The results of fourteen combined stress tests with variable stress ratios on annealed commercially pure aluminum are reported. These tests were made on a combined tension-torsion testing machine of a dead-load type. It is shown that the loading path carries a pointed vertex with it. However, the direction of the plastic-strain increment vector differs only slightly from the direction predicted by the Mises theory. Finally, the experiments indicate that the form of the yield surface at a considerable distance from the stress increment vector agrees much better with the prediction of the slip theory than it does with the prediction of the Mises theory.

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