Abstract

Crack bridging is an important source of crack propagation resistance in many materials and the bridging stress distribution as a function of crack opening displacement is widely believed to represent a true material property uninfluenced by sample geometry, loading conditions, and other extrinsic factors. Accordingly, accurate measurement of the bridging stress distribution is needed and many non-destructive methods have been developed. However, there are many challenges to accurately determining bridging stresses. A comparison of bridging stresses measured using R-curve, crack opening displacement (COD), and spectroscopy methods has been made using two bridging ceramics, Y2O3 and MgO doped Si3N4 and 99.5% pure Al2O3. The COD method is surface sensitive and gives a lower peak bridging stress compared to the R-curve technique which samples through the entire material thickness. This is attributed to a more compliant near surface bridging zone. Conversely, when R-curves rise steeply over the first few micrometers of growth from a notch, an effect of negative T-stress is expected to raise the R-curve determined peak bridging stress. Spectroscopy methods were only found to yield reliable bridging stress results if a reasonable through thickness volume of material is sampled. It was found that 2.5% of the specimen thickness achieved using fluorescence spectroscopy appears adequate for Al2O3 while 0.1-0.2% of the sample thickness achieved using Raman spectroscopy for Si3N4 appears inadequate. Overall, it is concluded that in the absence of T-stresses a bridging distribution can be determined that is a true material property. Also, a new method is proposed for determining the bridging stresses of fatigue cracks from (1) the bridging stress distribution for monotonically loaded cracks and (2) experimental fatigue data. (C) 2012 Elsevier Ltd. All rights reserved.