Z-fibre pinning is a new method of through-thickness reinforcement of laminated composites. This paper presents an experimental test and theoretical analysis on how and why these pins can improve the resistance to impact loading and post-impact compression. Specimens were made of carbon/epoxy T300/914C with nominal thicknesses of 2, 4, and 6 mm. For the specimens tested in this study, z-pinning reduced impact damage area by 19-64% depending on the specimen thickness and impact energy. Experimental results obtained also indicate that z-pins can significantly increase the compression-afterimpact (CAI) strength by about 45%. In this paper all of these observations are discussed in the context of theoretical and numerical models that have been developed previously to predict the critical impact force and z-pinning performance in terms of the laminate in-plane stiffness and mode I delamination fracture toughness with the z-fibre bridging effect.

The mechanical properties of z-pinned composite laminates were examined numerically. Finite element calculations have been performed to understand how the through-thickness reinforcement modifies the engineering elastic constants and local stress distributions. Solutions were found for four basic laminate stacking sequences, all having two percent volume fraction of z-fibres. For the stiffness analysis, a micromechanical finite element model was employed that was based on the actual geometric configuration of a z-pinned composite unit cell. The numerical results agreed very well with some published solutions. It showed that by adding two percent volume fraction of z-fibres, the through-thickness Young's modulus was increased by 22-35 percent. The reductions in the in-plane moduli were contained within 7-10 percent. The stress analysis showed that interlaminar stress distributions near a laminate free edge were significantly affected when z-fibres were placed within a characteristic distance of one z-fibre diameter from the free edge. Local z-fibres carried significant amount of interlaminar normal and shear stresses.

Abstract:This paper presents an approach to predict the delamination of braided composite Tpiece specimen using cohesive models. As part of an investigation on simulation of delamination in T-piece specimens, cohesive elements from ABAQUS were employed in forming a cohesive model to study the progressive delamination. Predictions given by the model of single delamination together with experimental results are presented.These results suggest that prediction of progressive delamination using cohesive models is feasible. Finally this paper proposes future work for precise prediction of delamination of braided composite T-piece specimens.2

This paper presents a detailed numerical study of the mode I interlaminar fracture of carbon/epoxy composite laminates with z-fibre reinforcement. The study was performed using a double cantilever beam configuration. A finite element model was developed using thick-layered shell elements to model the composite laminates and non-linear interface elements to simulate the through thickness reinforcements. An existing micro-mechanical solution was employed to model the material behaviour of the interface element. The numerical analysis showed that z-fibre pinning were effective in bridging delamination when damage had propagated into the z-fibre field; these pins provided crack closure forces that shielded the delamination crack from the full delaminating force and moment due to applied loads. Therefore, the z-fibre technique significantly improves the crack growth resistance and hence arrests or delays delamination extension. The numerical results were validated against experimental data. With reference to structural integrity this technique can be used to design a more damage tolerant structure.