ASTM E587-15(2020)

4.3.1 Angle-Beam Longitudinal Waves—As shown in Fig. 4, angle-beam longitudinal waves with refracted angles in the range from 1 to 40° (where coexisting angle-beam shear waves are weak, as shown in Fig. 3) may be used to detect fatigue cracks in axles and shafts from the end by direct reflection or by corner reflection. As shown in Fig. 5, with a crossed-beam dual-transducer search unit configuration, angle-beam longitudinal waves may be used to measure thickness or to detect reflectors parallel to the examination surface, such as laminations. As shown in Fig. 6, reflectors with a major plane at an angle up to 40° with respect to the examination surface, provide optimum reflection to an angle-beam longitudinal wave that is normal to the plane of the reflector. Angle-beam longitudinal waves in the range from 45 to 85° become weaker as the angle increases; at the same time, the coexisting angle-beam shear waves become stronger. Equal amplitude angle beams of approximately 55° longitudinal wave and 29° shear wave will coexist in the material, as shown in Fig. 7. Confusion created by two beams traveling at different angles and at different velocities has limited use of this range of angle beams.

4.3.2 Angle-Beam Shear Waves (Transverse Waves)—Angle-beam shear waves in the range from 40 to 75° are the most used angle beams. They will detect imperfections in materials by corner reflection and reradiation (as shown in Fig. 8) if the plane of the reflector is perpendicular to a material surface, and by direct reflection if the ultrasonic beam is normal to the plane of the reflector (as shown in Fig. 9). Reflectors parallel to the examination surface (such as laminations in plate, as shown in Fig. 10) can rarely be detected by an angle beam unless accompanied by another reflector; for example, a lamination at the edge of a plate (as shown in Fig. 11) can be detected by corner reflection from the lamination and plate edge. Generally, laminations should be detected and evaluated by the straight-beam technique. Angle-beam shear waves applied to weld testing will detect incomplete penetration (as shown in Fig. 12) by corner reflection, incomplete fusion (as shown in Fig. 13) by direct reflection (when the beam angle is chosen to be normal to the plane of the weld preparation), slag inclusion by cylindrical reflection (as shown in Fig. 14), porosity by spherical reflection, and cracks (as shown in Fig. 15) by direct or corner reflection, depending on their orientation. Angle-beam shear waves of 80 to 85° are frequently accompanied by a Rayleigh wave traveling on the surface. Confusion created by two beams at slightly different angles, traveling at different velocities, has limited applications in this range of angle beams.

4.3.3 Surface-Beam Rayleigh Waves—Surface-beam Rayleigh waves travel at 90° to the normal of the examination surface on the examination surface. In material greater than two wavelengths thick, the energy of the Rayleigh wave penetrates to a depth of approximately one wavelength; but, due to the exponential distribution of the energy, one half of the energy is within one-quarter wavelength of the surface. Surface cracks with length perpendicular to the Rayleigh wave can be detected and their depth evaluated by changing the frequency of the Rayleigh wave, thus changing its wavelength and depth of penetration. Wavelength equals velocity divided by frequency.

Subsurface reflectors may be detected by Rayleigh waves if they lie within one wavelength of the surface.

4.3.4 Lamb Waves—Lamb waves travel at 90° to the normal of the test surface and fill thin materials with elliptical particle vibrations. These vibrations occur in various numbers of layers and travel at velocities varying from slower than Rayleigh up to nearly longitudinal wave velocity, depending on material thickness and examination frequency. Asymmetrical-type Lamb waves have an odd number of elliptical layers of vibration, while symmetrical-type Lamb waves have an even number of elliptical layers of vibration. Lamb waves are most useful in materials up to five wavelengths thick (based on Rayleigh wave velocity in a thick specimen of the same material). They will detect surface imperfections on both the examination and opposite surfaces. Centrally located laminations are best detected with the first or second mode asymmetrical Lamb waves (one or three elliptical layers). Small thickness changes are best detected with the third or higher mode symmetrical or asymmetrical-type Lamb waves (five or more elliptical layers). A change in plate thickness causes a change of vibrational mode just as a lamination causes a mode change. The mode conversion is imperfect and may produce indications at the leading and the trailing edges of the lamination or the thin area.