ISO/TTA 3:2001 pdf download

ISO/TTA 3:2001 pdf download.Polycry stalline materials – Determination of residual stresses by neutron diffraction.
At pulsed sources, each pulse provides a diffraction profile across a very large range of lattice spacings. As a fixed scattering angle is used, most instruments at spallation sources use radial focussing collimation that allows neutrons to be detected over a wider solid angle than would be possible using a traditional slit, yet ensuring that all collected neutrons come from the defined sampling volume. The signal from the individual elements of the focussing collimator is combined by software taking into account the slightly different diffraction angles. Multiple radial collimators are often used to enable more than one Q (strain) direction to be measured simultaneously. A typical diffractometer used for strain measurement in two perpendicular directions simultaneously at a pulsed spallation source is shown in Figure 4. A typical diffraction spectrum from such an instrument is shown in Figure 5 which also shows the result of a Rietveld profile refinement where a crystallographic model of the structure is fitted to the diffraction data using a least squares analysis.
3.5 Stress determination
Stress and elastic strain are second rank tensors which are related through the elastic constants of a solid. Since neutron diffraction can be used to determine the elastic strain within a defined volume in a crystalline solid, it is possible to calculate the stress in that volume provided the relevant material elastic constants are known. Determination of the full strain tensor requires measurements of the elastic strain along at least six independent directions. If the principal strain directions within the body are known this can be reduced to three orthogonal directions. For plane stress or plane strain conditions, a further reduction to two directions is possible. For uni-axial loading measurement along one direction is sufficient.
The need to measure the strain in a given volume along a number of directions leads to the requirement that the specimen is accurately positioned with respect to the neutron beam collimation and the detectors. This is usually accomplished by mounting the specimen on motorised linear translation and rotation tables. By sequentially moving the specimen through this volume, which is fixed in space by the intersection of the incident beam and detector collimation, the spatial variation in elastic strain and, following measurement in other directions, stress can be mapped within a specimen or component.
4 Calibration procedure
A number of steps shall be carried out prior to experimental measurements being made on a sample.
4.1 Calibration of diffractometer
It is necessary to align and calibrate the diffractometer. At a monochromatic source, the detector angular response should be calibrated. At a time-of-flight source, the combined flight path and detector angular response should be calibrated. In both cases this is done using a typically stress-free sample with a known and well defined lattice parameter, typically silicon, ceria or alumina. These materials are chosen because they scatter neutrons well, and produce sharp diffraction peaks since they have small intrinsic sample peak widths. At a time-of-flight source it is also necessary to calibrate the detector efficiency as a function of wavelength. One way of doing this is to use a sample which predominantly scatters neutrons by a diffuse process such as vanadium. Quantification of the uncertainty in strain measurement is important. Repeated measurement on a reference sample is a method of quantifying the uncertainty in strain measurement.
4.2 Choice of diffracting plane
For the determination of macroscopic elastic strains, consideration shall be given to intergranular anisotropy. Due to the anisotropy of individual crystallites in a material the different lattice reflections exhibit a range of responses to a macroscopic stress field. In the macroscopically elastic regime these effects are linear. This means that any lattice reflection can be chosen for macroscale stress determination within this region. The stiffness of the plane (i.e. the diffraction elastic constant for the particular lattice plane) will not be the single crystal stiffness, but some polycrystafline average (see also clause 7). In essentially randomly textured materials the KrOner approach is often used [2j. A good approximation to the KrOner value is frequently the average of the Reuss [31 and Voigt [4] values. A description of these approaches is given in [5]. In non-randomly textured materials, a self-consistent approach to stiffness determination can be used, see for example [6].
In the plastic regime, a non-linear elastic strain to stress response can occur with certain diffraction peaks introducing more anisotropy between the behaviour of different crystallographic lattice planes. When determining macroscale stresses it may be desirable to minimise such effects, in others cases they may be the subject of study. Therefore for various classes of symmetry materials, advice is given as to the most appropriate lattice reflections to choose to provide small intergranular strains’, and therefore good representations of the macroscale stress field. In general a linear elastic strain response with macroscopic applied stress is desired for the measurement plane when macrostresses are to be determined.