Identification of breathing type disbonds in stiffened panels using non-linear lamb waves and built-in circular PWT array

A non-linear Lamb wave based active sensing technique for identification of breathing type disbonds in stiffened metallic plates has been presented. In this technique, a built-in dual transmitter and multi receivers (DTMR) clock-like array of piezoelectric wafer transducers (PWT) is used. The central part of the array consists of two transmitters, which are placed concentrically on either side of the plate. A Hanning window modulated sine pulse has been used for excitation. Opposite polling is applied to the transmitters to generate a predominant anti-symmetric (A0) Lamb mode to encourage breathing in the disbond. It is observed that the contact acoustic nonlinearity (CAN) of breathing disbond generates higher harmonics when a primary Lamb wave interacts with the disbond. Since these harmonics carry damage information, a bandpass filter is used to extract the second harmonic component from the received signals followed by wavelet transformation. Finally, the transformed signals are used to device a baseline free damage detection algorithm for locating the disbond. Experimental investigations are carried out to interrogate a full width disbond that is located either at the middle of the T-stiffener or at an offset from its center. The damage index (DI) map obtained for the entire plate show higher values at the disbond location. Once the disbond is localized, a damage quantity (DQ) curve is devised to determine the extent of disbond with a high degree of accuracy. Extensive numerical simulations are carried out using commercially available finite element package, ANSYS 14.0, in an effort to examine the effectiveness of the damage detection algorithm and to further establish the DQ curves. The numerical results are found to be in good agreement with the experimental findings that affirm the robustness of the nonlinear active sensing technique for detection and assessment of breathing type disbond in stiffened aluminium panels without requiring the baseline signals.