Analysis and experimental characterization of static and fatigue delamination in L-shaped laminates: Towards slow-growth damage tolerance

The development of slow-growth design approaches for the fatigue and damage tolerance of composite aircraft structures containing pre-existing damage remains a major challenge to this day. Delamination is particularly critical, due to difficulties in detection and the high sensitivity of laminated composites to this damage mode. The aim of this work is to advance the adoption of slow-growth approaches to damage tolerance by investigating propagation regimes in thick, predelaminated L-shaped laminates, and to propose numerical approaches to predict damage growth under quasi-static and fatigue loading. A cohesive zone-based finite element fatigue modelling framework is implemented in an explicit finite element code and verified by modelling standard fracture specimens. The method is subsequently applied to simulate the quasi-static and the fatigue response of L-shaped specimens exhibiting both stable and unstable delamination regimes. The evolution of energy release rate with crack growth is analysed with a semi-analytical approach to interpret the observed propagation phenomena. The numerical-experimental correlation suggests that the method presented can elucidate the delamination propagation under cyclic and monotonic loading, in a variety of significantly more complex scenarios than those of characterization coupons.