Energy-based prediction of failure in general symmetric laminates

This paper describes a new homogenisation technique for general symmetric laminates that enables progressive ply crack formation to be predicted in any number of plies having a variety of orientations. The approach involves (i) the analysis of non-uniformly spaced discrete ply cracks having a single orientation, (ii) a novel technique to homogenise the properties of the cracked ply so that discrete ply cracking can be analysed in plies having a different orientation, and (iii) the use of energy based methods to predict the progressive formation of ply cracks in any number of plies during loading. The analysis takes full account of the effects of thermally induced residual stresses. A key feature of the approach is the inclusion of a shear coupling term in the stress-strain relations for homogenised plies that ensures that the homogenised laminate has exactly the same effective properties as the laminate having a ply with discrete cracks in place of one of the homogenised plies. The model is applied to the prediction of the significant dependence of ply thickness and ply lay-up on laminate strength, and results for carbon fibre reinforced plastic laminates are compared favourably with published experimental results.