The purpose of this paper is to report the results of a series of special biaxial-stress tests made to check the validity of certain assumptions used in theory of plasticity. In these tests the stresses produced were tensile biaxial stresses introduced in a tubular specimen by applying an internal pressure and axial tension. The stresses produced were in the plastic range and the material tested was a hot-rolled aluminum alloy designated as 14S-T6. A test to determine the validity of the distortion-energy criterion, used in formulating the plastic stress-strain relations by the flow theory, showed that the test results did not support this assumption. Special tests were made to determine whether plastic-deformation requirements as predicted by the slip theory were correct. In so far as these tests were concerned, the results supported the slip theory. Finally, tests were made to check the assumption of constancy of volume and constancy of incremental change in volume as required by the deformation and flow theories. These test results do not support the assumptions made in these theories relative to volume change.

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