Detection of fatigue cracks in train structures using nonlinear lamb waves

Envisioning the rapid development of high-speed railway network worldwide but envisaging potential threats to the safe operation of the network due to continuous ageing of train structures, a fatigue damage detection technique for train structures was developed. For metal structures used in high-seeped trains, fatigue crack may take place under cyclic loads, causing unexpected accident. However inspection of fatigue damage can be a difficult task during the normal operation of the train using tradition nondestructive evaluation techniques. In this study, nonlinear Lamb waves were used to interrogate train structures. Higher-order harmonic wave fields were generated, to observe that under certain conditions the second harmonic was cumulative. The fatigue damage modulated the nonlinearity of the structure due to the plastic deformation or micro-cracks, influencing the cumulative character of the second harmonic Lamb wave. To quantify the degree of such cumulative second harmonic, a nonlinearity parameter was used. A short-time Fourier transformation-based signal processing was applied to the collected signals to extract the amplitudes of the first and second harmonics. Experiments on an aluminum plate containing fatigue damage validated the proposed method. The results indicated that the fatigue crack can break the cumulative character of the nonlinearity parameter whereby the fatigue damage can be detected. It is noteworthy that this method can be carried out without any baseline data or a benchmark structure, increasing the flexibility of the application.