This study identified fatigue cracks in a steel joint under vibration by nonlinear guided waves based on second harmonic generation. Piezoelectric transducers were utilised to excite S1 Lamb wave mode in experiment and the nonlinear ultrasonic testing was performed at different crack lengths. The contact acoustic nonlinearity (CAN) was measured in experiments and quantified by a nonlinear index to evaluate fatigue crack. Finite element method was utilised to verify the experimental results and they were in good consistence. The simulation was divided into two stages where the first stage was to model the vibration responses of the specimen and the second stage was to simulate the nonlinear ultrasonic testing to characterise the fatigue crack with the effect of vibration. A percent reduction in nonlinearity was proposed to evaluate the crack length based on the difference of nonlinear index measured at static and vibration conditions. It is observed that percent of reduction in nonlinearity increased with crack growth. The outcomes of this study can further advance the fatigue crack identification and qualification based on second harmonic generation under practical working condition of structures.

本研究通过基于二次谐波产生的非线性导波在振动下识别钢接头中的疲劳裂纹。实验中利用压电换能器激发S1兰姆波模式，对不同裂纹长度进行非线性超声检测。接触声非线性 (CAN) 在实验中测量并通过非线性指标量化以评估疲劳裂纹。采用有限元方法对实验结果进行了验证，结果一致性较好。模拟分为两个阶段，第一阶段是模拟试样的振动响应，第二阶段是模拟非线性超声测试，以表征受振动影响的疲劳裂纹。基于在静态和振动条件下测量的非线性指数的差异，提出了非线性减少百分比来评估裂纹长​​度。观察到非线性减少的百分比随着裂纹的增长而增加。本研究的结果可以进一步推进基于二次谐波产生的结构实际工况下疲劳裂纹的识别和鉴定。