This dissertation proposes a stress-based, multi-scale and multi-stage modelling approach, developed to predict the effect of the fibre orientation distribution on the fatigue life of an injection moulded short-glass fibre reinforced nylon 6,6 composite (vf=30%). The main objectives of this research were: to analyse the effect of the fibre orientation distribution in the material tensile and fatigue behaviour; to develop a stress-life (S-N), multi-stage computer-aided engineering (CAE), modelling methodology to produce sensible (±3std. dev.) fatigue life predictions; and to understand the damage mechanisms that incentivise crack initiation and propagation under constant amplitude cyclic loading. The novelty of the present work also included the full experimental validation at every step of the modelling phase to assess the accuracy of the numerical simulation and the effect of the propagation of these inaccuracies on the final fatigue life prediction.
The composite material presented highly anisotropic behaviour. This is a product of the variation of the fibre orientation through the thickness, produced during the injection moulding process. This non-homogenous fibre distribution showed a skin-shell-core microstructure.
Good agreement, qualitative and quantitative, was obtained between the finite element simulations and the experimental results in terms of: the fibre orientation tensor, full strain-fields and fatigue lives. However, a limitation of the stress-life based approach was revealed for fully reversed stress ratios, R=σ_min/σ_max =-1. Therefore, a new energy approach was investigated, allowing the introduction of a fatigue parameter derived from the analysis of the cyclic stabilised hysteresis loops, herein expressed in terms of Dissipated Energy vs Life curves. Initial results of this approach revealed a reduction in the spread of the fatigue data that was seen with the S-N method.
Finally, the analysis of specimens' fracture surfaces, via optical and scanning electron microscopy (SEM); together with high-speed camera images, digital image correlation results, and in conjunction with the stress and strain modelling results; suggested that the development of damage in the material follows a multi-crack nature, with initiation likely occurring below the surface, in locations where the maximum stress/energy is present. Additionally, evidence of several crack planes were observed, which implied the presence of multiple cracks developing and propagating at different points in the fatigue life of the specimens.
