Ring fractures generated by a steel ball indenting a flat glass plate occur outside the contact circumference, not at the circumference as the theory predicts. The proposal used as a basis for this work is that the position of ring fracture is not determined by the locus of maximum stress but by the presence of a flaw which is severe enough to initiate fracture under the local condition of stress. The ring fracture radii thus locate flaws of various sizes at various distances from the centre of contact. A new statistical method is introduced for calculating the distribution of such flaws with respect to both size and surface density of flaws. Application of this analysis to data taken from indentations of HF-etched surfaces shows that flaw density ranges from 15 to 20/mm2/5 a depth interval (150 to 300 flaws/mm 2).
