Research on the Weakening Process at the Interface of Bonded-Layer Composite Structures Using Ultrasonic Longitudinal Waves

The interface weakening process of bonded-layer composite structures is calculated, simulated, and experimentally investigated using ultrasonic longitudinal waves. Firstly, the reflection coefficients of echoes are calculated theoretically. Subsequently, a time-domain simulation model of bonded-layer composite structures is established. The propagation law of ultrasonic waves in bonded-layer composite structures is obtained. The relationship between different bonding interface states and the ultrasonic reflection characteristics are investigated through ultrasonic experiments on bonded composite structures. The theoretical calculation, simulation, and experimental results are as follows: when the bonding strength of the bonding layer changes from weak to strong, the amplitude of the first echo gradually decreases, the amplitude of the second echo progressively increases, and the amplitude of the third echo is basically unchanged; when the bonding strength of the upper interface changes from weak to strong, the amplitudes of the first and the second echoes are same as in the previous variation whereas the amplitude of the third echo slightly increases; when the bonding strength of the lower interface changes from weak to strong, the amplitudes of the first and the third echoes remain essentially unchanged, but the amplitude of the second echo progressively increases in the experiment compared with the theoretical calculation and simulation. In addition, the time of the first echo remains broadly unchanged, and the times of the second and the third echoes gradually decrease under all conditions.