A NUMERICAL-ANALYSIS OF FRACTURE AND HIGH-TEMPERATURE CREEP CHARACTERISTICS OF BRITTLE-MATRIX COMPOSITES WITH DISCONTINUOUS DUCTILE REINFORCEMENTS

The role of the material parameters and interface characteristics in the fracture and creep behavior of discontinuous ductile fiber-reinforced brittle matrix composite systems was investigated numerically. To simulate the fracture behavior, the ductile fibers were modelled using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. The matrix was assumed to be elastic and fails according to requirements of a stress criterion. The debonding behavior at the fiber interfaces was simulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. Results indicate that for rigid interfaces between the ductile reinforcing phase and the matrix the contribution of the ductile reinforcement to the work-of-fracture value (toughness) of the composite increases with less exhaustion of its work-hardening capacity before the onset of matrix failure. Therefore the failure strength and elastic modulus values of the matrix become important material parameters. In the case of interfacial debonding the load transfer to the discontinuous reinforcements after matrix failure should be somehow maintained for utilization to full capacity of the reinforcement and interfacial behavior. In the creep regime, for rigidly bonded interfaces the creep rate of the composite is not significantly influenced by the material properties and geometric parameters of the ductile reinforcing phase owing to the development of triaxial stress state and constrained deformation in the reinforcement. For debonding interfaces the geometric parameters of the reinforcing phase become important; however, even with very weak interfacial behavior, low composite creep rates can be achieved by suitable selection of the geometric parameters of the ductile reinforcing phase. Significant increases in room temperature fracture toughness can be achieved without extensively sacrificing the creep strength by ductile discontinuous reinforcements.