Non-destructive testing method of micro-debonding defects in composite insulation based on high power ultrasonic

It is crucial to ensure the adhesive quality of the interface in composite insulation equipment for power grid security. Due to viscoelasticity of silicone rubber and roughness of interface, the thickness of the local air layer in the defective interface is <100 mu m. This kind of defect is called micro-debonding defect in this paper, and it is significantly smaller than the millimetre-scale air gaps which could be defected by the x-ray, THz, and traditional ultrasonic method. It remains challenging to detect the micro-debonding in composite insulation by non-destructive testing methods. In this paper, the acoustic model and bi-linear stiffness model for micro-debonding in different stages are established. Accordingly, a conjecture is proposed that the bonding property of composite insulation is related to the acoustic impedance of interface and non-linear distortion of its constitutive relation. The result obtained from numerical simulation with and without defect is compared with the high-power ultrasonic experimental data to validate the correctness of the theoretical model. In addition, it can be concluded that the non-linear distortion of high-power ultrasonic wave can be effectively used to diagnose micro-debonding defect at the interface of composite insulation in its early stage (1-20 mu m).