4.1 An electrical pulse is applied to a piezoelectric transducer which converts electrical to mechanical energy. In the angle-beam search unit, the piezoelectric element is generally a thickness expander which creates compressions and rarefactions. This longitudinal (compressional) wave travels through a wedge (generally a plastic). The angle between transducer face and the examination face of the wedge is equal to the angle between the normal (perpendicular) to the examination surface and the incident beam. Fig. 1 shows the incident angle φ_i, and the refracted angle φ_r, of the ultrasonic beam.

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.