Glass surface crack propagation

The temperature distribution during crack propagation is shown in Fig. 15. As the crack advances, the heat continues to diffuse along the normal to the craze surfaces but the size of the hot zone remains comparable to that of the craze thickness. The maximum temperature increase is located at the crack/craze interface, where the craze thickening and related heat flux into the bulk are maxima. At this location, the temperature reaches the glass transition temperature Tg but plasticity is not enhanced in the bulk, which remains primarily elastic during crack propagation. 

The temperature distribution at craze breakdown and during crack propagation is shown in Fig. 17. As the crack advances, the temperature reaches the glass transition temperature at the location of the crack-craze transition, where plastic dissipation caused by craze thickening is maximum. However, this remains confined to a small volume around the crack-craze surfaces (see Fig. 17) so that no plasticity on a larger scale is promoted. 

There are two possible approaches to the selection of materials from the standpoint of thermal shock resistance. The first is suitable for glass and fine dense ceramics and was discussed in Section II. 5. 2. With these materials, it is necessary to avoid formation of primary cracks which originate at the surface and propagate rapidly into the interior where they are the cause of extensive fracture. In this case, the favourable properties include high strength and high thermal conductivity, and low elasticity modulus and expansion coefficient values. 

The introduction of surface compressive layers can strengthen ceramics and is a well-established technique for glasses (see Sec. 13.5 for more details). The underlying principle is to introduce a state of compressive surface residual stress, the presence of which would inhibit failure from surface flaws since these compressive stresses would have to be overcome before a surface crack could propagate. These compressive stresses have also been shown to enhance thermal shock resistance and contact damage resistance. 

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