Abstract :
Laminates have a propensity to delaminate; the
mathematical plane between adjacent plies offers a preferred
path for crack propagation, irrespective of the nature
of the stress field that gives rise to the elastic strain energy
released. This is because the plane between plies is characterised
by a specific fracture surface energy significantly
lower than those for internal surfaces that intersect fibres.
In the second of his two classical publications on fracture,
A.A. Griffith showed how crack rotation in two-dimensional
stress fields occurs. This suggests how, in a laminate,
pre-existing flaws are able to seek out the plane of lamination;
here, crack rotation under the influence of, for
example, shear stress is examined in the context of laminates
designed for use in aerospace. One physical
consequence of Griffith’s calculation is the prediction of
crack propagation in elastic solids subjected to bidimensional
compression with strongly unequal principal
stresses. A simple bidimensional compression rig has been
devised to investigate this prediction. To obviate the risk of
delamination, it will be necessary to move away from
anisotropic lay-ups, and further develop three-dimensional
weaves and methods for weaving three-dimensional
weaves. A method whereby a three-dimensional fibre
weave, which has cubic symmetry and no zero-valued
shear moduli, might be weaved is outlined
