Simulation and Experimental Evaluation of Mixed Mode Delamination in Multidirectional CF/PEEK Laminates under Quasi-Static and Fatigue Loading

Kurzfassung

The need for light weight structural materials with good resistance to fatigue has led designers of aerospace industry to increasingly employ CFRP structures, when especially cyclic loading is of primary concern. The composite laminate in this study is composed of multidirectional Carbon Fiber Reinforced Plastic plies (CFRP) with varying fiber orientations. The first section of the study focuses on the numerical modelling of quasi-static mixed mode damage of the multidirectional CFRP laminates. The effect of varying fiber orientations, different stacking sequences and combination of inter- and intralaminar damage modes on the mixed mode delamination failure of the constituent have been investigated numerically and experimentally. In the numerical FE model, each CFRP lamina is assumed as an orthotropic homogenized continuum under plane stress, permitting the modelling of damage initiation in each ply under the combination of longitudinal (in fiber direction), transverse and shear stress states. The interface elements, lying in the delmaination plane, are represented via the cohesive zone concept. A general constitutive law connects the traction vector to the vector of displacement discontinuities and an isotropic damage variable degrades this interfacial traction until the interface element totally fails [1]. This interfacial damage behaviour is implemented as a user defined element routine (UEL) in ABAQUS. The second part of the study includes the simulation of mixed mode delamination of multidirectional laminates under cyclic loads. Under cyclic loading, the interface damage model must count for subcritical stiffness degradation and damage accumulation during each unloading-reloading step [2]. This is achieved by adding the fatigue damage law, established by the evolution of the damage variable in terms of the crack growth rate (da/dN), to the constitutive behaviour of the cohesive element developed previously (the mentioned UEL). The obtained numerical results are validated successfully by comparison with the conducted experiments. Finally, scanning electron microscopy was also used for distinguishing the features of the fracture surfaces and to establish the differences between static and fatigue fracture, as well as the differences between the various modes of fracture in different multidirectional lay-ups.