of knowledge in applicable materials, fabrication and product technology sufficient to enable the selection of NDT methods and techniques and to assist in the establishment of test criteria where none are otherwise available. General familiarity with other NDT methods.

Level 3 certificated personnel may be authorised to carry out, manage and supervise PCN qualification examinations on behalf of the British Institute of NDT. Where Level 3 duties require the individual to apply routine NDT by a method(s) within a particular product or industry sector, the British Institute of NDT strongly recommends that industry demand that this person should hold and maintain Level 2 certification in the applicable methods and sectors. Training Table 1 Minimum required durations of training.

Level 1 hours Level 2 hours1 Level 3 hours 40 40 40 16 24 24 16 24 32 40 80 72 N/A 56 N/A 40 80 72 16 24 24 16 N/A N/A N/A 24 N\A (Direct access to Level 3 examination 80 parts A- C) Note 1: Direct access to Level 2 requires the total number of hours shown in Table 1 for Levels 1 and 2 and direct access to Level 3 requires the total number of hours shown in Table 1 for Levels 1-3. Up to one third of the total specified in this table may take the form of OTJ training documented using form PSL/42 provided it is verifiable and covered practical application of the syllabus detailed in CEN ISO/TR 25107:2006. NDT method ET PT MT RT RI UT VT BRS RPS Basic knowledge



Industrial NDT experience 





Industrial NDT experience in the appropriate sector may be acquired prior to or following success in the qualification examination. In the event that the experience is sought following successful examination, the results of the examination shall remain valid for up to two years. Documentary evidence (in a form acceptable to the British Institute of NDT, ie on PCN form PSL/30) of experience satisfying the following requirements shall be confirmed by the employer and submitted to BINDT AQB prior to examination, or directly to BINDT prior to the award of PCN certification in the event that experience is gained after examination.

Table 2 Minimum duration of experience for certification.

NDT method ET MT PT RT UT RI VT

Experience, months Level 1 Level 2 3 9 1 3 1 3 3 9 3 9 6 N/A 1 3

Level 3 18 12 12 18 18 N/A

12

Work experience in months is based on a nominal 40hr/week or the legal week of work. When an individual is working in excess of 40hs/week, he may be credited with experience based on the total hours, but he shall be required to produce evidence of this experience. Direct access to Level 2 requires the total number of hours shown in Table 2 for Levels 1 and 2 and direct access to Level 3 requires the total number of hours shown in Table 2 for Levels 1-3 Qualification examination Table 3 Numbers of general questions.

NDT method ET PT MT RT RI UT VT

Level 1 40 30 30 40 N/A 40 30

Level 2 40 40 40 40 40 40 40

BRS 30 N/A RPS N/A 20 plus 4 narrative Note: All Level 1 specific theory papers have 30 questions. All Level 2 specific theory papers have 30 questions.



Re-examination a

A candidate who fails to obtain the pass grade for any examination part (general, specific or practical) may be re-examined twice in the failed part(s), provided the reexamination takes place not sooner than one month, unless further training acceptable to BINDT is satisfactorily completed, nor later than twelve months after the original examination.

b

A candidate who achieves a passing grade of 70% in each of the examination parts (general, specific or practical) but whose average score is less than the required 80% may be re-examined a maximum of two times in any or all of the examination parts in order to achieve an overall average score of 80%, provided the reexamination takes place not sooner than one month, unless further training acceptable to BINDT is satisfactorily completed, nor later than twelve months after the original examination.

c

A candidate who fails all permitted re-examinations shall apply for and take the initial examination according to the procedure established for new candidates.

d

A candidate whose examination results have not been accepted for reason of fraud or unethical behaviour shall wait at least twelve months before re-applying for examination.

Summary The PCN scheme is managed and administered by the British Institute of NDT (BINDT) on behalf of its stakeholders. It meets or exceeds the criteria of EN ISO 9712. There are 6 appendices covering various industry and product sectors, 1 2 3 4 5 6

Aerospace. Castings. Welds. Wrought Products and Forgings. Pre and in-service inspection (multi sector). Railway.

There are many additional supporting documents varying from vision requirements PSL44 to renewal and recertification (Levels 1 and 2 – CP16; Level 3 – CP17) and so on. The document defines many terms used in certification of NDT personnel (PCN Gen Section 3) The certification body (BINDT) meets the requirements of ISO 17024 (PCN Gen section 5)



BINDT approves authorised qualifying bodies (AQBs) to carry out the examinations (PCN Gen Section 5) a

b c d e

f g h i j k l

m n o

The document sets out the Levels of PCN certification and what each level of personnel is qualified to do (PCN Gen section 6). There are 3 Levels of PCN certification. Candidates for examination must have successfully completed a BINDT validated course of training at a BINDT authorised traini ng organisation (PCN Gen Section 7). Table 1 shows the minimum required duration of training for all Levels and methods plus a section of notes. Table 2 gives the minimum duration of experience for each Level and method. A candidate is required to have a vision test of colour perception and a near vision test (Jaeger Number 1 or N4.5). PCN Gen Section a – the near vision test to be taken annually. Examination applications are made directly with the AQB. PCN Level 1s and 2 initial exams comprise general; specific and practical parts. Table 3 shows the number of general questions at Levels 1 and 2 examinations. There are 30 specific questions on the Level 1 papers. There are 30 questions on the Level 2 specific papers. A variety of practical samples are tested depending on the method and sector. A Level 3 examination comprises a basic and a method examination – however the basic examination needs to be passed only once. Table 4 shows the number of basic examination questions. Table 5 shows the number of Level 3 examination questions. Pass is obtained where each part is 70% or over with an average grade of 80% or over. A PCN certificate is valid for 5 years. Renewal and recertification requirements are covered in CP16 for Level 1 and Level 2 and CP17 for Level 3.

Table 4 Number of basic examination questions.

Part

Examination

Number of questions

A

Materials technology and science, including typical defects in a wide range of products including castings welds and wrought products.

30

B

Qualification and certification procedure in accordance with this document

10

C

15 general questions at Level 2 standard for each of four NDT methods chosen by the candidate, including at least one volumetric NDT method (UT or RT).

60

Table 5 Main method examination.

Part

Subject

Number of questions

D

Level 3 knowledge relating to the test method applied

30

E

Application of the NDT method in the sector concerned, including the applicable codes, standards and specifications. This may be an open book examination in relation to codes, standards and specifications.

20

F

Drafting of one or more NDT procedures in the relevant sector. The applicable codes, standards and specifications shall be available to the candidate.



Section 1 The Principles of MPI

1

The Principles of MPI

1.1

Introduction MPI is a quick, simple, sensitive and inexpensive NDT method that can be used for the detection of surface (and under favourable conditions near surface) cracks in ferromagnetic materials (eg low alloy steels). It can be used on painted surfaces providing it is not thick and can be used on large or small components and is not restricted by component size providing that the test surface is accessible. It does not need elaborate pre-cleaning but for the best results two directions of near perpendicular magnetic flux flow are to be applied for a satisfactory test. Most MPI test methods need an electrical supply for shop or site work, and it is not always evident whether the magnetic field is sufficiently strong to give a good defect indication. Spurious or non-relevant indications are not unusual and therefore interpretation can be a skilled task. Some detecting media used, ie paints and particle suspension fluids can emit fumes and be a potential fire hazard and extreme care should be exercised when the method is used in confined spaces. Factors affecting MPI test sensitivity are given in section 6.4 of these training notes.

1.2

Types of Magnetisation - diamagnetism and paramagnetism All materials are affected by magnetic fields, to a greater or lesser degree. The change or orbital motion of the electrons in the atoms of the substance concerned relates to the degree of magnetisation. Those materials which are termed:   

Diamagnetic are repelled by a magnetic force and have a small negative susceptibility to magnetism. Paramagnetic are lightly attracted by a magnetic force and have a small positive susceptibility to magnetism. Ferromagnetic are strongly attracted by a magnetic field.

Table 1.1 Magnetic properties exhibited by selected materials.

Ferromagnetic Iron Steel Cobalt Nickel 1.3

Paramagnetic Platinum Palladium Most non-ferrous metals Oxygen

Diamagnetic Bismuth Antimony Most non-metals Concrete

Ferromagnetism and domain theory Magnetic Particle Inspection (MPI) is an NDT method used for the location of surface and subsurface discontinuities in ferromagnetic materials where, in the presence of a magnetic field, the discontinuity causes magnetic flux leakage that can be detected by the application of finely divided ferromagnetic particles to the test surface. Ferromagnetic materials are strongly attracted by magnetic fields. These are the materials that can be magnetised and thus tested by MPI. Ferromagnetism can be explained using the idea of the magnetic domain. Domains can be considered to be minute internal magnets, each perhaps comprising 1015-1020 atoms. In ferromagnetic atoms, the configuration dictates that more electrons spin one way than the other. The resultant magnetic moment of a group of atoms means that an internal polarity is created, simply, a very small internal magnet, having a north and south pole.

NDT30M-60615 The Principles of MPI

1-1


The general principles of MPI as applied to ferromagnetic engineering materials is given in BS EN ISO 9934-1, Non destructive testing – magnetic particle testing – part 1 general principles.

Unmagnetised state Domains randomly orientated.

Magnetic field Magnetised state. Domains orientated in external magnetic field.

Magnetic field Magnetic field Saturated state. Domains orientated in strong external field. Magnetic field

Residual state. Domains remaining orientated in absence of external field. Magnetic field Demagnetised state. Domains randomly orientated in opposing field. Magnetic field Figure 1.1 Stages of magnetisation.


1-2


1.4

Permanent magnetism When the external magnetising force is removed from a ferromagnetic material the domains will remain in a partial alignment dependent on a number of factors, such as:    

Alloying elements. Carbon content. Heat treatment. Temperature.

Strong permanent magnets used in MPI are commonly made of iron alloyed with aluminium, nickel and cobalt. Hence such trade names as: Alnico or Alcomax. If a bar magnet is placed under a flat sheet of paper and iron filings are sprinkled on to the paper, a visual field is created. This is called a magnetograph and the filings are orientated by the magnetic field created by the lines of force running between the poles of the bar magnet.

N

S

Figure 1.2a Permanent magnet.

Figure 1.2(b) Magnetic poles: Like poles repel; unlike poles attract.

For a horseshoe magnet the lines of magnetic flux flow between the magnets poles (see figure 1.2). If a ferromagnetic material (eg flat steel bar) were placed across the poles of the horseshoe magnet the magnetic flux would be contaminated fully within the magnet and the steel bar, no flux would be detected externally in air.


1-3


Below are a number of rules relating to lines of force:         

1.5

They never cross. Repel each other laterally. Are in a constant state of tension. Take the path of least magnetic resistance. All have the same strength. Are more numerous where the field intensity is greatest. Their density decreases when they move from an area of higher permeability to an area of lower permeability. Their density decreases with increasing d istance from the poles. By convention flow from north to south outside the material and south to north inside.

Electromagnetism When an electric current flows through a conductor, a magnetic field is set up around the conductor in a direction at 90  to the electric current. This is explained by the right hand rule.

Current Magnetic field (circular) Figure 1.3 Linear conductor.

If the thumb of the right hand is extended in the direction in which the current is flowing, then the direction of the magnetic field is represented by the fingers.

Figure 1.4 Right hand rule.


1-4


When the conductor carries an electrical current, strong magnetic flux lines are created, (also in the direction of the fingers as with the right hand rule) this is called circular magnetism. Circular magnetism is not polar and cannot be detected externally on a round symmetrical specimen. The magnetic field strength varies from zero at the centre of the conductor to a maximum at the conductor surface with the field strength outside the conductor being directly proportional to the current. For a long uniform conductor, the field strength deceases with radial distance from the conductor surface. A long rectilinear current carrying conductor will similarly have an associated magnetic field but in this case it will not be uniform but be distorted consistent with the shape of the conductor. Now, if the original conductor carrying the current is bent into a loop, the magnetic field around the conductor will pass through the loop in one direction.

Figure 1.5 Coiled conductor.

The field within the loop has direction: one side will be a north pole and the other a south pole. By increasing the number of loops, a long (relative to its diameter) coil, or solenoid, is created and the strength of the field passing through the coil is proportional to the current passing through the conductor in amperes multiplied by the number of turns in the solenoid. When a ferromagnetic specimen is placed in an energised coil, the magnetic field is concentrated in the specimen. One end of the specimen is a north pole and the other a south pole. This is called longitudinal magnetism. Longitudinal magnetism has polarity and is therefore readily detectable. Only one type of field can exist in a material at one time; the stronger will wipe out the weaker. Normally in magnetic particle inspection, circular tests are carried out before longitudinal ones. Magnetic field (longitudinal)

S

N

Figure 1.6 Coil magnetisation.


1-5


1.6

Magnetic hysteresis When a ferromagnetic material is influenced by an alternating magnetising force (H), the variation of magnetic flux density (B) in it is related to a phenomenon known as magnetic hysteresis. The word hysteresis is derived from the Greek word for delayed and is used to describe one quantity lagging behind another. The variation of B-H follows a hysteresis loop and is characteristic to a p articular ferromagnetic material. The figure below is a typical hysteresis loop where the co-ordinates represent magnetising force (H) on the horizontal axis and flux density (B) on the vertical axis.

Figure 1.7 Hysteresis loop.

When an unmagnetised ferromagnetic material is exposed to a gradually increasing magnetising force, the corresponding flux density can be plotted along the dotted line o-a (curve of first magnetisation). The level of flux density is increased until point a is reached and a further increase of magnetising force produces no increase in flux density. The specimen is saturated with flux and indeed, point a is called the saturation point.


1-6


The dotted line o-a is often referred to as the virgin curve. Point a towards point b is where the hysteresis loop begins. As the magnetising force is reduced the flux density does not fall back to zero but follows the line a-b. So at b there is a zero magnetising force but a flux density o-b remains. The flux is lagging behind the force and this is what gives ferromagnetic materials their permanent magnetism. To reduce the flux density to zero, or demagnetise the specimen, a negative magnetising force c has to be applied and maintained. So as the force increases to produce the relationship of B-H, it follows the line b-c. The force o-c required to demagnetise the specimen is called the coercive force. Increasing the negative magnetising force still further produces a B-H relationship along the curve c-d. Point d is exactly opposite point a and represents negative saturation. As the negative force is reduced, point e is reached, exactly opposite point b and reversal to a positive magnetising force achieves a zero flux density at point f , exactly opposite c. The loop is completed by increasing the magnetising force, giving a B-H ratio along curve f-a. Note that once the virgin curve is produced the hysteresis loop does not pass through o again. The specimen will not be demagnetised until special steps are taken to achieve that state. Hard and soft ferromagnetics As stated earlier, a hysteresis loop is characteristic to a particular ferromagnetic material. The inner of the two curves shown is characteristic of materials such as low carbon steel defined as a soft ferromagnetic, whilst the outer would be typical of a hard ferromagnetic material such as high carbon steel. Modern permanent magnets are generally made from the latter and are of low permeability/high retentivity alloys that have been subjected to large magnetising forces.

Hard ferromagnetic (high retentivity) Soft ferromagnetic

Figure 1.8 Hysteresis loop for hard and soft ferromagnetic materials.


1-7


Soft ferromagnetic Typically low carbon steel High permeability Easy to magnetise Low residual magnetism

Hard ferromagnetic Typically high carbon steel Lower permeability More difficult to magnetism High levels of residual magnetism

The various properties of a ferromagnetic material can be altered by the addition of various alloying elements. The table below gives examples of some of these and their effects. Alloying element

Hysteresis

Permeability

Coercive force

Silicon







Remanence



Chromium





Nickel with pearlitic steels





Aluminium





Tungsten









Cobalt



Loss of power



Molybdenum

 Increase (The more arrows the more intense the effect.)

 Decrease From the law of continuity the normal component of the electric flux density (BN) vector and the tangential magnetic flux density(HT) vector must be continuous across the boundary between the two media, ie the conductor carrying the current and the component subjected to the magnetic flux.


1-8


1.7

Definition of terms Knowledge of some of the physical terms related to magnetisation is essential. However, the following definitions are not meant to be exhaustive but only those which are considered relevant to understanding the practice of MPI. Comprehensive glossaries of terms relevant to MPI and NDT in general can be found in the following standards:  



BS EN ISO 1330-1 Non Destructive Testing – Terminology - Part 1 General terms. BS EN ISO 1330-2 Non Destructive Testing – Terminology - Part 2 Terms common to NDT methods. BS EN ISO 1330-7 Non Destructive Testing – Terminology - Terms used in magnetic particle testing.

Flux density The number of magnetic flux lines per unit area Symbol = B SI unit = Tesla = T It has replaced the Gauss and 1Tesla = 10,000 Gauss. Magnetising force The total force tending to set up a magnetic flux in a magnetic circuit. Symbol = H SI unit = ampere per metre = Am -1 Permeability The ease with which a magnetic field or flux can be set up in a magnetic circuit. 

  

B H



Absolute magnetic permeability in Henry per metre (). Magnetic flux density in tesla (T). Magnetic field strength in Amperes per metre ( ).

For air and non-magnetic materials,  is constant and denoted by o. o = 4 x




10

Henries/metre

1-9


For ferromagnetic materials it varies considerably according to the value of H. For convenience we use relative permeability r:

 

 

Relative permeability is therefore a dimensionless ratio that relates the permeability of the material to that of air. Saturation The stage at which any increase in the magnetising force H applied to a specimen, produces no significant gain in flux density B. Effectively it is at point a on diagram of the hysteresis loop. Saturation on a test specimen can be recognised by a high ink background caused by clumping, furring or blushing of the particles. Coercive force The reverse magnetising force required to remove residual magnetism from a material. On the diagram of the hysteresis loop it is represented by o-c. Remanence The magnetic flux density remaining in a material after the magnetising force has been removed. On the hysteresis diagram it can be any value of B, between b and e, when, H=0 Residual magnetic field The magnetic field remaining in a material after the magnetising force has been reduced to zero. Reluctance A measure of the degree of difficulty with which a component can be magnetised that is analogous to resistance in an electrical circuit. It is the reciprocal of permeability. Retentivity The magnetic flux density remaining in a material after the magnetising force has been removed, synonymous to remanence. However, McGraw-Hill, Dictionary of Scientific and Technical Terms, defines retentivity as the residual flux density corresponding to the saturation induction of a magnetic material. This corresponds to point b in the maximum remanence. Curie Point/Curie Temperature The Temperature above which ferromagnetic materials can no longer be magnetised or retain their residual magnetism. For iron the curie temperature is 770C.


1-10


1.8

Flux leakage A flux leakage is a break or a discontinuity in a magnetic circuit. Any abrupt change of permeability within a magnetic specimen will change the number of flux lines that can flow and thus there will be a diversion of the field.

Figure 1.9 Magnetic flux leakage.

Magnetic particle inspection relies on flux leakage fields being seen on the surface of a ferromagnetic specimen under test. All defects produce flux leakage but not all flux leakage fields are created by defects. Magnetic particle inspection relies on:   

Magnetising the specimen to an adequate flux density. Applying fine ferromagnetic particles over the surface of the specimen. Being able to see the magnetic particles that gather at flux leakage fields.

The magnetic field must run in a direction so that it can be interrupted by the defect, thus producing a flux leakage field. Also the degree of distortion at the leakage must allow the magnetic particles to provide an adequate degree of contrast between the leakage and the adjacent material surface, so that it is readily visible. Flux lines will take the path of least resistance, hence the highest permeability. The figure below shows flux lines flowing in a ferromagnetic bar but having to divert around an air gap, creating a flux leakage. However, if ferromagnetic particles are sprinkled on the bar they will start to form a magnetic bridge across the flux leakage and a highly preferred path. If the flux leakage is strong, such as a surface-breaking crack in the optimum direction, then the visual indication will be clear.


1-11


Figure 1.10 Magnetic Flux leakage due to a defect.

Whether a flux leakage is made into a visual indication depends on a number of factors, such as:       

1.8.1

Size of defect. Shape of defect. Volume of defect. Orientation of defect. Depth below surface. Permeability of material (hard or soft ferromagnetic). Coating thickness (MPI may be carried out through non-ferromagnetic coatings up to 50 microns thickness providing they are unbroken and tightly adherent).

Indications Indications are any particle indications that are seen on the specimen under test. Just as not all flux leakage fields are defects, not all indications are due to flux leakage. Indications can be further subdivided into:   

Relevant. Non-relevant. Spurious.

Prior to beginning MPI areas to be tested should be free from dirt, scale, loose rust, weld spatter, grease, oil and any other foreign matter that may affect the test sensitivity. The surface quality requirements are dependent upon the size and orientation of the discontinuity to be detected. The surface should be prepared so that relevant indications can be clearly distinguished from false ones.


