Effect of the injection moulding fibre orientation distribution on the fatigue life of short glass fibre reinforced plastics for automotive applications

This work presents an analysis of the effect of the fibre orientation distribution on the fatigue life of a 50wt% short glass-fibre reinforced nylon 6,6 composite produced through injection moulding. A multi-stage modelling approach is proposed to predict the fatigue life of specimens cut at various locations in an injection-moulded plaque. Models for injection moulding, material properties prediction, stress distributions in cyclically loaded specimens and fatigue life calculations have been developed and validated by state-of-the-art experimental techniques. Computed tomography was used to characterise the variation of the fibre orientation at a reference location in the plaque, away from the mould’s side walls. 3-D Digital Image Correlation was used to quantify full-field strain distributions during mechanical loading. Fatigue testing was performed to generate stress-life (S-N) curves of the material with different fibre orientation distributions. Results showed a highly anisotropic behaviour. Better material performance, in terms of strength, stiffness and fatigue resistance, was observed for specimens aligned with the injection direction. An effect of the proximity of the mould’s side walls on fibre alignment was observed with a tensile strength value 20% higher in comparison to that for specimens cut at the reference location. Predicted fatigue lives also improved with higher fibre alignment. Results of the fatigue model validation, at the reference location, showed good agreement between predictions and experimentation, in particular at intermediate lives where a difference of less than 10% was found.