Background A wire wire rope broke while lifting lifting a load of reinforcing reinforcing steel estimated estimated to weigh 2.5 - 3 tonnes. The precise sequence of events leading to the the failure were not known but the load did not drop because the rope !ammed in the gap between sheave and support bracket. The rope was 2 "ears old and was stated to be maintained b" regular lubrication. Bounce of the load # Activit" # Activit" $ % was possible during lifting. The rope sheave diameter was 52& mm and it was known to have been in service longer than the rope. The insurance loss ad!ustor #whose !ob is to advise the insurance compan" on the quantum of loss and whether the e'isting policies covered the causes and consequences of the failure% commissioned a failure investigation to ascertain whether the rope was overloaded at the time of failure or whether poor maintenance had been responsible for for premature breakage. The resolution of these issues issues would determine whether or not insurance cover e'isted for the various consequences of the accident and would also indicate if an increase in premium was necessar" to offset a possible higher level of risk. Three pieces of rope were supplied to assist in this investigation - these comprised the two broken ends together with a section of rope taken well awa" from the failed ends. The purpose of this latter piece was to check the load capacit" of the rope via tensile testing at the time of failure. (isual )bservations The rope was a general engineering $* strand non-spin t"pe designed as $2'+#,$%,'+#,$% - see the illustration below. This designation is a shorthand form which summarises the information contained contained in the image. Thus this rope had an inner la"er of , strands of wires with each strand comprising + wires wrapped as , outer wires around $ inner wire while the outer la"er is formed b" $2 strands of wires wrapped in the same wa". The inner core of the wire is fibre. esistance to rope spin is provided b" opposing twist directions of the inner la"er #anticlockwise% #anticlockwise% and outer la"er #clockwise% whereb" the load-induced torque tends to cancel out. /urther information on wire rope design damage and the effect of sheave si0e can be found b" following this h"perlink h"perlink and and looking at thisweb thisweb page. page. As received rope lubrication was was deficient to dr" dr" with slight corrosion evident on on the outside of the rope.Figure rope. Figure 1 shows 1 shows the 2 broken ends of the rope. 1lose inspection of the rope near to the fracture plane showed that a number of wires were cracked in outer and inner strands #Figures # Figures 2 & 3%. 1racks on both inner and inner la"ers were associated with flattened regions on the wires.1racking was also observed on wires well awa" from this region #Figure #Figure 4%. 4%.
Figure 1
Figure 2
Figure 3
Figure 4
roceed to second part of part of case stud".
Tensile Testing Before performing tensile testing of the rope it was necessar" to establish the original grade and si0e of the wire rope. This would indicate what degradation of properties had taken place over the service life and provide an indicator of the severit" of service and qualit" of maintenance. The onl" information that the operator could suppl" was that the rope was a $++& a grade. Thus it was necessar" to measure the diameter of wires near to the break #average appro'imatel" $.5 mm% and the rope diameter #appro'imatel" 2$.5 mm%. 4se the information contained in the wire rope manufacturers table of properties # Activit" 2% to find the most likel" original rope diameter and breaking force. Tensile testing was performed on a $.5 m length sample using a wire rope testing machine giving a measured failure load of 232 k6. 7t should be noted that this load did not represent complete failure of the rope but rather fracture of $$ strands #,, wires% out of a total of $* strands #$&* wires%. Two inner and five outer strands remained unbroken.8'amination of the ,, wires that broke indicated that onl" $& of them had a flat fracture surface which would be indicative of the presence of an initial fatigue crack with the rest showing ductile failure modes. eferring back to the information on original rope diameter and breaking force use Activit" 3 to compare this load with the observed value for breaking load in the tensile test. 9hat conclusions can be drawn regarding the likel" influence of pre-e'isting fatigue cracks: /ractograph" A number of individual broken wires were cut off the fractured ends and e'amined at low magnification using stereo binoculars and at high magnification in a scanning electron microscope #;8%. The total number of wires in all strands was $&* and 2& wires were selected from the outer strands and $$ from the inner strands. The wires were de-rusted and ultrasonicall" cleaned in a de-greasing agent. T"pical ;8 observations of the fracture surfaces are given below at both a low and a high magnification together with information on the number of wires in the sample which were similar. Type 1: Tensile cup-and-cone fracture - 3 occurrences - $ wire in outer strands 2 wires in inner strands.