1-12


1.8.2

Relevant indications Relevant indications are discontinuities or flaws, which in turn are unwanted imperfections. When it is considered that a relevant indication will affect the fitness-forpurpose of a test specimen, then it is classified as a defect, but not all defects are cracks. Product and process knowledge (knowledge of product technology and the processes that a test specimen has been through) is necessary to define and interpret defects more closely. It is perhaps safer, without that knowledge, to categorise indications by their:   

Size. Shape. Orientation.

BS EN ISO 9934-1 classifies indications as either linear or spherical based upon the ratio between their length and width.

1.8.3

Linear indications:

Length > 3 times width

Spherical indications:

Length < or equal to 3 times width

Non-relevant indications Non-relevant indications are true magnetic particle patterns formed and held in place by leakage fields. However, they are caused by design features and the structure of the specimen and only in exceptional cases will they affect the fitness-for-purpose of the specimen. Below is a non-exhaustive list:          

Tool marks. Scores and scratches. Key ways. Internal splines and drillings. Abrupt changes of section/geometry. Fine threads. Dissimilar magnetic material (HAZ or heat treated material). Forging flow lines. Grain boundaries. Cold working.

Figure 1.11 Flux leakage due to geometry.


1-13


1.8.4

Spurious Indications Indications that are not held on the surface by a flux leakage are termed spurious. Lint, scale, dirt, hairs, drainage lines are examples. However, there is one spurious indication called magnetic writing that is a little different. If two pieces of steel touch when one of them is in a magnetised condition local poles are created at the areas of contact. If magnetic particles are then sprinkled on the surface the local poles become visible as fuzzy lines.

1.8.5

Longitudinal field It has already been stated that magnetic flux lines must run in a direction so they can be interrupted at a defect causing a flux leakage. So to detect defects, the flux lines should ideally be at 90 to the direction of potential defects. In the figure overleaf the magnetic lines of force are longitudinal in a bar and thus the bar has magnetic poles. Transverse flaws will easily show; but longitudinal defects such as seams, which are very straight, will not show. However, it is accepted that flaws up to 30  from the flux lines will also be shown. In fact, longitudinal flaws having a transverse component, such as jagged cracks, will almost certainly show.

Figure 1.12 Longitudinal field.

1.8.6

Circular field The longitudinal magnetising field in the bar is now replaced by a longitudinal current, which creates a magnetic field at 90  to itself. In fact, the current has produced a circular non-polar field around the bar. Under normal circumstances the circular field is not detected due to it having no external poles, but a longitudinal surface flaw at 90  creates a flux leakage, creating miniature poles and is thus detectable with magnetic particles. The figure below shows the effect of flaw orientation in a circularly magnetised bar.


1-14


Figure 1.13 Circular magnetism.


1-15


Section 2 Methods of Magnetisation

2

Methods of Magnetisation The equipment used for MPI can be divided according to size and purpose. The magnetising force may be supplied by anything from a small permanent magnet to a highly sophisticated fixed installation, utilising high values of rectified current and finely calibrated meters. When electricity is introduced into a specimen in order to magnetise, it is usually transformed into a low voltage, high amperage supply. Therefore there is no danger from electrocution; however, specimens do get hot due to electrical resistance if the supply is applied for more than a couple of seconds. Magnetising equipment must meet the requirements of and be used in accordance with BS EN ISO 9934 Part 3. Equipment falls into two categories: portable and fixed.

2.1

Portable equipment

2.1.1

Permanent magnets Permanent magnets produce a longitudinal magnetic field between the poles. Modern variants of the horseshoe magnet have adjustable arms and may have variable geometry removable pole ends. Optimum defect detectability is at 90 to the poles. Permanent magnets do not usually achieve the flux levels required by BS EN ISO 9934 Part 1 and as such should only be used by agreement with the customer. 

N

S

Figure 2.1 Permanent horse shoe magnets.

2.1.2

Advantages No power supply needed Cling to vertical surfaces No electrical contact problems Inexpensive No damage to test piece

Disadvantages Direct field only Deteriorate with wear Have to be pulled from test surface No control over field strength Magnetic particles attracted to poles

Lightweight

Legs (poles) must have area contact May have to be recharged

Electromagnets Electromagnets are made from soft iron laminates to reduce eddy current losses, if powered by alternating current (AC). The yoke laminates are encased in a multi-turn coil usually powered by mains electricity. The legs of modern equipment are normally articulated to allow area contact on uneven surfaces.

NDT30M-60615 Methods of Magnetism

2-1


Figure 2.2 Electromagnet/yoke with adjustable pole pieces (legs).

Electromagnets produce a longitudinal field with the test area being a circle inscribed by the poles. Defect orientation is the same as when using a permanent magnet. Rectified AC current or DC current from a battery may be used. DC is not favoured as a magnetising method as it is not considered to achieve the flux levels required by BS EN ISO 9934 Part 1 within the specimen. The lift test should confirm that the electromagnet can lift 4.5kg at their recommended pole spacing (usually about 300mm). Advantages AC, rectified or DC operation Controllable magnetic field strength Run direct from mains electricity supply Can be switched on and off allowing easy removal No harm to test piece Lightweight Can be used to demagnetise on AC 2.1.3

Disadvantages Needs power supply Longitudinal field only Carry mains voltage Poles attract magnetic particles Legs (poles) must have contact

Prods Prods induce a circular magnetic field by sending a high amperage (typically 1000A) current through the test piece. The high amperage can cause arcing between the electrodes and test surface. Contact points must be carefully cleaned, and electrode materials chosen to prevent contamination of the test piece. They produce a distorted circular magnetic field with defects showing at a maximum when orientated along a line between the prod tips.


2-2


Figure 2.3 Current flow prod technique.

Advantages

Disadvantages Danger of arcing Danger of overheating Heavy transformer required

Variable field strength AC or DC fields Useful in confined spaces Low voltages

Possible to switch on without creating field Possible contamination of the test piece by the electrode Must have good electrical contact

No poles to attract particles Control of amperage

AAs with all current flow inspection techniques, particular care has to be taken to avoid surface damage of the component under test due to burning or contamination. Any areas where arcing or excessive heating have occurred are considered to be defects and if they require further inspection it will be by a different technique to that first used (not the prods again). 2.1.4

Flexible coil In this technique the current-carrying cable is wound tightly around the component. It is a longitudinal magnetisation method and will find defects lying parallel to the cable. The area to be tested is considered to lie between the turns of the coil.

Figure 2.4 Typical use of a flexible coil.


2-3


Split coils with quick release fasteners are commercially available to allow coils to be fixed and removed more quickly. Advantages Simple to operate No danger of burning AC and rectified current

Disadvantages Difficult to keep turns apart Limited inspection cover High current capacity sometimes

Magnetising force is the product of amps needed multiplied by turns Current is adjustable 2.1.5

Adjacent conductor Working from the basic principle that a current must create a magnetic field around the conductor, the flexible cable is a useful means of testing welded constructions, large castings and forgings. The technique requires one or more insulated cables to be laid parallel to the surface of the component, adjacent to the area to be tested and supported a distance d above it as shown below. The width of the test area is considered to be 2d and the return cable for the electrical current must be arranged so as to be greater than 10d from the testing zone.

d d d

Figure 2.5 Adjacent conductor technique.

When used as a single or a multi-turn threading cable, the conductor is passed through openings of interest and defects will be found radially around the hole or longitudinally in the bore. When used on a pipe, defects will be found parallel to the cable internally and externally, as well as radially on the ends. The parallel closed loop is a novel variation, which has found some favour in underwater inspection and the gas industry:   

Cables or conductors are kept apart by insulators. Direction of current in each cable must be complementary, not opposing. Defects will be found within the grid, parallel to the conductors.


2-4


Advantages Simple application Variable field strength Can cover large areas 2.1.6

Disadvantages May require long cables High current draw sometimes Difficult to keep cable in position

Clamps and leeches Where prods are not suitable because heat damage may be caused or the item is too large and awkward, it is often still possible to pass a current into a specimen. Special crocodile clips with copper woven braiding on them are one alternative. Another possibility is to use permanent magnets as leeches to clamp on the job so that the operator's hands are free to apply the ink or powder. The current is passed through the leeches and does not affect the permanent magnetism.

2.1.7

Mobile equipment As the name implies, mobile equipment is too bulky and heavy to carry and yet needs to be moved to the work. Some mobile units are capable of supplying output currents up to 20,000A, although 5000A is more normal. The current required to test a job may be quite low but losses due to cable length or bulk of specimen may mean that a portable set cannot produce enough. Sections 2.1.3-2.1.6 are relevant to mobile units as well as portables. In addition to the normal features on a portable unit, the mobile is likely to have better current control and a step control to allow demagnetising.

2.2

Fixed equipment

2.2.1

Bench units Bench units are fixed installations used to test large numbers of manufactured specimens. They range in size and output from those able to test small components at no more than a few amperes to large cranks and gun barrels capable of 10-20,000A. Among the features normally found on bench units are:     



    

Adjustable head and tailstock on a fixed bed. Agitated ink trough or reservoir. Recirculating ink supply from reservoir to spray gun. AC and rectified current facilities. Large area copper gauze covered electrodes on head and tailstock to allow current flow, for circular magnetisation. Magnetic flow solenoids on head and tailstocks and/or rigid coil on the bed, for longitudinal magnetisation. Calibrated meters. Controls to vary magnetising force values. Foot and hand switches to operate controls. UV-A (black light) and white lights (optional). Timers to adjust operating duty cycle (optional).


2-5


2.2.2

Magnetic flow Energised solenoids in the bench heads create a longitudinal magnetic field in a component, which is clamped between the heads, completing the magnetic circle. Defects, where the major axes lie transverse to a line joining the heads, are found best and the method is most applicable for are short simple shapes. The solenoids on bench equipment are energised by Full-wave rectified current.

Detection of transverse defects or discontinuity in component

Figure 2.6 Magnetic flow bench unit with test component inserter between head stocks.

Where there are large differences between the size of the bench heads and the ends of the component, shaped extenders may have to be used to ensure that the flux is smoothed into the ends of the component. If this is not done, clumping of magnetic particles on the component will prevent defect detection. 2.2.3

Axial current flow The component is fixed firmly between contact heads having soft conductive surfaces providing good electrical contact, such as copper braiding. A low voltage, high amperage current is passed through the component creating a circular magnetic field around it. The method favours detection of defects lying in line with the contact heads and not more than 60 from the ideal. The strength of the current used determined by the peripheral dimensions of the component to be inspected. 

Possible hazards include excessive heating, burning and arcing which can cause metallurgical damage to the component.

Detection of axial defects or discontinuity in component.

Figure 2.7 Current flow bench unit.


2-6


2.2.4

Threading bar Magnetisation by the threading bar technique is induced by passing current through an insulated non-magnetic conductor (aluminium, copper or brass are usual) which is placed in a bore or aperture in the component. Hollow components such as tubes and rings, are normally tested by the threading bar technique. In practice a number of small parts, such as rings, can be tested at the same time, providing they are not allowed to touch each other.

Figure 2.8 Threading bar bench unit.

The threading bar technique induces circular magnetisation and defects in the same direction as the current will be found, externally, internally, and on end faces. Defects deviating up to 60 from the ideal will also be found. 

2.2.5

Rigid coil The component is placed in a current-carrying rigid coil with its longitudinal axis at 90 to the direction of the windings on the coil. Four to eight turn coils are usual and the specimen is placed in the bottom of the coil wherever possible. 

Longitudinal magnetism is induced into the component, so the method basically favours transverse defects. Long and slender components are best tested in coils, although a long component may have to be re-tested along its length. 300mm is about the longest that one can expect to inspect at any one time. Pole extenders should be used on short components, having a length/diameter ratio less than 5:1.


2-7


Transverse discontinuity

Figure 2.9 Different rigid coil arrangements.

2.2.6

Induced current The induced current technique is not normally a feature of standard bench equipment but is applicable to particular components, such as high quality finish bearing races, where arcing would ruin the part. The technique induces a circumferential current flow in a ring specimen by making it the secondary winding of a transformer. Therefore only alternating current may be used and only surface defects are revealed.


2-8


Figure 2.10 Induced current into test object.

It is a novel but extremely useful technique, as it eliminates the possibility of overheating the component under test. There are many variations, as often the technique has to be tailored to suit a specific component's inspection need.


2-9


Section 3 Detecting Media, UV Light and Other Equipment

3

Detecting Media, UV Light and Other Equipment The magnetic particle test method may be classified as:  



3.1

Wet or dry (based upon the detecting media used). Continuous or residual (according to when the detecting media is applied with relation to the magnetising force). Visible or fluorescent (according to the nature of the viewing conditions).

Inks and powders BS EN ISO 9934 Part 2 defines mandatory and recommended tests that are to be carried out before or periodically during an inspection including a sensitivity check using a suitable reference test piece. Powders comprise finely ground ferromagnetic particles, often iron, coated or heated to a temperature which will give a distinctive colour. BS EN ISO 9934 Part 2 specifies that 90% of the particles in a sample of dry powder are larger than 40 µm. There is no specified upper limit, but it is typically 200µm. Ideally the particle shape should be elongated. However, to allow dry powders to flow from the dispenser, a mixture of rod shaped particles and globular ones is used. Typical colours for powders are:    

Black. Red. Grey. Yellow.

Dry powders are dispersed on to the test component either through a puffer or a dry spray can. The chosen colour is the one that gives the best contrast against the specimen background. Powders are usually applicable to site work such as welds and castings, often as an initial check on a weld root pass, where wet materials would cause contamination. Generally they can be used for testing hot components up to 300C but fluorescent powders may lose their brightness if heated, so should be used at ambient temperature. It is advisable that manufacturers' recommendations shall be followed. Invariably powders are treated as disposable and should not be re-used, due to the danger of contamination by dirt and moisture. To summarise:      

  

Iron powder or magnetic iron oxide (magnetite). 40-200 µm (typically, although not specified in BS 9934-2). Rounded and elongated shapes. Colours vary for contrast against component. Can be used on hot surfaces. Poor particle mobility, HWDC best, DC or permanent magnets must never be used. Greater operator skill required. Difficult to apply to overhead surfaces especially in field conditions. Generally less sensitive to fine surface discontinuities than wet particles.

NDT30M-60615 Detecting Media, UV Light and Other Equipment

3-1


Magnetic inks consist of finely divided coloured fluorescent particles in a suitable carrier fluid that forms a uniform suspension when agitated. The carrier fluid is usually either kerosene or water with corrosion inhibitors added in the latter case. Inks can be supplied as concentrates, or ready for use. Water-based inks are becoming more popular because of:   

Price. Odour reduction. Health and safety implications.

The ink is comprised of finely ground oxides of iron, having high permeability and low retentivity and BS EN ISO 9934 Part 2 specifies that the particle size shall be within the range of 1.5-40 µm. Inks are usually sprayed, flooded or ladled on to the specimen. Kerosenebased materials are also sold in aerosols. Water-based inks, when used on site, are often sprayed from garden dispensers and when used in bench machines the tanks should be stainless steel. Although a wetting agent and corrosion inhibitor is added to the concentrate, the effects of contamination of the ink by corrosion products cannot be ignored. Water-based inks are sold as a concentrate and then mixed. To enhance the contrast a white strippable contrast paint may be sprayed or painted on the specimen. If this is applied lightly, not more than 50 µm thickness, however sensitivity will be reduced. Kerosene-based inks are supplied in bulk but to maintain the solid content at the correct level a small amount of concentrate is added at intervals. It is not recommended that magnetic inks are made up with normal kerosene, especially fluorescent inks since: 

 

The fire risk is greater. The previous standard, BS 4069, stated a minimum flash point of 65C, BS EN ISO 9934-2 states that the flash point of the carrier fluid shall be measured by the open cup method and reported. There will be a higher odour level. Almost certainly there will be high background fluorescence under UV/A light.

Of paramount importance is the maintenance of the ink strength. The solid content must be constantly monitored as detailed in control checks and the ink must be constantly agitated to keep the solid content in suspension. Although not stated in current standards, previous standards stated that the solid content levels should be between 0.1-0.3% for fluorescent ink and 1.25-3.5% for visible (black) ink. Ink concentration is determined using a Sutherland Flask.


3-2


Figure 3.1 Settling flask to measure solids content in MPI inks.

BS EN ISO 9934-2 calls for in-service testing utilising a reference block and for the manufacturer to specify a maximum recommended particle content in grams per litre.      

Magnetic iron oxide (magnetite) or iron powder. 80% of particle to be between 1.5-40 µm. Rounded and elongated shapes. Colour contrast or fluorescent. Water or kerosene-based. Good particle mobility.

BS EN ISO 9934-2 specifies a number of requirements for magnetic detection media, these include:             

3.2

Particle colour. Viscosity of carrier fluid. Particle size. Mechanical stability. Temperature resistance. Foaming. Fluorescent coefficient and stability. pH (acidity or alkalinity). Fluorescence of carrier fluid. Storage ability. Corrosion properties. Solid content. Sulphur and halogen content.

Visible or fluorescent Fluorescence is the property of some materials to absorb electromagnetic energy of one wavelength and re-emit the energy at another. The ultraviolet and visible light section of the spectrum, which is of interest in MPI, lies between 100 and 800nm. (A nanometre (nm) is 1 millionth of a mm.) In MPI long wavelength ultraviolet (black light) light sources are used having a waveband between 315-400nm. This is UV-A radiation. Fluorescent inks absorb energy at approximately 365nm and re-emit at about 550nm.


3-3


Industrial radiography Microwaves

Ultraviolet

10-10 10-8

10-6

Infrared

Electric waves

TV

10-4 10-2 1cm 102 104 106 Wavelength

108

Figure 3.2 Electromagnetic spectrum.

3.2.1

Types of UV-A lamp By far the most common type of light source used to inspect components tested with fluorescent ink is the mercury vapour arc lamp. In fact, the mercury arc lamp is a street or workshop lamp which has a filter over it to reduce the visible light to a minimum but allow the UV-A to be transmitted. The filter is called a Woods in the UK and a Kopps in the US.

Figure 3.3 Mercury vapour arc lamp.

The mercury arc is drawn between electrodes enclosed in a quartz tube. The resistor limits the amount of current in the starting electrode. The quartz tube is mounted and enclosed in the outer glass envelope which serves to protect it and filter out any possible hazardous radiations.


3-4


NOTE: A MERCURY VAPOUR BULB EMITS UV(A), UV(B) & UV(C) !!!

What the different UV light can cause A – Ageing B – Burns C – Cancer

Figure 3.4 Electromagnetic spectrum (UV through to IR).

400W mercury vapour arc flood lamps can be used where very large components are tested or to give background illumination in an inspection area. However, UV strip lights can provide background light more economically in a darkened area. 3.2.2

Safety precautions and operating instructions Under normal working conditions, there are no known long term harmful effects arising from the use of UV-A (black light) sources, providing simple safety precautions and operating instructions are observed. The precautions and instructions in these notes are general. For full advice the manufacturers' data on a particular light should be followed. Safety precautions when using a UV-A me rcury vapour arc lamp:    

  

Avoid looking directly at the light source. The light must not be used without a correctly fitted filter. Do not operate the light with a chipped or cracked filter. Some people may experience temporary health problems such as eyeball fluorescence. (The human eye contains a jelly which begins to fluoresce if exposed for long periods to UVA light. It causes clouded vision but the effect is temporary.) Avoid contact with the lamp housing as it becomes hot. Keep the light cables away from liquids, to avoid contamination or shorting. Ensure that regular electrical earth continuity checks are carried out on the lamp unit.

When working with UVA light for prolonged periods, sodium goggles can protect the eyes. They block UV light while allowing visible light to pass.


3-5


Figure 3.5 Sodium safety googles.

3.2.3

Operating instructions for a UV-A lamp 











Allow 10 minutes warm up period after switch on to allow the light to reach full intensity before inspecting with th e lamp. If the lamp is switched off and then immediately switched on again, allow a minimum of 10 minutes before recommencing inspection. The bulb will not relight until its temperature reduces. Avoid repeated switching on and off, as this will reduce bulb life significantly. Angle the light with respect to the specimen being inspected, to avoid reflections which reduce inspection efficiency. Clean the lamp filter regularly, with lint-free material moistened with a mild detergent/water solution. Check the light output of the lamp regularly. This should be done in accordance with BS EN ISO 3059. Non-destructive testing. Penetrant testing and magnetic particle testing – viewing conditions. The lamp must achieve a UV-A irradiance level of 10W/cm2 (1000 W/cm2) at the testing surface, using a radiometer (irradiance meter) wh ich must be in calibration.

Radiometer

Combined meter UV/White light

Figure 3.6 Monitor for UV light.

