The strength of sintered alumina, when tested in the normal atmosphere, is shown to depend on the time for which the stress is applied. When stressed in bending under constant load, the average time to fracture increases from 1 sec to 10 6 sec when the stress is reduced by about 22%. This delayed fracture can be largely eliminated by heat treating and testing in a vacuum. The vacuum heat treatment does not change the strength of the material permanently since the delayed fracture effect reappears when the material is again exposed to the atmosphere. It is concluded that the effect is due to attack of the stressed material by constituents of the atmosphere.
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S. Pearson (1956) studied this question.
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