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Handbook for Damage Tolerant Design

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Section Ultrasonic Inspection

Ultrasonic inspection uses high frequency sound waves as a probing medium to detect subsurface, as well as surface cracks.  The sound waves travel through the part with attendant energy loss and are reflected at material-crack interfaces.  Ultrasonic inspection devices detect cracks by monitoring one or more of the following: (a) reflection of energy from interfaces or discontinuities within the metal; (b) time of transit of a sound wave through the test piece; and (c) attenuation of the beams by absorption and scattering within the test piece.

Ultrasonic inspection is one of the most widely used NDI methods.  Cracks, laminations, shrinkage cavities, bursts, flakes, pores, bonding faults, and other discontinuities that act as metal-gas interfaces can be detected.  Inclusions and other non-homogeneity in the metal being inspected can also be detected by causing partial reflection or scattering of the wave, even though they may not act as a metal-gas interface.  Although the primary application of ultrasonic inspection in metals is the detection and characterization of internal cracks, it is also used to detect surface cracks, define bond characteristics, measure extent of corrosion and, (much less frequently) determine physical properties such as structure, grain size, and elastic constants.  The penetrating power of ultrasound waves allows the detection of cracks deep within a part.  Due to the sensitivity of the instruments, very small cracks can be detected but, if the gain is set too high, at the expense of many false indications.  Ultrasonic methods provide greater accuracy than other NDI methods in determining the position of internal cracks, estimating their size, and characterizing their orientation, shape and nature.  The limitations of ultrasonic methods are governed by the requirement for experienced technicians, the difficulty in developing inspection procedures, the need for reference standards for equipment calibration, and the physical limitations of the hardware.  Since couplants (light oil or water) are needed to provide effective transfer of ultrasonic wave energy between transducers and material, parts that are rough or irregular in shape are difficult to inspect.  Similarly, parts that are very small are difficult to inspect.  Finally, since discontinuities in a shallow layer immediately below the surface may not be detectable, inspection results of very thin components are questionable.