MICROBUCKLE PROPAGATION IN FIBER COMPOSITES

The propagation of a microbuckle in a unidirectional long fibre compos ite has been investigated by the finite element method. The tip region of the microbuckle is modelled using alternating layers of fibre and matrix, while the microbuckle at the macro scale is treated as a slidi ng mode II crack. By assuming that the composite deforms in accordance with a deformation theory of plasticity, material history effects are neglected. The microbuckle propagation direction is predicted for a w ide range of material properties by finding a correlation direction of the deflected fibres ahead of the microbuckle: propagation angles of between 5 and 30 degrees are predicted, depending on the matrix shear yield strain and the strain hardening exponent. For the case of a carb on fibre-epoxy composite, the predicted value of 19 degrees is in good agreement with measured values in the range 20-30 degrees. A predicte d value of tip toughness G(t)/tau(y)d of 25 is also in reasonable agre ement with experimental measurements of 32-55, where G(t) is the mode II strain energy release rate, tau(y) is the longitudinal shear yield strength of the composite and d is the fibre diameter. An alternative couple-stress finite element calculation, in which the bending resista nce of the fibres is incorporated directly into the element formulatio n, predicts propagation angles in reasonable agreement with those foun d From the above layer finite element model over the range of material parameters considered, with excellent agreement for carbon fibre-epox y composites. Copyright (C) 1997 Acta Metallurgica Inc.