3.2.4

Field indicators These are devices used to check residual magnetism prior to and after Magnetic Particle Testing they are also known as gaussmeters or magnetometers with the SI measurement being made in the unit, Tesla. These are small mechanical devices that utilise a small soft iron vane that is deflected by a magnetic field. The vane is attached to a needle which rotates and moves a pointer set on a graduated scale. Field indicators can be adjusted and calibrated, so that quantitative information can be obtained.


3-6


Generally the greater the deflection of the needle the greater the residual magnetic force. The limited range of the Field Indicator shown in Figure 3.7 means that they are best suited for the detection and measurement of any residual magnetic field that may be present after demagnetisation. However the measurement range of a field indicator is usually small due to the mechanics of the device. Gauss meters are also called tesla meters (SI units). The minimum flux density in the component surface shall be 1 tesla (1 T). This flux density is achieved in low alloy and low carbon steels with high relative permeability with a tangential field strength of 2kA/m (2A/mm). For other steels, with lower permeability, a higher tangential field strength may be necessary. If magnetisation is too high, spurious background indications may appear, which could mask relevant indications.

Figure 3.7 Magnetic field indicators.

3.2.5

Flux density meters/gauss meter with hall effect probe These are normally battery powered instruments with analogue or digital direct reading dials, normally graduated in gauss. They determine whether a magnetic field is of adequate strength as well as showing its direction. Knowing the direction of a magnetic field is a fundamental requirement of Magnetic Particle Inspection, because the field should be as close to the perpendicular position of a defect indication as possible and no more than 45 degrees from the normal. The ability to show field strength and direction is especially important when carrying out Magnetic Particle Inspections when using a multi-directional testing machine, because when the fields are not balanced properly, a vector field will be produced that may not, detect some defects. A gauss/tesla Meter with a Hall Effect Probe is commonly used to measure the tangential field strength on the surface of the part being tested when a magnetising force is applied. The Hall Effect is the transverse electric field created in a conductor when placed in a magnetic field. The tesla is the SI unit for the measurement of magnetic field strength or magnetic flux density. 1 tesla is equal to 10,000 gauss. Advantages of the Hall Effect instruments are that they provide a quantitive measure of the tangential magnetising force at the surface of the item under inspection as well as being used for the measurement of residual magnetic fields and the instrument can be used repetitively.


3-7


The main disadvantages of Hall Effect instruments are that they require periodical calibration, cannot be used to establish the balance of fields in multidirectional applications and the Hall Effect probes can be very fragile and easily damaged and are very costly to replace.

Figure 3.8 Field strength meter (with hall effect sensor).


3-8


Section 4 Application Techniques and Demagnetisation

4

Application Techniques and Demagnetisation Before applying MPI a thorough knowledge of the component to be tested is essential in terms of its material composition and properties, surface condition and preparation, types of defects/discontinuities, manufacturing method (ie wrought, cast, weld) and in-service conditions. These important issues are covered in detail in the attached TWI 'Product Technology Training Course Notes'.

4.1

Continuous technique The continuous technique implies that the detecting media is applied before the magnetising force, to a component and continued during the period of magnetisation. However, ink or powder application should be stopped before magnetisation is stopped. Indeed, on low retentivity components it is important to inspect at the same time as magnetisation and ink application. A classic case of reporting a spurious indication as a defect is where ink is allowed to run down the toe of a weld after a test. The solid content forms a visible line, exactly conforming to the shape of the toe and this line is often enhanced by the residual magnetism of the heat affected zone. What is worse is that the inspector notes the indication as spurious but fails to see small toe defects that are now masked by that spurious indication. To avoid overheating the component and the equipment, magnetisation times should be limited to 2-3 seconds. In fact some equipment have shot timers on them to avoid the duty cycle being exceeded. The table below lists the steps in a one shot continuous technique. It should be pointed out that if full cover of a component is envisaged, a number of shots would be required. 1

Demagnetise if specified

2

Clean

3

Apply contrast paint if specified

4

Affix magnetising contacts

5

Apply detecting media

6

Apply magnetising force, 2-3sec duration

7

Stop detecting media

8

Stop magnetising force

9

Inspect - this should start at operation 5 and end at 8

10

Demagnetise, if specified

11

Clean

12

Protect

13

Report

NDT30M-60615 Application Techniques and Demagnetism

4-1


If demagnetisation is called for, circular magnetising tests are done first followed by longitudinal. This is so because it is probable that a residual circular field is not detectable but that residual field will be removed by longitudinal test applied later. Therefore, the final residual field to be removed is a longitudinal one, which is detectable with a field indicator. All current waveforms are applicable to continuous techniques, depending on the defect morphology. 4.2

Residual technique The residual technique (also known as the remanence technique) uses only DC or rectified forms of AC to magnetise a component because it is the residual flux density which is relied upon to attract magnetic particles to the flux leakage created by defects. Direct current and rectified AC produces a full cross-sectional magnetisation, whereas AC will only create an effective flux density in the skin, hence skin effect. Direct current (DC) and rectified AC provide a deeper level penetration of the magnetic field dependant on the material properties and magnetic strength. Thus, the residual field from AC is not considered adequate for the residual technique. Also, the residual technique is only applicable on components, which have high retentivity ie high carbon equivalent steels. It usually follows that components suitable for the residual technique are high tensile machine parts, when the types of flaws being sought are; corners, or thread roots etc. If the continuous technique is used on these parts there will be a high build up of detecting media across such features and these non-relevant indications are likely to mask an actual defect beneath them. For best defect sensitivity the detecting media is applied after magnetisation and to allow time for the particles to migrate. Inspection takes place a short time after that. The table lists the steps in a one shot residual technique. The magnetising values should be the higher ones recommended for aerospace, using the appropriate electrical current waveform. Again, circular magnetism shots should be carried out before longitudinal, as invariably demagnetisation will be necessary. 1

Demagnetise

2

Clean

3

Apply contrast paint if specified

3

Affix magnetising contacts

4

Apply magnetising force, not AC, 2-3sec

5

Apply detecting media, spray or dip

6

Wait, 30sec - 1 min

7

Inspect

8

Demagnetise

9

Clean

10

Protect

11

Report


4-2


4.3

Demagnetisation BS EN ISO 9934-1 recommends that demagnetisation should be carried out if specifically requested at the time of enquiry of order. In certain industries the consequences of not demagnetising can be catastrophic. Demagnetisation can be carried out:  

Before testing, if residual fields could affect test results. Between tests except for when a similar shot is to be applied but at a higher amperage. An exception can be made if a subsequent shot is to be applied at 90 to the original and the original field strength is to be exceeded. After testing, when applicable. 



Post demagnetising must be done: 









On aircraft parts, where magnetic compasses and electronic equipment may be affected. On rotating parts, where magnetic debris might adhere and cause excess wear. Where automatic arc or electron beam welding is to be carried out and arc wander may be caused by residual magnetic fields. If residual magnetic fields could affect subsequent machining processes. Reamers and taps become magnetic as well and thus can break in use, if swarf is not cleared from flutes. When a high quality finish, such as electroplating is to be applied. The particles attracted will prevent or reduce adhesion.

It is not usually necessary to demagnetise specimens that are to be heat treated, provided that the heat treatment is beyond the Curie point, about 700 C. At and above the Curie point, ferromagnetic materials become paramagnetic. 

Often it is not possible or practical to demagnetise a specimen completely, especially when it has been magnetised with using DC. Therefore a maximum residual field level must be agreed. An agreed maximum deflection on a magnetic field indicator is the most common method to ensure proper demagnetisation.

Figure 4.1 Magnetic field indicator.

For critical situations a compass test is recommended. The component under test is positioned at an agreed distance from a suitable compass and rotated through 360 . The compass needle must deflect by less than 1 . (No longer in BS EN 9934-1, detailed in previous standard BS 6072). 




4-3


4.4

Principle of demagnetisation Looking at a typical hysteresis loop for a ferromagnetic material, after the initial magnetising force is applied and then removed, it is virtually impossible to end the test with a zero flux density. Even if a negative coercive force is applied it will only keep the flux density at zero, as long as it continues to be applied. The figure shows that the key to demagnetisation is that a reversing and reducing magnetising force must be applied, so that the hysteresis loop reduces until all the parameters achieve zero. There are a number of ways to achieve this. Magnetic flux density (B)

Field strength (H)

Figure 4.2 Demagnetisation process.

4.5

Methods of demagnetisation

4.5.1

Aperture coil, removal The component is passed through an aperture type coil, which has its major axis aligned in an east-west direction and is carrying AC. The component is removed from the coil to a minimum distance of 1.5m before the current is switched off. Special demagnetisers of this type are usually multi-turn coil, working directly from a single phase AC supply. However, a hand held coil made from a portable unit cable may be adequate for site use. If the component cannot be passed through the demagnetising coil there is no reason why the coil should not be passed over the component to achieve the same result. When using AC to demagnetise by any of the techniques listed the initial field strength should be equal to or greater than that used for magnetisation.


4-4


4.5.2

Aperture coil, reducing AC Where it is not possible to remove either the component or the coil from the influence of each other, then the AC can be reduced to zero to achieve the same demagnetising effect. Modern units use a capacitor discharging to achieve an almost instantaneous result.

4.5.3

Aperture coil, reversing DC Sometimes if a component has been magnetised using DC or rectified AC, it is nearly impossible to reduce the residual flux d ensity to a satisfactory level using AC. This is especially true if the component is a complex shape. Therefore, a reversing and reducing DC, or more usually full-wave rectified and smoothed AC, is used. The component is usually left in the coil but with long components the operation is carried out several times along its length. Each reduction of current should be 50% of the preceding one, down to a reasonable minimum.

4.5.4

Electromagnet, reversing DC The same principles apply as with the reversing DC aperture coil method, but in this case the component is clamped between the poles of an electromagnet in a field strong enough to saturate it magnetically. The field is then reduced and reversed in 50% increments to near zero.

4.5.5

Electromagnet, AC yoke A most useful way to remove local residual fields on components in situ, on a structure than cannot easily be moved or removed is by means of a portable AC powered electromagnet. The energised yoke is pulled over and off the component, to a distance of about 450mm and then switched off. If the level achieved is not adequate, the operation is repeated in the same way and direction until the residual field is removed. Approximately 450mm then switch off

Figure 4.3. Demagnetisation using an AC yoke.


4-5


Section 5 Current Waveforms

5

Current Waveforms It has been explained that different current wave terms are used in MPI, but not why. Alternating current is simple to transform, and employ when taken from the electrical mains. Because the polarity is changing fifty times a second the magnetic particles are constantly reversing their direction and this causes them to migrate or walk to areas of flux leakage. This is excellent because it gives bright clear indications. However, because of the skin effect phenomenon the magnetism is concentrated near to the surface of a component. Therefore only surface defects can be found using alternating current. If it is considered that sub-surface defects are critical and it is believed that they are likely to be orientated in a way that makes detection possible, then DC or rectified current must be used. HWR (half-wave rectified) circuits will give full depth magnetic penetration with a pulse effect to help the particles migrate. Complicated machined specimens with fine threads or key ways might be difficult to interpret due to flux leakage across changes of section. It is often possible to use the residual magnetism to produce fine line indications and reduce the incidence of non-relevant indications. This is called the residual technique and when employed, rectified current or DC must be used. Thus, before selecting a magnetising value and waveform for a job, the type, orientation and depth of likely defects must be deduced. It is the value of peak current that creates the maximum magnetising force and therefore the most drive to the magnetic particles to migrate to a flux leakage. However, few ammeters are calibrated in peak values. In fact they read some other quantity such as root mean square (RMS), mean or average. For timevarying current forms such as alternating or half wave rectified the RMS value, not the peak, is the required quantity. The table below shows the relationship between the peak current and the RMS, value for the various waveforms.

NDT30M-60615 Current Waveforms

5-1


Table 5.1 BS EN 9934-1:2001.

A meter can read the apparently simple ampere in many different ways and it is necessary to be aware of this. It is intended to look at the more common current waveforms and usual ways of reading their outputs. In view of the many ammeter variations, the safest thing for operators to do is to check with the equipment manufacturer as to what type of ammeter is fitted, then print the peak to actual readout ratio on the meter scale. BS EN 9934-1 stipulates that where time varying currents are used, then RMS is to be used.


5-2


5.1

Direct current (DC) An electrical current flowing in one direction only and effectively free from pulsation. Therefore, after a small build-up period the current is at a constant peak value and this is what the meter reads.

Figure 5.1 Direct current.

Direct current is either supplied from a battery pack or a DC generator. In the early days of MPI, DC was almost universally used. This is not so today. Advantages Sub-surface defects Availability from batteries

Disadvantages No agitation Less sensitive to surface defects

h t g n e r t s d l e i F

Distance Flux Leakage

Figure 5.2 Magnetic flux distribution for DC.


5-3


5.2

Alternating current (AC) Alternating current is a form of electricity which, after reaching a maximum value in one direction, decreases, reverses direction and reaches a maximum in the opposite direction before returning to zero. It is cyclic and the cycle is repeated continuously. Peak current Root Mean Square (RMS)

0

Figure 5.3 Alternating current.

It is, of course, the peak current which creates the maximum magnetising force, but in reality the meter reads the RMS value as the current is reversing between equal but opposite peak values. It is therefore impossible to measure the mean value. By plotting the squares of the current values we can find an average, since negative as well as positive values become positive. To measure the square of the current we use a moving iron ammeter. This type of ammeter consists of two iron rods which are forced apart as they are magnetised. Their level of magnetisation is proportional to the current and therefore the force between them is roughly proportional to the square of the current. The meter is calibrated to read the root of the mean of the square values and is therefore non-linear. The peak current is given as the meter reading x 1.414. Advantages

Disadvantages

Availability

Will not detect sub-surface defects

Sensitivity to surface defects Agitation of particles Demagnetisation

The phenomenon that causes the magnetisation produced by alternating current to be contained near the surface of a ferromagnetic component is called skin effect. Therefore, if the magnetic field produced by AC only exists at or just under the surface of the component AC will reveal only surface-breaking defects.


5-4


h t g n e r t s d l e i F

Skin Effect

Distance Flux Leakage

Figure 5.4 Magnetic flux distribution for AC.

If sub-surface defects are of interest, rectified or DC current must be used because they produce an even flux density through the cross-section of the component. 5.3

Half-wave rectified (HWR) [or HWRAC] This is a pulsed unidirectional current produced by clipping a half-cycle from single-phase alternating current. As a result there are intervals when no current is flowing. It is the least expensive form of rectification, used on cars and motorcycles. In the UK it is common in portable, mobile and bench units.

Figure 5.5 Half-wave rectified current.


5-5


Advantages Penetration like DC Agitation 5.4

Disadvantages Lower sensitivity to surface defects than AC

Full-wave rectified current (single phase) (FWRAC) This is a form of current where the negative half-wave of an alternating current is converted into a positive wave, so that both halves of the swing are able to deliver unidirectional current. Full-wave rectified equipment is unlikely to be even nominally portable, due to the weight of electrical equipment within them. Bench units using this waveform are most likely to be found where codes from the USA prevail.

Figure 5.6 Full-wave rectified current.

Advantages Penetration like DC Agitation 5.5

Disadvantages Lower sensitivity to surface defects than AC

Converting between RMS and peak values The amperage calculations described in BS-EN-9934-1 Annex A in most cases give results in Amps RMS. While some items of MPI equipment give the RMS reading, others provide the Peak Amp reading. In order to utilise calculated values a conversion is required. From Peak to RMS… I

RMS

=I

PEAK

x 0. 707

From RMS to Peak… I

PEAK

I



RMS

(orI PEAK =I



0.707

RMS

x

1.414 )

Point to remember: The Peak value is always higher than the RMS value.


5-6


Section 6 Assessing Magnetising Force and Amperage

6

Assessing Magnetising Force and Amperage Magnetic particle inspection practice in the UK and Europe is based on research that recommends that a minimum flux density of 1 tesla must be achieved. This flux density is achieved in low alloy and carbon steels, with relatively high permeability, with tangential field strength of 2000 Amps per metre (2kA/m). For steels with lower permeability higher tangential field strength may be necessary. Too high a magnetisation could, however, lead to spurious background indications that could mask out relevant indications.

6.1

Portable equipment

6.1.1

Permanent magnets and DC electromagnets As mentioned earlier according to the latest European standards permanent magnets and DC electromagnets should only be used by agreement with the customer at the time of enquiry and order due to their inability to meet the magnetisation requirements laid down in those standards (eg EN ISO 9934-1). Previous standards laid down criteria for their performance based upon their lifting power specifying that they should be able to lift at least 18kg of ferritic steel with the poles between 75 and 150mm apart.

6.1.2

AC electromagnets The performance of AC powered portable electromagnet can be determined by measuring the tangential field strength produced at the midpoint between the two poles. Periodic functional checks on such equipment may also be carried out by this means or by a lift test. AC electromagnets should be capable of lifting a mass of 4.5kg with the poles at their recommended spacing.

Figure 6.1 Electromagnet.

6.1.3

Prods The current to be used depends upon whether the test zone to be inspected between the prods is considered to be rectangular or circular as shown. To inspect a rectangular test zone I = 2.5 H x d Where:

I = Current in amperas H = Tangential Field Strength (kA ) d = Prod spacing (mm) for ‘d’ upto 200mm

NDT30M-60615 Assessing Magnetising Force and Amperage

6-1


To inspect a circle inscribed between the two prods: I=3Hxd

In the case of a circular test zone this excludes the area within 25mm of each prod and in both cases the formulae are only reliable when the radius of curvature of the inspection surface exceeds half the prod spacing. If a flat area is to be tested then a pattern similar to that in the figure below is used.

CF1 CF1

CF3

CF2

CF2

CF3

Figure 6.2 Prod test pattern.

6.1.4

Flexible coil BS EN ISO 9934-1 specifies: Using direct or rectified current, the RMS value of the current flowing in a cable shall have a minimum value of: I = 3H [T + (Y 2 /4T)] Where:   

I = RMS current. T = wall thickness (in mm) or radius of component if round. Y = the spacing (in mm) between adjacent windings in the coil.


6-2


Using alternating current, the RMS value of the current flowing in the cable shall have a minimum value of: I = 3H[10 + Y2 /40] Where:  

I = rms current. Y = the spacing (in mm) between adjacent windings in the coil.

Figure 6.3 Coil formed by flexible cable.

6.2

Alternative standards The UK system requires the cable windings to be spaced. But in the USA it is accepted that spacing the windings is extremely difficult and thus the formula in ASME V shown below, applies to flexible close turn coils: NI

K 

L 

D

2

Where:     

I = coil current. N = number of turns in the coil or cable wrap. L = part length. D = part diameter. K = 35000.

Note: The maximum L/D ratio for calculations is limited to 15:1. The effective field extends on either side of the coil to a distance approximately equal to its radius. 6.2.1

Adjacent conductor To achieve most efficient magnetisation the cable should be mounted at a distance from the test surface. The width of the effective inspection area on each side of the cable centre-line is then also d and is related to the peak current value (I) in the cable by: I = 4 d H Where I = rms current (amperas) d = distance of cable above surface (mm) H = Tangential Field Strength (k )


6-3


Figure 6.4 Adjacent conductor technique.

When testing cylindrical components or radiused corners the cable can be wrapped around the surface of the component and several turns may be bunched in the form of a close wrapped coil. In this case the inspected area lies within a distance d of the windings where: d = NI / 4  H Where 

NI = ampere turns.

Figure 6.5 Multiturn cable wrap coil.

6.3

Fixed equipment

6.3.1

Magnetic flow Magnetic flow with benches may be achieved either by an electromagnetic yoke or a fixed coil. There are no formulae for calculating the required magnetising force rather the tangential field strength within the item under inspection should be measured at its mid-point.

6.3.2

Axial current flow The formula current flow application is given below. When components having varying cross-section are tested, a single current value can be used if the current values to inspect the largest and smallest sections are within a ratio of 1.5:1. The large diameter governs the value.


6-4


Figure 6.6 Axial current flow for varying cross-section.

If the cross-section variation is greater than 1.5:1 then each section is tested in turn, starting with the smallest. I = H x perimeter Where:   

I H p

= Current in amperes. = Tangential field strength in kilo-amperes per metre. = Perimeter in millimetres.

Note: For low alloy carbon steels the tangential field strength is taken as 2 kiloamperes/metre. 6.3.3

Threading bar When the threading bar is placed centrally the current is calculated by the formula below. I = H x perimeter Where:   

I H p

= Current in amperes. = Tangential field strength in kilo-amperes per metre. = Perimeter in millimetres.

