Abstract
Welded steel tube which outside diameter is from 6 inches to 20 inches is popularly manufactured by cold forming and electric resistance welding. This cold forming consists of uncoiling, leveling, roll forming, sizing and straighting. These producing processes can be idealized to primary strain states. For example, flat steel sheet which was uncoiled and leveled, is bent to the circumferential direction in the state of plane strain at the roll forming process, and free from the constraint of the longitudinal direction. Assuming such simplified strain state of each process, the locked in stress induced by cold forming was analized by the mathematical theory of plasticity. Prandtl-Reuss equation was used to the stress-strain relationship of the steel material in plastic range. And isotropic hardening and independent hardening were adopted as hardening rules. But welding thermal residual stress was not considered in this analysis. Results from this calculation were compared with the released strain obtained by saw cutting of 318.5φ×10.3t grade STK 41 and STK 50 steel tubes. Finally, nominal stress-strain relationships in stab-column tests were calculated concerning the plastic strain history and locked in stress obtained from this analysis. these results were also compared with the experiments 356φ×6.3t, 356φ×11.0t grade STK 41 steel tubes.
