| The techniques for evaluating the static fracture behavior of monolithic materials are well established. Applying these same techniques to evaluate the fracture behavior of multilayered and/or gradient composites requires consideration of the changing microstructural influence on fracture. In the current study, single-edge-V-notched-beam (SEVNB) testing was used to evaluate the R-curve behavior of multilayered gradient alumina-zirconia composites. Crack initiation and extension from the V-notch tip were observed via in situ optical microscopy. The V-notch tip was positioned near specific microstructural features within the composite and during loading short cracks (5 to 75 mum) were initiated from the V-notch tip and extended stably in ∼10 mum increments. The influence of gradient microstructures, layer-layer interfaces, platelike alumina additions, residual stresses, and the direction of crack propagation on the resulting R-curves were investigated.;The fracture mechanics weight function was used to estimate the fracture behavior based on the stress distribution (applied bending stress and residual stresses). The results were compared to the measured R-curves and the weight function analysis was observed to underestimate the measured KR. These differences were likely due to bridging stresses within the samples that were not accounted for in the weight function analysis. Initial attempts to account for bridging stress within the weight function analysis were in good agreement with the measured R-curves for the monolithic sample and for the samples having a step-wise change in residual stress. However, the same bridging function was not applicable for the samples having a composition gradient within each layer. |
