低強度部を含む丸棒継手の降伏強さの評価手法に関する検討

抄録

The objective values of the yield strength and fracture toughness are respectively more than 1200 MPa and 200 MPa√m at 4K on the structural material for the superconductnig magnet container in the Fusion Reactor. It should be desireable to assure the above value for the properties of the weld metal used for the containers.
However, it is very difficult to assure the both properties for the same weld metal, especially made by electron beam welding which has the very good cost efficency. It should be reasonable to use a little bit undermatched weld metal in order to assure the fracture tonghness. So, it is understood to be very important to know the applicability and permitted limit based on the design condition of loading and deformation for the superconducting magnet container. From this view point, the initial yielding behavior of the undermatched joint is investigated in this paper.
Results are as follows. (1) On the nominal stress vs. nominal strain relationship at the central section of the undermatched metal of the joint, the stress increases higher than the yield stress of undermatched metal itself then gradually comes up to a constant value, after the whole area of the undermatched metal starts to generate plastic deformation as the stress reaches the yield stress of the metal. (2) The increase in the stress is caused by the restraint effect of the base metal on the plasticity, and the stress becames larger with the larger strength ratio of the undermatched metal to the basemetal and with the smaller relative thickness of the joint. (3) The strain does not distribute uniformly at the central section of the undermatched metal along the radial direction during the initial yielding process of the joint. (4) In the case that the yield strength of the joints is considered as the proof stress desided for a constant offset strain, the relationship between the dimensionless yield strength of the joint to the yield stress of the base metal and the relative yield strength of the undermatched metal to the base metal cannot be not established con-sistently. (5) The assumption that the base metal is rigid, causes the excessive evaluation of the joint yield strength of theoretical analysis. (6) As the quantitative evaluation method, the stress vs. strain relationship and then the algebraic expression has been formulated for the initial yielding behavior of the joint.