Laser-generated thermoelastic waves in an anisotropic infinite plate: Fem analysis

In recent years, the study of thermoelastic waves generated by focused laser beams has been undertaken by several researchers because the technique provides a means for noncontact generation of ultrasonic waves. Laser-generated ultrasonic waves have diverse applications ranging from material characterization to nondestructive testing of defects. Transient ultrasonic guided waves generated in an anisotropic infinite plate by a pulsed laser beam are investigated in this study. A semi-analytical finite element method (SAFEM) is adopted for this purpose. In this method, the plate is divided into parallel layers through its thickness and the displacement and temperature in each layer are approximated by quadratic polynomials in the thickness direction (z). They are assumed to be continuous functions of time and in-plane cartesian coordinates (x, y). Transient response is calculated using Fourier transformations in time and space variables (x, y). The analysis technique is applicable to a generally anisotropic plate. Results for dispersion of guided waves and transient displacement in infinite silicon nitride (Si ₃N₄) plates are presented. Numerical results show that for the assumed form of heat deposition by the laser mainly the lowest Lamb modes, namely, the lowest symmetric (S₀) and antisymmetric (A₀) modes, are excited. They also show that the transient response is dominated by the antisymmetric mode A₀, which shows characteristic dispersion. This study provides a quantitative model for laser-generated thermoelastic waves in an anisotropic plate and can be used for nondestructive evaluation.