Fatigue damage and micro-cracks growth have been proved to be closely related to the nonlinear effect of ultrasonic Lamb waves propagation in materials. In this paper, the relationship among fatigue life, length and width of micro-cracks, and the acoustic nonlinearity parameter was established. The variation of the acoustic nonlinearity parameter caused by fatigue damage was analyzed based on micro-cracks growth. Considering the multimodal nature of nonlinear Lamb waves, we selected the low-frequency mode pairs S0–s0 and S1–s2 to evaluate fatigue damage in 7075 aluminum alloy plates. Both FE simulations and experiments show that the mode pair S1–s2 is more suitable than S0–s0 for the assessment of fatigue damage. It was found that with increasing fatigue cycles, the relative acoustic nonlinearity parameter A₂/A₁² firstly increases because the micro-cracks length extends while the micro-cracks width does not change much. The relative acoustic nonlinearity parameter A₂/A₁² reaches its peak at about 60% of the fatigue life. Then the relative acoustic nonlinearity parameter A₂/A₁² decreases because of the rapid increase in the micro-cracks width. Our findings clarify the mountain-shape curve between the relative acoustic nonlinearity parameter A₂/A₁² and the fatigue cycles.

疲劳损伤和微裂纹的生长已被证明与超声波兰姆波在材料中传播的非线性效应密切相关。本文建立了疲劳寿命、微裂纹长度和宽度与声学非线性参数之间的关系。基于微裂纹扩展分析了疲劳损伤引起的声学非线性参数的变化。考虑到非线性兰姆波的多模态特性，我们选择了低频模态对 S0-s0 和 S1-s2 来评估 7075 铝合金板的疲劳损伤。有限元模拟和实验都表明，模式对 S1-s2 比 S0-s0 更适合评估疲劳损伤。发现随着疲劳周期的增加，相对声学非线性参数A ₂/ A ₁ ²首先增加，因为微裂纹长度增加，而微裂纹宽度变化不大。相对声学非线性参数A ₂ / A ₁ ²在疲劳寿命的 60% 左右达到峰值。然后由于微裂纹宽度的快速增加，相对声学非线性参数A ₂ / A ₁ ²减小。我们的研究结果阐明了相对声学非线性参数A ₂ / A ₁ ²与疲劳循环之间的山形曲线。