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 sapphire spheres has been investigated, under conditions of slow uniaxial compression and free impact. A wide range of metallic and ceramic materials was used for the compression platens and targets. The spheres fractured into large fragments, and the conditions of load or impact velocity under which fracture occurred depended strongly on the properties of the platen or target. With hard platens or targets, which deformed 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 which plastic indentations were formed, glass spheres failed from surface flaws at a critical value of the maximum surface tensile stress. The behaviour of sapphire spheres compressed between plastically deforming platens was more complex, and was not consistent with failure at a critical value of any single stress component.