Numerical Prediction Of Creep Crack Growth Rate And Its Constraint Effect For A Wide Range Of C~*

In the present creep life prediction, assessment and design of high temperature materials and components, the stress dependent creep constitutive model, creep ductility and constraint effect have not been effectively considered. Thus, the non-conservative (unsafe) or over-conservative (uneconomic) results will be produced. In this paper, the material used was a Cr-Mo-V steel (Chinese 25Cr2NiMo1V steel), and its stress-regime dependent creep model and ductility were implemented in a ductility exhaustion based damage model and ABAQUS finite element software, and then the technology of creep crack growth (CCG) simulation was developed. The reliability of the simulations was verified by comparing the simulated CCG rate with experimental data in existing literature. The effects and mechanical mechanism of stress dependent creep model, creep ductility and specimens with different constraints on CCG rate for a wide range of C* were investigated by the FE simulations. These researches have academic significance and practical value for developing advanced and accurately numerical prediction technology of CCG rate, understanding the relationship between stress dependent creep model, creep ductility and creep crack growth behavior, ascertaining the effect and mechanical mechanism of crack tip constraint on CCG rate and developing constraint-dependent CCG rate equations in a wide range of C*. The main work and conclusions obtained are as following:(1) By using the CCG simulation technology developed, the daldt-C* curves of Cr-Mo-V steel over a wide range of C* have been simulated by finite element analysis. The results showed that the daldt-C* curves are composed of four line segments with three different slopes, and the mechanical mechanism of turning points were ascertained. The effect of line segments in daldt-C* curves on the creep life assessments of high temperature components was analyzed. It shows that if the extrapolation CCG rate data in high or transition C* region were used in CCG life assessment of the components at lower C*, the non-conservative or over-conservative results will be produced.(2) The effect and mechanical mechanism of stress dependent creep ductility model parameters (such as transition region size, position, the value of upper/lower shelf creep ductility and so on) on the line segments in daldt-C* curves were investigated. In terms of the stress dependent creep ductility of materials, the basic principles for improving the creep crack growth property of materials were given. By decreasing transition stress levels and transition region sizes of creep ductility and increasing the lower shelf and upper shelf creep ductility values, the CCG resistance of materials can be improved.(3) By using the CCG simulation technology developed, the creep crack growth behavior of specimens with different constraints have been predicted successfully over a wide range of C*. The results showed that in low and transition C* regions, the crack-tip constraint has obvious effect on CCG rates, and the CCG rates increase with increasing specimen constraint. The mechanical mechanism of specimen constraint effects on creep crack growth behavior was analyzed by the distributions of equivalent stress and stress triaxiality ahead of crack tips at different C* values. The results showed that the constraint effects on creep crack growth behavior for a wide range of C* mainly arise from the interaction of crack-tip stress states and stress dependent creep ductility of the steel in different C* levels.(4) The effects of different in-plane, out-of-plane constraints and their interaction on CCG rates were examined in a wide range of C* by finite element simulations. The results showed that there exists interaction between in-plane and out-of-plane constraint in terms of their effects on CCG rate, and the higher in-plane constraint strengthens the out-of-plane constraint effect on CCG rate, and higher out-of-plane constraint also strengthens the in-plane constraint effect on CCG rate. Thus, for accurate CCG life assessments of high temperature components, both in-plane and out-of-plane constraint effects on creep crack growth behavior need to be considered.