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The relationship between indentation pressure produced by cones, pyramids and spheres, and the mechanical properties of elastic-plastic materials is considered, based on previous work which uses the expansion of a cavity in an elastic-plastic material. According to the earlier model, there are three zones: a hydrostatic 'core' of which the indenter is embedded; a hemispherical shell, where plastic flow is taking place; and beyond this the elastic hinterland. An improved correlation with experimental results is obtained if it is assumed that the hemispherical core is a region in which the stresses are changing from purely hydrostatic to values which satisfy the Von Mises yield criterion. A further improvement results if a correction is made for the lack of geometric similarity of the strain field in indentations by spheres.

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