The static and fatigue failure of co-cured composite joints with two-scale interface toughening

This research investigates the static and high-cycle-fatigue behaviour and failure mechanisms of co-cured composite single-step joints with two-scale interface toughening. Resin infusion followed by out-of-autoclave curing is used to manufacture the co-cured carbon/epoxy composite single-step joints without a structural adhesive. The co-cured composite joint interface region is toughened by one of the following three routes: (a) nano-scale toughening by core-shell rubber (CSR) nanoparticles added to the resin at a concentration of 10 wt%, (b) micro-scale toughening by micro-fibre polyphenylene sulfide (PPS) veils with an areal density of 20 g/m² included in the layup, and (c) two-scale hybrid toughening (i.e. CSR&PPS) with CSR nanoparticles (10 wt%) and PPS micro-fibre veils (20 g/m²). The static and fatigue failure behaviour of the untoughened (i.e. Baseline) and toughened joints are investigated by conducting tensile tests under quasi-static loading and different levels of cyclic loading. The fatigue tests are conducted in constant amplitude sinusoidal load control mode with a frequency of 10 Hz at a load ratio of 0. The debonded interface of the joints after tests is examined for failure mechanisms. The results show that the two-scale toughening strategy is effective in the improvement of the static strength and fatigue life of the co-cured joints. The single-scale toughening route is either adverse (by CSR nanoparticles) or less efficient (by micro-fibre PPS veils) compared to the two-scale toughening route. The nano-scale toughening mechanisms and micro-scale toughening mechanisms have a synergistic effect on improving the static and fatigue performance of co-cured joints.