SUBCRITICAL CRACK-GROWTH BEHAVIOR OF BOROSILICATE GLASS UNDER CYCLIC LOADS - EVIDENCE OF A MECHANICAL FATIGUE EFFECT

Amorphous glasses are generally considered immune to mechanical fatigu e effects associated with cyclic loading. In this study surprising new evidence is presented for a mechanical fatigue effect in borosilicate glass, in both moist air and dry nitrogen environments. The fatigue e ffect occurs at near threshold subcritical crack-growth rates (da/dt < 3 x 10(-8) m/s) as the crack extension per cycle approaches the dimen sions of the borosilicate glass network. While subcritical crack growt h under cyclic loads at higher load levels is entirely consistent with environmentally assisted crack. growth, lower growth rates actually e xceed those measured under monotonic loads. This suggests a mechanical fatigue effect which accelerates subcritical crack-growth rates. Like ly mechanisms for the mechanical fatigue effect are presented.