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A in-plane stress constraint factor Ciand a out-of-plane stress constraint factor Coare introduced to study the three-dimensional stress state and the elastic-plastic constraint effect of the crack tip. The deep crack specimens(a/W=0.5) and the shallow crack specimens(a/W=0.1) with different thicknesses are performed in the present study. By using the finite element analysis, the result shows that the thickness effect of the deep crack specimens is more obvious than that of the shallow crack specimens. For the deep crack specimens, the crack constraint enhances with the specimen thickness increase. When the specimen thickness is equal to the specimen width, the constraint levels in the middle 2/3 thickness of the specimen approach to those from the plane strain analyse. For the shallow crack specimens, the crack constraint is almost fixedness when the specimen thickness changes from three times of a crack length to a specimen ligament length. Namely, increased thickness is helpless to enhance the crack constraint effect. Therefore, a shallow crack specimen may be designed by the lesser thickness than the standard deep crack specimen.

A model of the nut connection components of a 600MW supercritical steam turbine was established by finite element analysis to investigate the influence of creep on the nut connection components. Due to the complex structure and the multi-axial loadings, Cocks-Ashby coefficients were used to depict the multi-axial creep behavior. Multi-axial creep deformation criteria (MCDC) and multi-axial creep rupture criteria (MCRC) were used to assess the structure reliability. In addition, sealing performance of the nut connection components in the creep time of 2×105 hours was numerically investigated in terms of the contact pressure and the radial contact length. The results demonstrated that the creep strength of the nut connection components after 2×105 hours could maintain the safety operation of the nut connection components. The contact pressure distribution and the radial contact length in circumferential could prevent the leakage steam flow from the high pressure zone to the low pressure zone.

A best-estimate assessment model was established to predict the entire life time of the material due to crack development and propagation in aggressive environment. Stress corrosion cracking (SCC) growth rate was modeled to investigate the influence of corrosion environment on SCC. Uncertainties with respect to material parameters, aggressive medium and mechanical loadings were taken into account. In addition, mean value estimation method (MVM) and monte carlo simulation method (MCS) were used to solve the established theoretical model. The studies demonstrated that the results obtained by using MVM reached agreement to those by using MCS. And the life time of the material due to the crack development and propagation in the aggressive environment was accurately predicted.