Thermoelastic Structures for High Density Ultrasonic Energy

The use of lasers to generate thermoelastic waves has received considerable attention since the publication of the theoretical work of White.1 The author derives equations for the propagation of longitudinal elastic waves produced by surface heating of a semi-infinite medium with harmonic thermal (laser) excitation. The importance of the boundary conditions in determining the magnitude of the resulting elastic waves is described. Two conditions are imposed and the resulting amplitudes derived: (1) Laser absorption at x = 0 with elastic amplitude u=0 at x = 0 for all times t ≥ 0 (perfectly clamped or constrained surface) and (2) for the stress σ = 0 at x = 0 for t ≥ 0 (free surface). Propagation of the elastic wave is in the x direction