Fiber/matrix interface crack propagation in polymeric unidirectional composite

Abstract: Fiber/matrix interface cracking plays an important role in determining the final failure of unidirectional composites. In the present study, energy release rate (ERR) for fiber/matrix interface debond growth originated from fiber break in unidirectional composite is calculated using 5-cylinders axisymmetric and 3-D FEM models with hexagonal fiber arrangement. In the model the debonded fiber is central in the hexagonal unit which is surrounded by effective composite. The effect of neighboring fibers focusing on local fiber clustering on the ERR is analyzed by varying the distance between fibers in the unit. Two different scenarios are considered, one is the steady-state debond where debond are long and thus there is no interaction between debond tip and fiber break; the other case is when debond are relatively short when debond tip interacts with fiber break. The steady-state ERR is calculated from potential energy difference between a unit in the bonded region far away from the debond front and a unit in the debonded region far behind the debond front. The ERR for different modes of crack propagation is obtained from a FEM model containing a long debond by analyzing the stress at the debond front. For very short debonds, the ERR was calculated by both the J integral and the Virtual crack closure technique (VCCT).For steady-state debond growth, results show that in mechanical axial tensile loading fracture Mode II is dominating, it has strong angular dependence (effect of closest fibers) but the average ERR is not sensitive to the local fiber clustering. In thermal loading the Mode III is dominating and the average ERR is highly dependent on the distance to neighboring fibers. For short debod growth, results show that the debond growth is Mode II dominated and that the ERR strongly depends on the angular coordinate. The local fiber clustering has larger effect on the angular variation for shorter debonds and the effect increases with larger local fiber volume fraction. Finally, the ERR values from 5-cylinder axisymmetric model could be considered as upper bound for the 3-D hexagonal model.