ABSTRACT
The elastic properties of ceramics and relationships between flaw size and strength as determined by flexure and tensile testing were discussed in Chapter 8. In flexure and tensile strength tests, the load is applied rapidly and fracture occurs from single flaws or groups of flaws initially in the material. However, in real life engineering applications stresses are applied for long duration and often at high temperature. Material degradation mechanisms can occur under these conditions that result in failure of the material at a stress lower than was measured during “fast fracture” strength testing. These “time-dependent” degradation mechanisms plus other environmentally induced sources of failure are discussed in this chapter:
• Creep — plastic deformation at elevated temperature • Slow crack growth (static fatigue, stress rupture) — flaws initially in the material grow
with time and result in fracture at a lower stress than the fast fracture stress • Chemical attack — chemical reactions with gases, liquids, or solids that come into con-
tact with the material and either cause recession of the material or a change in the flaw size
• Mechanically induced effects — recession due to erosion and wear and new surface flaws due to stress concentrations at localized points of contact
• Thermal shock — new material flaws caused by thermally induced stress
The term creep is normally used to refer to deformation at a constant stress as a function of time and temperature. Creep is plastic deformation rather than elastic deformation and thus is not recovered after the stress is released. A typical creep curve has four distinct regions, as shown in Figure 9.1. The secondary creep region is the most useful for predicting the life of a ceramic component. It is typified by a constant rate of deformation and is often referred to as steady-state creep.
