Effect of atmospheric humidity on the fatigue crack propagation behavior of short cracks in silicon nitride

The effect of the environment on crack growth processes in silicon nitride was studied by investigating the static and fatigue crack-growth behavior o f small surface cracks, as influenced by testing (i) in the ambient environ ment, (ii) in distilled water, (iii) under vacuum, and (iv) in toluene, A p rincipal finding was that testing under cyclic conditions led to crack-grow th rates that were much higher in air than in toluene, whereas testing unde r static conditions in air or toluene led to minor differences in the rate of static fatigue crack growth. This difference in sensitivity to the envir onment under static and cyclic loading conditions was attributed, in part, to a much-greater extent of microcracking at the surface ahead of the main crack in air under cyclic conditions, in comparison to that in other enviro nments. This propensity for microcracking at the surface in air under cycli c conditions also was reflected in the aspect ratios of the crack shapes th at developed.