The risk of fracture does exist even in the carefully designed ceramics components under excess stress unexpected in design procedure such as high stresses caused by restraining the deformation at the contact region of the component. To assess the safety and the reliability of ceramics components under such circumstances, it is indispensable to take the damage tolerance of the material into consideration. Ceramics is thought to be perfectly brittle and have no damage tolerance. But, though very little, it has the damage tolerance. The important problems open to us now are to estimate the extent of the damage tolerance of ceramics quantitatively and to clear the relation between the damage tolerance of the material and the reliability of the component. In this paper, we demonstrate the existence of the damage tolerance in the ceramics by tests using specimens of porous cordierite. The nonlinear stressstrain curve of the brittle material is thought to be a reflection of the damage tolerance of the material and it is shown that the nonlinear stress-strain curve obtained can be simulated by the distributed micro cracks model developed here. The relation between the damage tolerance and the reliability is also discussed through several simulation works. These simulations show that when the damage tolerance of the material is getting larger, the reliability of the component becomes higher. This conclusion is supported by the fracture test results using notched specimens of porous cordierite where the notch sensitivity is shown to be very little in this material. This means that this material has the high reliability under the stress concentration.
High cycle fatigue tests and computational studies of a coarse-grained Ni alloy were carried out to investigate fatigue crack nucleation and propagation behavior. Fatigue tests of a rectangular sample were performed by four-point bending under load control. Several interruptions were made to observe fatigue crack nucleation and propagations in the gauge region. Grain distribution and orientation of the tensile surface were characterized using electron beam backscattering diffraction pattern (EBSP) technique. A size of characterizing area was determined to sufficiently cover the inner span of four-point bending. Polycrystalline finite element analysis was made to investigate stress distributions of the specimen. Stresses were computed in global axis, such as longitudinal stress, and local axis, such as resolved shear stresses in slip systems, for whole grains. Longitudinal stress distributions tend to be affected by inter-grain relationships. Fatigue crack initiations tend to be occurred in areas indicating relatively high resolved shear stresses in slip systems.
Evaluation of fatigue crack nucleation and propagation behavior in anisotropic metals is necessary for improving an accuracy of structural integrity assessment. In this paper, high cycle fatigue tests and polycrystalline FE analyses were carried out to investigate a relationship between crack nucleation positions and stress distribution. Fatigue tests of a rectangular specimen were conducted by four-point bending under load control. Fatigue crack nucleation and propagation in the gauge region was carefully observed during the tests. Grain morphology and crystallographic orientation of tensile surface were characterized using electron beam backscattering diffraction pattern (EBSP) analysis. The size of characterized area was determined to sufficiently cover the gauge region of four-point bending inner span. Stress distribution of the specimen was computed by polycrystalline finite element analysis. Resolved shear stresses were calculated in expected twelve slip systems. Fatigue cracks were tend to be initiated in areas indicating relatively high resolved shear stresses in several slip systems.
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