Mechanical characterisation of composite materials with 3D woven reinforcement architectures

The use of traditional two-dimensional (2D) fibre preforms can be associated with poor out-of-plane and interlaminar mechanical performance, particularly in response to impact loads. Such preforms comprise multiple plies which necessitate labour-intensive ply cutting and assembly steps. 3D woven textiles, due to the incorporation of through-thickness yarns, have been found to exhibit superior out-of-plane mechanical properties whilst simultaneously reducing ply-assembly time and cost (single-piece preform construction). Their delamination resistance and damage tolerance have been extensively investigated over the last number of years; however, there is a paucity of published work on their in-plane and out-of-plane mechanical properties when compared to their 2D counterparts. Thus, this research details a comprehensive mechanical characterisation of an orthogonal 3D woven composite in comparison with a suitable 2D laminate. Composite panels have been manufactured with Henkel’s Loctite BZ9130 benzoxazine resin by means of the EADS-patented vacuum assisted process (VAP®). In-plane compressive performance, impact damage resistance, damage tolerance, and out-of-plane tensile behaviour have been evaluated for both reinforcement architectures. Coupons were subjected to an energy level of 6 Joules per millimetre of laminate thickness by means of a drop-weight impact tower. Damage resistance was quantified as a function of impact damage area using penetrant-enhanced x-radiography (PEXR). Combined loading compression (CLC) tests were performed to complement compression after impact (CAI) testing which was conducted to evaluate the materials’ damage tolerance. Furthermore, out-of-plane tensile (OPT) testing has been performed on cruciform coupons using a novel test method. The experimental data revealed a 7.5% higher compressive strength for the 2D material, which was expected due to the presence of crimp and yarn misalignments within the 3D woven material. In contrast, the 3D material performed better than the 2D laminate for all other tests with 13% lower damage area, 15.5% higher residual compressive strength and 36.8% higher mean out-of-plane tensile strength. It can thus be concluded that the incorporation of through-thickness yarn components can lead to significant improvements in damage resistance and tolerance by arresting the growth of delamination and limiting the growth of localised damage. Furthermore, when loaded in the out-of-plane direction, as with the OPT test configuration, much of the load is borne by the z-binder bundles rather than the much weaker fibre-matrix interface (as is the case with 2D laminates), and consequently, the OPT strengths are higher in the 3D woven composites. Limitations of this test method have been identified and recommendations for further development are presented in the concluding remarks.