Quantitative identification of delamination at different interfaces using guided wave signals in composite laminates

This article presents the development of a quantitative method for identifying interfacial delamination in composite laminates. Splitting and merging occur as Lamb waves pass through the delamination. It is demonstrated from the dispersion curve of Lamb waves that the group velocities of A(0) and S-0 modes in the sub-laminates above and below the delamination area are different when the delamination interface is not at the thickness center of a symmetrical layup laminate. Changes in the time of flight (ToF) of A(0) and S-0 modes are caused by the parallel propagation of Lamb waves. Based on those changes in the ToF, delamination size can be evaluated even though the delamination location through the thickness is not pregiven. Numerical and experimental studies are conducted to assess this method. Continuous wavelet transform is applied to extract the ToF of A(0) mode waves from the overlapped signals. Mode conversion is illustrated for the interaction of Lamb waves and delamination. The sensitivity and applicability of the proposed method are validated numerically and experimentally in two asymmetric layup composite laminates. The limitation of this method for symmetric layup composite laminates is analyzed and a potential improvement approach is further discussed.