If the component to be tested is a hollow pipe or ring the current is calculated according to the outside surface when inspecting the outer surface and the inside diameter when testing the inner surface. Alternatively, and when the threading bar is offset from the centre, the surface under test shall lie totally within a circle centred on the threading bar. When the conductor is non-central the field strength will be verified by measurement. When large rings, etc have to be tested a number of shots, equi-distant around the circumference may be necessary.


6-5


Test 1 Test 2

Test 5 Test 3 Test 4

Figure 6.7 Threading bar test coverage.

6.3.4

Rigid coil The formulae given by BS EN ISO 9934-1 is: NI

0. 4 HK 

L

or I

D

O. 4 HK 

L

xN

D

Where:        

I is the current value. H is the tangential field strength. N is the number of coil turns. L is the test piece length. D is the test piece diameter. K = 22000 for an AC source (RMS value). K = 32000 for a full-wave rectified current [FWRAC] (mean value). K = 11000 for half-wave rectified current [HWRAC] (mean value).

Note: For FWRAC and HWRAC, the answer to calculation is in given Mean Amps. To use the formula, the following conditions apply: 

 



 



The cross-sectional area ( πr2) of the test piece must be less than 10% of the cross-sectional area ( πr2) of the coil aperture. The test piece should lie against the side or bottom of the coil. L/D ratio of the part must be greater than 5:1 if not pole extenders can be clamped to the ends of the test piece. (see diagram). If the L/D ratio exceeds 20, then the ampere turn value for a 20:1 ratio should be used. The test should be repeated at coil length intervals. The major axis of the test piece should be parallel with the axis of the coil. When using rigid coils of helical form the pitch of the helix shall be less than 25% of the coil diameter. BS EN ISO 9934-1 implies that only the section in the coil is tested and the test must be repeated at coil length intervals. In US instructions the test area extends 6” beyond the coil on each side.


6-6


Figure 6.8 Using pole extenders.

6.3.5

Induced current A clamp meter is required to find out the value of current induced into a component. If one is available then the current values used for current flow apply. However, if the correct type of ammeter is not to hand, a flux indicator is the alternative. The required current is given by the formula: I = H x perimeter Where:   

I = Current in amperes. H = Tangential field strength in kilo amperes per metre. p = Perimeter in millimetres.

Note: For low alloy carbon steels, the transential field strength is taken as 2 kilo amperes/metre. 6.4

Verification of magnetisation BS EN ISO 9934-1 specifies that adequacy of the surface flux density should be established by one of the following methods: 

  

Testing a component containing fine natural or artificial discontinuities in the least favourable locations. Measuring the tangential field strength as close as possible to the surface. Calculating the tangential field strength for current flow methods. Other methods based on established principles.

Field strength meters based on the Hall Effect are the best way of ascertaining adequate field strength at the surface of a test component. However, they are expensive and the probes used tend to be fragile. Portable flux indicators are a common, simple to use alternative, giving a clear visual indication of the direction of the surface field. They provide only a guide to the magnitude and direction of the tangential field strength and as such should not be used to verify the acceptability of the field strength. They are a rough guide to the magnitude of the surface field. (This is only true if the flux indicator abuts intimately with the test specimen.)


6-7


Flux indicators consist of a magnetic material that is interrupted by nonmagnetic spacers. When the flux indicator is placed on the surface of a magnetised specimen, flux is induced in it. The non-magnetic spacers behave as artificial flaws. If the magnetic field at the surface of the specimen is sufficiently high, leakage flux above the artificial flaws can be detected by the application of a magnetic particle ink or powder. Flux indicators are made with high permeability magnetic materials with low coercivity and low remanence so that a flux can be easily induced into them, yet without permanently magnetising them. Opinion differs on their efficacy when used with permanent magnets and DC electromagnets. In every case when a permanent magnet or electromagnet is used, good area contact of the poles is imperative or the flux indicator is useless. Results may be misleading when indicators are used in a coil. Flux indicators may be divided into two main types:  

6.4.1

Segment type. Foil type.

Segment type Four or eight identical segments of ferrous metal are joined with non-magnetic compound of even thickness into the shape of a flat disc. One surface of the disc is covered with non-magnetic foil to prevent magnetic particles getting to the surface and giving misleading indications. The eight segment type, with a fixed foil is popular in the US. A four section indicator with an adjustable foil, giving a varying air gap between them is called a Berthold penetrameter. Berthold penetrameter This is a device that has been designed to indicate flux direction and sensitivity (field strength).The central, cylindrical iron piece is cut into quadrants to provide indications at 0 and 90 o. This piece is capped with a thin non-magnetic foil that is mounted on an adjustable screwed spacer, allowing the surface of the penetrameter to be raised off the surface of the item being examined. The penetrameter is mounted on a handle which allows the Inspector to place it on the area under examination. When the penetrameter is placed on a magnetised test surface, magnetic lines of flux will pass through the cut quadrants’ of the iron cylinder. These cut lines will then be visible, when using either wet or dry MPI testing media. Maximum indication direction can be achieved by rotating the penetrameter about its axis so that the cut lines in the penetrameter will be at right angles to the direction of the magnetic field. The sensitivity of the testing media can be determined by slowly turning the outside knurled ring of the penetrameter, increasing the distance off the test surface. The amount of lift off at the point where the indication just appears, gives a measure of the magnetic field test sensitivity.


6-8


Figure 6.9 Berthold Penetrameter.

ASME Pie Gauge (pie field indicator) This is a device that is used as an aid to determine the direction of magnetic fields for the detection of defects in ferrous materials. It is an octagonal shaped piece made with a low retentive steel material which has eight bonded segmented pieces, similar to portions of a pie. The octagonal shaped piece is mounted on a handle so the Inspector can place it on the area being magnetised. With an adequate amount of magnetising current and proper testing media application, the Pie Gauge will show indications in the same direction as defect indications would actually appear. Its principal application is on flat surfaces, such as welds or steel castings where dry powder is used with electromagnetic yokes or prods. Advantages of these gauges are that they can be easily used and if looked after carefully, will have a long working life. Disadvantages of these gauges are that they are not recommended for use on precision parts with complex shapes, for wet method applications, for proving field magnitudes, they have to be de-magnetised after each use because of the retentive steel material used in their construction, they can only be used on relatively flat surfaces and they cannot be reliably used for determination of balanced fields in multi directional magnetisation. These gauges conform to ASME V, ASTM E1444 and NAUSEA 250-1500-1

Figure 6.10 Pie gauge.

6.4.2

Foil type The most common foil type indicator is the Burmah Castrol strip or as it is more correctly now called, a magnetic flux indicator. These indicators consist of a magnetic foil containing linear slots of different widths to simulate discontinuities, sandwiched between non-magnetic foils. Non-magnetic foils are either brass or stainless steel depending on whether they are for general or aerospace use.


6-9


The simulated discontinuities in a magnetic flux indicator are arranged in three parallel lines. These foils are less than 0.2mm thickness and flexible, which gives them a significant advantage over the segment type. They are placed on the test object as it is being inspected, ideally at 90o to the possible defect orientation. The number of linear indications and thickness of the slot indications produced, on the strips, gives the Inspector a general idea of the magnetic field strength in that particular area. They are relatively easily applied to the component and can be successfully used with both wet and dry inspection media, using the continuous method of magnetisation. The results are fairly repeatable as long as the same orientation of the magnetic field is applied and maintained. Disadvantages of these strips are that they cannot be bent to complex shapes and are not suitable for multi-directional field systems since they only indicate defect indications in one direction only. They should not be used with DC fields and permanent magnets as the indicator will become permanently magnetised and give false readings.

Figure 6.11 Foil type magnetic flux indicator.

6.5

Factors affecting MPI sensitivity The sensitivity of the magnetic particle test will depend on several factors, some of which will be within the control of the inspector and others not. The sensitivity Sensitivity of an MPI Test Contrast 1

2

Surface Cleaning

Lighting

Ambient Light

Definition 3

4

Ink Condition

Black Light

Field Strength

5

6

Ink Condition

7 Efficiency of the Magnetic Field

Geometry of the Work Piece

Figure 6.12 Sensitivity of an MPI test.


6-10


The above Figure shows the main factors that will affect the quality of the MPI test performance. Following is an explanation for each of the numbered points from the above figure that affect the sensitivity of a magnetic particle inspection. Surface condition (1) The effectiveness of the cleaning processes to produce a bright finish is an important factor here. A contrast paint background would help for visualisation of magnetic particle indications. Lighting (2) If the ambient lighting is too high, for example because of bright sun light, the tests must be done at night when fluorescent inks are used. The inspector should regularly monitor the intensity. Glare should be avoided. Ink condition (3) Important factors here include the best colour to attract the attention of the inspector and the right size of particle. This is tightly controlled by the appropriate standard, there could be some consideration given to larger size particles that may give better contrast. The physical condition of the ink is also important. It should be a finely divided suspension of particles that is delivered to the work piece and so the inspector should check to see that the agitator is working continuously. This should be a regular part of the routine. Field strength (4) This must be high enough to hold the ink to the surface of the defect. Factors affecting definition The following factors are important when considering definition. Ink condition (5) As can be seen from the chart the ink condition has an effect on both contrast and definition. The definition can be improved if the defect outline is picked out by fine magnetic particles. Geometry of the work piece (6) Test component geometry may be complicated (eg complex castings) and a full understanding of the extent of the individual test(s) is required in order to obtain full diagnostic coverage. Efficiency of the magnetic field conditions (7) The effectiveness with which the inspector can set up the magnetic field condition is the heart of the magnetic particle inspection. Factors like current flow, field direction, electrical contact of the prods, coil fill factor, need to be considered carefully.


6-11


6.6

Assessment and reporting of indications and test procedure. Geometric and physical imperfections in the main product categories of cast, wrought (including rolled and forged) and welds are covered in the Product Technology Course Notes that accompany these Magnetic Particle Inspection (MPI) Testing Notes. The Product Technology Course Notes also covers the influence of manufacturing processes and materials on the types of discontinuities to be found and their assessment. The application codes and standards detail the information required (a) prior to test (b) safety precautions (c) personnel qualification requirements (d) surface condition and preparation requirements (e) MPI detection media – inks and powder(s) properties (f) MPI equipment and magnetic field type to be applied and the required checks and verification of the test parameters (g) MPI technique details including area of test (h) the recording of indications (i) the test report. The Quality Control of documentation must ensure that standards, codes, specifications and Test Technique Procedures and correct and to the latest relevant revision that will enable their full traceability and that the Written Instructions and resultant report are correctly distributed and archived for future reference and traceability. The following standards cover the required product categories: 



 

BS EN 10228-1: Non-destructive testing of steel forgings. Magnetic Particle Inspection. BS EN ISO 17638: Non-destructive testing of welds. Magnetic Particle Testing. BS EN 1369: Founding – Magnetic Particle Testing. BS EN 12062: Non-destructive examination of welds. General rules for metallic materials.

A detailed Test Report will normally be produced for each item of test and will cover all of the salient parameters that affect the quality and integrity of the test as laid out in the Test Procedure (see below) that must be made available to the Test Technician/Operator prior to starting the test along with Written Instructions detailing the components to be tested, the specific Test Procedure, Specifications/Standards and Acceptance criteria to be applied along with any special instructions that might apply (eg PPE to be used, use of photographs etc.). The Test Report will normally include an assessment of the condition of the component against the specified acceptance criteria (see Product Category Standards above).


6-12


Specific details that may be included in the Test Procedure are as follows:  





 











Title of Test Procedure. Description (including sketch/drawing if relevant) of components including materials and surface condition. Scope detailing general requirements of the test (eg type of applied magnetisation – ie continuous techniques - current flow, magnetic flow, residual/remanence technique, detection media – ink (wet), powder (dry) – visual, fluorescent to be used and field strength measurements and checks to be taken). Reference Documents (eg codes, standards, client requirements, personnel qualifications). Definitions and abbreviations used. Responsibilities (personnel involved in the test sequence including identification of test component, carrying out the test and making safe the area of test. Personnel Qualifications (technician undertaking the test, evaluating the results/indications and procedure preparation). Technique procedure, equipment and settings (if applicable), initial cleaning, surface preparation and demagnetisation if required, MPI equipment and detection media to be used. Examination details, diagnostic area and overlap. Application of MPI equipment, temperature limits and viewing conditions to BS EN 3059 – NonDestructive Testing - Penetrant Testing and Magnetic Particle Inspection – Viewing Conditions. Interpretation of results and evaluation of indications against the acceptance criteria (see Product Category Standards/Codes) with a sketch showing the positions of indications if required. Post test requirements including demagnetisation.


6-13


Section 7 Control and Maintenance Checks

7

Control and Maintenance Checks In order to ensure that the equipment ancillaries and materials are up to standard it is necessary to carry out a number of control checks. It is also important to make sure that the system performs consistently each day. Common sense dictates that the equipment, etc is maintained properly. The checks covered in this section are meant to be guides to proper practice. In different organisations there will obviously be variations and therefore the code or standard specified for a particular job must be the overriding factor.

7.1

Detection media BS EN ISO 9934-2 specifies in-service checks for colour and performance are carried out. Colour checking involves visually comparing the detecting media under working conditions with a type test sample (Type 1 shown below). Performance sensitivity checks are recommended before and periodically during testing, using reference blocks described in the standard. The results produced are compared with photographs of those produced by reference detection media.

Figure 7.1 Type 1 reference piece.

7.2

Fluorescent ink intensity Fluorescent inks should be discarded if there is evidence of fluorescence in the carrier fluid (supernatant liquid). This can happen due to over-vigorous agitation, causing the fluorescent dye to break off the magnetic particles. A sample of unused agitated ink should be compared visually with a sample of quinine sulphate solution and show no more fluorescence when irradiated with UV-A light of at least 10W/m 2 (1000µW/cm2).

NDT30M-60615 Control and Maintenance Checks

7-1


7.3

Overall performance check This test is carried out to find any changes that may have occurred during the day-to-day use of the equipment or materials. The test is carried out before the start of work or at shift change. BS EN ISO 9934-1 states that the most reliable method of checking this is to inspect a representative part containing natural or artificial discontinuities of known size, type, location and distribution. The part is to be demagnetised and free from indications from previous tests. Should such samples not be available fabricated test pieces with artificial discontinuities may be used.

7.3.1

Current flow test piece Used to assess the axial current flow performance of a bench unit. Process: Thoroughly degrease and magnetize the test piece - clamp within head and tailstock of the test bench - apply magnetic ink while the current is increased - establish the current required to make the hole nearest to the outer surface of the ring visible on the outer surface - further increase the current to establish indications from the other two holes on the outer surface of the ring.

Figure 7.2 Current flow test piece.

7.3.2

Magnetic Flow Test Piece Process: Thoroughly degrease and demagnetize the test piece - clamp between the poles of the test bench (magnetic flow) or, alternatively, place it centrally in the coil parallel to the coil axis - energize the equipment and establish that the transverse hole in the middle of the test piece shows a strong indication.


7-2


Figure 7.3 Magnetic flow test piece.

7.4

Viewing efficiency The output of the ultraviolet (UV-A) lamps used in MPI will deteriorate with age. In addition, the output can vary due to:   

Displacement and tarnishing of the reflector. Dirt and other contaminants on the filter. Variations of the voltage to the lamp.

It is therefore necessary to check the output of all UV-A lamps regularly. This check involves the use of a radiometer which will respond to radiation in the UV-A range (400-315nm) (nm = 1 nanometre = 10 -9m). The test procedure is as follows. Position the radiometer with the detector at a distance of 400mm, or working distance, from the front surface of the lamp. If the reading at this distance exceeds the full scale of the meter, use longer distances to bring the reading to approximately 2/3 scale. Move the detector in a plane normal to the axis of the beam from the lamp until a maximum reading is obtained. Record on the lamp calibration label the radiometer reading, the distance of the lamp from the radiometer if greater than 400mm and the date. This test, repeated at regular intervals, will reveal any deterioration in performance or the need for maintenance of the lamp. Ultraviolet, UV-A, lamps should be changed if the output at working distance falls below 1.0mW/cm2 or 1000 W/cm2 (mW = milli watt, W = micro watt) at test surface. The background light in an inspection area should be darker than 20lux. If black ink is being used in white light conditions, the level of light at the work face should exceed 500lux. This is equivalent to an 80W strip light at 1 metre. 7.5

Magnetising units This is a general check for wear, abuse and general cleanliness.

7.6

Tank levels A surprising amount of ink, including solid particles, is carried off on components during testing. Ink level and strength checks are underrated items and are ignored at the inspector's peril.


7-3


7.7

Ultraviolet lamp maintenance A considerable loss of light output can be experienced because of dirty filters. Before condemning a lamp, clean it and the filter in a detergent solution.

7.8

Ammeters Must be checked and calibrated regularly with a meter traceable to national standards. Most major manufacturers will provide a service if ownership of a master meter is not considered economic.

7.9

Demagnetiser Often forgotten until something goes wrong. BS EN ISO 9934-3 they are capable of demagnetising to a specified level between 0.4-1.0kA/m-1.


7-4


Glossary

8

Glossary Active particle magnetic ink. A magnetic ink where the particles are agitated by the action of evaporation of alcohol out of the carrier fluid which is water. The ink also contains a wetting agent and corrosion inhibitor. Adjacent cable technique. A technique of magnetisation in which an insulated, current carrying cable is laid close to the surface of the component, adjacent to the area to be tested. Alternating current. An electric current that alternately reverses its direction in a circuit in a periodic manner. Alternating-current magnetisation. Magnetisation by the magnetic field induced when alternating current is flowing. Ampere/meter (A/m). The field strength in air at the centre of a single turn circular coil having a diameter of 1m, through which a current of 1A is flowing. Note: This is the SI unit of field strength which has replaced the Oersted (1 Oersted = 79.58 A/m). Ampere turns. The product of the number of turns (N) of a coil and the current in amperes (I) flowing through the coil. Aperture type coil. An alternating current carrying coil constructed in such a way that components may be passed through it for the purpose of demagnetisation. Arc. A luminous high temperature discharge produced when a current of electricity flows across a gap. Background. The general appearance of the surface on which defect indications are viewed. Background paint. See contrast aid. Berthold penetrameter. A magnetic flux indicator that contains an artificial flaw in the shape of a cross, mounted below an adjustable cover plate. Note: It is placed on a magnetised component during magnetisation to check the magnetising technique and/or the ink. Black light. See UV-A. Burning. Local overheating of the component at the electrical contact area arising from high resistance or the production of an arc or prolonged contact. Captive fluid indicator. A device comprising a quantity of magnetic ink sealed in a transparent container, the ink behaving in the same way on a magnetised component as free magnetic ink. Carrier fluid. The fluid in which ferromagnetic particles are suspended to facilitate their application. Central Conductor. See threading bar.

NDT30M-60615 Glossary

8-1


Centrifugal tube settlement flask. A settlement flask used to determine the solids content of magnetic flaw detection inks. Circular magnetic field. The magnetic field surrounding an electrical conductor, resulting from the passage of a current through the conductor. Circular magnetisation. Magnetisation in a component resulting from current passed through a threading bar. Circumferential magnetisation. Magnetisation that establishes a flux around the periphery of a component. Clip-on meter. Portable instrument for measuring current flowing in a conductor without breaking the circuit. Coagulation. The agglomeration of ferromagnetic particles in a fluid. Coercive force. The reverse magnetising force required to remove residual magnetism from a material. Note: The corresponding field intensity value is indicative of the ease or difficulty of demagnetisation. Coil technique. A technique of magnetisation in which part or the whole of the component is encircled in a current-carrying coil. Note: The use of the term is usually restricted to instances in which the component does not form part of a continuous magnetic circuit for the flux generated. Coloured magnetic inks. Fluids containing ferromagnetic particles treated so as to produce an indication other than black. Compass test. A test for demagnetisation carried out by placing the component in specified positions in relation to a magnetic compass needle and ascertaining whether the consequent deflection exceeds a specified maximum. Concentrates. Magnetic flaw detection inks supplied in concentrated form for dilution with the appropriate carrier fluid. Conditioning agent. A soluble additive to water-based magnetic inks that imparts specific properties such as surface wetting, particle dispersion or corrosion resistance. Contact heads. The electrodes, fixed to the machine, from which the magnetising current flows. Contact pads. Metal pads, usually of copper braid, placed on electrodes to give good electrical contact, thereby preventing damage to the component under examination. Contrast. The difference in reflectivity or colouration between the component under examination and the indications as shown by the ferromagnetic particles. Contrast aid. A coating or film applied to a surface to improve contrast by providing a more suitable background (eg white contrast paint for black magnetic particle ink).


