Mitsuyoshi Tsunori

Residual Stress Evaluation of Hydraulically Expanded Tube-to-Tubesheet Joint

An evaluation of a residual stress of hydraulically expanded tube-to-tubesheet joints is important in strength evaluation of certain devices such as heat exchangers, steam generators, etc. In this study, we carried out the analytical and experimental investigation in order to develop a model for the prediction of residual stress.The experiment of the tube expansion was carried out with nickel-base alloy tubes and a low-alloy steel tubesheet. The residual stress of each tube was evaluated by sectioning method where the released strain was measured after each tube was cut into thin strips. As a result, the tensile residual stress was detected at the transition zone between the expanded zone and the non-expanded zone on each tube. A finite element analysis was also performed with a three dimensional model for the tube-to-tubesheet joint. The accuracy of the analysis, however, needed to be improved. Therefore FEM models were developed where the following factors were considered: (i) work-hardened layer on the tube before expansion, (ii) friction between the inner surface of the tube and the mandrel used for hydraulic expansion. The accuracy improvement of the analysis was confirmed through the comparison of the numerical and experimental results.Copyright

Numerical Simulation of Residual Stress in Multi-Pass Butt-Welded Stainless Steel Pipe

It is necessary to obtain an accurate welding residual stress distribution for the evaluation of stress corrosion cracking (SCC) behavior. However, a welding residual stress simulation for pipes is often performed by a two dimensional axisymmetric model because this type of simulation requires significant time to analyze the complicated inelastic behavior. This approximation deteriorates the modeling accuracy since the welding heat input and the structural response are approximated by axisymmetric responses although they are originally three dimensional. The authors propose “a virtual additional stiffness method” in order to improve the accuracy of the axisymmetric model. With this method, the difference between the axisymmetric model and a three dimensional behavior was greatly reduced. The virtual additional stiffness method was used to reproduce three dimensional constraints that were not taken into account in the axisymmetric model. In the case of the axisymmetric model, an unrealistic large thermal expansion was observed because of simultaneous heating along a hoop direction of the whole pipe. In order to compensate this unrealistic deformation, a virtual additional stiffness was added in axial and radial directions on the axisymmetric model. This stiffness was added by using spring elements whose positions and spring constants were determined by comparing the two and three dimensional models. Results obtained by this new method in the multi-pass butt-welded stainless steel pipe were in very good agreement with measurements of the mock-up specimens.Copyright