NXstress

Status:

application definition, extends NXobject

Description:

Application definition for stress and strain analysis of crystalline material defined by the EASI-STRESS consortium.

When a crystal is loaded (applied or residual stress) its crystallographic parameters change.

Stress and strain analysis calculates deformation (strain) and the associated force (stress) from diffraction data.

This application definition essentially standardizes the result of diffraction pattern analysis from different types of diffraction experiments for the purpose of stress and strain analysis. The analysis is typically some form of diffraction peak indexing and fitting. The experiments are for example

  • energy-dispersive X-ray powder diffraction

  • angular-dispersive X-ray powder diffraction

  • angular-dispersive neutron powder diffraction

  • time-of-flight (TOF) neutron powder diffraction.

Sample and detector positions can be defined with NXtransformations. If you don’t specify the direction of gravity and the direction of the beam then the standard NeXus Coordinate System is used.

Symbols:

nX: Number of diffractogram channels.

nD: Number of diffractograms. For example the number of energy-dispersive detectors or the number of azimuthal sections in an area detector.

nPeaks: Number of reflections.

xUnit: Diffractogram X units.

yUnit: Diffractogram Y units.

cUnit: Converted diffractogram X units (could be the same as xUnit).

Groups cited:

NXcollection, NXdata, NXdetector, NXentry, NXinstrument, NXnote, NXprocess, NXsample, NXtransformations

Structure:

ENTRY: (required) NXentry

definition: (required) NX_CHAR

Official NeXus NXDL schema to which this file conforms

Obligatory value: NXstress

title: (optional) NX_CHAR

Extended title for the entry.

experiment_identifier: (recommended) NX_CHAR

Unique identifier for the experiment as defined by the facility (e.g. DOI, proposal id, …).

experiment_description: (optional) NX_CHAR

Brief summary of the experiment, including key objectives.

collection_identifier: (optional) NX_CHAR

User or Data Acquisition defined identifier from which the content of this application definition is derived.

collection_description: (optional) NX_CHAR

Brief summary of the collection, including grouping criteria.

INSTRUMENT: (optional) NXinstrument

name: (required) NX_CHAR

Name of the instrument.

@short_name: (optional) NX_CHAR

Short name for the instrument, perhaps the acronym.

DETECTOR: (optional) NXdetector

Zero or more of these groups describe the detectors used in the experiment.

description: (required) NX_CHAR

name/manufacturer/model/etc. information

distance: (required) NX_NUMBER {units=NX_LENGTH}

This is the distance to the previous component in the instrument; most often the sample. The usage depends on the nature of the detector: Most often it is the distance of the detector assembly. But there are irregular detectors. In this case the distance must be specified for each detector pixel.

depends_on: (optional) NX_CHAR

The axis on which the detector position depends may be stored anywhere, but is normally stored in the NXtransformations group within the NXdetector group.

TRANSFORMATIONS: (optional) NXtransformations

This is the recommended location for detector goniometer and other related axes.

SAMPLE: (optional) NXsample

depends_on: (optional) NX_CHAR

The axis on which the sample position depends may be stored anywhere, but is normally stored in the NXtransformations group within the NXsample group.

gauge_volume: (optional) NXcollection

Defines the probed volume in the sample

primary_vertical: (optional) NX_NUMBER

Defines the primary beam size in the vertical direction.

primary_horizontal: (optional) NX_NUMBER

Defines the primary beam size in the horizontal direction.

secondary_horizontal: (optional) NX_NUMBER

Defines the depth of the probe volume as determined by detector collimators.

TRANSFORMATIONS: (optional) NXtransformations

This is the recommended location for sample goniometer and other related axes.

PROCESS: (optional) NXprocess

Zero or more groups to describe the data processing steps to obtain the content of this application definition.

notes: (optional) NXnote

User description of the data acquisitions. A description of data analysis goes in the NXprocess group(s).

peaks: (required) NXdata

This group contains all diffraction peak parameters that could be needed for stress-strain calculations. These parameters are derived from peak_parameters and additional metadata.

h: (required) NX_INT (Rank: 1, Dimensions: [nPeaks]) {units=NX_UNITLESS}

First Miller index.

k: (required) NX_INT (Rank: 1, Dimensions: [nPeaks]) {units=NX_UNITLESS}

Second Miller index.

l: (required) NX_INT (Rank: 1, Dimensions: [nPeaks]) {units=NX_UNITLESS}

Third Miller index.

lattice: (optional) NX_CHAR (Rank: 1, Dimensions: [nPeaks])

Crystal lattice systems (cubic, hexagonal, …)

space_group: (optional) NX_CHAR (Rank: 1, Dimensions: [nPeaks])

Crystallographic space group (Fm3m, Im3m, …)

phase_name: (optional) NX_CHAR (Rank: 1, Dimensions: [nPeaks])

Name of the crystallographic phase (hematite, goethite, a-Al2O3, …).

qx: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_DIMENSIONLESS}

First component of the normalized scattering vector Q in the sample reference frame. The sample reference frame is defined by the sample transformations.

qy: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_DIMENSIONLESS}

Second component of the normalized scattering vector Q in the sample reference frame. The sample reference frame is defined by the sample transformations.

qz: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_DIMENSIONLESS}

Third component of the normalized scattering vector Q in the sample reference frame. The sample reference frame is defined by the sample transformations.

center: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=cUnit}

Diffraction peak position.

center_errors: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=cUnit}

Uncentrainties on center.

center_type: (required) NX_CHAR

The space in which center is defined. Note that cUnit must correspond. For example if center_type=”two-theta” then cUnit must have an angle unit (degrees, rad, …).

