6. Hardness testing on brittle materials

Under the load of an indenter, a brittle material can only deform plastically to a very limited extent; it reacts by cracking. Cracks appear at the edges of the indenter. In the case of a ball, the crack takes the shape of a cone - the Hertzian cone crack. In the case of a pyramidal indenter, cracks propagate from the four corners of the indentation. The indenter acts on these cracks by causing them to open under conditions of imposed displacement. As a result, they are stable and propagate only as the penetration depth increases (although this is only true in the absence of environmental effects).

Propagation occurs if the stress intensity factor exceeds the material's toughness K _Ic , but stops when the cracks have reached a certain characteristic length. This is because the stress intensity factor decreases as crack...