Nonlocal effects in torsional deformation

The stress distribution in torsionally deformed cylindrical polycrystalline copper samples was investigated. The strain in torsional deformation changes with distance from the torsional axis and thus the local extent of work hardening and consequently the local flow stress changes as well. The variation of the flow stress with radial position in the samples was measured by microhardness tests. It was found that except for the near-axis region the local flow stress can be correctly calculated by the Nadai evaluation from the experimentally measured torque versus torsional angle function. Hardening is observed in the axis of the torsionally deformed samples in spite of the fact that the shear strain is zero here. The hardening obtained in the axis depends approximately linearly on the torsional angle, and it increases with grain size. This effect is interpreted by introducing nonlocal effects in the continuum description.