*4.2. Classification of Ultrasound Waves*

Elastic waves (ultrasonic, sonic and subsonic) can be classified into two groups—body waves, also known as bulk waves and surface waves, also known as guided waves. Body waves propagate through the interior of the material (bulk material), while surface waves propagate along the material's surface [41].

Bulk waves are classified into two types—longitudinal waves and shear waves. According to Coramik et al. [61], longitudinal waves are also known as compression or extension waves, since the material undergoes compression and extension when the wave propagates through it. In shear waves, the motion of the particles is perpendicular to the direction of the wave propagation. Shear (transverse) waves, when compared to longitudinal waves, have shorter wavelengths and slower dispersion. In terms of elastic constants, the wave velocities of these two types of waves (longitudinal and shear) are given by Equations (5) and (6), respectively [41].

$$c\_p = \sqrt{\frac{\lambda + 2\mu}{\rho}} = \sqrt{\frac{E(1-v)}{\rho(1+v)(1-2v)}}\tag{5}$$

$$c\_s = \sqrt{\frac{\mu}{\rho}} = \sqrt{\frac{E}{2\rho(1+\upsilon)}}\tag{6}$$

where *cp* and *cs* are the longitudinal and shear velocities, respectively. ρ is the density, λ is the first Lame constant, μ is the shear modulus or second Lame constant, *E* is Young's modulus and *v* is Poisson's ratio. It should be noted that *cp* is always greater than *cs*, since longitudinal waves propagate faster than shear waves, longitudinal waves are called primary waves or P-waves and shear waves are called secondary waves or S-waves.
