Stiffness degradation induced by multilayer intralaminar cracking in composite laminates

Abstract

A theoretical model for the prediction of the elastic properties of a general symmetric laminate containing multilayer matrix cracks is proposed. A five-layer equivalent constraint model (ECM) laminate [SL/ϕp/θq/φr/SR]s and seven others degenerated from such a model laminate by eliminating some of the sublaminates, [ϕp],[φr], SL and SR, are designed for the analyses of the degraded stiffnesses of the cracked laminae in the laminate; the crack configuration in the θ-lamina is explicitly represented (designated by underline `_') while the primary and secondary constraining effects are taken into account by the stiffness-equivalent homogeneous layers, [ϕp] and [φr], as well as SL and SR. A sublaminate-wise first-order shear deformation laminate theory is developed to analyze the stress and displacement fields in the ECM laminates under the combined tension–shear loading. The in-situ damage effective functions (IDEFs), Λ22 and Λ66, for characterizing the in-plane stiffness reductions of a cracked lamina constrained, are then expressed as explicit functions of the transverse ply crack spacing in such a lamina, as well as of the stiffness properties and the geometric parameters of the constraining layers by using the obtained stress field. The theory's predictions are in a good agreement with the experimental E-modulus data for [0/90/±45]s glass fiber reinforced epoxy composite laminates under fatigue.