8-2


Continuous technique. A technique where the ferromagnetic particles are applied to the component while the magnetising force is present. Core. Of an electromagnetic circuit. That part of the magnetic circuit which is within the winding. Crow-receiver. A free-standing, graduated measure which is mainly cylindrical but tapered towards the bottom to allow greater accuracy in reading small volumes. Curie point/curie temperature. The temperature above which ferromagnetic materials can no longer be magnetised or retain their residual magnetism. Note: Examples of such temperatures are Nickel 358 oC, Iron 770oC and Cobalt 1127oC. Current flow technique. A technique of magnetisation by passing a current through a component via, pads, contact heads or clamps. Note: The current may be alternating or direct. Current flow (prods) technique. A technique of magnetisation by passing a current through a component via prods. Note: the current may be alternating or direct. Demagnetization. The process by which a component is returned substantially to an unmagnetised state. Demagnetising coil. See aperture type coil. Demagnetising factor. In coil magnetisation the reduction of the field created by the coil due to the magnetic poles which can be considered to exist at the ends of the test piece. Note: It is a function of the length/diameter ratio of a given component and can be calculated for components having the shape of ellipsoids of revolution. For other shapes it has to be measured experimentally. Detecting medium. The powder or suspension of ferromagnetic particles that is applied to a magnetised test surface to determine the presence or absence of discontinuities. Diffuse indications. Indications that are not clearly defined, ie indications of sub-surface flaws. Direct current. An electric current flowing in one direction only and free from pulsation. Dry powder. Finely divided ferromagnetic particles suitable selected and prepared for magnetic particle inspection. Dry powder technique. The application of ferromagnetic particles without the use of a liquid carrier. Dry out Time. The time allowed for carrier fluid to evaporate leaving ferromagnetic particles in a dry condition.


8-3


Effective magnetic permeability. In coil magnetisation. The ratio of the flux density in the component to the applied magnetic field which would exist in the absence of the component. Note: The effective magnetic permeability of a component is not solely a material parameter as it is effected by the demagnetising factor. Electrode. A conductor by means of which an electric current passes into or out of the component under examination. Electromagnet. A soft iron core surrounded by a coil of wire that becomes a temporary magnet when an electric current flows through the wire. Energising cycle. The period of application of a magnetising force to the component under test. Examination medium. See detecting medium. Extenders. Parts made from ferromagnetic materials that are added to the ends of a component to increase its effective length for magnetisation purposes. False indications. Indications resulting from leakage fields not caused by imperfections or defects. Ferromagnetic. Having a magnetic permeability greatly in excess of unity and varying with flux density. Note: Iron and steel are the most common ferromagnetic materials. Ferromagnetic particles. Finely divided ferromagnetic materials used as an aid to the detection of leakage fields on magnetised components. Fill factor. In the coil technique of magnetisation. The ratio of the crosssectional area of the component within the coil to the cross-sectional area of the coil. Flash point. The temperature at which a liquid, heated in a Cleveland cup (open test) or in a Pensky-Martens apparatus (closed test), gives off sufficient vapour to flash momentarily on the application of a small flame. Flexible cable technique. A technique of magnetisation in which either (a) a current-carrying cable is wound around the component or (b) the cable is laid close to the surface of the component, adjacent to the area to be tested. Fluorescence. The absorption of radiation of a particular wavelength by a substance and its re-emission as light of a greater or visible wavelength. Note: With many substances ultraviolet radiation produces visible fluore scence. Fluorescent magnetic ink. A liquid containing ferromagnetic particles coated with fluorescent material, which will render discontinuities visible when a magnetised component is viewed under UV-A radiation. Fluorescent magnetic particle inspection. fluorescent magnetic ink as a detecting medium.

A

technique

that

utilises

Fluorescent powder. Finely divided fluorescent ferromagnetic materials.


8-4


Flux density. See magnetic flux density. Flux indicator. Small devices, generally in the form of metal strips or discs, containing artificial flaws and which are used to determine when correct magnetising conditions have been achieved and/or the field direction. Note: The indicator is placed in contact with the component being inspected. Flux-leakage field. See magnetic leakage field. Flux lines. See lines of force. Fluxmeter. See magnetic field strength meter. Flux penetration. The depth at which a magnetic flux is effective in a component. Full wave rectified current. Sensibly direct current produced by rectification of either three-phase or single phase alternating current, the former method producing a smoother ripple effect. Functional test. A test method designed to assess the efficiency of magnetic inks and powders or the performance of equipment. Furring. Build-up of ferromagnetic particles due to excessive magnetisation of the component under examination. Gauss. The CGS system electromagnetic unit of magnetic flux density and equal to one line per cm sq. Note: The gauss has been replaced by the tesla. Gauss meter. An instrument designed to measure magnetic flux density. Half wave rectified current. Pulsed unidirectional current produced by clipping a half cycle from single phase alternating current. As a result there are intervals when no current is flowing. Hall effect. A potential difference developed across the conductor, which is at right angles to the direction of both the magnetic field and the electric current, when a current flows along a rectangular conductor subjected to a traverse magnetic field. Hysteresis. The lagging of magnetic flux behind the magnetising field. Immersion procedure. A procedure whereby the component being tested is immersed in a bath of magnetic ink during the magnetisation cycle and subsequently removed for inspection. Indications. A detectable accumulation of ferromagnetic particles resulting from a distortion of the magnetic field and which require assessment to determine their significance. Indirect magnetisation. Magnetisation induced into a component by a curr ent passing through a conductor that is not in electrical contact with the component.


8-5


Induced current flow technique. A technique whereby a circumferential current flow is produced in a ring component by effectively making it the secondary of a mains transformer. Induced field. The field induced in a component by indirect magnetisation. Induction (magnetic). The magnetism produced in a ferromagnetic material by an external magnetising force. Keeper. A piece of ferromagnetic material placed across the poles of a permanent magnet when it is not in use in order to complete the magnetic circuit and thereby prevent loss of magnetism. Laminated pole pieces. Pole pieces consisting of separately adjustable magnetic elements to enable irregular component profiles to be accommodated. Leakage field. See magnetic leakage field. Lifting power. The ability of a permanent or electro-magnet to lift a piece of ferritic steel by magnetic attraction alone. Lines of force. A conceptual representation of magnetic flux derived from the pattern of lines produced when iron filings are sprinkled on paper laid over a permanent magnet. Longitudinal magnetisation. Magnetisation in which the flux lines traverse the component in a direction essentially parallel to its longitudinal axis. Magnetic circuit. The complete closed path followed by any group of lines of magnetic flux. Magnetic field. The region in the neighbourhood of a permanent magnet or a current-carrying conductor in which magnetic forces exist. Magnetic field distribution. The distribution of field strength in a magnetic field. Magnetic field indicator. See flux indicator. Magnetic field leakage. The loss of magnetic field strength due to discontinuities and changes in section in a magnetic circuit. Magnetic field strength (H). The intensity of a magnetic field at a given point. Note: Formerly measured in Oersteds but is now measured in the SI units of ampere/metres. Magnetic field strength meter. An instrument designed to measure magnetic fields. Magnetic flaw detection ink. A detecting medium consisting essentially of ferromagnetic particles in a carrier liquid. Magnetic flow technique. A technique of magnetisation in which the component, or a portion of it, closes the magnetic circuit of an electromagnet or permanent magnet.


8-6


Magnetic flow coil test piece. piece . A standard test piece designed for checking magnetic flow equipment and coils. Magnetic flux. flux. The total number of lines of force existing in a magnetic circuit. Magnetic flux density (B). The (B). The strength of the magnetic field, defined as the normal magnetic flux per unit area. Magnetic hysteresis. hysteresis. See hysteresis. Magnetic indication. indication. See indicators. Magnetic ink. See ink. See magnetic flaw detection ink. Magnetic leakage field. field . The magnetic field that leaves or enters the surface of a component due to the presence of a discontinuity and which is capable of detection by ferromagnetic particles. Magnetic particle flaw detection. detection . A method of detecting surface or nearsurface discontinuities in magnetic materials by the generation of a magnetic flux within a component and the application of suitable ferromagnetic particles to its surface so as to render the discontinuity visible. Magnetic particle flaw detector. detector . Equipment providing essentially current or flux for the purpose of magnetic particle flaw detection, and usually has facilities for holding components of varying dimensions and for adjusting and reading the magnetising current. Magnetic particles. particles. Finely divided ferromagnetic materials capable of being individually magnetised and attracted to distortions in a magnetic field. Magnetic permeability (μ). ). The ratio of the magnetic induction (B) to the external magnetic field (H) causing the induction. Magnetic poles. poles. The points in a magnet that are the apparent seat of the external magnetic field. Magnetic powder. powder. Ferromagnetic particles in dry powder form, of suitable shape and size for flaw detection purposes. Magnetic rubber. rubber. A special formulated medium, containing ferromagnetic powder, used to obtain replica castings of component surfaces, with any discontinuity present being reproduced within the replica by a suitable magnetising technique as a result of migration of the powder within the medium to the position of the discontinuity. Magnetic saturation. saturation. The stage at which any further increase in the magnetic field applied to a magnetised component will fail to show any significant increase in the magnetic flux within that component. Magnetic writing. writing. A form of non-relevant indication arising from random local magnetisation that is generally caused when the surface of a ferromagnetic particle comes in contact with another p iece of ferromagnetic material. Magnetising current. current. The flow of either alternating or direct current used to induce magnetism into a component being inspected.


8-7


Magnetising force. force. The magnetising field applied to a ferromagnetic material to induce magnetisation. Magnetising tongs. tongs. An accessory consisting of two insulated conductors crossing each other at a common pivot. On one side of the pivot they form the two halves of a single turn magnetising coil and, on the other, two handles whereby the coil is made and broken and is connected to the source of the current. Note:: Two turn and three turn tongs are also used. Note Magnetometer.. See magnetic field strength meter. Magnetometer Magnetomotive force. force. The circular integral of the magnetic field strength, H round a closed closed path. It is measured in amperes. Multidirectional magnetisation. magnetisation. The imposition on a component, sequentially and in rapid succession, of two or more magnetic fields in different directions. Note: Magnetic particle indications are formed when discontinuities are located Note: Magnetic favourably with respect to the directions of each field and will persist as long as the rapid alterations of field direction continue, thus enabling discontinuities with differing orientations to be detected in one operation. Non-relevant indication. indication. An indication not produced by a discontinuity but which is the result of spurious effects such as magnetic writing, changes in section, or the boundary between materials of different magnetic properties. Oersted.. The CGS system unit of magnetic field strength. Oersted Note: It has now been replaced by the SI unit ampere/meter. Ohm's law. law. The electric current I in a conductor is directly proportional to the potential difference V between its ends, other quantities (especially temperature) remaining constant. Parallel conductors. conductors. Insulated, current-carrying conductors laid parallel to each other and close to the surface to be inspected but so arranged that the current flows in the same direction through each conductor, thereby producing a substantially uniform magnetic field in the space between the conductors. Particle content. content . The apparent volume ratio of ferromagnetic particles to carrier fluid in magnetic flaw detection ink. Peak current. The current. The relevant quantity used for the calculation of magnetic field strength and which is the maximum instantaneous value of the direct or periodic current obtained during excitation. Note 1: 1: Usually with a dc battery source or with 3 phase full wave rectified AC it will be approximately that indicated indicated by the ammeter. ammeter. With AC or full wave rectified single phase AC it will be 2 x the RMS current, which is the current normally indicated by the ammeter. With half-wave rectified AC it will be approximately 1/2 x the RMS current. Note 2: 2: Ammeters fitted to half-wave equipment are usually calibrated to take account of the doubling factor (x2) and therefore indicate equivalent AC RMS values.


8-8


Permeability.. See magnetic permeability. Permeability Permanent magnet. magnet . A magnet that retains a high degree of magnetisation virtually unchanged over a long period, this being a characteristic of materials of high retentivity. Pole. See Pole. See magnetic poles. Polymer technique. An technique. An examination technique in which a polymer is used as the particle suspension vehicle. Portable flux indicator. indicator. See flux indicator. Powder. See Powder. See dry powder. Powder blower. blower. A compressed air device, operating at low pressure, used to apply dry powder over the surface of a component undergoing inspection. Prods. Hand held electrodes attached to wander cables to transmit the Prods. magnetising current from the source to the component under examination. Pull-off force. force. The force that has to be applied to one pole of a magnet to break its adhesion to a ferritic steel surface, leaving the other pole piece still attached. Rectified alternating current. current . An electric current obtained by rectifying alternating current without the deliberate addition of smoothing to remove the inherent ripples. Reference pieces. pieces . Specimens containing controlled artificial defects or natural defects used for checking the efficiency of magnetic particle flaw detection processes and/or equipment. Relevant indication. indication. An indication produced by the presence of a discontinuity and which requires assessment to determine its significance. Reluctance. A measure of the degree of difficulty with which a component can Reluctance. be magnetised that is analogous to the resistance in an electrical circuit. Note: In a material of length l, cross-sectional area A and permeability Note: reluctance is given by l/A μ.

μ,

the

Remanence. The magnetic flux density remaining in the material after the Remanence. magnetising force has been removed. Remanent magnetism. magnetism . See remanence. Remanent magnetisation tests. tests . Tests to ascertain, either qualitatively or quantitively, the degree of demagnetisation of a component. Residual magnetic field. field . The magnetic field remaining in a material after the magnetising force has been reduced to zero. Residual magnetism. magnetism. See residual magnetic field.


8-9


Residual magnetisation technique. A technique whereby ferromagnetic particles are only applied to a component being inspected after it has been magnetised and the magnetising force removed or discontinued. Note: The technique relies for its effectiveness on the strength of the residual magnetic field. Resultant field. The field produced when two or more magnetising forces operating in different directions are applied simultaneously to a ferromagnetic material. Note: The direction of the field is determined by the relative strengths and directions of the magnetising forces applied. Retentivity. See remanence. Rigid coil technique. A technique in which the coil turns are constructed from a non-flexible material and are secured so as to prevent relative movement between them if constructed from cable. RMS current. The Root Mean Squared value of an alternating current. Note: It is the square root of the mean value of the squares of the instantaneous current value taken over a complete cycle, and is almost invariably used for measuring alternating currents. Saturation, magnetic. See magnetic saturation. Self demagnetisation. An effect occurring in any magnetised component which possesses adjacent free poles (ie a ring with a gap) that is due to the field between the poles opposing that of the magnetising force. Note: The effect reduces the strength of the internal field in short components magnetised by the coil method. Sensitivity. The degree of capability of a magnetic particle flaw detection technique to indicate surface or near surface discontinuities in ferromagnetic materials. Settling time. The time allowed for settlement of ferromagnetic particles in a sample of magnetic ink prior to the assessment of particle content volume. Skin effect. The phenomenon that causes the magnetisation produced by alternating current to be contained near the surface of a ferromagnetic component. Solids content. The volume of ferromagnetic particles, including adherent nonmagnetic pigments, contained in a magnetic ink. See also total solids. Solenoid. A multi turn coil of wire wound on a ferromagnetic former. Note: When carrying a direct current it behaves like a bar magnet. Split coil. A single or multi turn coil constructed with plug connections to allow it to be opened for positioning over components having no free ends for normal coil access.


8-10


Spurious indication. A non-relevant indication. Sub-surface discontinuity. A discontinuity situated wholly below the surface of a component but sufficiently close to the surface to produce a visible indication during magnetic particle flaw detection. Surface field. The magnetic field at the surface of the component under examination. Suspension. A system in which visible, denser particles are distributed throughout a less dense carrier medium, settling being hindered by the viscosity of the carrier medium. Sutherland flask. A flask used for measuring the apparent proportion of solids separating under gravity from a known volume of magnetic particle flaw detection ink. The ungraduated upper portion, shaped like an inverted pear, is constricted at the top to receive a stopper and blended at the bottom into a graduated tube of small uniform cross section. Strippable lacquer. A quick drying easily removable paint sprayed on to a component to give a visual contrast with the magnetic particles. Swinging field magnetisation technique. A technique that utilises a form of multidirectional magnetisation to enable discontinuities having different directions to be detected in one operation. Note: Generally, longitudinal magnetisation is generated by one phase of a 3phase ac supply and traverse magnetisation by a different phase. Standard current flow and coil or flux flow techniques are used. Temporary magnet. Commonly a piece of soft steel or iron that is readily magnetised but retains only a very small field after removal of the external magnetising force. Tesla. The SI unit of magnetic density equal to 1Wb/m sq. Test piece. A specimen containing known artificial or natural defects used for checking the efficiency of magnetic particle flaw detection techniques. Threading bar. A current-carrying conductor passed through a hollow component and used to produce circular magnetisation within the component. Threading bar technique. A technique of magnetisation in which a current carrying bar, cable or tube is passed through a bore or aperture in a component under examination. Threading cable technique. A form of threading bar technique utilising a flexible cable to carry the current. Threading coil technique. A development of the threading bar technique in which a magnetising coil, rather than a straight run of bar or cable, is threaded through a bore or aperture in a component. Total solids. The ferromagnetic particle content of a magnetic ink plus any other solid constituent present that make up the total solids content of the ink.


8-11


Ultraviolet radiation. Radiation for which the wavelength of the monochromatic components are smaller than those for visible radiation and more than about 1nm. Note: The limits of the spectral range of ultraviolet radiation are not well defined and may vary according to the user. The International Commission on Illumination (CIE) distinguishes the following spectral range: UV-A 315-400nm. UV-B 280-315nm. UV-C 100-280nm. UV-A. Ultraviolet radiation having a wavelength in the range 315-400nm, used for exciting fluorescence. Vehicle. A liquid medium for the suspension of particles. Wet technique. An examination technique in which the particles are suspended in a liquid medium. Yoke. Those parts of an electromagnet that are extensions of the core, not being surrounded by windings, and which form the pole pieces. Note: The term is, however, often applied to an electromagnet as a whole. Yoke magnetisation. A longitudinal magnetic field induced in a component, or part of a component, by means of an external electromagnet.


8-12


MT PRESENTATION SLIDE (THEORY)

Course Objectives   

Magnetic Particle Testing (MT) Level 2



To explain the basic principles of magnetic particle inspection methods. To carry out magnetic particle inspection. To write clear and concise inspection instructions and test reports. To meet syllabus requirements for CSWIP/PCN Level 2.

NDT30M



Introduction

Introduction

NDT30M-60615

Requirements before examination:     

Provide two passport size photographs of yourself (sign and date on reverse). Current valid eye test for colour perception (Ishihara) and near visual acuity (Jaeger). Evidence of experience for Certification award (can be achieved after examination). Fully completed and signed TWI enrolment from. Completed PSL/57A form (PCN only).

Any questions before we start?



Introduction

Introduction

Sections: Introduction to NDT. 1. Principles of MPI. 2. Methods of magnetisation. 3. Detecting media, UV light and other equipment. 4. Application techniques and demagnetisation. 5. Current waveforms. 6. Assessing magnetising force and amperage. 7. Control and maintenance checks.


Introduction to NDT


1

Introduction to NDT

Non Destructive Testing

PCN examination now sets questions on the History of NDT and the PCN Scheme. This information can be found within the preliminary pages of your course notes. You also need to be aware of the capabilities and requirements for all the different methods of NDT that are available and the best method for that particular inspection.

Definition Non-destructive testing is the ability to examine a material/component without degrading it.





A quick overview of NDT Aside from visual testing we will consider the five main methods of NDT:

What can we expect to detect?  

1. 2. 3. 4. 5.

Liquid penetrant (PT). Magnetic particle (MT). Eddy current (ET). Ultrasonic (UT). Radiography – X and Gamma (RT).

  



Non Destructive Testing There are advantages and disadvantages to selecting any particular method to carry out an inspection. The factors affecting the choice of method are:  The reason for the inspection (cracks, material sorting, check assembly).  The likely orientation of planar discontinuities.  The type of material.  The likely position of discontinuity.  The geometry and thickness of object to be tested.  Accessibility. Copyright © TWI Ltd

Liquid penetrant – surface breaking flaws in almost any non-porous materials. Magnetic particle – surface and slightly subsurface flaws in ferromagnetic metals. Eddy current – surface and far surface flaws in conductive materials. Ultrasonic – surface, far surface and internal flaws in many materials. Radiography – surface and internal flaws in most materials.

Magnetism Defined as the phenomena of some materials to attract or repel certain other materials.    

Magnetism is a mysterious force. Hang a magnet up and it always tries to point in the same direction. Put it near different materials, and it will attract some but not others. Whatever magnetism is, it seems to be present in every atom.


2

Magnetism     

Some natural materials strongly attract pieces of iron to themselves. Such materials were first discovered in the ancient Greek city of Magnesia. Magnets were utilised in navigation. Oersted found a link between electricity and magnetism. Faraday proved that electrical and magnetic energy could be interchanged.

