Application of laser ultrasound imaging technology in the frequency domain based on Wigner–Ville algorithm for detecting defect

• Finite element modeling results demonstrated that propagation image could detect the flaws.
• The Wigner–Ville algorithm as a frequency selectivity method is proposed.
• The experiments demonstrated that damage propagation could be clearly quantitatively visualized.
• The frequency-selectable Wigner–Ville transformed ultrasonic propagation imaging method could be applied to automatic NDE.

According to the thermoelastic and heat conduction equations of laser ultrasonic, ultrasonic propagation images are obtained by solving finite element solution equations. Based on thermoelastic mechanism, a Wigner–Ville transform ultrasonic propagation imaging method of stronger frequency selectivity was proposed. An aluminum plate with defects was scanned by a pulsed laser, the ultrasonic waves produced by a pulsed laser were received by an ultrasonic sensor, and then ultrasonic propagation imaging on the aluminum plate was analyzed in detail. In order to isolate a damage-related ultrasonic wave, a Wigner–Ville algorithm as a frequency selectivity method was proposed to convert a complex time domain multiple wavefield into a simple frequency domain single wavefield. At last, we compared the conventional ultrasonic propagation imaging method with the Wigner–Ville transform ultrasonic propagation imaging method, which demonstrated that the Wigner–Ville transform ultrasonic propagation imaging method can effectively and intuitively evaluate the sizes of damages or flaws without any reference data, which should promote the application of the technique in the industry.