Interest in indentation fracture goes back to the empirical finding (Auerbach, 1891) that the load required to form a crack is proportional to the radius of the spherical indenter for indentations of radius <i£ 1 cm. Although spherical and pyramidal indenters are both used, only the latter will be discussed as the former method is applicable only to transparent material (e.g. glasses). The most distinctive feature of the indentation of brittle materials by the Vickers or pyramidal indenter is the appearance of cracks emanating from the corners of the indent (Fig. 18) and it is from the measurement of the lengths of these cracks that it is possible to determine the critical stress intensity factor for indentation cracking or kc.
fig. 18 Schematic diagram showing cracking around a Vickers indent. C0, crack length; d, length of diagonal of indent; a, length of half-diagonal of indent.
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