Magnetism 

If a small bar magnet is dipped into iron filings, the filings cling in clumps around its ends.



The magnetic force pulling the filings seems to come from two points, known as the poles of the magnet.



Magnetism 

Materials such as iron and steel are attracted to magnets because they themselves become magnetised in the presence of a magnet.



The magnet is said to induce magnetism in both metals, and a polarity test on each shows that the induced pole nearer the magnet is the opposite of the pole at that end of the magnet. It is the attraction between these unlike poles that holds each piece of metal firmly to the magnet.

Magnetism 

If the steel is pulled well away from the magnet, it keeps some of the induced magnetism, and itself becomes a permanent magnet.



Magnetism induced in the steel is only temporary however, and is virtually all lost when the steel is pulled well clear of the magnet.



Section 1

Magnets 

Materials such as brass, copper, aluminium, and non-metals, are commonly described as non-magnetic because they aren't attracted to small magnets and cannot apparently be magnetised.



Experiments with very strong magnets however indicate that even these materials are influenced by magnetism to a slight extent.

Principles of Magnetic Particle Inspection



3

Types of Magnetism

Types of Magnetism

On the basis that all materials can be magnetised in some way, materials can be divided into three groups:

Diamagnetic: Weakly repelled by a magnetic field.  Examples: Gold, Copper, Water.

1. Diamagnetic. 2. Paramagnetic. 3. Ferromagnetic.

Paramagnetic: Weakly attracted by a magnetic field.  Examples: Aluminium, Tungsten. Ferromagnetic: Very strongly attracted by a magnetic field.  Examples: Iron, Cobalt, Nickel. Copyright © TWI Ltd


Theory of Magnetism – Atomic Structure

Theory of Magnetism – Domains     

When an electric current flows there is an associated magnetic field. An electric current consists of a flow of electrons through a conductor. The electrons in any atom are in constant motion. This motion causes an associated magnetic field. In most materials this field is cancelled by the movement of electrons in opposing directions.



Theory of Magnetism – Domains Electron spin


Theory of Magnetism – Domains 

According to the generally accepted theory of magnetism, each electron acts as a tiny magnet as it spins and moves around the nucleus of an atom.



In some materials, the electron motions are such that the magnetic effects normally cancel out. In others, they do not cancel, and each molecule therefore behaves as a tiny magnet.



Ferromagnetic materials are made up of molecular magnets of this type. Copyright © TWI Ltd

4

Theory of Magnetism – Domains Ferro-magnetic materials


If a magnetised steel strip is broken into pieces polarity tests show that each piece is itself a magnet.



If the strip is broken into very much smaller pieces, these too are found to be magnets.



There is evidence to suggest that the smallest magnets of all lie within molecules themselves.






In a ferromagnetic material, the molecular magnets line up with each other in groups called domains.

Domain Theory  

A domain is a minute internal magnet. Each domain comprises 1015 to 1020 atoms – typically several million domains exist in each individual grain.

Within any one domain, the magnetic axes of the atoms all lie in the same direction, but this direction varies from one domain to the next if the material is unmagnetised.

Unmagnetised state



Domain Theory

Domain Theory

Magnetising force

Magnetising state

Domains randomly orientated

Magnetising force

Domains aligned in external field


Saturated state

All domains fully aligned with external field


5

Domain Theory

Domain Theory

Unmagnetised

Magnetising force

Magnetised

Saturated Magnetising force removed

Residual magnetism remains

Residual



Magnetic Fields

Permanent Magnet



In order to understand how magnets interact with one another the concept of a magnetic field is used.



The idea of a magnetic field is based on the patterns made by magnetic particles when they are placed in a magnetic field.


Magnetograph

UV(A)


Magnetic Fields

Magnetic fields are thought to consist of lines of flux.



6

Magnetic Flux

Lines of Flux

Magnetic flux is defined as: The total number of lines of flux in a magnetic field or circuit.



Lines of Flux

Properties of Lines of Flux  

Unlike poles attract.

 

Iron filings show distribution of lines of force from a bar magnet.

 

They flow from a North pole to a South pole outside a magnet. They flow from a South pole to a North pole inside a magnet. They They They They

form closed loops. repel one another. never cross. follow the path of least resistance.

Like poles repel.

 

 



Electromagnetism

Electromagnetism

Oersted discovered that when an electrical current flows a magnetic field is produced. Faraday investigated the relationship between electricity and magnetism.



The magnetic field produced is always at 90° to the direction of electrical current flow. The flux density produced is proportional to the magnitude of the electric current.

Direction of current flow



A current flows through a conductor and sets up a magnetic field around it. Field is at 90° to the direction of the electrical current.

Direction of magnetic field



7

Electromagnetism – Right Hand Rule

Coil Magnetisation

S

 

N

Changes circular field into longitudinal. Increases the strength of the field.



Coil Magnetisation – Revealed

Hysteresis     

Hysteresis comes from a Greek word that means lagging behind. Ferromagnetic materials resist being magnetised. But once magnetised, they resist being demagnetised. They oppose change. This is best explained by the Hysteresis Loop.



The Hysteresis Loop – Terms

The Hysteresis Loop +B

Flux density (B) The number of magnetic flux lines per unit area - S.I. unit: Tesla (old unit was Gauss). Magnetising force (H) The force tending to set up a magnetic flux - S.I. unit: Ampere per meter (Am-1).

a Saturation

b

Residual Magnetism

Virgin Curve c o

-H

e

-ve Saturation d

-B Copyright © TWI Ltd

f

+H

Coercive Force H = Magnetising force B = Magnetic flux density


8

The Hysteresis Loop HARD ferromagnetic

SOFT ferromagnetic

Magnets Magnetic and non-magnetic materials  Materials which can be magnetised strongly, and are therefore strongly attracted to magnets, all contain at least one of the metals iron, nickel and cobalt.  Strongly magnetic materials are known as ferromagnetics.  They are classified as magnetically hard or soft depending on how well they retain their magnetism when magnetised.



Magnets

Magnets



Hard magnetic materials such as steel and alcomax (a steel-like alloy) are the most difficult to magnetise but do not readily lose their magnetism.

Soft magnetic materials, such as iron and mu-metal (a nickel-based alloy) are relatively easy to magnetise but their magnetism is only temporary.



They are used to make permanent magnets.





They are used in electromagnets because in this case they remain magnetised only as long as a current is passing through a surrounding coil. Unlike permanent magnets, electromagnets can be switched on and off.



The Hysteresis Loop Hard ferromagnetic

Soft ferromagnetic


Definitions 

Magnetic field Region in which magnetic forces exist.



Magnetic flux The total number of lines of force in a magnetic circuit.



Permeability – The relative ease with which a material may be magnetised.



Reluctance – a measure of the degree of difficulty with which a material can be magnetised (opposite of permeability). Copyright © TWI Ltd

9

Definitions 

Saturation – the point at which which an increase increase in magnetising force produces no significant gain in flux density.



Residual magnetism – magnet magnetic ic field remaining remaining after the magnetising force has been reduced to zero.



Remanence – magnetic flux density density remaining remaining after the magnetising force has been removed.



Coercive force – rever reverse se magnetising magnetising force required to remove residual magnetism.



Flux leakage – break or discontinu discontinuity ity in a magnetic circuit.

Permeability (μ) Permeability (μ) can be defined as the relative ease with which a material may be magnetised. 

 

It is defined as the ratio of the flux density (B) produced within a material under the influence of an applied field to the applied field strength (H). (H). μ =B/H (the gradient of the line). From the hysteresis loops in the previous slides it can be seen that permeability is not a constant.



Relative Permeability (µr) This is the permeability of any material relative to the permeability of free space. 

Free space is basically air.



Permeability of free space = µo = 1.0.



Relative Permeability (µr) = µ/µo.

Relative Permeability (µr) On the basis of relative permeability materials can be divided into three groups: 1. Diamagnetic 2. Paramagnetic 3. Ferromagnetic

-

Slightly < 1 Slightly > 1 High 240 +

Absolute permeability is difficult to measure.



Relative Permeability (µr) Permeability is affected by:   

The Basics of MPI Testing 

Chemical composition. Heat treatment. The shape of the component.



The opposite of permeability is reluctance reluctance.. 


The basis of MPI is that the material under test is magnetised, a magnetic ink or powder is applied to the medium surface and the resultant indications are evaluated. The formation of the indications is dependent on the difference in magnetic properties between the discontinuity and the material under test. Generally the discontinuity is non-magnetic therefore it’s magnetic properties differ to the surrounding area.


10

The Basics of MPI Testing

Flux Leakage

Magnetic field around a bar magnet, flux lines expand away from the ‘N’ pole.

Magnetic broken in two creates two magnets. Magnetic flux field expands in air from ‘N’ pole and then is drawn to the ‘S’ pole. Partial break. Magnetic flux field expands in air away from ‘N’ pole then drawn back to ‘S’ pole. Magnetic flux leaks above surface of material. This flux leakage can be revealed by ferrous particles.

Flux leakage between the poles of a magnet

When the poles are brought closer together the flux leakage becomes more localised and concentrated

If the magnet is formed into a closed loop the lines of force are still present but there is no flux leakage



Principles of MPI Flux Leakage

Principles of MPI Flux Leakage

No defect present

Defect present Leakage field



Magnetic Particles Attracted by Flux Leakage

Flux leakage occurs at defect

Detecting media attracted to the flux leakage forming an indication that is larger than the defect

Principle of MPI Flux Leakag Leakage e - Defect No defect

Defect

Lines of flux follow the path of least resistance. Copyright © TWI Ltd


11

Principle of MPI Flux Leakag Leakage e - Depth Surf rfac ace e defe fec ct

Sub-s -su urf rfac ace e de deffec ectt

Permanent Magnet  

Longitudinal flux field between poles. Maximum sensitivity for defects orientated at 90° to a line line drawn between between poles. poles.

Lines of flux follow the path of least resistance. Copyright © TWI Ltd


Defect Orientation

Defect at 90° to flux: Maximum Maximum indication. indication.

Defect Orientation

Defect > 30° to flux: Acceptable Acceptable indication. indication.


Defect Orientation


Sub Surface Defect Orientation

Flux leakage

Defect < 30° to flux: Weak Weak indication. indication.



12

Leakage Fields

Visibility of Flux Leakage Depends on:  Depth of defect.  Orientation of defect.  Shape of defect.  Size of defect.  Permeability of material.  Applied field strength.  Contrast.

Magnetic saturation Copyright © TWI Ltd

Indications


Magnetic Field Descriptions

Relevant indications - Indications due to discontinuities or flaws.  Examples: Cracks, lack of fusion, pores.

Magnetic field… Longitudinal – along

Non-relevant indications - Indications due to flux leakage from design features.  Examples: Rivets, splines, threads.

Defects… Transverse – across

Spurious indications - Indications due incorrect inspection procedures.  Examples: Hairs, lint, magnetic writing, scale. Copyright © TWI Ltd


Magnetic Field Descriptions

Section 2

Magnetic field… Longitudinal – along Circular – around Methods of magnetisation Defects… Transverse – across Longitudinal – along Radial – from centre



13

MPI Equipment Portable  Permanent magnet.  Electromagnet.  Prods.  Flexible coil.  Flexible cable.  Clamps and leeches.

Fixed  Current flow.  Magnetic flow.  Threader bar.  Rigid coil.  Induced current.


Portable equipment Permanent magnet and electromagnet – DC Yoke (Magnetic flow)



Permanent Magnet and DC Yoke  

Longitudinal field between poles. Maximum sensitivity for defects orientated at 90° to a line drawn between poles.

Permanent Magnet/DC Yoke 



DC N

S

N



A permanent magnet/DC yoke suitable for MPI should be capable of lifting a steel weight of 18kg. Flux indicators become permanently magnetised in a DC field and are unreliable when used with permanent magnets or DC yokes. Permanent magnets/DC yokes are not generally permitted by BS EN 9934-1 (they can be used if the contracting parties agree).

S



Permanent Magnet Advantages  No power supply.  No electrical contact problems.  Inexpensive.  No damage to test piece.  Lightweight.  Can detect subsurface defects.

Disadvantages  Direct field only – unreliable sensitivity surface defects.  Deteriorate over time.  No control over field strength.  Poles attract detecting media.  Tiring to use.


DC Yoke Advantages  No electrical contact problems.  Relatively inexpensive.  No damage to test piece.  Lightweight.  Operates on a low voltage (12 V).  Can detect subsurface defects.

Disadvantages  Direct field only unreliable sensitivity for surface defects.  Not suited for use with dry powder detection media.


14


UV(A)

Portable equipment Electromagnet – AC Yoke (Magnetic flow)



Electromagnet

Electromagnet

Magnetic field induced in core by electric current passing through coil

Soft iron laminated core

Adjustable legs & pole pieces

Maximum defect sensitivity at 90 degrees to the magnetic flux field

Area adjacent to poles is not suitable for carrying out inspection due to saturation and particle migration.


Electromagnet (AC Yoke) Advantages  AC, DC or rectified.  Controllable field strength.  No harm to test piece.  Can be used to demagnetise.  Easily removed.

Disadvantages  Power supply required.  Longitudinal field only.  Electrical hazard.  Poles attract particles.  Legs must have area contact.




Portable equipment Prods


15

Prods

Prods

Electrical current passes between prods through components.



Field produced is taken as two deformed circles between prods.





 

Defects found at 90° to magnetic field.

Steel or aluminium tipped prods should be used. Copper or lead tipped prods are not permitted. Galvanised prods are not permitted. Flux density can be confirmed using a flux indicator. Generally limited to the inspection of rough castings - overheating at the contact points can cause cracking.


Prods Advantages  AD, DC or rectified.  Controllable field strength.  No poles attract particles.  Excellent sensitivity.  Easy to use on complex shapes.

Disadvantages  Arcing/damage to work piece.  Heavy transformer required.  Current can be switched on without creating field.  Good contact required.  Usually a two man operation. Copyright © TWI Ltd

Flexible Cable



Portable equipment Flexible cables


Closely Wrapped Coil

Current passed through a flexible cable. Used as:  Flexible coil.  Threading cable.  Adjacent cable.



16

Cable: Adjacent – Parallel Cable

Flexible Cable Advantages  Simple to operate.  No danger of burning.  AC, DC or rectified.  Current adjustable.  Suited to underwater applications.

Disadvantages  Difficult to keep cables in place.  High currents required.  Transformer required.

A single parallel cable lying on the surface can be considered as a coil of one turn. The inspection zone being ‘d’ mm each side of the cable.



Cable: Adjacent – Twin Parallel Cable

Flexible Cable

Kettle element

Advantages  Simple to operate.  No danger of burning.  AC, DC or rectified.  Current adjustable.  Suited to underwater applications.

Disadvantages  Difficult to keep cables in place.  High currents required.  Transformer required.

Where d = half the distance between the cables in mm.




Bench Unit

Portable equipment Magnetic bench unit



17


Magnetic Flow Component clamped between headstock solenoids. Solenoids energised to produce strong magnetic field across component.

Bench – Magnetic flow




Magnetic Flow


Bench – Axial current flow



Axial Current Flow

Axial Current Flow Component clamped between headstocks. Electrical current passed through component produces an encircling magnetic field. Defects found at 90° to magnetic field.



18

Axial Current Flow

Axial Current Flow  

 



The length of a component has no effect on the required current value. Field strength can be assessed using a flux indicator, a cracked component or a standard test piece. If the component is not tightly clamped overheating may occur. This method is not suitable for some awkwardly shaped components.


Threader Bar

Bench – Threader bar



Threader Bar

Threader Bar



Threader bar clamped between headstocks. Electrical current passed through threader bar produces an encircling magnetic field. Defects found at 90° to magnetic field.

19


Coil

Bench - Coil



Coil

Coil

Coil can either replace the headstock or clamp between. Electrical current passed through coil produces a longitudinal magnetic field through coil. Defects found at 90° to magnetic field. Copyright © TWI Ltd


Section 3

Dry Detecting Media  

Dry particles are available in a wide variety of colours. Using the right colour it is usually possible to work without contrast aid paint.

Detecting Media, UV light and Other Equipment



20

Dry Detecting Media        

Iron powder or magnetic iron oxide (magnetite). 40-200 microns, rounded and elongated shapes. Colours vary for contrast against component. Can be used on hot surfaces. Poor particle mobility, HWDC best, DC or permanent magnets must never be used. Greater operator skill required. Difficult to apply to overhead surfaces especially in field conditions. Generally less sensitive than wet particles.

Dry Detecting Media Advantages  Virtually no lower limit on temperature.  Upper temperature limit 65°C for fluorescent particles and 315°C for other colours.  No fire or explosion hazard.  Residues easily removed.  Using HWDC prods - dry particles provide the best sensitivity for sub-surface defects.  Fluorescent powder is available but rarely used.



Dry Detecting Media

Dry Detecting Media

Disadvantages  Poor particle mobility.  Reduced sensitivity for surface breaking defects.  Not suited for use with a permanent magnet or DC magnetic field.  Not suited to residual testing.  Greater operator skill is needed.  Difficult to use on overhead surfaces.  Difficult to use in windy conditions.



Centreline and Toe Crack in Butt Weld

Wet Detecting Media        

Magnetic iron oxide (magnetite) or iron powder. 1.5-40 microns rounded and elongated shapes. Colour contrast or fluorescent. Water or kerosene based. Concentration important. Good particle mobility. Easier to use. More sensitive.

Using Colour Contrast Dry Powder Copyright © TWI Ltd


21

Wet Detecting Media  





Water or paraffin based inks are available. Water based inks contain a detergent to improve wetting ability, a corrosion inhibitor and an anti-foaming agent. Paraffin based inks are less affected by adverse surface conditions and generally more effective. Paraffin may be a problem if the future application is (for example) potable water. It may also cause problems in any subsequent painting operation.

Wet Detecting Media Advantages  When magnetic particles are suspended in a liquid carrier particle mobility is greatly improved.  Wet methods provide the best sensitivity for surface defects.  Magnetic ink is suited for use with a permanent magnet or DC magnetic field.  Suitable for residual methods.  Less operator skill is needed.



Splined Shaft With Service Induced Crack

Wet Detecting Media Disadvantages  Limited temperature range 

 

Using fluorescent ink

Water and paraffin evaporate quickly at high surface temperature (exceeding 50°C or so) and freeze at sub-zero temperatures.

Possible fire or explosion danger if using paraffin based ink. Residues can be a problem for subsequent processing.



Special Detecting Media Magnetic rubber  Can be used to preserve a magnetic particle indication.  Largely superseded these days by digital cameras.


Detecting Media BS EN ISO 9934-2: 2002 Magnetic Particle Testing - Detection Media 

Specifies the requirements for magnetic inks, magnetic powders and contrast aid paints.


22

Detecting Media

Detecting Media

Halogen/sulphur content For products designated as low sulphur - low halogen:   

UV light

Sulphur content shall be less than 200ppm. Halogen content shall be less than 200ppm. The accuracy of testing shall be  10ppm.



Electromagnetic Spectrum X-rays & Gamma

Radio Waves

Microwaves Ultra violet

Infra Red

Electromagnetic Spectrum

TV

Light

10-10 10-8

10-6

10-4

10-2

1cm

102

104

106

108

Wavelength



Electromagnetic Spectrum

Black Light (or UV Light) 

 


The black light sources used in fluorescent penetrant inspection use a mercury vapour arc lamp. This type of lamp emits – visible light, UV(B), UV(C) in addition to UV(A). With a properly fitted woods filter (which must be in good condition) only UV(A) and a low level of visible light are emitted.


23

Black Light (or UV Light) Warnings 

Never look directly at a black light.



Do not use if filter is cracked, damaged or incorrectly fitted.



Avoid unnecessary skin exposure.

Black Light (or UV Light) Precautions  UVA radiation is relatively safe to work with.  It may cause temporary health problems such as Eyeball Fluorescence.  The human eye contains a jelly which begins to fluoresce if exposed for long periods to UVA light.  This fluorescence causes clouded vision but the effect is temporary.  Sodium goggles can reduce the risk.



Fluorescence and the Electromagnetic Spectrum

Black Light (or UV Light) With a properly fitted filter a black light emits: 

UVA radiation in the wavelength range 



315 to 400 nanometres.

The principal wavelength emitted is 

365 nanometres.


Fluorescent v Colour Contrast        

Fluorescent methods are more sensitive. Less operator fatigue with fluorescent. Background lacquer is not required.


Fluorescent v Colour Contrast Black particles

Flourescent particles

Fluorescent properties will degrade if exposed to UV light, acids, alkalis or high temperature. Background fluorescence is a problem on rough surfaces. Some oils will produce strong background fluorescence. Low background light levels are required. Not suited to site work. Copyright © TWI Ltd


24

Visible Methods Advantages  No special lighting needed.  Easier to use on rough surfaces.  Coloured particles are stable at surface temperatures of up to 315°C.


