Samples were prepared as standard single-edge notched beams (SENB), and mirror polished before testing . 3 × 6 × 36 mm samples were used for fluorescence spectroscopy and SEM characterization. The fluorescence of trivalent chromium (Cr 3+ ), a ubiquitous impurity in alumina-based polycrystalline ceramics, was used to map stresses in the composite . Even in very low quantities (< 0.5 wt%), Cr 3+ causes an intense fluorescence characterized by two sharp and intense lines, noted R 1 and R 2 , whose energy shifts with applied stress .