Abstract

Mechanical strength of glass decreases with time when stressed in an atmosphere containing water. Also, the greater the tensile stress on the glass, the shorter the time to failure if the stress is held constant. This can limit the service life of various glass products such as optical wave guides made of silica glass fibers. Often this mechanical fatigue is explained by slow crack growth [1]. Namely, it is postulated that a small crack exists on the glass surface and that this crack propagates slowly under an applied stress leading eventually to a failure. The failure time, then, is the time required for a crack to grow to a critical length. The role of the water in the environment is believed to be the acceleration of the crack growth rate. According to this explanation, one can estimate the service life-time of a glass under stress using data on slow crack growth rate. Thus there have been numerous measurements of the crack growth rate of glass under varied stress and environment. In this explanation of mechanical fatigue based upon slow crack growth, it is tacitly assumed that the crack tip geometry of the specimen which shows mechanical fatigue is the same as that of the propagating crack observed in the crack growth measurement. If this assumption is not valid and the crack tip geometry of the sample is different from that of the propagating crack, then the explanation of the mechanical fatigue based upon the slow crack growth would not be valid. In this paper, therefore, first, the crack tip geometry of silica glass (and high silica glass) after various thermal and chemical treatment will be discussed. Subsequently, the fatigue characteristics of these treated glass will be described and will be related to the crack tip geometry.

© 1987 Optical Society of America