Due to the strong expectation for the application of structural health monitoring, many studies have been conducted to evaluate the relationship between the degree of damage and nonlinear acoustic characteristics in polymer-based fiber-reinforced composites. Fiber-reinforced composites are known to have more complex fracture aspects than conventional materials, and various scales of damage ranging from fiber-matrix interface delamination at the microscopic level to interlaminar delamination at the larger scale. This indicates that each damage morphology may have a different effect on the acoustic nonlinearity. For practical non-destructive testing based on nonlinearity, the relationship between the damage morphology of CFRP laminates and the acoustic nonlinearity is required to be clarified. The purpose of this study is to evaluate the effect of damage morphology on acoustic nonlinearity in CFRP laminates. Tensile tests were conducted on specimens with different stacking sequences. During the tests, optical observations and ultrasonic measurements of material nonlinearity were continuously carried out. The relationship of the density of the transverse cracks and the nonlinear parameter β’ were experimentally evaluated. The observation shows that there is a negative correlation between the transverse crack density and the magnitude of change in the nonlinear parameter in the cross-ply CFRP laminates, indicating that the transverse crack itself is not the dominant factor of the nonlinear parameter. The effect of the 0°/90° delaminations associated with the transverse cracks on the nonlinear parameters is also discussed.
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