On the static failure behaviour of co-cured composite joints with enhanced interlaminar fracture energies via multiscale toughening

This paper is focused on the multiscale toughened co-cured composite interfaces of carbon/epoxy laminates using vacuum infusion and out-of-autoclave curing—with an emphasis on the role of enhanced interlaminar fracture energies on the static failure of laminate joints. To toughen co-cured interlaminar regions, three toughening routes are used: (i) epoxy matrix toughening with 10 wt% core–shell rubber (CSR) nanoparticles, (ii) 10 g/m2 polyphenylene sulfide (PPS) non-woven micro-fibre veil toughening, and (iii) multiscale toughening by combining epoxy matrix toughening with 10 wt% CSR nanoparticles and 10 g/m2 PPS veils. Double cantilever beam and end-notch flexure tests are conducted with the toughened interfaces for mode-I and mode-II fracture energies. Moreover, three co-cured joint configurations (viz. double-notch single lap, stepped scarf and skin-stiffener joints with the three toughening routes) are tested to characterise the role of enhanced interlaminar fracture energies on the static failure of laminate assemblies. The results show that the three toughening routes have significantly altered the R-curve behaviour and interlaminar fracture energies and that the degree of enhancement in mode-I and mode-II fracture energies is sensitive to the type of toughening route. Based on the material systems tested, the mode-I fracture initiation energy is enhanced by ∼150% with multiscale toughening, ∼65% with particle toughening, and ∼60% with veil toughening; whereas, the mode-II fracture initiation energy is enhanced by ∼85% with veil toughening and ∼35% with multiscale toughening—but reduced by ∼20% with particle toughening. The joint tests show that the toughening routes affect the static failure of the three joint configurations and that the strength and damage evolution of the co-cured joints depend on the interlaminar fracture energies. The strength of the co-cured single lap joints is increased by ∼50% with multiscale toughening and ∼40% with veil toughening; whereas, the strength of the co-cured skin-stiffener joints is enhanced by ∼35% with multiscale toughening and ∼45% with veil toughening. The failure mechanisms observed on the fracture surfaces indicate that the strength/failure behaviour of the co-cured joints depends on the dominant interlaminar fracture mode and the corresponding fracture energies. Overall, it is shown that the multiscale interlaminar toughening routes that are used to address the issue of delamination can also be effectively explored to enhance the static failure of co-cured laminate assemblies by toughening the co-cured interfaces/bondlines.