PNNL

Abstract

A dynamic, crack-growth model has been developed to model slow crack growth in ceramic composites containing nonlinear, creeping fibers in an elastic matrix. The model uses mechanics for frictional bridging and nonlinear fiber-creep equations to compute crack extension dynamically by integrating the resulting time-dependent equations. Discrete two-dimensional fiber bridges are employed, which allows separate bridge "clocks", to compute slow crack-growth rates for composites containing Nicalon-CG and Hi-Nicalon fibers. The model predicts activation energies, time-temperature exponents, crack lengths, and crack-velocity data for composites in bending at 1173K to 1473K in inert environments that are in good agreement with experimental data. In addition, calculated creep strains in the model bridges agree with experimental damage-zone strains. The effect of multiple-matrix cracking is discussed and accounted for in displacement ca