CYCLIC FATIGUE OF A MICA-CONTAINING GLASS-CERAMIC AT HERTZIAN CONTACTS

Fatigue damage in a mica-containing glass-ceramic is examined using Hertzian contact tests. For the material in its base glass state, such tests indicate that fatigue occurs solely by chemically enhanced cone crack extension. In the glass-ceramic, fatigue is evident as an expansion of a macroscopic subsurface microfracture zone. Comparative observations of the subsurface damage in static and cyclic loading, and tests in different environments, indicate that the fatigue in the glass-ceramic is mechanical in origin, although it is enhanced by moisture. This result is reinforced by load-point-displacement data, which reveal significant hysteresis in the glass-ceramic but not in the base glass. Flexure tests on Hertz-indented glass-ceramic specimens show only a slight loss of strength, <5%, over 10(5) cycles. This contrasts with the base glass which, although of higher laboratory strength, is subject to abrupt and severe strength degradation from cone crack pop-in. High magnification examination of the subsurface damage in the glass-ceramic suggests the underlying cause of the mechanical fatigue mechanism to be attrition of frictional tractions at closed microcrack interfaces.