TIME-DEPENDENT FAILURE IN FIBER-REINFORCED COMPOSITES BY MATRIX AND INTERFACE SHEAR CREEP

The inelastic response of fiber-reinforced ceramic and metal matrix co mposites under fixed load at elevated temperature is due to the comple mentary effects of creep and damage in the constituents. After matrix cracking or tensile creep relaxation in a short time, subsequent defor mation and failure are driven by shear stress relaxation in the matrix and at the fiber-matrix interface around broken fibers. The shear cre ep causes stress redistribution to unfailed fibers, causing Further fi ber breakage and shear relaxation, culminating in abrupt failure of th e composite. This sequence of events is modeled both analytically and numerically within the Global Load Sharing (GLS) approximation previou sly utilized for quasi-static loading. Analytically, a unit cell model is used to obtain simple closed-form relationships For the time-depen dent relaxation of the shear at the interface. This relaxing shear str ess is then incorporated into a simulation model which follows the evo lution of slip and fiber damage up to failure. The slip lengths and fa ilure times are predicted vs matrix creep exponent n, fiber Weibull mo dulus in, applied load and, interestingly, physical specimen length. A n analytic model for failure shows good agreement with the simulation results and so can be used for qualitative estimates of lifetime. Appl ication to Ti-MMCs is discussed. (C) 1997 Acta Metallurgica Inc.