Chapter 6 - Guided Waves

Abstract

This chapter will deal with an important class of waves that have widespread applications in SHM – the class of guided waves. Guided waves are especially important for SHM because they can travel large distances in structures with only little energy loss. Thus, they enable the SHM of large areas from a single location. The chapter starts with Rayleigh waves, a.k.a. surface acoustic waves (SAW), which travel close to a free surface with very little penetration in the depth of the solid. For this reason, Rayleigh waves are also known as surface-guided waves. The chapter continues with shear-horizontal (SH) waves in plates. The particle motion of SH waves is polarized parallel to the plate surface and perpendicular to the direction of wave propagation. The SH waves are multimodal and dispersive (with the exception of the very fundamental mode, SH0, which is nondispersive). Next, the chapter discusses Lamb waves in plates, which are dispersive and multimodal. For relatively small values of the frequency–thickness product fd, only the basic symmetric and antisymmetric Lamb wave modes (S0 and A0) exist. As the fd product approaches zero, the S0 and A0 modes degenerate in the basic axial and flexural plate modes discussed in Chapter 5. At the other extreme, as fd→∞, the S0 and A0 Lamb wave modes degenerate into Rayleigh waves confined to the plate surface. Lamb waves are studied in both rectangular and circular coordinates. The chapter ends with a brief review of guided waves in tubes and cylindrical shells in which dispersive longitudinal (axial), flexural, and torsional multi-mode guided waves can simultaneously exist.