Other MPI equipment

Disadvantages  Less contrast - less sensitive.  Contrast aid paint may be required.  Tiring to use - not suited to batch inspection.


Flux Indicators


UV(A)

Used to check for adequate flux density and correct flux orientation. (Do not use with permanent magnets or DC electromagnets.)


Flux Indicators – Common Types 

Burmah castrol strips



ASME pie gauge


ASME V Magnetic Flux Indicator

Consists of 8 steel pie segments brazed together with copper faceplate. 

Berthold penetrameter



25

ASME V Magnetic Flux Indicator

Burmah Castrol Strip

Strong well defined indication at 90 degrees to flux Weaker slightly fuzzy indications oblique to flux No indication parallel to flux


Field Indicators


Section 4


Flux Density Meter


Application Techniques MPI may be performed using:  Either a continuous or residual method.  Either fluorescent or visible detection media.  Detection media which is either wet or dry.  Not all combinations of the above are effective.


eg Dry detection media is not suitable for residual methods.



26

Continuous or Residual? Continuous method  Detecting media applied immediately prior to and during magnetisation. Residual method  Detecting media used after the applied field has been removed.  Component must have high retentivity.  Less sensitive than continuous method.  Useful for components like ball bearings.

Fluorescent Methods Advantages  More contrast = more sensitive.  Less operator fatigue – suited to batch inspection.  No contrast aid paint required.



Visible Methods Advantages  No special lighting required.  Easier to use on rough surfaces.  Colour particles are stable at surface temperatures up to 315°C.

Disadvantages  Less contrast = less sensitive.  Contrast aid paint may be required.  Tiring to use – not suited to batch inspection.

Disadvantages  Special lighting required.  Not suited to rough components – high background colour reduces contrast.  Fluorescent coating on detecting media is easily damaged by high temperature, acids and sunlight.

Types of Indication There are three types of indication that can be found during magnetic particle inspection 1. Relevant indications. 2. Non-relevant indications. 3. Spurious (false) indications. 1. Relevant indication – an unwanted imperfection due to the flux leakage from a discontinuity or flaw. If it is considered to affect the fitness-for-purpose of the component, it is classified as a defect.


Types of Indication 2. Non-relevant indications Due to flux leakage but arising from design features:    

Changes in section. Changes in permeability. Grain boundaries. Forging flow lines.



3. Spurious indications Not due to flux leakage:     

Lint. Scale. Dirt. Hairs. Magnetic writing.


Demagnetisation


27

When to Carry Out Demagnetisation?

Why Carry Out Demagnetisation?



Before: To remove existing residual fields.





During: When carrying out shots at different orientations.

 



After: To remove residual magnetisation.

  

Aero parts – may affect compasses and electronic equipment. Rotating parts – magnetic debris will cause premature wear. Before welding processes – may cause arc blow or drift. Before machining processes – magnetised swarf not cleared from flutes. When high quality finished are to be applied – electro plating, power coating. It is not required if the component is to undergo heat treatments above the Curie point (700°C).



Principle of Demagnetisation Demagnetisation is based upon the principle of a reversing and reducing magnetic field.

Practical Methods It can be achieved in a number of ways for practical applications. 1. Aperture Coil: 2. Aperture Coil: 3. Aperture Coil:

The initial force applied must exceed the existing coercive force.

Remove part to 1.5m. Reducing AC to zero. Reversing DC and reducing current by 50% each time until zero.

4. Electromagnet: Reversing DC as above. 5. Electromagnet: AC yoke pulled over and away from the part to a distance of 450mm and turned off.



Demagnetisation

Demagnetisation

A reducing AC magnetising force works well.


But a stepped reversing/reducing DC field is the most effective.


28

Axial Current Flow

Using a Permanent Magnet

Problems  The circular magnetic field produced by axial current flow cannot be detected using a magnetometer/field indicator.  If a component has a permanent circular magnetic field problems may occur in any subsequent machining operation.  A powerful coil shot will destroy any permanent circular field and replace it with a longitudinal one.  The longitudinal field is then easy to detect and easy to remove.

Twist the magnet as it is removed from the surface to a distance of 1.5 metres.



Section 5

Current Values and Waveforms Magnetising equipment employs different method forms of electricity to generate a magnetic field. 

Current Waveforms

Direct current (DC) and alternating current (AC).

The peak current generates the maximum magnetising force but magnetic bench units rarely have ammeters that display peak values. MPI bench units also change a mains AC current in a variety of ways to produce different waveforms. Copyright © TWI Ltd

Current Types 

Direct current (DC).



Alternating current (AC).



Half wave rectified current (HWDC or HWRAC).



Full wave rectified (FWDC or FWRAC).



Conversion Factors (BS EN 9344-1)


29

Root Mean Square (RMS) The RMS value is the effective value of a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect. The value of an AC waveform is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again.

Root Mean Square (RMS) Clearly for most of the time it is less than the peak voltage or amperage, so this is not a good measure of its real effect. Instead we use the Root Mean Square amps (IRMS) which is 0.707 of the peak amps (Ipeak): I RMS = 0.707 × I peak

or

I peak = 1.414 × I RMS



Waveform

DC – Field Distribution

Direct current (DC) Advantages  Sub-surface defects.  Availability from batteries.

Field strength

Disadvantages  No agitation.  Less sensitive to surface defects.

In order to achieve the same sensitivity to shallow defects a DC field must be far more powerful than a corresponding AC field.

Limited flux leakage

Distance



Waveform – Alternating Current (AC)

AC – Field Distribution Field strength

Skin effect

Flux leakage


Distance


30

AC Versus DC – Skin Effect

Waveform – Half Wave AC (HWAC)

Leakage

No leakage

In order to achieve the same sensitivity to shallow defects a DC field must be far more powerful than a corresponding AC field. Copyright © TWI Ltd

Waveform – Full Wave Rectified AC (FWRAC)


Waveform – 3 Phase Rectified AC



Root Mean Square (RMS) – Explained

Root Mean Square (RMS) – The Maths Take the current at each instant in turn, square it, add up the squares (which are now all positive) and divide by the number of samples to find the average square or mean square.

With an infinite number of intervals, the result is 0.70710678… or 1/ √ 2 Copyright © TWI Ltd


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Conversion Factors – AC

Section 6

From Peak to RMS…

I RMS = I PEAK x 0.7071 Assessing Magnetising Force and Amperage From RMS to Peak…

I PEAK = I RMS x 1.414 or

I PEAK = I RMS ÷ 0.7071









Measurement of Flux Density

Measurement of Flux Density

The flux density achieved during magnetic particle inspection largely determines the sensitivity of the test. BS EN 9934-1 requires an RMS flux density of at least 1 Tesla in the surface of the component. In low carbon steel with high permeability this is generally achieved with an applied tangential field strength, H, of… 



In practice flux density is difficult to measure.



The use of a hall effect probe is generally accepted to be the best method for measurement of flux density.



Even this equipment, which is fragile and expensive, measures the flux density outside the component – not the actual flux density achieved within the component.

2000 Amps per meter (2kA/m). Copyright © TWI Ltd


Assessing Magnetising Values

Permanent Magnet and Electromagnet – DC Yoke (Magnetic Flow)

Permanent Magnet/DC Yoke

N

S DC N


S


32

Permanent Magnet/DC Yoke The lift test A permanent magnet or DC yoke should be…

Permanent Magnet/DC Yoke 

Flux indicators become permanently magnetised in a DC field and are unreliable when used with permanent magnets or DC yokes.



Permanent magnets and DC yokes are not generally permitted by BS EN 9934-1 (but they can be used if the contracting parties agree).

DC

capable of lifting a steel weight of 18kg with a pole spacing of 75-150mm.



Magnetic Flow 

Adequacy of field strength can be assessed using a flux indicator, a cracked component or by using a standard test piece.



The rigid coil method is generally better if the component geometry is suited to coil magnetisation.


Electromagnet – AC Yoke (Magnetic Flow)



Permanent Magnet/DC Yoke The lift test An AC yoke suitable for MPI should be…

AC Yoke – Flux Indicator

AC

capable of lifting a steel weight of 4.5kg with a pole spacing recommended by the manufacturer (usually 170mm).

 


Flux can also be checked using a flux indicator. Area adjacent to poles is not suitable for carrying out inspection due to saturation and particle migration. Copyright © TWI Ltd

33


Prods Electrical current passes between prods through components. Field produced is taken as two deformed circles between prods.

Prods




Prods Current passed through sample, typically:

Prods Rectangular test area

Circular test area

IRMS = 2.5 H d (valid up to d = 200mm)

IRMS = 3 H d

5 or 6 amps (RMS) per mm of prod spacing. Rectangular area: 2.5 x H x d Circular area: 3 x H x d Where H = 2kA/m tangential field strength. D = prod spacing.

Where IRMS H d

is root mean square current in amps. is the tangential field strength in amps per metre. i s the prod spaci ng in mm (max 200mm).


Prods Example: Calculate RMS half-wave rectified AC current value for the prod technique using a prod spacing of 150mm if the inspection zone is a circle drawn through the contact points.



Flexible Cables Flexible coil

IRMS = 3 H d

IRMS = 3 x 2 x 150 = 900 amps Copyright © TWI Ltd


34

Closely Wrapped Coil

Close Wrapped Coil Closely wrapped coil of N turns d= 4

NI H

Where N=4 H=2 d = 4I 25.14

.

.



Flexible Cable – Close Wrapped Coil


Closely wrapped coil of N turns Assuming we need to test an area Extending 150mm from the coil. d

 . .

I=

..

Flexible Cables Adjacent cable



So the required current = 943 Amps Note: Where I is RMS AC, HWDC or FWDC current.


Adjacent Cable (Single Parallel Cable) A single parallel cable lying on the surface can be considered as a coil of one turn.


Adjacent Cable (Single Parallel Cable) Assuming we need to test an area extending 100mm from the cable… 

 .

.

  . 

So the required current = 2514 Amps The inspection zone being d mm each side of the cable 

Note: Where I is RMS AC, HWDC or FWDC current.

 .



35

Adjacent – Twin Parallel Cable Kettle Element

Adjacent – Twin Parallel Cable Kettle Element Example: 2d = 100 mm

I =4

I = 4

dH

dH

I = 4 x 3.14 x 50 x 2 = 1256 Amps

Where d = half the distance between the cables in mm

Where I is RMS AC, HWDC or FWDC current




Magnetic Flow Component clamped between headstock solenoids. Solenoids energised to produce strong magnetic field across component.

Magnetic Bench Unit Magnetic flow




Magnetic Flow


There are no formulae for calculating the required magnetising force when using magnetic flow. A flux indicator should be used to determine the magnetic field strength at the mid-point of the component under test.


Magnetic Bench Unit Axial current flow


36

Bench Unit – Axial Current Flow

Axial Current Flow Typical values AC or HWDC current:

Component clamped between headstocks. Electrical current passed through component produces an encircling magnetic field.



2 amps/mm perimeter IRMS = H x perimeter



When non-circular components are tested the flux density is not continuous over the surface of the component. eg for a square section the flux density is higher at the centre of the faces than it is at the corners.




Axial Current Flow – Irregular Shapes

Axial Current Flow

Square section

Typical values AC or HWDC current:

Flux density is higher at the centre of the faces than it is at the corners

IRMS = H x perimeter Where…

IRMS = 2

H = 2 kA/m perimeter =  x diameter d



Axial Current Flow Example: Calculate peak AC current value for 30mm diameter round bar stock. IRMS = 2

Axial Current Flow – Conditions  

d 

Answer: So I RMS = 2 x 3.142 x 30 = 188.52 amps and I PEAK = 188.52 x 1.414 = 267 amps




The length of a component has no effect on the required current value. Field strength can be assessed using a flux indicator, a cracked component or a standard test piece. With varying cross section a single value may be used where the ratio is less than 1.5:1, using the value of the larger cross section. Where the ratio is exceeded, two or more shots will be required. Copyright © TWI Ltd

37


Central Conductor (Threading Bar) Threader bar clamped between headstocks. Electrical current passed through threader bar produces an encircling magnetic field.

Magnetic Bench Unit Central conductor or threading bar




Central Conductor (Threading Bar) Typical values AC or HWDC current: 



Central Conductor (Threading Bar) Increase the current (I) to increase the radius (R) of the test zone.

2 amps/mm perimeter. IRMS = H x perimeter When non-circular components are tested the flux density is not continuous over the surface of the component. eg For a square section the flux density is higher at the centre of the faces than it is at the corners.

If the test piece is a hollow pipe or tube the current shall be calculated according to the outside diameter when testing the outside surface. The ID when testing the inside surface.



Central Conductor (Threading Bar) Example: Calculate peak HWRAC current value for a 100mm hexagon nut, threading bar technique.

Central Conductor (Threading Bar) 



IRMS = H x perimeter

Where a large ring requires testing the magnetic field may not enclosed the complement. More shots will be required, turning the component between shots to ensure coverage.

or IRMS = 2 p p = 6 x 58 = 348mm So IRMS = 2 x 348 = 696 Amps (RMS) Therefore, Ipeak = 696 x 2 = 1392 Amps (peak) Copyright © TWI Ltd


38


Bench Unit – Coil Coil can either replace the headstock or clamp between. Electrical current passed through coil produces a longitudinal magnetic field through coil.

Magnetic Bench Unit Coil

Defects found at 90° to magnetic field. Copyright © TWI Ltd


Rigid Coil 



The required current is inversely proportional to the number (N) of turns in the coil and the length to diameter ratio of the component (L/D). Typical current calculation formula:

Rigid Coil – Conditions   

NIRMS = 0.4 HK L/D 

This method is not generally applicable if L/D is less than 5, otherwise extenders are required.



Cross sectional area (CSA) of test piece <10% of Coil (Fill Factor). Test piece must lie against side or bottom (where the magnetic field is strongest). BS EN 9934-1 implies the test zone is the part of the component which lies within the coil (but in US instructions may extend up to 150mm beyond coil). L/D ratio must be between 5-20:  

If >20 use 20 as the ratio. If <5 pole extenders should be used to increase the length.



Rigid Coil – Pole Extenders

Rigid Coil

Pole extenders must be ferromagnetic material and approximately similar diameter to component under test.

NIRMS = 0.4 HK L/D

It artificially extends component length making L/D valid for use.

N= K=

L/D = H=


or

IRMS = 0.4 HK : N L/D

Number of turns in coil. 32,000 for DC (typical). 22,000 for AC or FWRAC (typical). 11,000 for HWRAC (typical). Length/diameter ratio. Tangential field strength, 2kA/m.


39

Rigid Coil – AC Rigid coil of N turns (Fill Factor < 10% CSA) FOR AC L/D 5 Note: If L/D > 20 assume L/D = 20

NIRMS = 0.4 HK L/D

Where IRMS is Root Mean Square current in Amps. H is the Tangential Field Strength in Amps per Metre. K is a constant. L is the component length in mm and D is the component diameter in mm. (non-circular components, effective D = perimeter/ )

AC: K = 22,000

Rigid Coil Example 1: Calculate peak AC current value for 50mm diameter by 300mm long round bar using a 200mm diameter rigid coil of 5 turns. What is the fill factor? Coil cross section =  x 1002 = 31,416 mm2 Component cross section =  x 252 = 1,963 mm2 Fill Factor = (1,963 / 31,416) x 100 = 6.25% (ie <10% so ok) NIRMS = 0.4 HK L/D

= 0.4 x 2 x 22000 = 2933 Amp turns 300/50


Rigid Coil Example 1: continued


Rigid Coil – FWRAC or HWRAC Rigid coil of N turns (Fill Factor < 10% CSA)

NIRMS = 0.4 HK = 0.4 x 2 x 22000 = 2933 Amp turns L/D 300/50

For a coil of 5 turns: The required current (RMS) = 2933/5 = 587 Amps Using AC current: The required peak current value is therefore: 587 x 1.414 = 830 Amps

FOR FWRAC or HWRAC L/D 5 Note: If L/D > 20 assume L/D = 20

NIMEAN = 0.4 HK L/D

Where IRMS is Root Mean Square current in Amps. H is the Tangential Field Strength in Amps per Metre. K is a constant. L is the component length in mm and D is the component diameter in mm. (non-circular components, effective D = perimeter/ ) HWRAC: K = 11,000 FWRAC: K = 22,000



Rigid Coil Example 2: Calculate peak HWRAC current value for a 25 mm square section by 800 mm long component using a 150 mm diameter rigid coil of 5 turns. L/D = 800/32 = 25 Therefore use an assumed value of 20 Coil cross section = p x 75 2 = 17,671 mm 2 Effective component diameter = 4 x 25/p = 32 Eff. Comp. cross section = p x 162 = 804mm2 Fill factor = (804/17,671) x 100 = 4.55% (ie <10% so ok)

Rigid Coil Example 2: continued NIMEAN = 0.4 HK = 0.4 x 2 x 11000 = 440 Amp turns L/D 20

For a coil of 5 turns: The required current (MEAN) = 440/5 = 88 Amps Using HWRAC current: The required peak current value is therefore: 88 ÷ 0.318 = 276 Amps

NIMEAN = 0.4 HK = 0.4 x 2 x 11000 = 440 Amp turns L/D 20 Copyright © TWI Ltd


40

Section 7

Electromagnets Lift test

DC Yoke Control and Maintenance Checks

– 18kg steel weight

AC Yoke – 4.5kg steel weight


Control and Maintenance Checks Equipment performance check   


Performance Check – Magnetic Flow Magnetic Flow Test Piece clamped between headstock solenoids.

Magnetic flow test piece. Current flow test piece. Cracked component.

Solenoids energised to produce strong magnetic field across component. The sub-surface defect should be visible.



Performance Check – Magnetic Flow

Performance Check – Magnetic Flow

Magnetic flow test piece

1. Thoroughly degrease and demagnetise the test piece. 2. Clamp it between the poles of the test bench (magnetic flow) or place it centrally in the co il parallel to the coil axis.

Indication Steel Bar

3. Energise the equipment and establish that the transverse hole in the middle of the test piece shows a strong indication.

Through drilled holes



41

Performance Check – Axial Current Flow

Performance Check – Axial Current Flow

Current flow test piece clamped between headstocks. Electrical current passed through component produces an encircling magnetic field. Cross drilled holes should be visible.



Performance Check – Axial Current Flow 1. Thoroughly degrease and magnetize the test piece. 2. Clamp it within head and tailstock of the test bench. 3. Apply magnetic ink while the current is being increased. 4. Establish the current required to make the hole nearest to the outer surface of the ring visible on the outer surface. 5. Further increase the current to establish indications from the other two holes on the outer surface of the ring.


Lighting Requirements (BS EN 3059) Visible methods  Minimum white light Luminance – 500 Lux (measure using a photometer or Luxmeter).  Glare must be avoided.  Monochromatic light sources such as sodium vapour lamps are not permitted.  Sunlight is best: Tungsten filament bulbs and fluorescent strip lights are OK.


Lighting Requirements 

Adequate lighting is crucial if the best test sensitivity is to be obtained in MPI.



BS EN 3059 Specifies lighting requirements for visible and fluorescent methods.


Lighting Requirements (BS EN 3059) Fluorescent methods  Minimum black light (UVA) irradiance 1000mW/cm2 (measure using a Radiometer).  Maximum white background lighting - 20 Lux.  Photosensitive spectacles must not be worn.  Allow the lamp to warm up for 10 minutes.  Allow 5 minutes dark adaptation.


42

Control and Maintenance Checks Ink settlement test…

Magnetic inks - ISO 9934-2 (amongst other things) 100ml

1 0 0

Decant 100ml of ink into the Sutherland flask and allow particles to settle. Fluorescent Ink  0.1 - 0.3 %

4 3. .0 20 .

Non-Fluorescent Ink  1.25 - 3.5 %

Detecting Media

0 . 5

1 . 0

0

1.0ml 0.5ml

Requires:  Fluorescent Particles - minimum level of fluorescence.  Carrier Liquid - maximum level of fluorescence and maximum viscosity.  Magnetic Particles - size distribution, maximum particle size (40m), concentration (per supplier’s recommendation).  Test blocks and tests.

Typical values as BS EN 9934-2 does not specify. Copyright © TWI Ltd


Detecting Media BS EN ISO 9934-2 describes three categories of tests for detection media. 1. Type testing: Tests intended to be performed by the supplier. 2. Batch testing: Tests intended to be performed by the supplier. 3. In-service testing: Tests which should be performed by the inspector prior to and during actual magnetic particle testing.

