Environmentally Affected Small Cyclic Fatigue Crack Growth Behavior of Silicon Nitride in Atmospheric Air and Toluene

To investigate the effect of atmospheric humidity on cyclic fatigue crack growth behavior, fourpoint bending fatigue tests were performed on silicon nitride in laboratory air and toluene. Crack aspact ratios for surface cracks in laboratory air were clarified to be lower than in toluene, The relationship between crack growth rate and maximum stress intensity factor for small surface cracks was obtained using the above crack aspect ratios. This result indicated that crack growth rate was accelerated by atmospheric humidity in the low stress intensity factor range, but not in the high range. This acceleration was clarified to be causes by the mutual action of cyclic stressing and atmospheric humidity. Microcrack initiation ahead of the main crack, the cutting process of ligament between the micro and the main crack, and cyclic loosening of crack bridging at wake of the crack tip were ciarified to be dominant factors which influence the crack growth behavior in both environments.