Edge disbond detection of carbon/epoxy repair patch on aluminum using thermography

Abstract The composite patches are used to repair the damaged metal and composite structures in different industries, especially in the aerospace industry. One of the most dangerous types of defects are the disbond defects in repaired structures because of cutting the flow of stress to the patch. Disbond defects are often initiated at the edge of the repair patch and propagated to the center by degradation in service. Flash thermography inspection at the edge of repair patch is a challenging issue due to the edge effect. In the present study, the disbonds at the edges of composite repair patch have been investigated by step heating thermography method. Since, raw thermal data are usually not suitable for quantitative defects evaluation and so, different image processing methods, including Fourier transform (FT), one- and two-dimensional Daubechie's wavelet transforms have been performed. Furthermore, for the first time, the three dimensional wavelet transform has been used to optimize the results in terms of signal to noise ratio and time processing. Moreover, the simulation of the thermography inspection process has been carried out using finite element modeling (FEM). The model enables a better understanding of the heat transfer process and provides a means of establishing the experimental set-up parameters required for inspecting appropriately the edge of the repair patch on metal structures. Results provided some suggestions in selecting more suitable post processing for interrogating of adhesive bonded composite with different states.