Model to predict effects of triaxial loading on ply cracking in general symmetric laminates

This paper first considers the mechanics for predicting stress transfer in a general symmetric laminate, having a uniform distribution of ply cracks in a single orientation, subject to combined general in-plane and through-thickness loading. The effects of residual stresses arising from thermal expansion mismatch are taken into account. The paper extends an existing stress transfer model so that the effect of through-thickness loading can be considered. The second step is the extension of an existing framework for the formulation of criteria that can be used to predict the progressive formation of ply cracks during complex loading. The effective stress/strain relations for a damaged laminate are shown to be identical in form to those of an undamaged laminate. By considering ply crack closure conditions, a series of very useful inter-relationships between thermo-elastic constants for damaged and corresponding undamaged laminates are derived. In particular, it is shown that a single damage dependent function controls the dependence of all thermo-elastic constants on the state of ply cracking. This function has been determined from stress transfer analysis for the special case of uniform ply crack distributions having a single orientation. The inter-relationships between thermo-elastic constants enable the derivation of crack formation criteria that are relatively simple in form.