Creep strength of fiber and particulate composite materials: The effect of interface slip and diffusion

It is well known that the creep resistance of metal and intermetallic matrix composites containing fibers or participates arises from the constraint of the matrix by the reinforcement. Experimental results indicate that, in general, at modest temperatures the creep strength of these composites is better than that of the matrix material alone. However, at temperatures higher than approximately half of the melting temperature of the matrix the composite strength is limited and in some cases the strengthening imparted by the reinforcements is completely lost. The quantitative effect-of diffusional relaxation and interface slip along the matrix-reinforcement interface, on the matrix constraint and thereby on the creep resistance, is studied. The composite behavior is modeled by coupling the slip and diffusional mass transport along the interface with power law deformation of the matrix with the reinforcement considered as rigid. The transverse creep resistance of a continuous fiber composite material and the axial creep strength of a discontinuous fiber composite are investigated in plane strain and axial symmetry respectively. The relevant unit cell boundary value problems are solved by the finite element method. Results indicate that slip at the interface alone can reduce the transverse creep strength of a continuous fiber composite to levels below that of the pure matrix material. On the contrary, analysis of the axial creep resistance of discontinuous fiber composites indicates that diffusion is necessary in addition to the free slip for the same result.