Level III UT Study Note 1. The velocity is determined by the material’s elastic moduli and density. 2. Guided waves are guided along the respective surfaces and their velocities are dependent upon elastic moduli, density, thickness, surface conditions and relative wavelength interactions with the surfaces. 3. Rayleigh waves (surface waves) useful depth of penetration is restricted to about one wavelength below the surface. It travels at a velocity slightly less than shear waves and its mode of propagation is both longitudinal and transverse with respect to the surface. 4. The amount of sound energy is reflected is dependent upon the difference in acoustic impedances between the adjacent materials. (another word, a material’s acoustic impedance is used to determine the relative amounts of sounds energy coupled through and reflect at an interface) 5. Acoustic impedance = material density X velocity 6. Critical angle is an incident angle at which the refracted angle equals 90 degrees. 7. Diffraction……………. 8. Resonance occurs when the thickness of the item equals half a wavelength or its multiples, T=V/2F. 9. Attenuation due to geometrical spreading (beam spread), scattering and absorption. It is measured in terms of energy loss ratio per unit length, Db/in or Db/m. Attenuation caused by scattering increases with increase in frequency and grain size. 10. Beam spread multipliers: a. 1.2 for theoretical null b. 1.08 for 10% peak c. 0.88 for 32% d. 0.7 for 50% 11. Broadband pulse is a short time-duration pulse of only a few cycles.(Low frequency, good penetration) 12. Backing material is bonded to the rear face of the transducer to provide strength and damping for the transducer element. 13. To improve near surface resolution and to decrease noise, standoff devices and dual crystal units may be used. 14. Shear waves are introduced into test materials by inclining the incident L-wave beyond the first critical angle, yet short of the second critical angle. 15. Ultrasonic sound waves travels poorly in air and not at all in vacuum. 16. Liquid transmit longitudinal sound waves rather well, but because of their lack of any significant shear moduli, they do not transmit shear wave. 17. Couplants must have many desirable properties including: wettability, proper viscosity, low cost, removability, noncorrosive and nontoxic properties, low attenuation and an acoustic impedance that matches well with the other materials. For stainless steel and high-nickel alloys, couplants containing sulphur or chlorine is prohibited for reason of stress corrosion cracking. 18. Advantages of concave lenses focusing: (have shorter near field)
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a. The energy at the focal point is increased which increases the sensitivity or signal amplitude b. Sensitivity to reflectors above and below the focal point is decreased which reduces noise c. The lateral resolution is increased because the focal point is normally quite small, permitting increased definition of the size and shape of the reflector. Mosaic assemblies, e.g., paintbrush transducers. Reject control is to discriminate against low-level signals, through use of a threshold, below which no information is made available to the operator. LITHIUM SULFATE a. Advantages: one of the most efficient receivers of ultrasonic energy. Barium titanate a. The most efficient sound transmitter Natural quartz a. Advantages: mechanical and electrical stability, insolubility in liquids, resistance to aging b. Disadvantages: the least effective generator of ultrasonic energy of all commonly used materials Ceramics transducers a. Advantages: One of the most efficient generators of ultrasonic energy Hash: a term used to describe numerous small indications on the instrument display resulting from test part structure, numerous small discontinuities, or both. Marker: a separate time base line imposed on the viewing display of some ultrasonic testing instruments that permits measurement of distances. A type of data presentation most likely to be used with a high speed automatic scanning system is a C scan. An amplitude type gate is necessary for all automatic examinations. It is called the amplitude gating technique. Most basic pulse echo ultrasonic instruments use an A scan presentation, not an automatic read-out. A B scan display shows the relative distance a discontinuity is from the transducer and its length in the direction of transducer travel. Longitudinal wave velocity in water is appro. ¼ the velocity in metal, the minimum water path should be ¼ the test piece thickness plus ¼ inch. Fundamental resonant frequency formula: F=V/2T In immersion testing, to remove the second water reflection from between the entry surface signal and the first back reflection, you should increase the water path. Both longitudinal and shear waves may be simultaneously generated in a second medium when the angle of incidence is between normal and the first critical angle. When an ultrasonic incident angle is set at 5 degree from normal, the refracted wave has two components, one of which will be the same mode as the incidence wave. Some of the applications of ultrasonic techniques include: a. Determination of a material’s elastic modulus b. Study of a material’s metallurgical structure c. Measurement of a material’s thickness
37. As the radius of curvature of a curved lens is increased, the focal length of the lens increases. 38. The ultrasonic technique in which finger damping is most effective in locating a discontinuity is the surface wave technique. (Surface wave also has the shortest wavelength compared to compression wave and shear wave.) 39. When using focused transducers, nonsymmetry in a propagated sound beam may be caused by a. Backing material variations b. Lens centering or misalignment c. Porosity in lens d. Small imperfections in electrodes and bonding 40. On an A scan, the “dead zone” refers to the distance covered by the front surface pulse width and recovery time. 41. In the FAR FIELD ZONE, the amplitude of an indication from a given discontinuity diminish exponentially as the distance increases. 42. As transducer diameter decreases, the beam spread increases. 43. A diagram in which the entire circuit stage or sections are shown by geometric figures and the path of the signal or energy by lines and/or arrows is called a BLOCK DIAGRAM. 44. THE INDEX OF REFRACTION is the ratio between the wave speed in one material and the wave speed in a second material. 45. MAGNETOSTRICTION is the expansion and contraction of a magnetic material under the influence of a changing magnetic field. 46. The resolving power of a transducer is directly proportional to its bandwidth. 47. Acoustic lens elements with cylindrical curvatures permit focusing the sound energy to enter cylindrical surfaces normally or along a line focus. 48. In the basic pulse echo instrument, the synchronizer, clock, or timer circuit determines the pulse repetition rate. 49. In general, discontinuities in wrought products tend to be oriented in the direction of grain flow. 50. In cases where the diameter of tubing being inspected is smaller than the diameter of the transducer, a collimator can be used to confine the sound beam to the proper range of angles. 51. Increasing the length of the pulse used to activate the transducer will increase the strength of the ultrasound but decrease the resolving power of the instrument. 52. When maximum sensitivity is required from a transducer, the piezoelectric element should be driven at its fundamental resonant frequency. 53. The sensitivity of an ultrasonic test system depends on the transducer, pulser, and amplifier used. 54. Transducer sensitivity is most often determined by the amplitude of the response from an artificial discontinuity. 55. The ability of a test system to separate the front-surface echo and the echo from a small discontinuity just below the surface depends primary upon the pulse length generated from the instrument.
