The main aspect which needs to be considered before an experiment is the gauge volume (GV) dimensions used. The GV is defined by primary slits on the incident beam and a pair of secondary slits in front of the detector. The aspect ratio of the GV is highly dependent on the diffraction angle used. Due to the high energies, this angle must be kept below ~12o which leads to aspect ratios of at least 10:1, with the longer direction aligned with the incident beam. Under typical operational conditions the smaller dimensions of the GV range from 0,05 x 0,05 mm² to 0,5 x 0,5 mm², it’s longer dimension can be estimated on the graph below. Smaller GV dimensions are possible under special circumstances. Please contact the beamline staff if smaller GVs are needed.
The optimal GV size and shape is ultimately a compromise between the required spatial resolution and optimal beamsize and 2th. The latter ones are chosen based on sample thickness, composition and microstructure. Higher electronic density or thicker samples mean higher absorption on low energies and less overall intensity. As such, measurement of high absorption samples may require shifting the spectra to higher energies, therefore using lower 2th, resulting in a larger aspect ratio. Depending on intensity, high absorption samples require an increase of GV volume for achieving reasonable measurement times. The microstructure affects the gauge volume mostly due to grain size. If an insufficient number of grains is diffracting, the measurements will not have sufficient statistics for analysis, requiring the GV to be larger or the sample to be wiggled. In general, grain size should be at least 10 times smaller than the beam size.
The first step of any experiment is to optimize the measurement conditions, bearing in mind the science case addressed. The beamline staff generally performs this with the users. When preparing a proposal, please allocate at least half a shift for this optimization.
Measurement times:
Measurement times are severely affected by the GV volume, diffraction angle, sample thickness and composition. In general, counting times range from 0.5 to 50 s per exposure. As such, measurement times can vary widely from experiment to experiment. Below are some examples of short optimized exposure times for different samples/measurement conditions:
| Material | Thickness (mm) | 2θ angle (o) | Incident beamsize (mm2) | Acquisition exposure time (s) |
|---|---|---|---|---|
| Al 1100 | 30 | 7.4 | 0.15 x 0.15 | 0.5 |
| Martensitic stainless steel | 2 | 7.8 | 0.1 x 0.1 | 0.1 |
| Martensitic stainless steel | 2 | 7.8 | 0.02 x 0.02 | 5 |
| Low alloy steel | 20 | 7.1 | 0.15 x 0.15 | 5 |
| Low alloy steel | 40 | 4.8 | 0.2 x 0.2 | 50 |
Position resolved stress determination
The beamline can be programmed to scan samples in a variety of different ways, allowing stress determination through three different strategies:
- Principal direction scanning (Strain scanning): The sample is mapped with detectors aligned with the principal directions. Fastest possible measurement approach, but with low statistics. Accuracy is reduced.
- Position resolved sin2ψ measurements: For each position within the map, the sample is measured at several ψ (and possibly φ) positions, allowing for position resolved sin2ψ analysis.
- Position resolved strain pole figures: For each position within the map, a full pole figure is collected. Highly time-consuming approach, but ideal when severe variations in texture, plastic deformation, principal directions, etc. exist within the sample.
If measurement times allow, position resolved sin2ψ measurements are the recommended approach. sin2ψ measurements usually use ~5o step scans between 0o and 90o ψ. Scans can be repeated at different φ positions to obtain different stress components. The table below indicates the stress information which can be derived depending on the φ positions used:
| φ positions (o) | Stress components |
|---|---|
| 0 | σ11 - σ33 |
| 0, 90 | σ11 - σ33, σ22 - σ33 |
| 0, 180 | σ11 - σ33, σ13 |
| 0, 90, 180, 270 | σ11 - σ33, σ22 - σ33, σ13, σ23 |
If d0 is known, σ11, σ22 and σ33 can be separated from the stress differences. The beamline has custom made software available to perform point-resolved sin2ψ analysis.
The limiting factor for measurement precision is mostly dependent on sample microstructure (grain statistics). For high grain statistics measurements, 5 to 20 MPa error bars can be expected.
