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 50×50 μm2 with the scanning interval of 12.5 or 25 μm. 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, Kapp. 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.

Numerical Fracture Mechanics Analysis of Cracked Fibre-Metal Laminates with Cut-Outs

The boundary element method (BEM) for two-dimensional numerical stress analysis is employed to investigate crack-face bridging of cracked fibre-metal laminates (FML) with cut-outs in this study. The fracture mechanics prediction of crack growth in these perforated laminates involves the interaction of the geometry and crack size, the delamination between the pre-peg and metal layers, and the extent of fibre-bridging of the crack flanks with the stress field caused by the cut-out. The present work investigates the effects of a stress concentration on the fibre-bridging stress and the stress intensity factor of a bridged crack in fibre-metal laminates. A number of cracked configurations are analyzed and the FML, ARALL2, is considered. The bridging stresses on the crack flanks are modeled in the 2-D analysis using power-law expressions and with the mechanical properties of the laminate homogenized through the thickness. An iterative scheme is employed to solve for the bridging stresses as they are not known a priori. Three dimensional finite element method (FEM) analyses are also carried out to confirm the validity of the 2-D BEM models. FML's with circular cut-outs will contain high bridging stresses near the cut-out resulting in fibre failure there, causing a reduction of the extent of fibre bridging of the crack. Results of the study show a likelihood of fibre failure near the edge of the cut-out and this could lead to a reduction of the bridging length. Comparison of the BEM with the FEM stress intensity factors for the range of problems analyzed reveals that the percentage difference is generally less than about 6%, except for a few cases when the power-law index of 0.5 is assumed. The BEM results indicate an increasing bridging stress and stress intensity factor with decreasing bridging length and the benefits of the fibre bridging of the crack are clearly demonstrated. This numerical study confirms that the 2-D BEM models employed can indeed be used to provide a quick and reasonable estimate of the stress intensity factor for a bridged crack in a FML with a circular cut-out.