Crack-Wake Plasticity and Time-Dependent Bridging During Subcritical Crack Growth in CVI-SiC Reinforced with Nicalon Fibers

Abstract

Time-dependent crack growth, referred to as subcriticai crack growth or slow crack growth (SCG), likely controls the long-term life of continuous-fiber-reinforced ceramic matrix composite (CFC) structural materials at elevated temperatures. The use of CFCs in systems where long-term stability is required demands resistance to time-dependent crack growth, as well as high-temperature corrosion. Subcriticai crack growth, or static fatigue, in glass1, 2, glass-ceramic3, and monolithic ceramics, such as alumina and silicon carbide4–11, have been extensively studied. Data for crack propagation shown on a log-log plot with crack velocity V as a function of applied stress intensity K typically show three distinct regions, referred to as regions I, II, and III. Region I exhibits a strong dependence of crack velocity on K such that the relationship V = AK n is obeyed once a threshold value of K, denoted as K th , is exceeded. Values of n can range from 4 to 30 or higher and n is a function of temperature, microstructure, and chemical environment.7–11 Higher values of n are associated with increased resistance to crack growth and n decreases with increasing temperature, decreasing grain size, and increasing corrosion rates. Region I behavior can be modeled by reaction rate theory and appears to be reaction rate limited. Region II, in contrast, exhibits a weak dependence on K and is observed in systems where diffusion of a critical chemical species to the crack-tip is rate controlling. Region II is not always observed in ceramics, especially at elevated temperatures.5–10 In region III the crack velocity again depends strongly on K and is, typically, independent of environment as K approaches K C and rapid crack growth ensues. For the majority of ceramics at elevated temperature the entire range of crack propagation can be represented by a single curve obeying V = AK n.