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 t he fracture and creep behavior of discontinuous ductile fiber-reinforc ed brittle matrix composite systems was investigated numerically. To s imulate the fracture behavior, the ductile fibers were modelled using a constitutive relationship that accounts for strength degradation res ulting from the nucleation and growth of voids. The matrix was assumed to be elastic and fails according to requirements of a stress criteri on. The debonding behavior at the fiber interfaces was simulated in te rms of a cohesive zone model which describes the decohesion by both no rmal and tangential separation. Results indicate that for rigid interf aces between the ductile reinforcing phase and the matrix the contribu tion of the ductile reinforcement to the work-of-fracture value (tough ness) of the composite increases with less exhaustion of its work-hard ening capacity before the onset of matrix failure. Therefore the failu re strength and elastic modulus values of the matrix become important material parameters. In the case of interfacial debonding the load tra nsfer to the discontinuous reinforcements after matrix failure should be somehow maintained for utilization to full capacity of the reinforc ement and interfacial behavior. In the creep regime, for rigidly bonde d interfaces the creep rate of the composite is not significantly infl uenced by the material properties and geometric parameters of the duct ile reinforcing phase owing to the development of triaxial stress stat e and constrained deformation in the reinforcement. For debonding inte rfaces the geometric parameters of the reinforcing phase become import ant; however, even with very weak interfacial behavior, low composite creep rates can be achieved by suitable selection of the geometric par ameters of the ductile reinforcing phase. Significant increases in roo m temperature fracture toughness can be achieved without extensively s acrificing the creep strength by ductile discontinuous reinforcements.