Identification of fatigue crack under vibration by nonlinear guided waves

Abstract

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.