Surface tensile stresses around indentations in NaCl

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Abstract
The circumferential tensile stress distribution around plastic-elastic indentations (100) surfaces is a significant function of indenter shape: radial cracks form around sharp conical indenters with included angles of 40 degrees but not around Vickers pyramid indenters. These cracks form well away from (100), and are not exactly crystallographically aligned, but propagated along regions of intense surface bending about (110) around the conical indentations. This (110) bending has a much smaller curvature at diamond pyramid indentations. The non-cubic cracks are of the 'angel-wing' type characteristic of semi-brittle solids (rather than the median-event and lateral cracking found in strong brittle materials) and initiate in the surface layers. It is suggested that this morphology is produced by the high local tensile stresses due to the (110) bending. Small cubic crystals, about 5 mu m edge length, accidentally compressed during indentation, were found to be completely ductile, a behaviour explained in the Appendix in terms of the scaling law of fracture mechanics.

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