T.O. 33B-1-15-85.1.4.4 LambWaves.Lamb waves, also known as plate waves, propagate within thin plates, a few wavelengths thick. Lamb waves may begenerated in an infinite variety of modes whose velocity is dependent on the incident angle, the frequency, the platethickness, and the properties of test material. Calculation of Lamb wave modes is complex, and is generally notrequired. However, when Lamb waves are generated in material thinner than one wavelength, the velocity can bedetermined by noting the relative positions of the echoes as a function of distance.5.1.5 RefractionandModeConversion.5.1.5.1 Snell’sLaw.When a sound beam passes from one medium into another with a different sound velocity, at an angle not normal to theinterface separating the two media, refraction (a change in propagation direction) occurs (see Figure 5-10). Theincident and refracted angles follow Snell's law:sinsinff1212=vvWhere:f1(phi) = angle between the normal to the interface surface and the incident sound beamf2= angle between the normal to the interface surface and the refracted sound beamv1 = velocity of incident sound beamv2 = velocity of refracted sound beamFigure 5-10.Sound Beam Refraction.5.1.5.2 MultipleRefractedBeams.When an incident longitudinal beam is normal to the test part surface (f1 = 0°), the longitudinal sound beam istransmitted straight into the test part and no refraction occurs. When the incident angle is other than normal,refraction and mode conversion occur. A portion of the longitudinal incident beam is refracted into one or more wavemodes traveling at various angles and intensities depending on the incident angle of the longitudinal beam. The angles
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