Geometric modeling of 3D woven preforms in composite T-joints

A common method to fabricate net-shaped three-dimensional (3D) woven preforms for composite T-joints is to weave flat 3D preforms via a standard weaving machine with variation in binder yarn path and then separate the preform in the form of a bifurcation. Folding introduces fiber architecture deformation at the 3D woven bifurcation area. In this paper, a geometric modeling approach is proposed to represent the realistic fiber architecture, as a preprocessor for finite element analyses to predict composite structural performance. Supported by X-ray micro-computed tomography (mCT), three important deformation mechanisms are observed including yarn stack shifting, cross-section bending, and cross-section flattening resulting from the folding process. Furthermore, a set of mathematical formulae for simulation of the deformations in the junction region are developed and satisfactory agreement is observed when compared with mCT scan results

Impact and post-impact behavior of composite laminates reinforced by Z-Pins

Delamination, i.e. the separation between layers that occurs by failure of the resin-rich interlaminar interface, is a direct consequence of the lack of out-of-plane reinforcements and, undoubtedly, the most common damage mode in laminated composites. Delaminations induced by low-velocity impacts are of primary concern in structural applications, since impact damage, often miss-detected, may propagate, impairing the load bearing capacity of the component, particularly under compressive loads. Preventing delamination, as well as delaying and limiting its propagation, are thus key issues in the design of composite structures. Over the last decades, many strategies have been proposed to address this problem. Among them, the introduction of through-thickness reinforcements (stitching, pinning and stapling) has proven to be effective in improving the interlaminar properties of composite materials. In particular, recent studies have shown that Z-pinning, which consists in inserting high stiffness pins through the thickness of uncured laminates, may significantly enhance the delamination resistance of laminated components. In this study, conventional and Z-pin reinforced [02/902]s graphite/epoxy laminates were subjected to low-velocity impact and compression after impact (CAI) tests in order to examine the effect of the reinforcements on the impact response and the residual post-impact properties of the laminate. The results show that Z-pinning significantly reduces the extent of delamination induced by impact, while, on the contrary, it appears to only marginally improve the post-impact compressive strength of the laminates