Abstract
SiC-based fibers are sensitive to delayed failure under constant load at high temperatures in air. Static fatigue at intermediate temperatures < 800 °C was attributed to slow crack growth from flaws located at the surface of fibers, driven by the oxidation of free carbon at grain boundaries. The present paper examines the static fatigue behavior of SiC-based Hi-Nicalon fibers at high temperatures up to 1200 °C and Hi Nicalon S fibers at intermediate temperatures (500–800 °C). The degradation of stress- rupture time relation of multifilament tows with increasing temperature was investigated. Predictions of tow lifetime based on critical filament-based model of tow failure were compared to experimental stress-rupture time diagrams. Critical filaments are characterized by strength–probability relation. The critical filament-based model was found to describe satisfactorily the static fatigue behavior of fiber tows at these temperatures. The influence of various factors on lifetime as well as the origins of variability is analyzed.
Highlights
Continuous fiber reinforced ceramic matrix composites (CMCs) exhibit a combination of superior properties over monolithic ceramics which makes them attractive for high temperature applications.This results from the presence of fiber reinforcement that is responsible for damage tolerance, high resistance to fatigue, to creep, and to fracture, and reliable behavior
The experimental stress-rupture time diagrams were compared to the static fatigue behavior of particular single filaments identified by their reference strength measured in the absence of slow crack growth and environmental effects
The paper has shown that the static fatigue behavior of SiC Hi Nicalon tows at high temperatures in air is dictated by the delayed failure of a critical filament
Summary
Continuous fiber reinforced ceramic matrix composites (CMCs) exhibit a combination of superior properties over monolithic ceramics which makes them attractive for high temperature applications. Filament tows that form composite preform (one-dimensional (1D), two dimensional (2D), three-dimensional (3D)) are fundamental constituents that warrant attention with respect to the above considerations Their mechanical behavior under various conditions of loading, environment and temperature must be well documented. Delayed failure was attributed to the formation of a silica layer on fiber surface; the thickness of this layer was introduced in place of the critical flaw size in the linear fracture mechanics equation of strength. An outcome of this model is that fiber bundle strength decreases with time as t−1/4 , which corresponds to a fiber type independent stress exponent (denoted n in the paper) n = 4. The influence of various factors on lifetime as well as the origins of lifetime variability is analyzed
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