Internal Structure

T.O. 33B-1-1 5-19 5.1.7.4 Internal Structure. Discontinuities inherent in the test article, such as grain boundaries, affect the ultrasonic test by scattering the ultrasonic energy. This reduces the energy available for finding detrimental discontinuities and causes “noise” in the waveform presentation. Effects on an inspection increase as the frequency is increased and are most noticeable in materials with relatively large grain size. In certain applications, the loss in ultrasonic energy caused by internal scattering can be measured to evaluate metallurgical structures. 5.1.8 Discontinuity Variables. Ultrasonic beams can be reflected at various angles at the discontinuity interface and can also spread or focus depending on the shape of the discontinuity. 5.1.8.1 Size and Shape. When discontinuities smaller than the sound beam are oriented with one surface perpendicular to the incident sound beam, the amplitude of a reflected ultrasonic beam from a discontinuity increases as the area of the surface normal to the incident sound beam increases. An irregularly shaped or round discontinuity reflects sound energy at many angles; thus, a flat discontinuity perpendicular to the sound beam reflects the greatest amount of sound energy back to the search unit. 5.1.8.2 Orientation. For discontinuities with surfaces oriented at angles other than perpendicular to the sound beam, only a portion (if any) of the sound beam is reflected back to the search unit. If discontinuities are suspected to be located at angles other than parallel to the entry surface, consider angle beam inspection or straight beam inspection from another surface (if the discontinuity is expected to be parallel to that surface). To help in detecting discontinuities oriented at angles to an incident straight beam, it is helpful, when geometry permits, to monitor the back surface reflection. A sudden decrease in back reflection when scanning may indicate a discontinuity or possibly a number of small discontinuities. If a discontinuity signal is observed which is proportional to the loss in back reflection, the discontinuity is probably flat and oriented normal to the incident sound beam. If the discontinuity signal is small in relation to the loss of back reflection signal, the discontinuity is probably turned at an angle to the incident sound beam or is rounded. A decrease in back reflection accompanied by multiple discontinuity signals or a general increase in the noise level may indicate the presence of multiple discontinuities. 5.1.8.3 Acoustic Impedance. When an ultrasonic beam strikes a boundary between two different materials, part of the energy is transmitted to the second medium and a portion is reflected. The percentage of sound energy transmitted and reflected is related to the ratio of the acoustic impedances of the two materials. Acoustic impedance can be calculated as follows: Z = rv Where: Z = acoustic impedance of a material r (rho) = material density v = velocity of sound in the material 5.1.8.3.1 Table 5-2 includes acoustic impedance values for several materials. Acoustic impedance can be used to calculate the theoretical reflected and transmitted energy for an interface. The greater the difference in acoustic impedance across the interface, the greater amount of sound reflected. The theoretical reflection at a water-steel interface is 88 percent; at a water-aluminum interface it is 72 percent. However, the actual reflection often differs significantly from the calculated theoretical reflection. Surface roughness is one of the variables besides acoustic impedance that affects the percentage of reflection. The acoustic impedance of the discontinuity material in relation to the acoustic impedance of the test part is important. The reflections from an air interface such as a crack or void are large due to the acoustic impedance ratio. If a discontinuity had acoustic impedance close to the acoustic impedance of the test material, the acoustic impedance ratio would be small, and very little reflection would occur. The following formula is used to determine the amount of reflected energy that occurs at an interface.