Nonlinear analysis of misaligned unidirectional carbon fibre-epoxy compression specimens

Finite element analysis of shear instability of short gauge length compression specimens with a uniform misalignment between the loading axis and fibre direction has been carried out. The nonlinear shear response of the composite and the nonlinear geometry effect of the rotation of the fibres due to shear were included in the analysis. The results are compared with predictions from a simple analytical model assuming a uniform stress field with no constraint over a range of misalignment angles. The finite element analysis predicts a greatly reduced effect of misalignment on the compressive strength because of the lateral constraint of the fixture. This explains why deliberate misalignment of specimens has not been found to have such a dramatic effect on measured compressive strength as indicated by simple models assuming a uniform unconstrained stress field, and demonstrates the limitations of such models when applied to real compressive failures.