F. Yao

An investigation on the transition from creep damage to creep fracture

To predict a complete failure evolution process due to creep, the transition from creep damage to creep fracture is explored based on the moving jump forms of conservation laws. With the use of the strain-based rate-independent damage surface in a combined damage/viscoplasticity model, a continuum tangent stiffness tensor is formulated to identify the transition. It is shown that the rate-independent damage surface can predict the rate-dependency of limit stress. Sample problems are considered to illustrate the proposed procedure for simulating the failure evolution process due to creep.

Static and cyclic fatigue behaviour of crack-healed Si3N4/SiC composite ceramics

Abstract Si 3 N 4 /SiC composite ceramics were sintered in order to investigate their fatigue strength behavior after crack healing, Y 2 O 3 and Al 2 O 3 power was added as sintering additives to enhance its sintering property. Three-point bending specimen (sized 3×4×40 mm) was hewed out according to JIS standard from sintered compact obtained. About 100 or 200 μm semi-circular surface cracks were made on the center of the tension surface of the three-point bending specimen using Vickers indenter. After the crack-healing processing of 1200 and 1300°C, 1 h, in air, cyclic and static fatigue strength behavior of these crack-healed specimen were determined systematically at room temperature and high temperature of 800, 1000, 1200 and 1300°C. The results show that Si 3 N 4 /SiC composite ceramics possess remarkable crack-healing ability, and crack-healed specimens showed similar cyclic and static fatigue strength behavior as smooth specimens at high temperature, not only at room temperature. Crack-healed zones had sufficient fatigue strength even though at high temperature environment, and most fractures occurred outside the pre-cracked zone in those crack-healed specimens.