Neutron Residual Stress Mapping Facility (HB-2B)
| Monochromator: | vertically and horizontally focussed Si |
| Monochromator angle: | 88°,  = 1.452  (511); 1.731 (331); |
| 1.886 (400); 2.275 (311) |
|
| Detector angle settings | 30 – 150° |
| Detection system: | 7 Position-Sensitive Detectors |
| X-Y-Z translation ranges: | X ± 200 mm, Y ± 100 mm, Z ± 100 mm |
| Omega goniometer mount: | X ± 25 mm, Y± 25 mm |
| Nominal Gauge volume: | 0.2 mm x 10 mm x 0.2 mm to 3 mm x 20 mm x 3 mm |
| Position-sensitive detector range: | 7° 2Θ at 800 mm sample-to-detector distance |
| Detector resolution: | 1.8 mm |
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. Strain scanning studies of co-extruded piping for Kraft paper process liquor boilers were instrumental in the selection of alternative alloys for improved boiler performance. 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.
Two diffractometers are available for residual stress measurements at the HB-2B beam port. The X-Y-Z instrument 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 second instrument is based on a Kappa goniometer that can be used in strain scanning of large grained materials and single crystals. The instrument can measure preferred grain orientation in polycrystalline materials as well. The two instruments are designed for convenient interchange at the HB-2B beam port. (Only the X-Y-Z instrument is shown in Fig. 2B.5.)
HB-2B Schematic
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 monochromator device consists of two silicon crystal assemblies with differing crystallographic orientations.
The scattering from the test sample is recorded with five 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. In such cases, it is advisable to use a comparison of strain measurements to calculated strains for validation of residual stress calculations.
