Effect of off-axis ply orientation on 0°-fibre microbuckling

The aim of the present work is to study both experimentally and theoretically the compression failure mechanisms in multi-directional composite laminates, and especially the effect of the off-axis ply orientation on fibre microbuckling in the 0 degrees-plies. The critical mechanism in the compressive fracture of unidirectional polymer matrix composites is plastic microbuckling/kinking. In multi-directional composites with internal 0 degrees-plies, catastrophic failure also initiates by kinking of 0 degrees-plies at the free-edges or manufacturing defects, followed by delamination. When 0 degrees-plies are located at the outside, or in the case of cross-ply laminates, failure rather tends to occur by out-of-plane buckling of the 0 degrees-plies. T800/924C carbon-fibre-epoxy laminates with a [(+/-theta/0(2))(2)](s) lay-up are used here to study the effect of the supporting ply angle theta on the stress initiation of 0 degrees-fibre microbuckling. Experimental data on the compressive strength of laminates with theta equal to 30, 45, 60 or 75 degrees are compared to theoretical predictions obtained from a fibre kinking model that incorporates interlaminar shear stresses developed at the free edges at (0/theta) interfaces. Initial misalignment of the fibres and non-linear shear behaviour of the matrix are also included in the analysis. (C) 1999 Elsevier Science Ltd. All rights reserved.