Interlaminar Stresses in Composite Notched and Unnotched Laminates

Abstract

The purpose of this paper is to determine the interlaminar stress distribution at curved and straight free edges for quasi-isotropic and zero-dominated laminates. The interlaminar stresses are calculated using a three-dimensional finite-element code with error control. For straight free edges the results are compared to existing semi-analytical methods. Another objective with this investigation is to evaluate a quadratic failure criterion for prediction of initial load for delamination and the most likely position for the delamination. In this paper it is found, for quasi-isotropic stacking sequences, that it is not possible to tailor a stacking sequence so that o, becomes compressive near hole boundaries or at straight free edges. On the contrary both tensile and compressive stresses appear in a periodic manner through the thickness. It can also be concluded that interlaminar stresses exhibit a thickness effect for repeating bundles. The singularity of the interlaminar components is concentrated to a narrow region, approximately 1-2 fibre diameters in thickness and radial direction. Since the interlaminar stresses are so localised, a continuum approach will not provide useful solutions in this area. Consequently the material cannot be treated as homogeneous in this region, instead the heterogeneous nature within each ply should be modelled. The smallest element size in the present analysis for the refined mesh is less than 1 pum, i.e., far less than a fibre diameter. Utilising a quadratic failure criterion, for the studied quasi-isotropic laminates, in conjunction with obtained FE-results, it is shown that global strain to initiate delamination does not depend on the stacking sequence, but so does the number of possible initiation points. For quasi-isotropic layups with a circular cut-out, initiation of delaminations occurs at approximately the same global strain, 0.25%. For unnotched zero-dominated layups initiation and fracture strain are almost the same, i.e., -1.2%. The investigation also shows that the number of ±450/00 or ±45°/900 alterations should be kept at a minimum to minimise the interlaminar stresses and possible damage initiation points. It is likely that the primary cause for delamination initiation for notched plates are the high interlaminar shear stresses and not the normal stresses. Finally it is demonstrated that available semi-analytical methods do not correctly describe the interlaminar stress distribution.