Plastic Flow of Mild Steel Under Proportional and Non-Proportional Straining at a Controlled Rate

In the present investigation, thin tubular specimens of annealed mild steel (En8) were tested under combined straining, using a newly developed tension-torsion, closed-loop, servo-controlled, electro-hydraulic testing machine. Two types of straining program were used, proportional straining at constant strain rate and nonproportional straining in which the specimen was loaded to yield in pure torsion and then extended with the twist held constant. The resulting load and torque components were continuously recorded during each test, as functions of time. A remarkable feature of the second type of test is that it was possible to obtain almost the whole positive quadrant of the initial yield locus of this material from a single test, since the load-torque trajectory followed closely the yield locus. Both proportional and nonproportional straining paths gave results in good agreement with the von Mises yield condition. The work included two theoretical analyses of the present problem, one using Perzyna’s constitutive laws for rate-sensitive but nonworkhardening material and the other using a rate-independent theory. Comparison with the experimental results of the nonproportional straining program indicates that the time history of the neutral loading path was reasonably well matched by the prediction of both the rate-dependent and rate-independent theories. Some preliminary experimental investigations of the hardening behavior in the proportional paths are also reported.