FRACTURE OF BRITTLE SPHERES UNDER COMPRESSION AND IMPACT LOADING .2. RESULTS FOR LEAD-GLASS AND SAPPHIRE SPHERES

The fracture of 0.7-0.8 mm diameter lead glass and single-crystal sapp hire spheres has been investigated, under conditions of slow uniaxial compression and free impact. A wide range of metallic and ceramic mate rials was used for the compression platens and targets. The spheres fr actured into large fragments, and the conditions of load or impact vel ocity under which fracture occurred depended strongly on the propertie s of the platen or target. With hard platens or targets, which deforme d only elastically, failure initiated at an effectively constant value of internal shear stress, although complete disruption followed under the action of tensile stresses. With softer platens or targets, in wh ich plastic indentations were formed, glass spheres failed from surfac e flaws at a critical value of the maximum surface tensile stress. The behaviour of sapphire spheres compressed between plastically deformin g platens was more complex, and was not consistent with failure at a c ritical value of any single stress component.