Influence of reinforcement positioning on tension-tension fatigue performance of quasi-unidirectional GFRP laminates made of stitched fabrics

In quasi-unidirectional (quasi-UD) laminates reinforced with stitched non-crimp fabrics (NCF), axial fibre bundles in neighbouring fabrics can lie on top of each other or they may be overlapped. Also, the stitch phase can vary between the neighbouring fabrics. In this study, the effect of mutual location of axial fibre bundles and stitch phase difference on tension-tension fatigue performance of quasi-UD laminates reinforced by glass-fibres was experimentally investigated. Damage development and accumulation were monitored by photographing and thermal-imaging in the specimens' gauge section during the fatigue tests. Internal structure characterisation of the laminates based on micrographs was performed to find possible reasons that might affect the fatigue life. The study revealed that the stitch phase difference clearly affects the damage progression, fatigue life and the slope parameter of fitted S-N curves, but the effect of the mutual location of axial fibre bundles on these parameters is minor.