On the propensity of laminates to delaminate
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.
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