INDENTATION FATIGUE - A SIMPLE CYCLIC HERTZIAN TEST FOR MEASURING DAMAGE ACCUMULATION IN POLYCRYSTALLINE CERAMICS

A simple Hertzian contact procedure for investigating cyclic fatigue d amage in brittle polycrystalline ceramics is described. Repeat loading of a spherical indenter on a coarse alumina ceramic produces cumulati ve mechanical damage. The mode of damage is one of deformation-induced intergranular microfracture, leading ultimately at large numbers of c ycles and high contact pressures to severe grain dislodgement. In cont rast to the classical Hertzian cone cracks that form in more homogeneo us materials in the regions of tensile stress outside the contact circ le, the damage in the coarse-grain alumina develops in a zone of high shear stress and hydrostatic compression beneath the contact circle. T he fatigue damage is evident in inert environments, confirming the mec hanical nature of the process, although exposure to moisture accelerat es the effect. The relatively modest degradation in failure stress wit h number of repeat contacts for indented flexure specimens suggests th at conventional strength and toughness testing procedures may not alwa ys provide sensitive indications of the extent of damage that can be i ncurred in concentrated loading.