MECHANICAL PROPERTIES OF HYBRID COMPOSITE LAMINATES - Management of failure modes in Z-pinned composite structures

The thesis presents results of quasi-static tensile tests of thin-walled, patch-joined, butted I-sections manufactured from woven carbon fabric (T300/914). The work concentrates on the performance of z-pins as a means of strengthening a lap joint between two carbon fibre reinforced polymer composites (CFRP) I-sections placed end-to-end. These I-section test joints have been tested under tensile load in an attempt to best mimic the loading conditions of a representative aerospace structure. For the benefit of this work the author has attempted to produce a failure map for the joint in question, incorporating the effects of overlap length and z-pin reinforcement on the load capability of the joint. The effect of through-the-thickness reinforcement (TTR) by Z-pinning of the patch on the maximum load is minimal, but the failure mode changes from patch delamination to a laminate failure in the flanges and the web of the I-section. The delamination limiting effect of the Z-pinning is also seen in post-impact tensile tests on similar I-sections. The thesis examines also the relationships between the macromechanics of the failure of the I-sections and the micromechanics of failure of Z-pinned laminates. The failure management consists of adjusting the various joint geometries to induce the wanted failure mode. The results presented show that Z-pinning is effective in retarding or stopping delamination crack propagation and its use can in some cases lead to a slightly increased joint failure load. There is a knock down in tensile strength for the Z-pinned laminates which is seen for the joints which fail in the laminate. The post impacted joints which fail in the laminate do not display this knock-down suggesting a damage tolerance improvement through the use of Z-pins