
The indentation behaviour of hard materials which deform plastically and also exhibit surface fracture is discussed. It is shown that both radial and circumferential fracture can develop around the indentation and that this can be explained in terms of a transition from elastic/plastic behaviour. In general the circumferential cracks form at lower loads but remain short and shallow, and their initiation is affected by the grain size of the material. Experiments with a hard spherical indenter showed that radial cracks in water-quenched EN44B steel develop at loads above approximately 40 F*, where F* is the applied load to cause the first plastic deformation. Residual stresses in the surface layers can cause these cracks to continue growing after load removal. Finally, an equation relating contact area and pressure is developed from earlier work of K L Johnson (1970) and it is shown that this gives good agreement with experiment.
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