Micro-crack initiation and propagation in fiber reinforced composites

Abstract: Predicting micro-damage initiation and evolution is one of the key challenges for safe design of fiber reinforced polymer composites. Micro-scale damage such as, for example, single fiber break may be unnoticeable and negligible during the initial service life of composite, however, with many loading and unloading cycles this initially micro-scale damage may propagate forming macroscopic scale cracks that can significantly reduce the service lifetime or even lead to unforeseen catastrophic failure of the composite structure. The objective of this Doctoral thesis is to develop methodology for prediction of micro-crack propagation in fiber reinforced polymer composites. Fracture mechanics concepts of strain energy release rate are applied for crack growth analysis. Analytical modeling combined with numerical FEM calculations are used to obtain the values of energy release rate. Parametric analysis is performed to evaluate the significance of the applied load and various material properties on the micro-crack growth rate. Calculation results are implemented into Paris law relation, to predict the crack growth in fatigue loading. In Paper I fiber/matrix interface debond growth starting from single fiber breaks in unidirectional (UD) polymer composites is studied. Analytical solution for Mode II energy release rate GII is found and parametric analysis is performed in the self-similar debond crack propagation region. When the fiber/matrix interface debond crack is short, the self-similarity condition is not valid. Due to interaction with fiber break, GII is magnified. In Paper II, numerical FEM modeling is performed to calculate GII for short debond cracks. The findings from GII analysis for self-similar and short debond cracks are summarized in simple expressions. Simulations of fiber/matrix interface debond crack growth in tension-tension fatigue using Paris law are performed. In Paper III, debond growth in single fiber (SF) composites subjected to tension-tension fatigue is analyzed. Using the same procedure as for UD composites, first, an analytical solution for Mode II energy release rate GII is found for self-similar crack growth region and then FEM modeling is performed to obtain magnification profiles for short debond cracks. In Paper IV modeling methodology described in Paper III is advanced further and the modeling results are compared with experimental data for interface debond crack growth in SF composites subjected to tension-tension fatigue loading. The Paris law constants are extracted from the best fit between the experimental and modeling results. Validation of results prove that Paris law can be implemented to characterize micro-crack growth in fatigue of polymer composites. In Paper V fiber/matrix interface debond growth on the surface of a UD composite subjected to tension-tension fatigue is analyzed. 3-D FEM modeling is performed to account for the non-axisymmetric stress state due to the edge effect. Modeling results for debond growth in fatigue are compared with experimental data available in the literature. Finally, in Paper VI fracture mechanics concepts of energy release rate are used to model micro crack initiation and propagation in a carbon fiber, which, apart from the load bearing function, also serves as an electrode in a novel lithium-ion rechargeable battery. When subjected to lithium ion intercalation, carbon fiber experiences a non-uniform swelling that leads to development of high mechanical stresses. In many cycles of charging-discharging these stresses can introduce damage and reduce the mechanical and electrochemical properties of the battery. FEM modeling using thermal analogy is performed to solve the transient ion diffusion and mechanical stress problem. Different crack initiation and propagation scenarios are compared.