=igh magnification fractograph from the central region of the cup-and-cone fracture.
Type 2: Flat twisted failure -2 instances in outer strands.
Type 3: Flat semi-elliptic regions present - 2, cases> $+ in outer strands ? in inner strands
Example 1
=igh magnification fractograph from f lat semi-elliptic region shown with arrow.
Example 2 4sing the fractographic information from Activit" @ determine the mechanism of failure indicated b" t"pes $-3.
;ummar" and 1onclusions The information gleaned so far f rom this case stud" is summarised below $. )verall the strength of the rope was reduced b" the presence of fatigue cracks - this is evidenced b" the observed tensile strength of 232 k6 compared with the manufacturers stated breaking load of 332 k6. 2. The observed breaking load is still ver" much higher than the stated load being lifted at the time of failure #some 25 - 3& k6%. Thus the rope must still have failed through application of an overload relative to its current strength level. The failure was not solel" due to the presence of fatigue cracks #whose e'istence is fairl" normal in wire ropes and e'plains the requirement for regular maintenance and high factors of safet"%. 3.
The cause of this overload is not clear but bouncing of the load might have allowed the rope to !ump from its groove and !am between sheave and boom during winding. 7f this state of affairs could e'ist for a short time undetected and the rope winding was continued a ver" significant overload could be applied to the rope.
@.
The cause of the fatigue cracks needs clarification. The" can initiate as a result of bending stresses induced b" too small a sheave diameter. The recommended diameter is $*' rope diameter which equals @32 mm for the resent rope. The actual sheave diameter was 52& mm which should have been sufficient. As the sheave was older than the rope however it is possible that wear of the sheave groove has had an influence. /atigue cracks can result from deformed surface regions where ductilit" thus becomes e'hausted particularl" if surface damage from abrasion occurs. This would be e'acerbated b" an" decrease in sheave groove diameter which could occur b" wear during service and b" poor lubrication practice #which was apparentl" the case%.
The conclusion to be drawn from this investigation is that the presence of fatigue cracks has lowered the breaking load of the rope b" some 3&. =owever the breaking load is still 232 k6 ver" much higher than the stated lifting load of 25 - 3& k6. The fractographic work has indicated ductile fracture in all wires demonstrating that the rope metallurg" is up to specification. The most likel" cause of the fracture seems to be rope !amming between sheave and groove probabl" due to bounce during lifting. The cause of the bouncing is unknown. 7n insurance terms poor maintenance is not a prime cause of the failure which would have been Csudden and une'pectedC when it occurred. 1over should e'ist for such circumstances.ecommendations An" good failure investigation leads to recommendations aimed at avoiding the problem in the future or at least reducing its likelihood of occurring. ecommendations in the present case are $. 8nsure adequate lubrication is maintained in the rope. 2. e-groove the sheave at regular intervals and particularl" when the rope is replaced b" a new one.
3.
1ontrol lifting to avoid bouncing and install detectors which are activated b" rope coming off the sheave.
@.
onitor condition of rope b" surface inspection and tensile testing.
eferences - to fatigue of wire ropes. $. D
6/ 1ase" and 9F
@.
Alani and aoof #$??+% 8ffect of mean a'ial load on a'ial fatigue life of spiral strands 7nternational Dournal of /atigue (ol. $? 6o. $ pp$-$$
5.
F 1oultate #$??+% agnetic attraction of wire rope testing aterials 9orld (ol. 5 ;eptember $??+ pp5$?-52&.
,.
F ;chrems and G aclaren #$??+% /ailure anal"sis of a mine hoist rope 8ngineering /ailure Anal"sis (ol. @ 6o. $ pp25-3*.
+.
G Furuppu A T"tko and T; Eolosinski #2&&&%
*.
