| Many pressure vessels and pressure equipments used in petrochemical industry, power generation, aviation and other systems usually work under high temperature and cyclic loading. With the development of industrial technique in China, the working conditions of these equipments become more and more aggressive. Therefore, the credible life prediction for these equipments is a very important problem. Under high temperature and cyclic loading, the typical failure modes of pressure vessels and pressure equipments mainly include fatigue, creep and their interaction. In this thesis, the low cycle fatigue behavior of 316L steel at elevated temperature and the fatigue-creep interaction behavior of 1.25Cr0.5Mo steel at high temperature are researched. Based on this research, the new methods for the life predictions of fatigue and fatigue-creep interaction have been developed. Besides, according to the damage mechanics, the damage evolutions of fatigue and fatigue-creep interaction in materials are also studied in this thesis. And the corresponding methods for the damage assessment have been presented. The main contents of this thesis are shown as follows:(1) The low cycle fatigue behavior of the typical pressure vessel steel 316L at elevated temperature under stress control has been researched. Significant results are obtained, which include the laws of cyclic hardening and softening, the ratcheting under stress control and its influencing factors, the changing rules of the plastic strain range, the Masing characteristic of material, and the calculation of the plastic strain energy density, etc. These results provide the experimental data and analyzing base for the life prediction and damage assessment of 316L steel.(2) The fatigue-creep interaction behavior of the typical pressure vessel steel 1.25Cr0.5Mo at high temperature under stress control has been studied. Some meaningful results are acquired, which include the cyclic properties of material at high temperature, the changing rules of the mean strain and non-elastic strain range, etc. These results provide the experimental data and analyzing base for the life prediction and damage assessment of 1.25Cr0.5Mo steel.(3) The stress-life (S-N) curve of 316L steel at 420℃ has been obtained. Compared with the low cycle fatigue design curves in the analysis and design criterion of steel pressure vessels JB4732-95 and British criterion BS1515, the low cycle fatigue design curve of 316L steel at 420℃ is acquired.(4) According to the law of the energy conservation and the momentum conservation principle, the failure process of objects in the motion has been researched. In this research, the change of the internal energy is used as a key parameter connected with the life, and then a new model for the life prediction of objects is proposed. This model is developed from the laws of thermodynamics. Therefore, it is suitable to the life predictions of low cycle fatigue and fatigue-creep interaction. This model only needs several parameters and its expression is very simple. Based on this model, the assessments for the low cycle fatigue of 316L steel at elevated temperature and the fatigue-creep interaction of 1.25Cr0.5Mo steel at high temperature have been performed. A good agreement is found between the predicted results and the experimental data. In addition, from this model, the linear accumulation damage principle of fatigue is also discussed. Then, a new linear accumulation damage principle is presented. Using this principle, the residual life prediction of 316L steel at elevated temperature is carried out. A good agreement is noted between the predicted and experimental results.(5) Based on continuum damage mechanics (CDM), the damage evolutions of low cycle fatigue, creep and fatigue-creep interaction have been researched. And corresponding damage models are obtained. Using the suitable damage variables, the low cycle fatigue damage curves of 316L steel at elevated temperature and the fatigue-creep interaction damage curves of 1.25Cr0.5Mo steel at high temperature are obtained based on the damage models mentioned above. The values of damage calculated from the suitable damage variable definition are in good agreement with these damage curves. This result shows that with suitable damage variables the damage models presented in this thesis can describe the damage evolutions of low cycle fatigue and fatigue-creep interaction well. Besides, according to the fatigue-creep interaction damage model mentioned above, this thesis also discussesthe fatigue-creep interaction failure assessment of 1.25Cr0.5Mo steel at high temperature.The innovative points of this thesis are given as follows:(1) The low cycle fatigue behavior of 316L steel under stress control at elevated temperature has been fully researched. Significant results are obtained, which include the laws of cyclic hardening and softening, the ratcheting under stress control and its influencing factors, the Masing characteristic of material, etc. Besides, it is also found that the dynamic strain aging strengthening is remarkable in the material at 420℃.(2) The fatigue-creep interaction behavior of 1.25Cr0.5Mo steel under stress control at high temperature has been fully researched. Some meaningful results are acquired, which include the cyclic properties of material at high temperature, the changing rules of the mean strain and non-elastic strain range, etc.(3) According to the law of the energy conservation and the momentum conservation principle, a new model for the life prediction has been developed. This model is applicable to the life predictions of both low cycle fatigue and fatigue-creep interaction. Experimental and calculated results show that this model has the good effects on the life predictions of low cycle fatigue and fatigue-creep interaction. In addition, from this model, a new linear accumulation damage principle is presented. Using this principle, the residual life prediction of 316L steel at elevated temperature is carried out. A good agreement is noted between the predicted and experimental results.(4) Based on continuum damage mechanics, a new damage model for low cycle fatigue has been presented. The change of the stress-displacement curve area is chosen as the damage variable instead of the change of the plastic strain energy density. With this damage variable, the damage model presented in this thesis can describe the low cycle fatigue damage evolution of 316L steel at 420℃ well.(5) According to continuum damage mechanics, a new damage model for fatigue-creep interaction has been developed. Defining the change of mean strain as the damage variable, the fatigue-creep interaction damage curves of 1.25Cr0.5Mo steel at high temperature are obtained based on the new damage model. The values ofdamage calculated from the damage variable definition mentioned above are in good agreement with these damage curves. This result shows that with the damage variable mentioned above the damage model proposed in this thesis can describe the fatigue-creep interaction damage evolution of 1.25Cr0.5Mo steel at high temperature well. Besides, in the experiments, the mean strain values of different loading conditions always increase more quickly at the last stage. According to this phenomenon, there must be a critical cycle in the fatigue-creep interaction damage evolution of 1.25Cr0.5Mo steel at high temperature. Based on the above damage model, the mean strain value at the critical cycle can be obtained. Then, a method to prevent the rupture of engineering components is proposed in this paper.
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