Simultaneous use of neutron diffraction and attenuation based transmission Bragg edge imaging for strain measurements is demonstrated in this paper using the pulse neutron source at Los Alamos National Laboratory. Cylindrical samples made from ferritic steel have been subjected to in-situ elastic loading in tension and torsion. Lattice strains were investigated for both deformation modes by time-of-flight (TOF) neutron diffraction using two detector banks at 2θ of ±90°. At the same time, the transmitted portion of the neutron beam was recorded with the goal to analyze the position and shape of the Bragg edges, using a novel time/energy resolved Microchannel Plate (MCP) detector with pixel size of 55 µm and a 28x28 mm2 field of view. Lattice strains obtained using neutron diffraction indicate that the deformation path (tension versus torsion) has important effect on their evolution and related results are summarized.
The emphasis of this paper is to explain the aspects of the experimental setup and data interpretation associated with neutron Bragg edge transmission technique for obtaining through-thickness averaged strain measurements. Implications of using transmission imaging based strain mapping for samples subjected to deformation under tensile loading (where stress at a given cross-section is constant) versus torsional loading (where stress varies linearly from center to outer radius) are discussed. In the case of samples subjected to tensile loading, analysis of the Bragg edge shifts provides the strain value in the direction of the transmitted beam. Thus, three strain components are measured simultaneously when performing Bragg edge imaging in addition to diffraction using two detector banks. For specimens subjected to pure shear by torsion, the Bragg edge transmission technique cannot readily provide quantitative strain information as the mid-point of the Bragg edge does not shift uniformly due to external loading, but results in a broadening of the Bragg edge. Such information can be used to describe the variation of strain distribution along the transmitted beam direction. Spatially resolved Bragg edge maps will be very helpful to detect d-spacing inhomogeneities within the illuminated volume, which may remain undetected when using diffraction only measurements.