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Chapter 4:
Stress Corrosion
So far, we have learned that flaws reduce the strength of glass and ceramics to below theoretical values and that residual stress present in those flaws reduces strength even further. Now we will see that time and moisture are the enemies of these brittle materials, further reducing strength. Before getting into that analysis, a few definitions are in order. Stress corrosion is a phenomenon in which a chemical reaction causes premature failure with time under constant load in the presence of a chemically active environment. Stress corrosion cracking is subcritical crack growth and, in the presence of a chemically active environment, is also known as slow crack growth, since growth is not sudden but time dependent. It is also known as static fatigue, since stress is constant with time, to distinguish it from cyclic fatigue, which varies with time. Cyclic fatigue is a phenomenon in which premature failure occurs with time in the presence of cyclic loading, both for full reversal of stress (tension to compression), and any oscillatory stress change. Corrosion fatigue is a combination of stress corrosion and cyclic fatigue. The prevailing phenomenon for crystals, ceramics, glass, and brittle materials is stress corrosion. Materials that are brittle at ambient temperature do not exhibit the same dislocation phenomena common in cyclic fatigue failures. Conversely, the prevailing phenomenon for metals (e.g., steel and aluminum), which are ductile at ambient temperature, is cyclic fatigue. To be sure, metals are subjected to corrosion (salts, etc.), and glasses and ceramics can be subjected to cyclic fatigue,1 but in the latter, stress corrosion is dominant. Thus, we concentrate on this type of failure only.
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