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

A simple Hertzian contact procedure for investigating cyclic fatigue damage in brittle polycrystalline ceramics is described. Repeat loading of a spherical indenter on a coarse alumina ceramic produces cumulative mechanical damage. The mode of damage is one of deformation-induced intergranular microfracture, leading ultimately at large numbers of cycles and high contact pressures to severe grain dislodgement. In contrast to the classical Hertzian cone cracks that form in more homogeneous materials in the regions of tensile stress outside the contact circle, the damage in the coarse-grain alumina develops in a zone of high shear stress and hydrostatic compression beneath the contact circle. The fatigue damage is evident in inert environments, confirming the mechanical nature of the process, although exposure to moisture accelerates the effect. The relatively modest degradation in failure stress with number of repeat contacts for indented flexure specimens suggests that conventional strength and toughness testing procedures may not always provide sensitive indications of the extent of damage that can be incurred in concentrated loading.