Breviary Technical Ceramics



 Crack Growth

A ceramic component must be designed not only with respect to its maximum load, but also for a certain lifetime. For this reason, knowledge of the relationship between strength and time is necessary. Failure results as a rule from single defects which (often) lie on or close to the surface, since the greatest stresses usually occur there. Assuming that the component is subjected to loads below its design strength, and particularly if water or moist air are present in its environment, a crack can start from a critical defect, growing very slowly at first, then accelerating under further load until the part finally fails. This behaviour is known as subcritical crack propagation (stress crack corrosion). It most often occurs in oxide ceramics, but also in non-oxide ceramics that possess an amorphous oxide grain boundary phase. This type of crack propagation behaviour is well described using a method known as "linear-elastic fracture mechanics".

Figure 88:
SPT-diagram for zirconium oxide

Taking subcritical crack propagation into account, the relationship between strength, probability and time can be illustrated for a ceramic material on what is known as an SPT-diagram.

Test procedures for determining the parameters of subcritical crack propagation are specified in DIN EN 843-3.


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