D-7 ;uh and ; 1hang #2&&&% 8'perimental stud" on fatigue behaviour of wire ropes 7nternational Dournal of /atigue (ol. 22 pp33?-3@+.
?.
Torkar and B Ar0ensek #2&&2% /ailure of crane wire rope 8ngineering /ailure Anal"sis (ol. ? 6o. 2 pp22+-233.
Failure Analysis of Wire Rope Brett A. Miller, Stork Technimet Inc. From: B.A. Miller, Failure Analysis of Wire Rope, Advanced Materials and Processes, Vol 1! "#o. $, May %&&&, p '()'* Abstract: Mechanical properties of +ire ropes, their chemical composition, an the failure analysis process for them are escri-e. The +ires are manufacture from hih/car-on, plain car-on steel, +ith hih/strenth ropes most often manufacture from AISI 0rae 1&!'. urin 2isual failure e3amination, the rope, stran, an +ire iameters shoul all -e measure. 43amination shoul also aress the presence or a-sence of lu-ricant, corrosion e2ience, an ross mechanical amae. Faile +ires can e3hi-it classic cup/an/cone uctile features, flat fatiue features, an 2arious appearances in/-et+een. 5o+e2er, +ires are often mechanically amae after failure. Most nonestructi2e e2aluation "#4$ techni6ues are not applica-le to +ire rope failures. 4lectron microscope fractoraphy of fracture surfaces is essential in failure analysis. Fatiue is the most important fracture moe in +ire ropes. Metalloraphic features of +ire ropes that faile -ecause of uctile o2erloa an fatiue are escri-e. Keywords: Fractoraphy7 Metalloraphy7 Wire rope Material: 1&!' "#onresulfuri8e car-on steel$, 9#S 01&!'& Failure types: uctile fracture7 Fatiue fracture
Introduction Among load-bearing steel constructs, wire ropes and cables are possibly the most widely used and most highly stressed. Wire ropes are assemblages of intertwined steel wires and wire strands for pulling or lifting. They serve in critical applications and in severe environments, and failures are common. One
particularly damaged rope that failed via fatigue is shown in Fig. 1. From a metallurgical failure analysis perspective, wire rope failures can appear deceptively mundane. owever, investigation of these failures involves many complicated and uni!ue considerations that are worthy of review.
Fig. 1 This photoraph of a se2erely cracke +ire rope e3hi-its +ire fractures that result from an improper ser2ice en2ironment.
This article includes a description of the mechanical properties of wire ropes, their chemical composition, and a detailed discussion of the failure analysis process.
Mechanical properties The mechanical properties of a wire rope r esult from the individual wires and their arrangement or construction. "nli#e most other metallic components, rope wires are stressed alternately in tension, compression, torsion, and shear. The wires and strands are designed to slide in relation to each other, distributing the complicated applied stresses more effectively. For this reason, wire rope is often called a $machine.% As machines, ropes re!uire prudent inspection, maintenance, and periodic replacement. The inherent friction coefficient between bare steel strands mandates presence of ade!uate lubrication to allow the re!uisite wire movement. Wire rope constructions are identified by the number of strands, the number of wires per strand, the type of core, the $lay% &length' direction, left or right handedness, and many other attributes. (ope grades are rated in tons of brea#ing strength. The individual wires provide strength, whereas the construction dictates service characteristics. )t is generally accepted that many smaller wires provide better fatigue resistance, while fewer, larger wires provide better abrasion resistance. )ndependent wire rope cores &)W(*' provide better crushing resistance than fiber cores. +ecause of the comple geometry of the assembled wires, the ultimate tensile strength is not e!uivalent to a large wire of e!ual cross-sectional area. A certain percentage of the load on individual wires produces shear stresses, rather than aial tensile stresses, and steels ehibit lower &approimately /' ultimate strength in shear. 0evertheless, a synergy of mechanical properties is produced, providing ecellent elastic bending properties.
