Numerical and Experimental Study of the Effect of the Evolution of Matrix Cracks in Composites on the Nonlinear Ultrasound Response

Early-onset damage in composite materials consists matrix cracking with certain regularity in distribution. However, the effect of the spatial arrangement of matrix cracks and the relationship between the direction of expansion of the cracks and the incident direction of ultrasound waves on the nonlinear parameter in nonlinear ultrasound detection is poorly understood. This study analyzes the nonlinear variation of matrix millimeter-scale and micron-scale cracks by experiment and finite-element-based numerical calculation to improve the damage evaluation of composite materials by nonlinear ultrasound and provide a reference for crack expansion prediction and imaging. The results show that millimeter and micron cracks follow the same trend, with more cracks and “dislocated” spatial arrangement, both providing positive contributions to the relative nonlinear parameter. However, when the incident direction of the ultrasound waves is orthogonal to the crack expansion direction, the evolution of the relative nonlinear parameter will follow an opposite trend with respect to the expansion size compared to when the incident direction and crack expansion direction are the same. We also propose a new nonlinear index (DI), the evolution of which can be used to identify the type of spatial distribution of the cracks in the matrix.