Experimental and numerical study of the effect of using polyurethane instead of Teflon strip to simulate debonding defect in composite patch repairs aluminum plate under thermography inspection

In order to evaluate the capabilities of a non-destructive testing (NDT) technique for detecting the potential defects, the simulation conditions of manufactured defects must be correspond to real defects from shape and material point of view. Debonding defects are one of the most dangerous types of damage in composite patches for repairing cracks and defects in aircraft structures. In the case of real debonding defect, a thin air layer is embedded between the aluminum and the epoxy resin adhesive. In general, an artificial debonding is simulated by inserting Teflon (Polytetrafluoroethylene) tapes within the joint. This paper proposes the application of polyurethane (PU) instead of polytetrafluoroethylene (PTFE) strip to simulate debonding in composite when the specimen is inspected by thermography technique. The effect of the simulation of debonding defect at the edge of composite patch using polyurethane, Teflon tapes, and air gaps under thermography inspection is studied. Finite element modeling (FEM) and experimental verification of debonding at the edge of composite patch show a different efficiency of thermography inspection due to different thermal properties of Teflon, polyurethane, and air gap (real debonding). Comparing the performance of thermography inspection of composite patch with debonding simulated by PU, PTFE and air reveal the importance of simulating defects with realistic industrial configurations. Finite element analysis and experimental results show that using the proposed polyurethane sheet instead of Teflon sheet can provide much more realistic simulation of debonding defect in composite patch subjected to thermography inspection.