Any of these values:

  • two-theta

  • energy

  • momentum-transfer

  • d-spacing

  • channel

sx: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_LENGTH}

First component of the sample position in the sample reference frame. The sample reference frame is defined by the sample transformations.

sy: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_LENGTH}

First component of the sample position in the sample reference frame. The sample reference frame is defined by the sample transformations.

sz: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_LENGTH}

First component of the sample position in the sample reference frame. The sample reference frame is defined by the sample transformations.

peak_parameters: (optional) NXdata

This group contains all diffraction peak fit parameters. This information is not required for stress-strain calculations. Note that as in any NXdata group, each field can have uncertainties associated to them (e.g. center_errors would be the uncertainties on the peak center).

title: (required) NX_CHAR

Diffraction peak profile.

Any of these values:

  • gaussian

  • lorentzian

  • voigt

  • pseudo-voigt

  • split pseudo-voigt

  • pearson VII

center: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=xUnit}

Diffraction peak position.

center_type: (required) NX_CHAR

The space in which the peak profile is measured. Note that xUnit must correspond. For example if center_type=”two-theta” then xUnit must be an angle unit (degrees, rad, …).

Any of these values:

  • two-theta

  • energy

  • momentum-transfer

  • d-spacing

  • channel

area: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=yUnit}

Diffraction peak area (not including the background).

height: (optional) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=yUnit}

Diffraction peak height (not including the background).

fwhm: (required) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=xUnit}

Diffraction peak full width at half maximum.

fwhm_left: (optional) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=xUnit}

Left-side FWHM for split profiles.

fwhm_right: (optional) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=xUnit}

Right-side FWHM for split profiles.

form_factor: (optional) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_DIMENSIONLESS}

  • Voigt or Pseudo-Voigt: Lorentzian fraction

  • Pearson VII: decay parameter

  • Other profiles: not applicable

azimuth: (optional) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_ANGLE}

Angle that define the position of the integrated sector in the diffraction cone for angular-dispersive diffraction or the position of the detector for energy-dispersive diffraction.

background_parameters: (optional) NXdata

This group contains all background fit parameters. This information is not required for stress-strain caluclations.

title: (optional) NX_CHAR

Diffraction background profile. Required when background parameters fields are present. For example: linear, 5-degree polynomial

A: (optional) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=NX_DIMENSIONLESS}

Background parameter(s). For example a second-degree polynomial will have fields A0, A1 and A2.

background_area: (optional) NX_NUMBER (Rank: 1, Dimensions: [nPeaks]) {units=yUnit}

The background area integrated over a confidence interval around the center (0.95 by default).

background_area_interval: (optional) NX_DIMENSIONLESS

Confidence interval from which the background counts are integrated. For example 0.95 means that the background is integrated over the range in which the integrated peak area is 95% of the total peak area.

diffractogram: (optional) NXdata

Diffractogram with fit results in peak_parameters and background_parameters. This information is not required for stress-strain caluclations.

@axes: (required) NX_CHAR

List of the two axes field names to be used by default.

@signal: (required) NX_CHAR

Default field name to be plotted.

Obligatory value: data

@auxiliary_signals: (required) NX_CHAR

List of additional field names to be plotted.

DAXIS: (optional) NX_CHAR (Rank: 1, Dimensions: [nD])

One or more fields that contain the values for the nD dimension. For example the azimuthal positions of different energy-dispersive detectors or the average azimuth of different azimuthal sections on an area detector.

XAXIS: (required) NX_NUMBER (Rank: 1, Dimensions: [nX]) {units=xUnit}

One or more fields that contain the values for the nX dimension. For example: MCA channels, scattering angle \(2\theta\) in degrees, scattering vector length q in \(\mathrm{nm}^{-1}\), …

diffractogram: (required) NX_NUMBER (Rank: 2, Dimensions: [nD, nX]) {units=yUnit}

Diffractogram counts (default signal)

@interpretation: (required) NX_CHAR

Obligatory value: spectrum

fit: (required) NX_NUMBER (Rank: 2, Dimensions: [nD, nX])

Diffractogram fit counts (auxiliary signal).

@interpretation: (required) NX_CHAR

Obligatory value: spectrum

background: (optional) NX_NUMBER (Rank: 2, Dimensions: [nD, nX])

Diffractogram background counts (auxiliary signal).

@interpretation: (required) NX_CHAR

Obligatory value: spectrum

residuals: (optional) NX_NUMBER (Rank: 2, Dimensions: [nD, nX])

Difference between diffractogram and fit (auxiliary signal).

@interpretation: (required) NX_CHAR

Obligatory value: spectrum

Hypertext Anchors

List of hypertext anchors for all groups, fields, attributes, and links defined in this class.

NXDL Source:

https://github.com/nexusformat/definitions/blob/main/contributed_definitions/NXstress.nxdl.xml