Time-frequency analysis of laser-generated ultrasonic wave in coating-substrate systems

Laser induced ultrasonic waves in coating-substrate systems are broadband and dispersive,and multiple propagation modes overlap in time domain.By using time-frequency analysis based on the Wigner-Ville distribution,the characters of modes and energy distribution are obtained after analyzing laser ultrasound signals in the coating-substrate system,which are obtained by means of the finite element method.Furthermore,time-frequency method provides a direct measurement velocity dispersion by delay estimation,whose advantage is that it only needs a single measured signal.