On the quantification of bridging tractions during subcritical crack growth under monotonic and cyclic fatigue loading in a grain-bridging silicon carbide ceramic

The mechanisms of cyclic fatigue-crack propagation in a grain-bridging ceramic, namely an in situ toughened, monolithic silicon carbide, is examined. The primary goal is to directly quantify the bridging stresses as a function of cyclic loading. To investigate the effect of the number of loading cycles on the strength of the wake bridging zone, crack-opening profiles of cracks grown at high velocity near the K-c instability (to simulate behavior on the R-curve) and at low velocity near the fatigue threshold (to simulate the cyclically-loaded crack) were measured in situ in the scanning electron microscope at a fixed applied stress intensity. Differences between the measured profiles and those computed for elastic traction-free cracks permit the estimation of the traction distributions. These are then used to simulate resistance curve and fatigue-crack growth rate date. Predictions are found to be in close agreement with experimental measurements on disc-shaped compact-tension specimens. The results provide direct, quantitative evidence that bridging tractions are indeed progressively diminished due to cyclic loading during fatigue-crack propagation in a grain-bridging ceramic. (C) 1998 Acta Metallurgica Inc.