Detection of disbonds in multi-layer bonded structures using the laser ultrasonic pulse-echo mode

•We investigate a quantitative method for evaluating disbonds at the adhesive layer.•We investigate the characteristics of reflected laser ultrasonic waves.•Laser ultrasonic methods with reflection can recognize a 3 mm size disbond.•The reflected shear waves are more sensitive to disbonds than others.•The actual sizes are in good agreement with the measurement results. The laser ultrasonic technique with a pulse-echo mode has been investigated to detect disbonds in a multi-layer bonded structure and a quantitative method has been proposed to evaluate the defect sizes. The simulations were carried out to analyze influences of spot sizes on the characteristics of laser ultrasonic reflected waves, the interaction of laser ultrasonic reflected waves with disbonds, and quantitative characterization on disbonds. A noncontact laser ultrasonic inspection system has been established to perform a series of experiments to verify the theoretical results. Laser ultrasonic C-scans based on reflected shear waves can clearly redraw appearances of disbonds in the adhesive bond. The proposed quantitative method can evaluate the sizes of disbonds within a reasonable error range. Moreover, the experimental data are in good agreement with the simulation results. Therefore, a combination of the laser ultrasonic pulse-echo mode and the proposed quantitative strategy is practical for accurate detection of disbonds in multi-layer bonded structures.