Abstract
Glass ceramic specimens having a controlled surface flaw introduced by Knoop indentation were tested under dynamic, static and cyclic loads by four-point bending of plates (uniaxial tension) and by diametral-compression of disks (tension-compression). All specimens showed a susceptibility to dynamic, static and cyclic fatigue failure. Equivalent time-to-failure obtained from the dynamic and cyclic fatigue data, using the assumption that there is no enhanced effect of cycle on the rate of subcritical crack growth, was plotted in the diagram between applied stress and time-to-failure. The crack propagation parameter, n, was estimated from the relation of time-to-failure and the maximum applied stress in the static and cyclic fatigue tests and the stressing rate dependence of the fracture strength in the dynamic fatigue tests. Consequently, it was found that the n value for diametral-compression loading became somewhat higher than that for four-point bending. The higher n value in diametral-compression could be mainly explained by the fact that a crack in a tension-compression stress state has a less freedom of motion than that in a uniaxial tension stress state, and such a strong orientation dependence for crack growth in a tension-compression stress state restrains the crack to grow around obstacles or grains.
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