A structural health monitoring approach based on contact acoustic nonlinearity and its application to quantitative evaluation of fatigue cracks

A damage characterization approach was developed in this study by exploiting the second harmonics generated owing to the interaction between incident Lamb waves and a "breathing" crack. The approach can be expanded to deployment of structural health monitoring, whereby fatigue cracks in a plate-like structure can be evaluated quantitatively. A dedicated analytical model, in conjunction with the use of a variational principle-based method and an elasto-dynamic reciprocity method, was established. Using the model, an insight into the modulation mechanism of the crack on Lamb wave propagation was achieved, and the contact acoustic nonlinearity (CAN)-induced second harmonic generation was interrogated. Two scenarios were considered in which the plate bearing a "breathing" crack was modelled in two-dimensional and three-dimensional scenarios, respectively, yielding a quantitative correlation between crack parameters and second harmonic-based nonlinearity index. Results obtained from the proposed approach were compared with those from finite element simulation, to observe good coincidence.