Papers by Author: Chang Yu Zhou

Abstract: The local wall thinning defect is very normal on pipes in power plants, which may result in stress redistribution of the pipes during the service process at elevated temperature. For the purpose of understanding the stress redistribution and strain accumulation of pipes with local wall thinning affected by load properties under creep condition, three groups of models were calculated, using three-dimensional elastic-plastic finite element analyses (FEA) based on FEA codes ABAQUS. In this study, the pipes has an identical defect of local wall thinning, the load properties and values are changed. Three groups of load properties, considering here, were monotonic internal pressure, monotonic moment and both internal pressure and moment, respectively. The numerical simulation conducted on P91 full-scale steel pipes at 625°C, with local wall thinning located at the inner surface. Then, von Mises stress and creep strain of pipes after 100,000h could be obtained corresponding to different models. Based on the analysis, the figures of creep stress and strain varying with load properties were plotted. Then, the stress and strain of pipes with local wall thinning affected by load properties were discussed. The results indicate that creep stress and creep strain increase with load properties. The variation laws have been summarized. The research results can provide the possibility on safety assessment and structure integrity analysis of the pipe with local wall thinning at high temperature effectively.

Abstract: The creep of materials makes it difficulty to determine the limit load of component at high temperature. In this paper, limit load was obtained by finite element simulation according to isochronous stress versus cumulative strain data and creep failure criterion at high temperature. Firstly, isochronous stress versus cumulative strain data of P91 steel was generated. In finite element analysis code ABAQUS, isochronous stress versus cumulative strain data was replaced by equivalent elastic-plastic constitutive relation. Then, sustained load versus cumulative strain curves at high temperature during service was obtained after finite element simulation. At last, limit load at high temperature during given working hours was determined based on the restriction of total strain at key point of specific component. The restriction of total strain which could also be regarded as long-term fracture strain was discussed in this paper. Finite element simulation of limit load of component at high temperature is simple and reasonable. Limit load of complex component at high temperature during given working hours can be obtained by this method. Using this method, limit loads of a pipe with local wall thinning defect and a branch junction at high temperature during given working hours were obtained as examples.

Abstract: At the present time, as the steam conditions and capability of the supercritical power unit increasing, the unit reliability is an important factor for the unit efficiency. High temperature, thick walled pipes are widely used in power plants and chemical plants. In this paper, life of the welded joint was predicted by the methods of skeletal point (SP) rupture stress rupture stress, which was calculated by finite element method (FEM). For the life prediction of welded component, the continuum damage mechanics was employed too. The life prediction of the welded joint by SP rupture stress was compared with the life prediction by the method of continuum damage mechanics (CDM). The research results showed that the two predicted methods were consistent. So, it can be concluded that the SP rupture stress can be used for predicting life of the high temperature welded component. The SP rupture stress method was used conveniently for the structure of power plant or other high temperature components.

Abstract: By the finite element analysis software ABAQUS and the function of coupling process between heat and stress, the welding residual stress of Cr5Mo and 20 steel joint was analyzed. In addition the heat treatment of dissimilar steel welded joint was simulated. The residual stress distributions of dissimilar steel welding and heat treatment after welding were obtained. The comparison of welding residual stress between the homogenous steel and dissimilar steel was carried out. The results indicate that the welding residual stress of the same steel is lower than that of dissimilar steel welded joint obviously. Because of the difference of thermal expansion coefficient for base metal and welding microstructure, the relatively higher residual stress is produced due to the bigger thermal expansion coefficient of base metal. The highest annular residual stress is in welding line root of internal wall, while the highest axial residual stress is in welding line surface of outer wall. The welding residual stress of dissimilar welded joint is reduced obviously after heat treatment. The research results provide the possibility for optimizing the welding procedure and improving the reliability of dissimilar steel welding joint.