The Neutron Residual Stress Mapping Facility at HFIR
The HB-2B beam port is optimized for strain measurement and determination of residual stress in engineering materials. The incident beam is delivered at a fixed angle of 88° by elastically bent silicon crystal monochromators. Both vertical and horizontal focusing are used. The wavelength is chosen from a variety of monochromator crystal settings with a selection of wavelengths from 1.2 to 2.4 Å. The sample goniometer is designed for spatial scanning of residual stresses at depths from a millimeter to several centimeters. Spatial resolution at a fraction of a millimeter is possible depending on the material.
The scattering from the test sample is recorded with seven linear-position-sensitive detectors. The detectors are stacked vertically and have a horizontal acceptance of 9 cm at a distance of 80 cm from the sample test point. The nominal scattering angle (at the center of the position-sensitive detector) can be set at 30 to 140°. The top and bottom detectors span ±10° out of the horizontal scattering plane.
In strain scanning, “gauge volume” is defined by the intersection of the incident and scattered beams. The dimensions of the incident beam collimator range from 1 cm to less than 1 mm. The scattering collimator having the same size range is set close to the test specimen. The location of these collimators controls the size of the gauge volume and also defines the translation range over which the test specimen can be scanned. The determination of residual stress from measurements of residual strain requires measurements of strain in at least three orthogonal strain directions. In some instances, it is not feasible to carry forward a complete stress determination, and in such cases, it is advisable to use a comparison of strain measurements to calculated strains for validation of residual stress calculations.
Available sample environments include a uniaxial (tension or compression) load frame, a Eulerian cradle, high-temperature furnaces, and a 5-Tesla magnet with an induction furnace insert. Custom-built sample environments can be accommodated on the XYZ positioning stage.
The penetrating power of neutrons is very useful in scanning residual stresses in engineering materials. Examples of past work include scans of welds, forgings, extrusions, bearings, and materials under applied stress as well as of piezoelectric materials under the influence of electrical fields. The study of post-weld annealing effects on residual stresses in welded steel plates provided a nondestructive and detailed delineation of stress pattern changes arising from the heat treatment.