Multi-directional hybrid laminates-studying the effect of fragmentation and dispersed delamination on stress-strain curves of unnotched laminates using analytical modelling.

Hybridisation is one of the approaches to introduce pseudo-ductility to brittle composite materials. In this approach, two or more different types of fibre are combined and if the configuration and material constituents are well selected, the tensile response shows a gradual failure and pseudo-ductile strain. Different types of hybrid composites with continuous layers have been studied to produce pseudo-ductile tensile behaviour. However, most hybrid material studies to date have been focused on UniDirectional (UD) laminates which are not usually applied in industry due to poor transverse mechanical properties. In this study, the behaviour of multi-directional hybrid laminates made with UD hybrid sub-laminates is studied. The final goal is to introduce pseudo-ductility to layups with wider industrial applications. The effect of layup as well as the UD building-block stress-strain curve on the final stress-strain curve of the laminate is also studied. A new analytical approach based on Classical Laminate Theory is introduced in which the effect of different damage modes in UD hybrid laminates (fragmentation and dispersed delamination) is taken into account. The output of this method is the non-linear stress-strain curve of a multi-directional laminate with UD hybrid sub-laminates. This method is then used to study the effect of different parameters such as the mechanical properties of the constituents (low and high strain materials) and layup on the pseudo-ductility

Demonstration of pseudo-ductility in quasi-isotropic laminates comprising thin-ply UD carbon/epoxy hybrid sub-laminates

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Quasi-Static Cyclic Response of Unidirectional Thin-Ply Hybrid Composites

Quasi-static cyclic loading of unidirectional (UD) thin-ply hybrid composites was conducted to assess the extent of stiffness loss with increasing applied strain. For this study, three types of hybrid configuration were examined: SG1/MR401/SG1, SG1/TR301/SG1, SG1/TR302/SG1, where SG is a high strength glass fibre and MR40 is an intermediate modulus carbon fibre while TR30 is a standard modulus carbon fibre. The strain at first carbon ply failure and the knee point strain (εk) for the SG1/TR301/SG1 hybrid is higher than for the SG1/TR302/SG1 hybrid. This is due to the ‘hybrid effect’ which provides a delay in damage initiation due to a constraint on broken carbon cluster development. For SG1/MR401/SG1 and SG1/TR302/SG1 configurations, the stiffness reduction over the course of loading was governed by fragmentation of the carbon plies and delamination between the carbon and glass plies. A smaller stiffness reduction for the SG1/TR301/SG1 configuration compared to the other hybrid configurations was observed with the fragmentation of the carbon ply as the main damage mechanism responsible for the reduction. With each loading cycle, there was a small amount of hysteresis and residual strain. The response of the UD thin-ply hybrid laminates are considered pseudo-ductile because the damage in the form of ply fragmentation and stable delamination, leads to gradual loss of stiffness. The stable delamination of this hybrid material is due to the low energy release rate of the thin carbon ply