The aim of the present work is to study both experimentally and theoretical
ly the compression failure mechanisms in multi-directional composite lamina
tes, and especially the effect of the off-axis ply orientation on fibre mic
robuckling in the 0 degrees-plies. The critical mechanism in the compressiv
e fracture of unidirectional polymer matrix composites is plastic microbuck
ling/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 degre
es-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-pl
ies. 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 dat
a on the compressive strength of laminates with theta equal to 30, 45, 60 o
r 75 degrees are compared to theoretical predictions obtained from a fibre
kinking model that incorporates interlaminar shear stresses developed at th
e free edges at (0/theta) interfaces. Initial misalignment of the fibres an
d non-linear shear behaviour of the matrix are also included in the analysi
s. (C) 1999 Elsevier Science Ltd. All rights reserved.