Ryota Higuchi

Study of Residual Stress Distribution at Start-Finish Point of Circumferential Welding Studied by 3D-Fem Analysis

Recently, the stress corrosion cracking (SCC) of internal cores and/or recirculation pipes of low-carbon austenitic stainless steel has become actualised. The SCC is considered to occur and progress near the welding zone because of the tensile residual stress due to welding. In the present work, the thermo-elastic-plastic analysis of the residual stress was performed in order to clarify the distribution of residual stress in the steady and unsteady welding regions by circumferential welding. The target was a butt-welded joint of SUS316L-pipes. The residual stress has been calculated using a three dimensional model and the results have been compared and discussed in detail.

FEM-MD Combined Method for Investigation on Effects of Microscopic Heterogeneity Near Weld Zone on Strength Characteristics

The microscopic heterogeneity of structural steels and their weld zone is one of the factors that influence the mechanical properties. It causes the different deformations of grains in the case of various loads applied to the steels, and the stress concentration around the triple junction occurs due to the mismatch of displacement that appears near the grain boundary. Therefore, it is important to estimate the stress distribution that is related to the mechanical behaviours. In this study, the numerical analysis method is used for the estimation of the stress distribution by considering the microscopic heterogeneity and the deformation near the grain boundary. The grain shapes reflecting the microstructure of steels are modelled by using Voronoi tessellation that is the method dividing the region into arbitrary polygons. Furthermore, the FEM-MD combined method is proposed for the estimation of the stress distribution. FEM applied to the analysis of the grain itself, and MD (Molecular Dynamics) applied to the analysis of the neighbourhood of the grain boundary, are combined to model the mismatch of the displacement near the grain boundary by considering the slip on the boundary. Then FEM-MD combined method is applied to investigate the influence of the change of grain diameter due to the heating and cooling conditions assuming the thermal history near the weld zone, and the difference of the stress distribution is seen. It is expected that FEM-MD combined method will be helpful in further investigating the influence of the microscopic heterogeneity on the mechanical properties.

Parametric FEM Evaluation of Residual Stress by Circumferential Welding for Austenitic Stainless Steel

Recently, stress corrosion cracking (SCC) of core internals and/or recirculation pipes of austenite stainless steel has been observed. SCC is considered to occur and progress near the welding zone because of the weld tensile residual stress. In the present work, thermo-elastic-plastic analysis of the residual stress was performed in order to clarify the effect of several parameters (diameter, thickness, number of multilayer welding) in the circumferential welding zone. Butt welding joint of SUS316L-pipes was examined. The residual stress was calculated by three dimensional-model and axisymmetric model and the results were compared and discussed in detail.Copyright

FEM–MD combined method for evaluation of microscopic stress distribution by considering microscopic heterogeneity and deformation in close vicinity to grain boundary of steels

Finite element method (FEM)–molecular dynamics (MD) combined method is proposed for the microscopic stress analysis of steels. In this numerical method, FEM is applied to the stress analysis inside grains, and MD is applied to the calculation of the atomic configuration near the grain boundary in order to consider the microscopic heterogeneity and the deformation near the grain boundary that influences the stress distribution. Slip length between two grains caused by the mismatch of the displacement near the grain boundary is calculated by FEM. Slip resistance, which is necessary to calculate slip length, is obtained by calculating the atomic configuration near the grain boundary by MD. The combination of FEM and MD is realized by using slip resistance in FEM and slip length in MD. The validity of modelling of the deformation near the grain boundary is investigated by comparing the deformation near the grain boundary calculated by FEM–MD combined method to that observed in the experiment in the case of a load applied to the specimen. Calculated slip length coincides with measured slip length. FEM–MD combined method is applied to the investigation of the influence of change in the grain shape caused by the thermal history such as the weld zone upon the strength characteristic. The high stress region tends to increase the incidence of larger grain diameter and it is indicated that grain coarsening due to the weld thermal history increases the possibility of the crack initiation. FEM–MD combined method is expected to be helpful in investigating the mechanism of fracture or the strength characteristic of the complicated microstructure such as the weld zone by evaluating the microscopic stress distribution.

A method for evaluating the stress–strain relationship of materials in the microstructural region using a triangular pyramidal indenter

A method for evaluating the microstructural stress–strain relationship of materials, using a triangular pyramidal indenter, is proposed in order to investigate the mechanical properties of steels and weld zones. An existing evaluation method, using a ball indenter, is correspondingly applied to the evaluation method using a triangular pyramidal indenter because the strain distribution under the indenter or the indentation curve on the unloading process between the ball and pyramidal indentation has a similarity. A corresponding ball indenter whose projection area is equal to that of the triangular pyramidal indenter is used to replace the triangular pyramidal indentation with the ball indentation, and the representative stress and strain that express the complicated deformation under the indenter are determined. The stress–strain relationships of single-phase steels in microstructural size are estimated by the proposed method, and on average correspond with those measured by macro-tensile tests. The difference in the stress–strain relationships due to the difference of the crystal orientation of each grain is possibly negligible with this method. It is expected to clearly estimate the difference in the stress–strain relationship of each phase in, for example, dual-phase steels by the proposed method.

Evaluation of Strength Characteristics Considering Microscopic Heterogeneity of Structural Steels and Weld Zone by Using FEM-MD Coupling Method

The strength properties of structural steels or their weld zone are influenced by the microscopic heterogeneity. It is important to investigate the stress distribution for clarification of the mechanism of fracture and the material design by considering a grain boundary or its neighborhood because either can be a zone where the stress concentration is likely to occur due to a mismatch of the displacement. For this purpose, FEM-MD coupling method is one of the useful methods because it can treat the mismatch of the displacement caused by the microscopic heterogeneity. In this study, FEM-MD coupling method is proposed and the influence of the microscopic heterogeneity of steels is investigated by using FEM-MD coupling method.Copyright

FEM-MD Coupling Simulation for Strength and Fracture Analyses Near Grain Boundary Zone of Structural Steels

FEM-MD coupling method is proposed for the evaluation of the strength properties of structural steels. It is important to investigate the stress distribution by considering the microscopic heterogeneity and deformation near the grain boundary for clarification of the mechanism of fracture and the material design. FEM-MD coupling method is used to estimate the stress distribution. Especially, the influence of microstructure of steel on strength properties is investigated by estimating the difference of stress distributions caused by different distributions of the grain shape, such as grain diameter, aspect ratio and grain orientation.Copyright