Damage accumulation and stiffness degradation in composite laminates

Sammanfattning: In the presented thesis analytical modeling of elastic properties degradation and modeling of damage evolution in composite laminates is performed. Transverse cracking initiation on a fiber/matrix level is briefly studied by testing model composites and cross-ply laminates. Analysis of obtained results show limitation in application of model composites. Models originally obtained for cross-ply laminates (shear lag, variational) are adapted for more general lay-up of laminates [S,90n]s, where S can be any balanced sub-laminate. Closed form expressions to calculate elastic properties changes as a function of crack density are derived. These models are verified by comparing prediction with experimental results. In this work damage evolution modeling is performed by Monte-Carlo simulation. Two approaches that use different failure criteria are executed. One method employs strength failure criteria and second uses fracture mechanics approach. Experimental data for [+Q/-Q/90/90/90/90]s laminates are used to verify correctness of the applied models. Influence of the thickness of 90-layer on damage evolution is also analyzed. The investigation of laminates containing off-axis layers is performed. The results show that degradation of the shear modulus occurs in those layers during the loading. Degradation of the shear modulus influences longitudinal modulus of the whole laminate as well as stress distribution in 90-layer.