CRACK INITIATION AND MECHANICAL FATIGUE OF SILICA GLASS

The characteristics of the static and dynamic fatigues of freshly abraded and water-soaked silica glass were investigated in water and in formamide. While freshly abraded samples showed static and dynamic fatigue both in water and in formamide, water-soaked samples showed an appreciable fatigue only in water and showed practically no fatigue in formamide. These behaviors can best be explained as follows: the fatigue of the samples with blunt cracks produced by soaking in hot water involves crack initiation which plays the dominant role, in addition to slow crack growth, while that of the freshly abraded samples with sharp cracks involves only the slow crack growth. While both water and formamide can cause slow crack growth of a sharp crack, only water is capable of causing crack initiation from a blunt crack under sub-critical stress. The static and dynamic fatigue equations were modified to account for the crack initiation in addition to the crack growth. The analysis based upon this model showed that for specimens with a blunt crack the crack initiation stress was almost identical to the failure stress and that most of the fatigue lifetime is the time required for crack initiation. © 1990.