Development of Advanced Creep Damage Constitutive Equations for Low CR Alloy Under Long-Term Service

Low Cr alloys are mostly utilized in structural components such as steam pipes, turbine generators and reactor pumps operating at high temperatures from 400℃ to 700℃ in nuclear power plants. For safe operation, it is necessary at the design stage to predict and understand the creep damage behaviour of low Cr alloys under long-term service conditions but under low stress levels. Laboratory creep tests can be utilized in the investigation of creep damage behaviour, however, these are usually expensive and time-consuming. Thus, constitutive modelling is considered here for both time and economic efficiency.
Existing constitutive equations for describing creep are mostly proposed based on experimental data for materials under high stresses. For low stress levels, the computational determination of a current state is extrapolated from those constitutive equations by simply using a powerlaw or sinh law. However, experimental observation has shown that this method is not satisfactory. The aim of the current research is to utilize continuum damage mechanics (CDM) to improve the constitutive equations for low Cr alloys under long-term service.
This project provides three main contributions. The first is a more accurate depiction of the relationship between minimum creep rate and stress levels. The predicted creep rates show good agreement with creep data observed experimentally for both 2.25Cr-1Mo steel and 0.5Cr-0.5Mo- 0.25V steel creep specimens. Secondly, it gives a more comprehensive description of the relationship between creep damage and creep cavitation. The CDM approach has been used and a reasonable agreement has also been achieved between predicted creep strain and experimental data for 0.5Cr-0.5Mo-0.25V base material under the critical stress of 40MPa at 640℃. Thirdly, it proposes a more accurate creep rupture criterion in the creep damage analysis of low Cr alloys under different stress levels. Based on investigation of creep cavitation for 2.25Cr-1Mo steel, the area fraction of cavitation at rupture time obviously differs under different stress levels.
This thesis contributes to computational creep damage mechanics in general and in particular to the design of a constitutive model for creep damage analysis of low Cr alloys. The proposed constitutive equations are only valid at low and intermediate stress levels. Further work needs to be undertaken when more experimental data are available.