Microcrack inspection in a functionally graded plate structure using nonlinear guided waves

The nonlinear guided wave technique based on contact acoustic nonlinearity (CAN) has been considered one of the best approaches for characterizing microcracks or fatigue cracks in structures. Through various mode selections, nonlinear guided wave testing has higher sensitivity that is suitable for detecting micro-defects at a very early stage. However, in most of the previous studies, this technique has been implemented in homogeneous materials such as aluminium, steel and layered composites. In recent years, functionally graded material (FGM) has received a great deal of attention because of its exceptional mechanical properties. Monitoring and evaluating the microstructural changes of FGM are essential for safety. In this study, the nonlinear interaction of the guided waves with a microcrack in an FGM plate is investigated numerically. The results showed that the generation of higher harmonics provides a sensitive means for detecting microcracks in FGM plates. Besides, the dependence of relative nonlinear parameters on microcrack length, sensor location and crack locations is studied through comprehensive finite element simulations. The study reveals that the nonlinear ultrasonic technique can be used effectively to detect microcracks in FGM plate structures.