ISO/TTA 3:2001

TECHNOLOGY ISO/TTA 3
TRENDS
ASSESSMENT
First edition
2001-09-15

Polycrystalline materials — Determination
of residual stresses by neutron diffraction
Matériaux polycristallins — Détermination des contraintes résiduelles par
diffraction neutronique




Reference number
ISO/TTA 3:2001(E)
©
ISO 2001

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ISO/TTA 3:2001(E)
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ISO/TTA 3:2001(E)
ISO/TTA

Page
FOREWORD . iv
EXECUTIVE SUMMARY . v
BACKGROUND . vi
REFERENCES . ix
Participants in VAMAS TWA 20. x
Participants in RESTAND . xi
Introduction. xii
1. Scope. 1
2. Symbols and abbreviations. 2
3. Summary of method. 4
4. Calibration procedure. 10
5. Materials characterisation. 16
6. Recording and measurement procedures . 18
7. Calculation of stress. 20
8. Reliability of results. 25
9. Reporting. 26
ANNEX A – Recording and measurement procedures . 28
ANNEX B – Determination of uncertainties in a measurand. 37
ANNEX C – Recommendations for a standard strain scanner base-plate. 42
Bibliography . 48



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ISO/TTA 3:2001(E)
FOREWORD

The International Organisation for Standardisation (ISO) is a worldwide federation of national
standards bodies. It is responsible for preparing International Standards through ISO
technical committees in association with other international organisations and national
governmental and non-governmental agencies.

The Versailles Project on Advanced Materials and Standards (VAMAS) supports trade in
high technology products through international collaborative projects aimed at providing the
technical basis for drafting codes of practice and specifications for advanced materials. The
scope of the collaboration embraces all agreed aspects of enabling science and technology
which are required as a precursor to the drafting of standards for advanced materials. The
VAMAS activity emphasises collaboration on pre-standards measurement research,
intercomparison of test results, and consolidation of existing views on priorities for
standardisation action.

ISO Technology Trend Assessment (ISO/TTA) documents are published under a
memorandum of understanding concluded between ISO and VAMAS. They enable the
technical innovations and developments emerging from a VAMAS activity to be published at
an early stage prior to their incorporation into a Standard. Whilst ISO/TTAs are not
Standards, it is intended that they will be able to be used as a basis for standards development
in the future by the various existing standards agencies.

This particular ISO/TTA reports the findings of a comprehensive ‘round-robin’ study which
was carried out by VAMAS Technical Working Area (TWA) 20 to investigate the feasibility
of measuring residual stresses in crystalline materials by neutron diffraction. It was supported
by a European (EU) project RESTAND (Residual Stress Standard using Neutron Diffraction)
aimed at demonstrating that the techniques developed can be applied to real components.


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ISO/TTA 3:2001(E)
EXECUTIVE SUMMARY

Neutron diffraction is a relatively new method for determining residual (and applied) stresses
in crystalline materials. It is similar to the X-ray technique for surface determinations.
However because neutrons are not charged, neutron diffraction can be used to obtain residual
stresses non-destructively to a depth of several centimetres in most materials of practical
interest. No standard is currently available for making these measurements.

An international project, under the auspices of VAMAS (Versailles Agreement on Advanced
Materials and Standards), Technical Working Area 20 (TWA 20) was initiated in January
1996 to carry out the under-pinning research necessary to develop a standard. The
investigation involved most of the neutron sources worldwide which are capable of making
the measurements. A series of ‘round-robin’ specimens including a shrink-fit aluminium
alloy ring and plug assembly, a ceramic matrix composite, a nickel alloy shot-peened plate
and a ferritic steel weldment were examined. This study was supported by a European (EU)
project RESTAND (Residual Stress Standard using Neutron Diffraction) which was started in
December 1997 to demonstrate the usefulness of the technique to a range of practical
applications and to develop confidence in the method for industry.

This document gives the background to the VAMAS TWA 20 and RESTAND projects. It
outlines the main findings and indicates the precautions that are required to achieve accurate
positioning and alignment of specimens (and components) in the neutron beam and the
analysis required to obtain reliable results. It also shows that special attention is needed in
dealing with near-surface measurements because of surface aberration effects. It is
demonstrated that, provided the recommended procedures are followed, a positional tolerance
-4
of ± 0,1 mm can be achieved with an accuracy in strain of ±10 to give a resolution in
residual stress of ± 7 to 20 MPa in most materials of practical interest.


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ISO/TTA 3:2001(E)
BACKGROUND

Neutron diffraction is a technique that can be applied for determining residual (and applied)
stresses in crystalline materials [1-3]. With the method, elastic strain is measured and stress
calculated using the elastic properties of the material.  The depth to which these
measurements can be made non-destructively within specimens or components depends on
their size and shape. It is also dependent on the neutron scattering and absorption
characteristics of the materials of which they are made. Typically the depths to which these
measurements can be obtained are up to a few centimetres in most materials of practical
interest.

