GB/T 26140-2023 Non-destructive testing - Standard test method for determining residual stresses by neutron diffraction
WARNING - This document does not purport to address the safety concerns, if any, associated with its use. It is the responsibility of the user of this document to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
1 Scope
This document describes the test method for determining residual stresses in polycrystalline materials by neutron diffraction. It is applicable to both homogeneous and inhomogeneous materials including those containing distinct phases.
The principles of the neutron diffraction technique are outlined. Suggestions are provided on:
——the selection of appropriate diffracting lattice planes on which measurements should be made for different categories of materials;
——the specimen directions in which the measurements should be performed, and
——the volume of material examined in relation to the material grain size and the envisaged stress state.
Procedures are described for accurately positioning and aligning test pieces in a neutron beam and for precisely defining the volume of material sampled for the individual measurements.
The precautions needed for calibrating neutron diffraction instruments are described. Techniques for obtaining a stress-free reference are presented.
The methods of making individual measurements by neutron diffraction are described in detail. Procedures for analysing the results and for determining their statistical relevance are presented. Advice is provided on how to determine reliable estimates of residual stresses from the strain data and on how to estimate the uncertainty in the results.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
EN 13925-3:2015, Non-destructive testing - X-ray diffraction from polycrystalline and amorphous materials - Part 3: Instruments
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
——ISO Online browsing platform: available at http:// www .iso .org/ obp
——IEC Electropedia: available at http:// www .electropedia .org/
3.1
neutron absorption
neutron capture by an atomic nucleus
Note: A table of nuclear capture cross-sections can be found in Reference.
3.2
alignment
adjustment of the specimen position and orientation and also of all the components of the instrument such that measurements can be performed precisely at the desired location in the specimen
3.3
anisotropy
dependence of material properties on the direction with respect to the sample
3.4
attenuation
reduction of the neutron beam intensity
Note: Attenuation can be calculated by using the so-called “total neutron cross-section”, which comprises neutron absorption (3.1) and different nuclear scattering processes. The attenuation length is the distance within the material for which the primary neutron beam intensity is reduced by 1/e.
3.5
background
intensity considered not belonging to the diffraction (3.13) signal
Note: Background dependence on the scattering angle or time-of-flight (3.34) is not uncommon and can have an influence on the peak position (3.11) resulting from data analysis.
3.6
beam-defining optics
arrangement of devices used to define the properties of a neutron beam such as the wavelength and intensity distributions, divergence and shape
Note: These include devices such as apertures, slits, collimators, monochromators and mirrors.
3.7
Bragg edge
sharp change in the neutron intensity as a function of the wavelength or monochromator take-off angle corresponding to the condition λ = 2dhkl, where hkl indicates an (hkl) diffracting lattice plane of the material under investigation
3.8
Bragg peak
intensity distribution of the neutron beam diffracted by a specific (hkl) lattice plane
3.9
peak height
maximum number of neutron counts of the Bragg peak (3.8) above the background (3.5)
3.10
peak function
analytical expression to describe the shape of the Bragg peak (3.8)
3.11
peak position
single value describing the position of a Bragg peak (3.8)
Note: The peak position is the determining quantity to calculate the strain.
3.12
peak intensity
integrated intensity
area under the diffraction (3.13) peak above the background (3.5), normally calculated from the associated fitted parameters of a selected peak function (3.10) and a background function
3.13
diffraction
scattering arising from coherent interference phenomena
3.14
diffraction elastic constants
Ehkl
νhkl
elastic constants associated with diffraction (3.13) from individual (hkl) lattice planes for a
polycrystalline material
3.15
diffraction pattern
intensity distribution of neutrons diffracted from a crystalline material over the available wavelength, time-of-flight (3.34) and/or diffraction (3.13) angle ranges
3.16
full width at half maximum; FWHM
width of the Bragg peak (3.8) at half the peak height (3.9) above the background (3.5)
3.17
full pattern analysis
determination of the crystallographic structure and/or strain from a measured (multi-peak) diffraction pattern (3.15) of a polycrystalline material
Note: In general, the full pattern analysis is termed after the method used (e.g. Rietveld refinement). See also single peak analysis (3.31).
3.18
gauge volume
volume from which information is obtained
3.19
lattice parameters
linear and angular dimensions of the crystallographic unit cell
3.20
lattice spacing
d-spacing
lattice plane spacing
distance between adjacent parallel crystallographic lattice planes
3.21
Type I stress
macrostress
stress that self-equilibrates over a length scale comparable to the structure or component, thereby spanning multiple grains and/or phases
3.22
Type II stress
stress that self-equilibrates over a length scale comparable to the grain size
Note: Stresses of Type II and Type III are collectively known as microstresses.
3.23
Type III stress
stress that self-equilibrates over a length scale smaller than the grain size
Note: Stresses of Type II and Type III are collectively known as microstresses.
3.24
monochromatic instrument
instrument employing a narrow band of neutron energies (wavelengths)
3.25
monochromatic neutron beam
monochromatic beam
neutron beam with narrow band of neutron energies (wavelengths)
3.26
orientation distribution function
quantitative description of the crystallographic texture (3.32)
