Comparison on damage tolerance of scarf and stepped-lap bonded composite joints under quasi-static loading

Scarf or stepped-lap repairs are desirable for composite structures in aerospace, especially where a flush finish is required. Damage tolerance is a critical issue in the repair design. This paper investigates and compares the damage tolerance of scarf and stepped-lap joints (repairs) under quasi-static loading using finite element analysis (FEA). The damage was represented by a flaw embedded in the bondline between composite adherents. 3D FE models were built for both scarf and stepped-lap joints. Both linear and nonlinear adhesive properties were simulated under room temperature (RT) and hot-wet (HW) conditions. The flaws with varying length and width were imbedded in the bondline, so that the sensitivity of damage tolerance to the flaw size can be analysed and compared between scarf and stepped-lap joints. Other parameters, which may affect the damage tolerance of both joints, include flaw location, number of steps of stepped-lap joint, stacking sequence of laminate adherend, out-of-plane boundary conditions and the presence of an external doubler. The results showed that the stepped-lap joint exhibited better damage tolerance than the scarf joint with linear elastic adhesive. The damage tolerance of both joints became similar when the adhesive became nonlinear under RT or HW conditions. The damage tolerance of both joints can be improved by attaching composite doubler reinforcement or constraining the out-of-plane movement.