Detecting Media Type testing/batch testing    

 

Colour. Particle size. Temperature resistance. Fluorescent coefficient/fluorescent stability. Carrier liquid fluorescence. Flash point.

     

Corrosion tests. Mechanical stability. Foaming. Acidity/alkalinity (pH). Storage stability. Sulphur and halogen content.



Detecting Media

Detecting Media

Particle size distribution

Magnetic powders:

lower diameter = d 1 average diameter = d a upper diameter = d u

d1 : generally greater than  40mm (BS EN 9934-2) du : not specified, but typically  200mm

   

No more than 10% shall be smaller than d 1 and No more than 10% shall be larger than d u

(previously stated in, now superseded BS 4069)

50% of the particles shall be larger than d a and 50% of the particles shall be smaller than d a Copyright © TWI Ltd


43

Detecting Media

Detecting Media

Magnetic inks:

In-service testing:

d1 : shall be greater than or equal to 1.5mm



For inks, powders and contrast-aid paint: Colour (By comparison).



For inks and powders: Performance testing.

du : shall be less than or equal to 40mm 1.5mm  d  40mm



Performance Checks – Detection Media Performance checks (for inks and powders) i.a.w. BS EN ISO 9934-2: 2002. Magnetic particle testing - Detection media. Either  Using test block Type 1 or Type 2 compare the indications produced with known results. or  As part of a system test using a component containing discontinuities similar to those which it can be expected the test will detect.

Performance Checks – Detection Media Reference block type 1 Grinding cracks

Stress corrosion cracks

  

Heat treated steel disk - Grade 90MnCrV8. Containing grinding cracks and stress corrosion cracks. Permanently magnetised using a central conductor - direct current, 1,000 Amps (peak).



Control Check Frequency         

Settlement test Test piece Tank levels Fluorescent intensity Magnetising units Viewing efficiency UV lamp Ammeters Demagnetiser

Daily Daily Daily Weekly Weekly Monthly Monthly 6 monthly 6 monthly

Magnetic Particle Testing

Any questions?



44

MT HANDOUT -CONTROL CHECK -PRACTICAL REPORT EXAMPLE -CALCULATION -WRITING INSTRUCTION EXAMPLE

Candidate name:

Date:

GENERAL PRACTICAL EXAMINATION MAGNETIC PARTICLE INSPECTION

.

Please use the space provided overleaf to record details & results of the test that you have been requested to perform. The time allowed for this part of the examination is 1 hour.

1.

Ink settlement test

−

perform a settlement test on the ink sample provided and report your results.

2.

Lift test (AC Yoke)

−

complete a lift test on the AC yoke provided and report your results.

3.

Lift test (DC Yoke or permanent magnet)

−

complete a lift test on the DC yoke or permanent magnet provided and report your results.

4.

UV-A illumination

−

check and record the levels of UV-A illumination in the area indicated to you by the invigilator.

5.

White light illumination

−

check and record the levels of white light illumination in the area indicated to you by the invigilator.

6.

System test for magnetic flow

−

using the magnetic flow test piece provided determine the equipment setting required to raise an indication from the drilled hole.

7.

System test for current flow

−

using the current flow test piece provided determine the HWDC current required to raise an indication from the first drilled hole.

1

1.

EQUIPMENT REQUIRED/USED Magnaflux Y6 Calibrated 4.5 kg test weight

2.

APPLICABLE SPECIFICATION & MINIMUM REQUIREMENTS BS EN 9934-3 paragraph 4.1.1

3.

METHOD 1. Set pole spacing to 170 mm. 2. Adjust pole pieces to ensure good contact with the test weight. 3. Place the yoke on the test weight and energise. With the magnet still energised try to lift the test weight. 4. The yoke passes the test if it is capable of supporting the test weight.

5. Record the test results on the daily log sheet.

5. RESULTS 4.5 kg test weight successfully lifted – the yoke has passed the test

2

Candidate name:

Date:


.


1.

Ink settlement test

−


2.

Lift test (AC Yoke)

−


3.


−


4.

UV-A illumination

−


5.


−


6.


−


7.


−


3

1.

EQUIPMENT REQUIRED/USED Radiometer / Photometer Magnaflux Mk VI (with valid calibration certificate) Black light – Magnaflux s/n 2719 Darkened area

2.

APPLICABLE SPECIFICATION & MINIMUM REQUIREMENTS BS EN 9934-1 Paragraph 10.2 & BS EN 3059 Background white light maximum 20 lux 2 UV-A irradiation 1000 µW/cm at the test surface

3.

METHOD Check meter for valid calibration certificate. Check filter for damage, if OK switch on & warm up for a minimum of 10 minutes. Check filter for correct fitting, if any UV leaks noted switch off and stop the test. Place the combined UV-A / white light sensor on the test surface. Press the white VIS – lux button and record the white light illumination measured. 2 Press the black UV mW/cm button and record the UV-A irradiation. Note that to convert mW to µW you need to multiply by 100

4.

RESULTS Calibration check – instrument number 6421 used calibration valid until 1709-2006. White light check – 17 lux at the test surface, this is less than 20 & therefore acceptable. 2 2 Black light check – 1.63 mW/cm = 1630 µW/cm which is greater than the 1000 required and therefore acceptable.

4

Candidate name:

Date:



1.

Ink settlement test

−


2.

Lift test (AC Yoke)

−


3.


−


4.

UV-A illumination

−


5.


−


6.


−


7.


−


5

1.

EQUIPMENT REQUIRED/USED Photometer Magnaflux Mk VI (with valid calibration certificate)

2.

APPLICABLE SPECIFICATION & MINIMUM REQUIREMENTS BS EN 9934-1 Paragraph 10.1 & BS EN 3059 White light minimum 500 lux at the test surface

3.

METHOD Check meter for valid calibration certificate. Place the white light sensor on the test surface. Press the white VIS – lux button and record the white light illumination measured.

4.

RESULTS White light illumination measured, result 950 lux at the test surface. This is well above the required 500 lux minimum required – ACCEPTABLE.

6

Magnetic Testing Report

WORLD CENTRE FOR MATERIALS JOINING TECHNOLOGY

Page : 1 of 2

Name :

Date :

Your Name

Specimen Description:

Sample no.: MPTL 001

Butt weld in mild steel plate Size : 300mm x 200mm x 12mm

Surface cont./Pre cleaning : As welded , slag removed

Visible wet continuous method

Technique

:

Values

: Not Available

Equipment used : AC energised electromagnet (AC yoke) Photometer Burmah Castrol Strip

Consumables

:

Solvent Cleaner SKS-C White Contrast Paint - WCP 712 Magnetic Ink - Supramor 4 Black Ink

Light Levels

: White Light Illumination =700 Lux

Component Field Strenght Assessment (Reading) 1) Pre Test 2) Post Test

: :

0 Gauss 0 Gauss

Component Flux Indication Assessment Post Test Requirement:

:

3 lines visible on castrol strip

Component cleaned and protected

Defect Record to Include : Datum, Size, Location and Defective Remarks

Signature :

:

See attached sketch

Date :

7

SARAWAK SAMPLE

MAGNETIC PARTICLE TESTING

MTPL 03 ABM SK - PLATE

SARAWAK SAMPLE

MAGNETIC PARTICLE TESTING

MTPL 03 ABM SK - PLATE

WORLD CENTRE FOR MATERIALS JOINING TECHNOLOGY

VISIBLE WET CONTINUOUS METHOD

TECHNIQUE:

A

B

2

1

CL

1 Defect:

2 Defect:

Branch Crack

From Datum A: Length/width:

Branch Crack

From Datum A: Length/width:

52 mm 42 mm

233 mm 43 mm

REMARKS:


WORLD CENTRE FOR MATERIALS JOININGTECHNOLOGY

Page :

Name :

Date : 30/08/03

Matt Flux


Casting component Size : 125mm x 20mm

Surface cont./Pre cleaning : As cast

Technique

:

Flourescent wet continous

Values

:

CF 1= 159amps ac RMS CF 2= 289 amps ac RMS

Sample no.: P 227

of


WORLD CENTRE FOR MATERIALS JOININGTECHNOLOGY

Page :

Name :

Sample no.: P 227

Date : 30/08/03

Matt Flux


of

Casting component Size : 125mm x 20mm

Surface cont./Pre cleaning : As cast

Technique

:

Flourescent wet continous

Values

:

CF 1= 159amps ac RMS CF 2= 289 amps ac RMS

Equipment used : Bench Unit SBU 3000 - AC energised

Consumables

:

Solvent - MX 325 Flourescent Magnetic ink - MXF 22

: 2530 µW/cm² on surface of component

Light Levels

Component Field Strenght Assessment (Reading) 1) Pre Test 2) Post Test

: :

2 Gauss 1 Gauss

Component Flux Indication Assessment Post Test Requirement:

:

3 lines visible on castrol strip

Component cleaned and protected

Defect Record to Include : Datum, Size, Location and Defective Remarks

Signature :

:

See attached sketch

Date :

31/08/04

9

Magnetic Particle Testing Report Datum 2

C

C A325M 325MA 325MA

20

1

35

1 373

47 60

BOTTOM VIEW

SIDE VIEW

TOP VIEW

7 3

FINDINGS: 1

- Seams

2

- Seams

Inspected by : Signature

3

- Seams

Qualification : :

TWI WORLD LEADERS IN MATERIAL JOINING

Date

TECHNOLOGY

:

11

Component 1 Question 1: Component 1 has dimensions a = 15 mm; D = 70 mm & L = 360 mm. (a). (b). (c). (d).

Calculate the RMS HWRAC current required to magnetise the octagonal section using axial current flow. Calculate the RMS HWRAC current required to magnetise the cylindrical section using axial current flow. Calculate the mean HWRAC current required to magnetise the octagonal section if using a coil of 5 turns. Calculate the mean HWRAC current required to magnetise the cylindrical section if using a coil of 5 turns.

Question 2: Component 1 has dimensions a = 17 mm; D = 65 mm & L = 375 mm. (a). (b). (c). (d).

Calculate the RMS HWRAC current required to magnetise the octagonal section using axial current flow. Calculate the RMS HWRAC current required to magnetise the cylindrical section using axial current flow. Calculate the mean HWRAC current required to magnetise the octagonal section if using a coil of 7 turns. Calculate the mean HWRAC current required to magnetise the cylindrical section if using a coil of 7 turns.

Question 3: Prod method: calculate the RMS AC current required to magnetise an inspection zone which is a circle 180 mm in diameter. Question 4: Adjacent cable method: calculate the RMS AC current required to magnetise an inspection zone which is 65 mm wide.

12

Answers Question 1: Component 1 has dimensions a = 15 mm; D = 70 mm & L = 360 mm. (a).

Calculate the RMS HWRAC current required to magnetise the octagonal section using axial current flow. p = 8x15 = 120; I RMS = Hp = 2x120 = 240 = 240 A

(b). Calculate the RMS HWRAC current required to magnetise the cylindrical section using axial current flow. p = 70π = 220; I RMS = Hp = 2x220 = 440 = 440 A (c).

Calculate the mean HWRAC current required to magnetise the octagonal section if using a coil of 5 turns. π = 38.2; L/D eff = 360/38.2 = 9.42 Deff = (8x15)/ π NI = (0.4x2x11000) (0.4x2x11000)/9.42 /9.42 = 934 I MEAN = 934/5 = 187 A

(d). Calculate the mean HWRAC current required to magnetise the cylindrical section if using a coil of 5 turns. L/D = 360/70 360/70 = 5.14 NI = (0.4x2x11000) (0.4x2x11000)/5.14 /5.14 = 1712 I MEAN = 1712/5 = 342 = 342 A Question 2: Component 1 has dimensions a = 17 mm; D = 65 mm & L = 375 mm. (a).

Calculate the RMS HWRAC current required to magnetise the octagonal section using axial current flow. p = 8x17 = 136; I RMS = Hp = 2x136 = 272 = 272 A

(b). Calculate the RMS HWRAC current required to magnetise the cylindrical section using axial current flow. p = 65π = 204; I RMS = Hp = 2x204 = 408 = 408 A (c).

Calculate the mean HWRAC current required to magnetise the octagonal section if using a coil of 7 turns. Deff = (8x17)/ π π = 43.3; L/D eff = 375/43.3 = 8.66 NI = (0.4x2x11000) (0.4x2x11000)/8.66 /8.66 = 1016 1016 I MEAN = 1016/7 = 145 A

(d). Calculate the mean HWRAC current required to magnetise the cylindrical section if using a coil of 7 turns. L/D = 375/65 375/65 = 5.78 NI = (0.4x2x11000) (0.4x2x11000)/5.78 /5.78 = 1522.5 I MEAN = 1522.5/7 = 217.5 = 217.5 A Question 3: Prod method: calculate the RMS AC current required to magnetise an inspection zone which is a circle 180 mm in diameter. I RMS = 3Hd = 3x2x180 = 1080 A Question 4: Adjacent cable method: calculate the RMS AC current required to magnetise an inspection zone which is 65 mm wide. I RMS = 4π dH = 4x3.14x65x2 = 1634 A

13

MPII Calcul MP Calcul ation Exer cis e 1

50mm

30mm

150mm

150mm Calculate Peak AC currents required to detect longitudinal and circumferential (transverse) faults. 1. 2.

Axial Current Current Flow Coil shot

NOTE: SHOW FULL WORKING OUT.

1. Current Flow Diameter ratio, 50:30 equates to 1.667:1 >>> requires 2 shots Shot 1. I = Hp I = H π d I = 2 x π x 30 I = 188.5 Amp RMS I = 266.6 Amp PEAK

Shot 2. I = Hp I = H π d I = 2 x π x 50 I = 314.2 Amp RMS I = 444.4 Amp PEAK

14

2. Coil Fill Factor Coil CSA Item CSA

2

πr 2 πr

π x 150 x 150 π x 25 x 25

1963.5 ÷ 70685.8 x 100% NIRMS

=

2

70685.8 mm 2 1963.5 mm

= 2.78 %

< 10% -- OK --

0.4 H K L/D

Where: L = 150mm ; D = 50mm ; H = 2 ; K = 22000 (AC) ; N = 5 L/D

= 300/50

=6

NIRMS

=

0.4 x 2 x 22000 150/50

=

17600 6

NIRMS

=

2933 Amp Turns

IRMS

=

2933 ÷ 5

=

586.7 Amp RMS

=

829.8 Amp PEAK

-- OK --

15

MPI Calcul ation Exer cis e 2

20mm

20mm

190mm

Calculate Peak AC currents required to detect longitudinal and transverse faults. 1. 2.

Axial Current Flow Coil shot (4 turn ; 200mm Internal diameter)


1. Current Flow Shot 1. I = Hp I = 2 x 20 x 20 I = 160 Amp RMS I = 226 Amp PEAK

16

2. Coil For non-circular components, the effective diameter is D = perimeter ÷ π Perimeter = 4 x 20mm = 80mm D = 80 ÷ π D = 25.46 mm Fill Factor Coil CSA Item CSA

2

πr 2 πr

π x 100 x 100 = π x 12.73 x 12.73 =

509 ÷ 31416 x 100%

= 1.62 %

L/D

= 190/25.46 = 7.46

NIRMS

=

2

31416 mm 2 509 mm

< 10% -- OK --

( > 5 ) -- OK --

0.4 H K L/D

Where: L = 190mm ; D = 25.46mm ; H = 2 ; K = 22000 (AC) ; N = 4 NIRMS

=

0.4 x 2 x 22000 190/25.46

=

17600 7.46

NIRMS

=

2358.4 Amp Turns

IRMS

=

2358.4 ÷ 4

=

589.6 Amp RMS

=

833.9 Amp PEAK

17

MPI Calcul ation Exer cis e 3

40mm

Inside Diameter 32mm 60mm

Calculate Peak AC currents required to detect longitudinal faults using a threader bar (10mm diameter) 1. 2.

Calculate current required to carry out complete inspection in ONE shot. Calculate current for diameter of component, and estimate number of shots required to ensure full coverage.


1. ONE shot I = Hp I = H π d I = 2 x π x 62 I = 389.6 Amp RMS I = 551 Amp PEAK 31mm

2. Using component O/D Shot 1

I = Hp I = H π d I = 2 x π x 40 I = 251.3 Amp RMS I = 355.5 Amp PEAK 3 shots required to cover item

Shot 3

Shot 2

18

WRITTEN INSTRUCTION FOR THE MAGNETIC PARTICLE TESTING OF BUTT WELDS IN FLAT PLATE. AC YOKE, WET CONTINUOUS METHOD Document Reference: WI/MPI/001 Issue: 0 Revision: 0 Written by: Name: Gary Masding

Qualification: PCN Level 2 MPI (Gen) Signature: th

Date: 18 August 2005 Approved by: Name: Malcolm Spicer

Qualification: PCN Level 3 MPI (Gen) Signature: th

Date: 18 August 2005 Component Identification & Details

This instruction applies to butt welds in flat low carbon steel plate. Plate identification MW001 Purpose of Test

To detect all relevant surface breaking defects in the weld cap and heat affected zone. Test Zone

The weld cap and at least 25 mm of parent material each side of the weld shall be tested 100%. Personnel Qualification

Personnel working in accordance with this written instruction shall as a minimum be qualified to EN473 Level 1 MPI (welds). Equipment

AC Yoke – Magnaflux Y6 or similar Photometer – with valid calibration certificate Burmah Castrol flux indicator – Type II Gaussmeter Wire brush 4.5 kg certified test weight Paint marker

19

Consumables Note: The use of bulk consumables is not permitted

White contrast paint – WCP2 (aerosol cans only) Black magnetic ink – 7HF (aerosol cans only) Solvent cleaner – Sonasol

Safety precautions

Appropriate personal protective equipment is to be worn at all times. MPI consumables can be hazardous to health if used inappropriately. All personnel should make themselves familiar with the relevant COSHH data sheets. These are held on file by the Inspection Supervisor. In particular MPI must not be performed in areas where there is a potential source of ignition and all areas where testing is performed must be well ventilated. Preparation for test.

Prior to testing the weld cap and 25 mm of parent material each side shall be free from weld spatter, slag, mill scale, loose corrosion products, dirt and grease. Sharp changes of contour shall be blended by grinding. 1. 2. 3. 4.

Solvent clean the weld cap and 25 mm of parent material each side. Wire brush to remove loose corrosion products etc. Carry out close visual inspection making note of any defects or irregularities. Perform a residual magnetisation test using the gaussmeter. If the reading obtained exceeds 2 gauss demagnetise by stroking with the yoke. Place the yoke on the weld, energise & while keeping the yoke energised drag it along the weld and withdraw to a distance of not less than 1.5 metres from the weld. 5. Check lighting levels using the photometer. A minimum level of 500 lux white light shall be present at the test surface. Acceptable types of lighting are daylight, tungsten filament bulbs and fluorescent strip lights. Monochromatic light sources such as sodium bulbs are not permitted. 6. At the start of each shift perform a lift test on the yoke using the 4.5 kg test weight with the yoke set at a pole spacing of 170 mm. If the yoke fails this test report immediately to the inspection supervisor. 7. As all consumables are pre-mixed aerosols it is not necessary to check the particle concentration.

20

Test Instructions

1. Apply a thin film of white contrast paint to the weld cap and a minimum of 25 mm of parent material each side. Shake the can well before use and hold the aerosol at least 300 mm from the surface. Use just enough paint to mask out the background colour. Thick layers of paint will reduce test sensitivity. Allow at least 2 minutes for the paint to dry. 2. Check for adequate magnetisation using the Burmah Castrol strip. Set the pole spacing to 150 mm and adjust the pole pieces to ensure good contact. Place the strip between the poles of the magnet at 90° to the expected field direction. Magnetise, then apply ink whilst continuing to magnetise. Three lines shall be clearly defined, if this is not the case, report immediately to the inspection supervisor. 3. The test area is to be magnetised in two directions mutually at 90 ° using a pole spacing of 100 to 150 mm. Pole positions are shown in figure 1 below. The inspection zone is a circle drawn through the poles. The continuous method shall be used. Ink shall be applied during magnetisation and magnetisation shall continue for a few seconds after the application of detection media ceases. Non-conformance

If for any reason the operator is unable to perform the test as described he or she must stop work immediately and report to the inspection supervisor. Recording Criteria

All indications having a dimension greater than 2 mm shall be investigated to determine whether they are relevant, non-relevant or spurious. The following shall be reported: 1. All relevant linear indications. 2. All relevant rounded indications having a maximum dimension of 3 mm or greater.

Reporting

For each item tested a report shall be prepared using the standard report format. Where recordable conditions are detected a dimensioned defect sketch shall be attached to the report. In order that defect positions can be reported the test item shall be marked with a datum point using a paint marker.

21

CSWIP MT Level 2 Textbook (2016).pdf

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