Fracture of a large-grained alumina polycrystal has been examined in situ by optical microscopy. Concurrently, local bridging stresses, as generated by friction or tension of unbroken ligaments in the wake of the crack path, were measured by piezospectroscopy. Stress measurements were performed both at fixed sites as a function of the external load and at a fixed external load along the crack profile. Frictional stresses were ≈50 MPa, while unbroken ligaments between the crack faces were found to support tensile stresses up to ≈100 MPa. The maximum bridging stress was dictated by the weak (intrinsic) interface bonding of the polycrystal. Average bridging stresses, either theoretically calculated from R-curve data or experimentally measured by piezospectroscopy on frictional/bridging sites, were similar. Such a circumstance enables us to explain the fracture behavior of polycrystalline alumina by considering crack-wake shielding as the main micromechanism contributing to toughening.