hemical composition Wire rope specifications are somewhat anachronistic compared with those of most steel components. Although the engineering community has become increasingly more specification-driven, no standards re!uire steel wire ropes to be manufactured from a particular composition, with identified elemental or impurity limits. The wires are manufactured from high-carbon, plain carbon steel, with high-strength ropes most often manufactured from A)) 2rade 134. Wire rope conformance is typically dependent upon mechanical properties alone, with composition dictated by the ability of the finished product to satisfy the re!uisite strength level after wire drawing and rope manufacture. Wire grades
5abeled grades of wires are of historical significance, but are somewhat vague. The following grades have been specified6 iron, traction steel, cast steel, etra strong cast steel &a.#.a. mild plow steel', plow steel, improved plow steel, etra-improved plow steel, and etra-etra-improved plow steel. 5owerstrength grades have been almost completely discontinued. )mproved plow steel, etra-improved plow steel, and etra-etra-improved plow steel are primarily used in engineering applications, with approimate comparative strength ratings of 161.1761.89, respectively. Lubricants
The lubricant applied to wire ropes also provides a measure of corrosion resistance in relatively benign service environments. For enhanced corrosion resistance, wire rope is available in a galvani:ed form, but with a 1/ reduction in mechanical strength from identically si:ed $bright% &non-plated' carbon steel rope, unless it is drawn after galvani:ing. Austenitic stainless steel grades are available for more severe environments, but can also be substantially lower in strength than plain carbon steel ropes, depending on wire si:e and rope construction.
Microstructure As with all engineering materials, the physical and mechanical properties of steel rope wires are a function of the microstructure, which is a function of processing. (ope wires are very heavily drawn, with severely cold-wor#ed microstructures. The typical longitudinal and transverse microstructures of a high strength wire rope are shown in Fig. 8. The structure consists of pearlite and ferrite grains that have been drawn down so far that the grain boundaries are not easily resolved. (elatively low levels of nonmetallic inclusions are re!uired in high !uality rope wires, to provide better fatigue resistance and more uniform mechanical properties.
Fig. ! Typical lonituinal "left$ an trans2erse "riht$ microstructure of hih strenth steel +ire rope. %: #ital etch "&&;$
Failure analysis process Preliminary Investigation
The preliminary portion of an investigation is of great importance in wire rope analyses, similar to all materials failure investigations. All possible information should be gathered, including the purchased rope specification, service history, service environment, estimated loads, and maintenance history. owever, it is not unusual to receive very little reliable information concerning a rope failure. This is often due to a general industrial misunderstanding of the compleity of wire rope constructions and service characteristics. After the available information has been collected, the failure analysis procedure should be planned, including identification of analytical tests and the location of test samples. Visual examination
ince a prudent investigator will progress systematically from least to most destructive analytical methods, a thorough visual evaluation is typically underta#en first. The visual eamination of wire rope and cable failures is analogous to other metallurgical failures, ecept for the fact that hundreds of individual fracture surfaces may be present. )nspection generally includes photographic documentation of all pertinent observations. The rope, strand, and wire diameters should all be measured. )t is especially important to measure all different wire si:es that may be present. This is necessary to identify the rope construction. ;amination should also address the presence or absence of lubricant, corrosion evidence, and gross mechanical damage. The identification of crac#s, #in#s, doglegs, and abrasion on the rope is also important. Failure features
Failed wires ehibit classic cup-and-cone ductile features, flat fatigue features, and various appearances in-between. owever, wires are often mechanically damaged after failure, as the separated wires are often dragged through sheaves or abraded against other components. )n addition, the etreme energy dissipation upon failure often results in considerable post-fracture damage that can be misleading to an investigator. (elatively rapid general corrosion can also accompany wire rope failures as the protective lubricant is removed and the natural passive oide layer on the wires is disrupted. Nondestructive examination
The mechanical strength of a failed wire rope cannot always be verified. >ery often, additional regions of the rope have been damaged or have yielded, providing unrealistic results for subse!uent tests. Tensile testing is the only strictly applicable mechanical test, as the brea#ing strength in tons is the sole guarantee provided by the supplier to the purchaser. Tensile strength of undamaged wire ropes can be verified in accordance with the re!uirements of AT< A ?1. The preferred tension test attachment method is soc#eting, which re!uires a measure of familiarity with wire rope handling. =irect clamping of wire ropes in >-shaped universal tensile machine @aws is not recommended, because of poor load distribution and crushing of the wires and strands. ometimes the wire rope has been so distressed prior to and after a failure, that only microhardness testing on cross sections can evaluate the strength of a rope. Although the approimate tensile strength of the wires can be interpolated, and the wire rope strength can be estimated, these are unavoidably imprecise appraisals, and cannot accurately discriminate between rope grades or be relied upon as an acceptancere@ection criteria. On occasion, ropes have been intentionally misrepresented or even counterfeited, but such incidents are eceedingly rare. Ongoing industry efforts to better regulate and specify wire ropes are of great importanceB however, no simple nondestructive method has been developed to confirm mechanical strength of a failed rope.
