Microcracking in fiber composites and degradation of thermo-elastic properties of laminates

Sammanfattning: The macroscopic failure of composite laminates subjected to tensile increasing load is preceded by initiation and evolution of several microdamage modes. The most common damage mode and the one examined in this thesis is intralaminar cracking in layers. Due to this kind of microdamage the laminate undergoes stiffness reduction when loaded in tension. For example, the elastic modulus in the loading direction and the corresponding Poisson’s ratio will decrease. The degradation of the elastic properties of these materials is caused by reduced stress in the damaged layer which is mainly due to two parameters: crack opening displacement (COD) and crack sliding displacement (CSD). At fixed applied load these parameters depend on the properties of the damaged and surrounding layers, on layer orientation and on thickness. When the number of cracks per unit length is high (high crack density in the layer) the COD and CSD are reduced because of to crack interaction. The main objective of the first paper is to investigate the effect of crack interaction on COD using FEM and to describe the identified dependence on crack density in a simple and accurate form by introducing an interaction function dependent on crack density. This interaction function together with COD of non-interactive crack gives accurate predictions of the damaged laminate thermo-elastic properties. The application of this function to more complex laminate lay-ups is demonstrated. All these calculations are performed assuming that cracks are equidistant. However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In the second paper, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis the COD and CSD are calculated using FEM, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution we obtain lower bond to elastic modulus. A “double-periodic” approach presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers, whereas for high crack density in surface layers it underestimates the modulus reduction. In the third paper, the thermo-elastic constants of damaged laminates were calculated using shear lag models and variational models in a general calculation approach (GLOB-LOC) for symmetric laminates with transverse cracks in 90° layer. The comparison of these two models with FEM was presented for cross-ply and quasi-isotropic laminates.