Nonlinear guided waves for fatigue crack evaluation in steel joints with digital image correlation validation

This paper presents a study of nonlinear guided-wave-based fatigue crack identification in a steel joint subjected to tensile fatigue loadings. The steel joint is considered as a section of a railway bogie, a vital component of the train structure. Finite element method and experiment were used to evaluate fatigue crack based on the second harmonic method. The symmetric mode (S1) of Lamb waves was selected as the input signal and excited in experiment by piezoelectric transducers installed on the upper surface of the joint. The sensing paths were aligned perpendicularly to and at various distances from the potential crack trajectory. When the waves interacted with the crack, higher order harmonics were produced owing to the phenomenon of contact acoustic nonlinearity. A nonlinear index based on the second harmonic characteristics was exploited to define the nonlinearity. To validate the outcomes based on nonlinear guided waves, a digital image correlation technique was implemented and the results were in good agreement. The findings indicated that the nonlinear guided-wave-based technique is capable of identifying crack initiation and growth in such joint type structures.