Residual strains of tubular specimens subjected to either torsion or tension using neutron diffraction

Residual stress is an important phenomenon that occurs commonly in materials due to thermal or elastic misfit at macro or micro level. Residual strain evolution under 1-D loading, typically uniaxial tension, by using neutron diffraction-based methods has been well studied. However, little research using neutron diffraction has been reported for pure torsion type loading or under 3-D multiaxial loading conditions. Torsion provides unique opportunity to probe mechanical behaviors of materials under pure shear stress, and in combination with axial load, provides a mechanism to rotate principal stresses in a controlled fashion. Thus, complete 3-D mechanical behavior can be investigated by controlling the major principal stress rotation to characterize anisotropic materials. In this study, identical tubular steel specimens were first subjected to either pure torsion or uniaxial tension that corresponds to a target equivalent strain invariant. These specimens then were examined using the second generation neutron residual stress mapping facility (NRSF2) at Oak Ridge National Laboratory (ORNL). Residual strains based on the changes in the lattice spacing were measured in radial, circumferential and axial directions with a gage volume of 1×2×1 mm³. Two wavelengths of 2.67 and 1.73 Å were used in this study. Results are analyzed for the (110) and (211) reflections, reported to be weakly affected by intergranular strain for tension stress path. Results indicate considerable differences in the measured residual strain variation between the tubular specimens subjected to torsion versus tension. This study is the first of its kind for evaluating residual strains under generalized loading conditions which also helped to demonstrate the need for having an in-situ multi-axial loading system (axial and torsional) for the anticipated residual stress testing facility (Vulcan) at the Spallation Neutron Source (SNS) in Oak Ridge, TN.