Fracture mechanics analysis of damage initiation and evolution in fiber reinforced composites

Sammanfattning: When a unidirectional (UD) fiber reinforced polymer composite is loaded in fiber direction in quasi-static or in a high stress cyclic tension-tension regime, many fiber breaks may occur in random positions already during the load increase in the first cycle. This is because fiber strain to failure in UD composites is lower than the polymer matrix strain to failure.In cyclic loading with constant amplitude we usually assume that fibers do not experience fatigue. Therefore the next step in damage evolution with increasing number of cycles may be development of interface cracks (debonds) growing along the fiber/matrix interface.Fracture mechanics concepts are applied and Mode II strain energy release rate GII related to debond crack growth along the fiber/matrix interface is used for damage evolution analysis.In Paper I analytical solution for Mode II energy release rate GII is found and parametric analysis performed in the self-similar debond crack propagation region. For short fiber/matrix debond cracks the self-similarity condition is not valid - due to interaction with fiber crack, GII is magnified. Thus in Paper II, numerical FEM simulations in combination with virtual crack closure technique are used in order to calculate GII for short debond cracks. The findings from GII analysis for self-similar and short debond cracks are summarized in simple expressions and then used in simulations of fiber/matrix interface debond crack growth in tension-tension fatigue using Paris law.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 obtained for self-similar crack growth region. Then FEM calculations are performed in order to obtain GII magnification profiles for short debond cracks. For SF composites it was additionally found out that equal GII magnification profiles are obtained no matter if purely mechanical, purely thermal or combined mechanical and thermal load is applied to the composite. Thus for SF composites even simpler expressions can be used for simulations of debond growth using Paris law relation.