Section 2: Physics of Ultrasound

The form is determined by whether the particle motion is symmetrical or
asymmetrical with respect to the neutral axis of the test piece. Each form is
further subdivided into several modes having different velocities, which can
be controlled by the angle at which the waves enter the test piece.
Theoretically, there are an infinite number of specific velocities at which Lamb
waves can travel in a given material. Within a given plate, the specific
velocities for Lamb waves are complex functions of plate thickness and
frequency.
In symmetrical (dilatational) Lamb waves, there is a compressional
(longitudinal) particle displacement along the neutral axis of the plate and an
elliptical particle displacement on each surface (Fig. 4a). In asymmetrical
(bending) Lamb waves, there is a shear (transverse) particle displacement
along the neutral axis of the plate and an elliptical particle displacement on
each surface (Fig. 4b). The ratio of the major to minor axes of the ellipse is a
function of the material in which the wave is being propagated.