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 stress es as a function of cyclic loading. To investigate the effect of the n umber of loading cycles on the strength of the wake bridging zone, cra ck-opening profiles of cracks grown at high velocity near the K-c inst ability (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 applie d stress intensity. Differences between the measured profiles and thos e computed for elastic traction-free cracks permit the estimation of t he 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 compa ct-tension specimens. The results provide direct, quantitative evidenc e that bridging tractions are indeed progressively diminished due to c yclic loading during fatigue-crack propagation in a grain-bridging cer amic. (C) 1998 Acta Metallurgica Inc.