The dependence of K,, on the notch-root radius has been examined for a notch radius as small as a few micrometers in a dense, fine-grained, polycrystalline alumina ceramic. The notch radius can be systematically varied by using a semimanual procedure in a special jig which polishes out rather than cuts the specimen. K,, is independent of the notch sharpness for notch-root radii 4 0 pm. The results are critically compared with those obtained by other standard techniques and discussed in terms of residual compressive stresses introduced during the notching procedure.

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Bridging stresses that result both from elastic tractions and frictional interlocking in the wake of an advancing crack have been evaluated quantitatively via in situ Raman microprobe spectroscopy in a toughened Si 3 N 4 polycrystal. Crack opening displacement (COD) profiles of bridged cracks also have been measured quantitatively via scanning electron microscopy to substantiate the piezospectroscopic determination of microscopic stresses via Raman spectroscopy. The highest spatial resolution of the stress measurement in the Raman apparatus was 1 µm, as dictated by the optical lens that was used to focus the laser on the sample. Measurements of the bridging stresses were performed both at fixed sites (as a function of the applied load) and along the profile behind the crack tip (under a constant load). Rather high stress values (i.e., 0.4-1.1 GPa) were measured that corresponded with unbroken ligaments that bridged the crack faces in elastic fashion, whereas frictional sites were typically under a lower tensile stress (0.1-0.5 GPa). Mapping the near-tip COD profile and the bridging stresses at the (normal) critical load for catastrophic fracture enabled us to calculate the crack-tip toughness and to explain the rising R-curve behavior of the material. From a comparison with conventional fracture-mechanics data, a self-consistent view of the mechanics that govern the toughening behavior of the Si 3 N 4 polycrystal could be obtained. In particular, crack bridging is proven to be, by far, the most important mechanism that contributes to the toughening of polycrystalline Si 3 N 4 materials.