A theoretical framework which allows determining the whole set of 2-D thermomechanical constants of a damaged laminate as a function of crack density in different layers is presented. In this approach, closed-form expressions, which contain thermoelastic ply properties, laminate layup, and crack density as the input information are obtained. It is shown that the crack opening displacement (COD) and crack face sliding displacement, normalized with respect to a load variable, are important parameters in these expressions influencing the level of the properties degradation. They are determined in this paper using generalized plain strain FEM analysis results for noninteractive cracks. The strong dependence of the COD on the relative stiffness and thickness of the surrounding layers, found in this study, is described by a power law. The methodology is validated and the possible error introduced by the noninteractive crack assumption is estimated by comparing with the 3-D FEM solution for a cross-ply laminate with two orthogonal systems of ply cracks. Experimental data and comparison with other models are used for further verification.

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The previously developed closed form expressions for thermo-elastic properties of laminates with intralaminar cracks (Lundmark and Varna, 2005) contain crack surface opening and sliding as main local parameters. The dependence of these parameters on various material and geometrical characteristics was in (Lundmark and Varna, 2005) described by power functions valid only for noninteractive cracks in a given layer (low crack density). In this article the 90-layer crack interaction in terms of its effect on the crack opening displacement (COD) is discussed. The effect on COD is described by the introduced ‘interaction function’ which is determined fitting results of finite element (FE) analysis for cross-ply laminates. To simplify the application in stiffness predictions, the numerically found weak dependence of the interaction on geometrical and material parameters is neglected and the interaction function is presented as a function of crack density only. Using the interaction function to determine the COD, the previously developed calculation scheme (Lundmark and Varna, 2005) has been used to predict stiffness reduction in the entire crack density region. The results are validated comparing with tests and FE simulations.

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