Analysis And Quantitative Characterization Of Creep Constraint Effect Induced By Specimen Geometries

In this paper, the crack-tip constraint induced by geometries of the CT specimen was investigated under creep conditions by using the finite element method (FEM). A new creep parameter R was defined, taking the stress field in CT specimen with high constraint (a/W=0.5) as the reference field. The creep crack-tip constraint was quantitatively characterized and analyzed in detail by the parameter R. The effects of specimen sizes, load levels and creep time on the creep constraint were.also examined. The main results obtained are as follows:(1) The creep crack-tip constraint induced by specimen geometries (including various crack depths, specimen thicknesses and widths) in CT specimens could be characterized and analyzed by the new constraint parameter R. The creep crack-tip constraint effect becomes more pronounced with increasing the load level (C*). During the early stages of creep, the crack-tip constraint induced by specimen geometries varies greatly with creep time, and then it gradually becomes independent on the time, that is the creep constraint parameter R approaches to a steady-state value.(2) The CT specimens with a/W=0.35-0.5 have highest creep constraint, and the constraint effect caused by crack depths at non-steady-state creep is more pronounced than that at steady-state creep. For specimens with larger width, the differences of creep crack-tip stresses between specimens with various crack depths are small, thus the creep crack-tip constraint induced by crack depths are negligible for large CT specimen. The creep crack-tip constraint also increases with increasing the specimen thickness. The creep constraint in 3D specimen lies between the two limit cases of the plane strain and plane stress state. There is a central region with higher creep constraint along the crack front in the 3D specimen, and the size of the region increases with increasing specimen thickness, and decreases with increasing the creep time.(3) The effects of crack-tip constraint on creep crack initiation time and creep toughness were investigated by the FEM with damage subroutine. It has been found that the creep crack initiation time decreases with increasing the constraint. It was also shown that, there is essentially no influence of the constraint R on the toughness Kmatc of creep brittle materials; while the toughness value of creep ductility materials will decrease with increasing the creep parameter R. (4) The effects of creep fracture process zone size on the crack-tip constraint were studied by FE simulations of models containing voids. The results show that the damage in specimen with deep crack depth exhibiting high constraint is larger than that in low constraint specimen with shallow crack depth. If the crack initiates after the steady-state of creep has been reached, then the constraint effect induced by crack depths will be found to become more obvious for the smaller size of creep fracture process zone.The results above will lay foundation for the establishment of constraint-dependent life prediction and safety assessment method for structures containing defects at high temperature.