Onset and growth of adhesion failure and delamination induced damages in double lap joint of laminated FRP composites

Three-dimensional non-linear finite element analyses have been carried out for graphite/epoxy laminated FRP composite double lap joints (DLJ) to study the onset and growth of adhesion failure and delamination induced damages. Out-of-plane stresses on different interfacial surfaces of the DLJ have been determined. The peel and shear stress distributions in the adhesive layer have been evaluated and compared with the existing analytical and numerical solutions showing good agreement. Tsai–Wu coupled stress failure criterion has been employed to identify the critical locations for the onset of adhesion failure and delamination induced damages. Modified crack closure integral (MCCI) technique based on the concept of linear elastic fracture mechanics (LEFM) has been followed to calculate the three individual modes of strain energy release rates (SERR) GI, GII and GIII along the damage fronts of pre-embedded damages due to the adhesion failure and the delamination induced damage for studying the damage propagation. The critical locations for the onset of the adhesion failure and the delamination induced damages have been found to be on the interfacial surfaces between the main adherend and the adhesive layer and on the surface between the first and second plies of the main adherend beneath the adhesive layer within the overlap region of the DLJ, respectively. The SERR in the shearing mode is the governing mode for the propagation of the adhesion failure and the SERR in the opening mode controls the propagation of the delamination damage. However, the contributions of GI and GIII along the adhesion failure front cannot be outrightly ignored. Therefore, it can be said that the adhesion failure would propagate under mixed mode condition, while delamination damage propagation would be predominantly due to the mode I. Also, it has been observed that the adhesion failure is more detrimental than the delamination damages for the failure of DLJ when the DLJ is embedded with both types of damages.