Zero-tension fatigue behaviour of co-cured composite step joints with multiscale toughening

The objective of this study is to investigate the effect of multiscale toughening with nano core-shell rubber particles and micro-fibre thermoplastic veils on the static and fatigue behaviour of co-cured composite step joints. Three toughening routes were used to toughen the interface region of the co-cured step joint: (a) 10 wt% core-shell rubber nano particles for matrix toughening, (b) 20 g/m2 polyphenylene sulfide micro-fibre veils for interface toughening and (c) multiscale toughening trough combining matrix toughening and interface toughening (i.e. 10 wt% core-shell rubber nano particles and 20 g/m2 polyphenylene sulfide micro-fibre veils). The co-cured composite joints without adhesive were manufactured by vacuum-assisted resin infusion with out-of-autoclave curing. Tensile tests under quasi-static loading and different levels of cyclic loading were conducted to investigate the static and fatigue behaviour of the untoughened and toughened joints. Scanning electron microscopy was used to characterise the damage mechanisms after the tests. The experimental results showed that the multiscale toughening routes were much more beneficial for the improvement of the static failure load and long cycled fatigue life of the co-cured composite joints in comparison to those of the untoughened and single material toughened composite joints. For the static behaviour, the average failure load of the co-cured joint was increased by ∼50% by multiscale toughening compared to that of the baseline joint, which was significantly higher than the single material toughening route. Through the S-N curves and two-parameter Weibull probability analysis, the fatigue life of the multiscale toughened joint was significantly increased with the same relative load level and reliability level. For example, the fatigue life of the multiscale toughened joint was increased by ∼230% under 55% relative load level with 90% reliability, while the core-shell rubber particles and polyphenylene sulfide veils only had ∼1% and ∼18% improvement, respectively.