Neutron Diffraction Study of Engineering Materials Subjected to Complex Loadings

No studies using diffraction have specifically aimed to study the (hkl) specific shear strain of lattice planes to the application of pure shear due to applied torsion. Also no measures of strain resultant from combined loads of tension and shear stress applied simultaneously have been performed. Current techniques simply measure the planar response to axial stress and cannot provide shear strains directly. Shear moduli are not necessarily related through traditional continuum mechanics to planar Youngs’ moduli (E_hkl) from these axial experiments for all materials. This is of significance as most engineering components fail under the influence of shear stress, or combination of shear and axial stress, rather than solely under axial stress. However, even though shear is the predominant loading case and yield mechanism, no exhaustive studies have yet been carried out describing the (hkl) specific shear strain in response to applied shear stress. A new technique that has been implemented as a proof-of–concept at the 2nd Generation Neutron Residual Stress Mapping Facility (NRSF2) at Oak Ridge National Laboratory (ORNL) that is able to indirectly measure the shear response of individual hkl plane. Torsion provides a unique opportunity to study the mechanical behavior of materials subjected to pure state of shear stress. An additional technique is presented that allows for the measurement of a full strain tensor in materials subjected to complex and combined proportional loads of axial and shear stresses. These new techniques are utilized to measure the planar response of a bcc ferretic steel alloy ex situ (post loading) and in situ (during loading). A comprehensive analysis of the strain tensors derived through neutron diffraction experiments on this material measured ex situ and in situ are presented. Also, a detailed description of gauge volume location through an advanced laser tracker system is demonstrated.