No standard or code of practice is available for making residual stress measurements by
neutron diffraction. As a consequence VAMAS TWA 20 (Versailles Agreement on
Advanced Materials and Standards, Technical Working Area 20) was set up in January 1996
with the aim of carrying out the under-pinning research necessary for preparing a standard.
The specific objectives of TWA 20 were to:

- establish accurate and reliable procedures for making non-destructive residual
stress measurements by neutron diffraction,
- examine a selection of samples in which residual stresses had been introduced
by different techniques,
- conduct inter-laboratory comparisons to establish reproducibility,
- assemble the necessary information for preparing a draft standard for making the
measurements.

A European (EU) project RESTAND (Residual Stress Standard using Neutron Diffraction)
was also started in December 1997 to demonstrate the application of the technique to
industrial situations. This ISO/TTA presents the findings of these two investigations. It
includes a draft procedure which can be used for making the measurements until a standard
has been developed. Relevant committees concerned with the preparation of this standard are
ASTM E 28.13, CEN/TC 138 AHG 7 and ISO/TC 135/SC 5.

Both the VAMAS TWA 20 and RESTAND investigations involved a series of ‘round-robin’
experiments. These were carried out by making measurements on the same samples at a
number of neutron sources. Most of the sources world-wide that are capable of making the
measurements participated. A list of the participants contributing to VAMAS TWA 20 is
given in Table 1. Those participating in RESTAND are included in Table 2.

For the VAMAS TWA 20 project, four types of ‘round-robin’ sample were examined. These
were a shrink-fit aluminium alloy ring and plug, a ceramic matrix composite, a nickel alloy
shot-peened plate and a ferritic steel weldment. These examples were chosen to establish the
range of application of the technique. They were investigated in the order mentioned. In each
case a protocol was specified which each participating group was required to follow.
Measurements were made at each neutron source independently. The results were then
collected together and statistical analyses carried out to determine the reliability of the

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ISO/TTA 3:2001(E)
measurements. Data were collected on steady state instruments which used a monochromatic
beam of neutrons and also on time-of-flight instruments which employed a pulsed
polychromatic beam. With a monochromatic source, measurements are made on specific
crystallographic planes; with the time-of-flight method the entire spectrum can be analysed
using profile refinement [4] to obtain strains. It has been found that comparable results are
obtained from each type of instrument.

The ring and plug assembly was the first specimen to be measured because residual stresses
had been introduced into it elastically, a discontinuity is obtained at the ring/plug interface
and comparisons can be made with theory. The ceramic matrix composite was chosen to
determine the feasibility of making measurements in a dual phase system containing fine
fibres. The shot-peened plate was selected to establish whether steep stress gradients (of the
order of 2000 MPa/mm) can be measured close to external surfaces and the ferritic weldment
to determine whether reliable results can be obtained through regions of different
microstructure (and possibly chemical composition).

The studies on the ring and plug assembly established the basic procedures that should be
followed. The findings are contained in VAMAS report no. 38 [5]. It has been found that it
is essential to ensure accurate positioning and alignment of a specimen in the neutron beam
for reliable results to be obtained. A suitable shape and size of ‘gauge volume’ over which
individual measurements should be made to achieve adequate resolution in regions of strain
3
gradients has been identified. It is recommended that a minimum size of 8 mm is adopted to
encompass sufficient grains and to give neutron counting times which are not too long. In
some cases cube-shaped sampling volumes are required. When there is no strain gradient in
one direction a ‘match-stick’ shape, with the axis aligned along the direction of zero strain
gradient, can be employed. If there is no strain gradient in two directions a plate-shaped
volume can be used. For the shot-peened plate it has been established that steep stress
3
gradients approaching 2000 MPa/mm can be measured with a 1x1x10 mm ‘match-stick’
sampling volume with its axis aligned in the plane of the plate. In regions away from
3
interfaces and steep gradients a 3x3x3 mm volume can be used. In the absence of stress
gradients, such as may exist in the presence of uniform applied loads, the entire specimen
may be bathed.

From statistical analysis of the data, it has been established in most cases, that a positional
accuracy with a standard deviation of 0,1 mm can be achieved provided proper alignment
procedures are adopted. It has also been ascertained that strains can be recorded away from
-4
surfaces to a tolerance of ±10 corresponding to a stress of ± 7 to 20 MPa in most materials.
Close to surfaces (or interfaces) and regions of variable microstructure, greater errors can be
expected. Where the volume from which neutrons are being counted is traversed through a
surface, there is the possibility that compensation is needed for the change in shape and size
of the volume of material being sampled affecting the position at which the strain measured
should be recorded. This is particularly important in the presence of steep stress gradients in
highly absorbing materials where there are significant differences in neutron path lengths
through different regions of the volume of material being examined. In this case, it is
necessary to establish the neutron intensity weighted centroid of the material cross-section
being measured and record the strain at this location. In traversing regions of variable
microstructure and/or chemical composition it may be required to make allowance for a
change in stress-free crystal lattice spacing with position.


