Median vent crack

Also referred to as median vent cracks, these are caused to pop-in by exceeding a critical indenter load. It is the pop-in phenomenon that is important to the development of this subject in ceramic science because the halfpenny crack has the surface trace which allows opaque materials to be analyzed by recording the radial crack size as a function of increasing load. There is, however, the implication that the surface must be prepared carefully by polishing to an optical finish in order to see the radial cracks. If necessary, samples must be annealed to remove polishing stresses. Radial cracks are the result of surface tensile stresses, (Xyy in equation (1.29). Such stresses are at a maximum at the elastic-plastic boundary. 

Considering the mechanisms listed above it is perhaps easiest to visualize (1) if we are dealing with an ideally brittle solid, for then the energy consumed is equal to total new surface area produced multiplied by the specific surface energy. Three components are present in this assessment. First, as Figure 2.18 shows, the pivot makes an indent similar to a Vickers diamond. Therefore there is the same projected area. Second, depending upon the load, there will be production of the median vent crack into the solid whereas the lateral vent cracks are not produced until the load is removed. Therefore, rocking the pendulum produces only one set of cracks which produce new surface. And, finally, as the pivot rocks and to some extent processes, crushed debris is produced. 

The erosion of brittle materials has been described in terms of interacting Hertzian crack systems, or by the formation of lateral and median vents for sharp indentors. This is indicated in Figure 9.4. It has also been observed that brittle materials are capable of undergoing plastic deformation particularly when under erosion by very small particles.This is important because, at high temperature, some oxides formed as scales may show a ductile reaction to erosive impact. 

Lateral vent cracking, estimated in Ref. 12, seems unusual since it occurs only on removal of the pendulum and therefore does not contribute to the absorption of energy. However, the median and lateral vents are related as shown in Figure 5.5 thus the estimate obtained by Fox and Freeman and shown in Table 2.1 can be used. 

Fig. 5.1. Cracks developed around a Vickers indent on a scandium silicide ceramic showing median and lateral vent cracks. The lateral crack shows a marked anisotropy arising from the structure.

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