Breviary Technical Ceramics



 Stress Intensity Factor

Ceramic materials are subject to brittle fracture. Ductile deformation, as observed in metals, is possible only at temperatures close to the softening temperature. The stress intensity factor KI has been adopted from fracture mechanics in order to determine the behaviour of brittle materials with respect to crack growth.

The critical stress intensity factor, KIC [MPa*m] is a measure of the susceptibility to cracking or the brittleness of the material (crack resistance). Crack growth behaviour is not dependent on loading alone, but on a combination of loading and crack size. Failure occurs when the critical value KIC is reached.

KIC is essentially the product of stress perpendicular to the plane of a crack caused by outside forces, , and the square root of the length of the most dangerous crack, a. Furthermore, there is also a dependency on the geometry of the part and that of the crack and therefore on a correction factor, Y.

Simply put, materials with a high KIC value (for example, SN or zirconium oxide PZT KIC = up to 10.5) have a high resistance to crack propagation.

Figure 86:
Scematic crack growth diagram for technical ceramics


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