Damage mechanics analysis of inelastic behaviour of fiber composites

Sammanfattning: Composite laminates under service loading undergo complex combinations of thermal and mechanical loading, leading to microdamage accumulation in the plies. The first mode of damage is usually intralaminar cracking with the crack plane transverse to the laminate middle-plane, spanning the whole width of the specimen. The density of cracks in a ply depends on layer orientation with respect to the load direction, temperature change, number of cycles in fatigue, laminate lay-up, ply thickness and, certainly, material fracture toughness. Relative displacements of crack surfaces during loading reduce the average strain and stress in the damaged layer, thus reducing the laminate stiffness. In this work, a theoretical framework which allows determining the whole set of 2-D thermo-mechanical constants of a damaged laminate as a function of crack density in different layers is presented. In this approach closed form expressions, which contain thermo-elastic ply properties, laminate lay- up 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 work using generalized plain strain FEM analysis results for non-interactive 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 non- interactive crack assumption is estimated by comparing with 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. The combined effect of mechanical and thermal loading on damage development is investigated experimentally, by subjecting CF/EP [02,904]s specimens to tensile testing while being cooled down in a climate chamber. The specimen edges and surfaces of cuts parallel to the loading direction were examined in a microscope in order to determine the crack density in the 90-layer corresponding to different applied mechanical stress levels. Comparison has been made to tensile tests at room temperature to see if the damage pattern and the crack density are affected by the testing temperature. The different crack types found were categorised and FE-calculations have been performed varying the lay-ups and the material systems (GF/EP and CF/EP) in order to investigate how each crack type affects the elastic modulus of the laminate. The crack-interaction within a layer is investigated using FEM and an interaction function that can be used to find the COD for interacting cracks is developed. It can be used for all material and lay-ups. Finally the derived constitutive model is used for damage in short fiber composites.