Mitigation of Free-Edge Effects by Meso-Scale Structuring

Abstract

We are developing a new class of fiber-reinforced polymer composite materials to facilitate the embedment of multifunctional features and devices in material systems and to manage interlaminar stresses at the external free edges and internal free surfaces of holes and cut-outs in composite laminates. The idea is centered on the introduction of one or more additional dimensions of design space by a tessellation of individual laminae into sets of discrete tiles, each possessing the same levels of design freedom normally associated with an entire lamina (material constituents, fiber orientation, and so on). In this work, we have focused on the development of tiling schemes that will allow blending of disparate laminates (lay-ups), where a lay-up suitable for suppressing interlaminar stresses could be substituted at necessary locations in place of another lay-up that may be more suitable for the global structural loads. This technique results in the inclusion of possibly detrimental matrix-rich tile-to-tile interface pockets in the plane of each lamina. Mechanical testing has shown that uniaxially reinforced tiled composites maintain stiffness levels near those of their traditional continuously reinforced counterparts, yet with a potential degradation of strength. We have used the finite element method to investigate the effects of resin-rich pocket size, the use of supporting continuous layers, tile size, and tile overlapping schemes (interface stacking geometry) on the distribution of stress and transfer of load around interfaces in uniaxially reinforced tiled composites. This was done with the aim to identify parameters controlling overall strength. We discovered that alignment of the resin-rich pockets through the thickness exacerbates stress-concentration and that outer continuous layers on the composite may help in better load transfer and more efficient material utilization. Failure analyses of the finite element results using three-dimensional Hashin-Rotem failure criteria [1] have shown the concept to be effective in the suppression of free-edge delamination in traditional quasi-isotropic and angle-ply laminates under tensile loading. Although each meso-scale structured solution must be tailored to the exact structural geometry and anticipated loads, the technique shows promise to have broad application