Assessing the effects of ply constraints on local stress states in cross-ply laminates containing manufacturing induced defects

Computational micromechanics is employed to assess the influence of ply constraints on the early stages of 90 degrees ply crack formation in a cross-ply laminate under tensile loading. Manufacturing induced defects, including nonuniform fiber spatial distribution, microvoids and microcracks, are explicitly simulated. Local distortion and dilatation energy density values in the matrix, associated with local yielding and brittle cavitation, are assessed within unconstrained and constrained 90 degrees plies. The effects of ply constraints are found to emerge when preexisting microvoids or microcracks are present, while dissimilar levels of constraining are observed in distinct regions of the ply, e.g. fiber clusters versus resin-rich zones. These important results reveal that for constrained plies variations in the local deformation fields within these regions and their interdependency directly influence crack nucleation and its subsequent evolution. Additional parametric studies show the influence of pre-existing microvoid and microcrack sizes on the degree of ply constraining. Furthermore, the effect of the constrained and adjacent ply thicknesses, ply interface resin-rich layer thickness, and post-cure thermal cooldown on the degree of constraining are also considered. The study provides an improved understanding of the early stages of ply crack formation in constrained plies, which is important for predicting damage evolution in laminates and for using the lowest damage thresholds for safe designs.