Nonlinear Lamb wave analysis for microdefect identification in mechanical structural health assessment

•A nonlinear Lamb wave detection system was established.•The cracks of aluminum plates under different tensile load cycle is investigated.•Influence about defects on nonlinear effects of Lamb waves is obtained.•Crack defects exacerbates the nonlinear effect of the test piece.•Macro-defects have a smaller effect on nonlinearity. In order to study the application of ultrasonic Lamb wave in the detection of defective plates, a nonlinear Lamb wave detection system was established. The system was used to detect aluminum alloy plates with different depth cracks and aluminum alloy plates with different tensile load cycles. The acquired time domain waveforms are analyzed by Fast Fourier Transform (FFT), and the influence of two kinds of defects on the nonlinear effects of Lamb waves is obtained. The test results show that for crack defects, the deepening of the defects will increase the ultrasonic nonlinear effect of the test piece. When the crack depth exceeds 4 mm, the non-linear effect of crack defects does not increase. For fatigue crack defects, the occurrence of crack defects will also exacerbate the nonlinear effect of the test piece, but when the test piece is fatigue fractured, its macro defects have a smaller effect on nonlinearity.