hemical analysis The chemical composition of a wire rope should be determined during the course of a failure investigation, but will not li#ely provide much insight. )n rare cases of ecessive, deleterious impurity content or possible counterfeitsubstitution, the composition may be very important. *hemical analysis of any surface residue or corrosion product can also be helpful, especially in cases of corrosion fatigue or severe pitting. Other useful analytical techni!ues include energy dispersive C-ray spectrometry, C-ray diffraction, or a comparable surface science methodology. )n !uestions concerning the presence or absence of lubricant, or if the identification of a wire rope lubricant is needed, infrared
spectrometry may provide answers. The fiber core could also be analy:ed for evidence of overheating, grease degradation, and organic contamination.
Fractographic analysis ;lectron microscope fractography of wire r ope fracture surfaces is essential in failure analysis. >isual eamination alone is not sufficient, as several hundred fracture surfaces are present and may not all be analogous fractures. )n overload, steel wire ropes typically ehibit classic cup-and-cone ductile overload features. Fatigue is the most important fracture mode in wire ropes because it generally appears at a fraction of the rated strength, below the yield strength, and often without warning. The fractures are flat and do not ehibit any nec#ing.
Metallographic e"amination ;amination of metallographic cross sections can clarify many failure characteristics. 5ongitudinal sections through representative failed wires can confirm the type of fracture mode indicated during visual and fractographic eamination. A typical ductile overload failed wire is shown in Fig. . ;ven if the fracture surface has been damaged or corroded, the telltale grain flow and nec#ing may be apparent.
Fig. # Metalloraphic cross section throuh a +ire that faile 2ia uctile o2erloa. %: #ital etch "<&;$
The longitudinal metallographic profile of a fatigue fracture is shown in Fig. 4. ometimes a whitish surface layer of abrasion-induced untempered martensite is observed at the origin of crac#s in severely abraded ropes. These profiles may also show grain flow evidence, suggesting cutting or pinching in cases
of flat fractures without fatigue crac#ing. The longitudinal microstructure near the fracture is shown in Fig. 7. evere disruption of the linear structure is sometimes evident and may be very important if the fracture surfaces have been obliterated by corrosion.
Fig. $ =ross section throuh a +ire that faile -y fatiue. Seconary cracks are e2ient. %: #ital etch "(%;$
Fig. % Some lonituinal microstructure isruption near the fracture surface in Fi. ' is apparent. %: #ital etch "&&;$
Transverse cross sections near the fracture surfaces are often useful, particularly when evaluating the potential contribution of corrosion and mechanical crac#ing to a rope failure. ;perience has shown that a steel hose clamp affied around the rope prior to metallographic mounting and polishing can avoid inadvertent destruction of the wire strand and wire configuration. This allows for easy wire counting,
although wire dimensions will be misleading because the cross sections are oblong due to the helical preforming.
&ata summation The final phase of the failure analysis is a logical summation of all laboratory data, service history information, and technical #nowledge. The data should indicate the failure mode and identify the contributing factors. )t may be helpful to delineate the factors that did not contribute to failure, as this information can also
