Two-dimensional mapping of crack-face bridging stresses in alumina using synchrotron micro x-ray beam

In order to understand an effect of crack-face bridging stress field of alumina ceramics on its fracture toughness, local residual stress distribution due to crack face grain bridging behind the crack tip was measured using synchrotron x-ray beam at SPring-8 in Japan. The SEPB (Single Edge Precracked Beam) specimens of two types of polycrystalline Al2O3 were used for stress measurement; one was pressureless sintered Al2O3 (AL1) and the other was hot-press sintered Al2O3 (TAL). Pop-in precracks were introduced by bridge-indentation method. Before residual stress mapping, the SEPB specimens were unloaded from a constant applied load to zero using four points bending device. Two-dimensional residual stress field was mapped by scanning a micro X-ray beam of 50x50 mu m(2) with the scanning interval of 12.5 or 25 mu m. As a result, in the case of AL1 having conventional fracture toughness and strength, the compressive residual stresses due to crack-face bridging were only observed in the close vicinity of crack tip. On the other hand, in the case of TAL having higher fracture toughness and strength, the compressive residual stresses were widely distributed behind the crack tip. Larger compressive stress was locally generated along the crack path at interlocked grains. The compressive bridging stresses distributed behind the crack tip were found to enlarge with a decrease in the crack opening displacement against a constant applied stress intensity factor, K-app. It was concluded that the difference in residual stress fields behind crack tip was attributed to the differences in its microstructure and microcrack propagation behavior, such as deflections and interlocked grains.