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ISO/TTA 3:2001(E)
A main aim of the RESTAND project was to develop industrial confidence in the application
of the neutron diffraction technique for residual stress measurement. Measurements have
been made on felt and fibre-reinforced composites for heat insulation and thermal shock
resistance, on deep-rolled crankshafts to represent complex shapes, a quenched component
and through fusion, linear-friction and friction-stir welds for power generation and aerospace
applications. For composites, with fibre and matrices of similar composition, it has been
found that it is sometimes necessary to separate out the effects of overlapping peaks. With
complex shapes such as crankshafts, care is needed to avoid orientations which involve long
neutron path lengths to minimise attenuation.

Similarly curved surfaces can exaggerate surface aberrations. In all cases, it has been
determined when using a monochromatic beam of neutrons, that measurements should be
restricted to those planes which give high peaks close to a diffraction angle of 90° and which
represent bulk material behaviour. It is also recommended that a check is made for force and
moment equilibrium, where possible, to provide additional confidence in the results.

The remainder of this document contains the proposed protocol for making the measurements.
It includes the scope of the method, an outline of the technique, the calibration and
measurement procedures recommended, and details of how the strain data should be analysed
to calculate stresses and establish the reliability of the results obtained.


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ISO/TTA 3:2001(E)
REFERENCES

[1] Allen, A, Andreani, C, Hutchings, M.T. and Windsor, C.G. ‘Measurements of internal
stress within bulk materials using neutron diffraction’ NDT International 14, 1981.

[2] Stacey, A., MacGillivray, H.J., Webster, G.A., Webster, P.J. and Ziebeck K.R.A.
‘Measurement of residual stresses by neutron diffraction’ J. Strain Analysis, I.MechE 20,
1985.

[3] Hutchings, M.T. and Kravitz, A.D. (Eds) ‘Measurement of residual and applied stress
using neutron diffraction’ Proc. NATO ARW Oxford, March 1991, Kluwer, Netherlands
1992.

[4] Rietveld, H.M. ‘A profile refinement method for nuclear and magnetic structures’ J. App.
Crystal 2, 1969, 65-71.

[5] Webster, G.A. (Ed) ‘Neutron diffraction measurements of residual stress in a shrink-fit
ring and plug’ VAMAS report no. 38, Jan. 2000.


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ISO/TTA 3:2001(E)
Participants in VAMAS TWA 20
G. A. Webster (Chairman) Imperial College, London, UK.
R. C. Wimpory (Secretary) Imperial College, London, UK.

P. Brand NIST, Gaithersburg, USA.
M. A. M. Bourke LANSCE, Los Alamos, USA.
M. Ceretti LLB, Saclay, France.
M. R. Daymond ISIS, Rutherford Appleton Laboratory, UK.
L. Edwards Open University, Milton Keynes, UK.
J. H. Fox CRL, Chalk River, Canada.
T. Gnäupel-Herold University of Maryland / NIST, Gaithersburg, USA.
T. M. Holden CRL, Chalk River, Canada.
C. R. Hubbard ORNL, Oak Ridge, USA.
D. J. Hughes University of Salford, Salford, UK.
M. T. Hutchings AEA Technology plc, Abingdon, UK.
M. W. Johnson ISIS, Rutherford Appleton Laboratory, UK.
A. Krawitz University of Missouri, Missouri, USA.
A. Lodini LLB, Saclay, France.
T. Lorentzen Riso National Laboratory, Roskilde, Denmark.
P. Lukas NPI, Prague, Czech Republic.
G. Mills University of Salford, Salford, UK.
N. Minakawa JAERI, Japan.
C. Ohms JRC, Petten, Netherlands.
M. Ono KURSS, Kyoto University, Japan.
J. Pang University of Manchester/UMIST, Manchester, UK.
E. A. Payzant ORNL, Oak Ridge, USA.
R. L. Peng Studsvik NRL, Studsvik, Uppsala University, Sweden.
T. Pirling ILL, Grenoble, France.
H. J. Prask NIST, Gaithersburg, USA.
H. G. Priesmeyer LANP, GKSS, Geesthacht, Germany.
A. Pyzalla HMI, Berlin, Germany.
W. Reimers HMI, Berlin, Germany.
R. B. Rogge CRL, Chalk River, Canada.
J. Root CRL, Chalk River, Canada.
S. Spooner ORNL, Oak Ridge, USA.
A. Venter Safari Atomic Energy Corporation, Pretoria, South Africa.
D. Q. Wang Studsvik NRL, Studsvik, Uppsala University, Sweden.
X. L. Wang ORNL, Oak Ridge, USA.
P. J. Webster University of Salford, Salford, UK.
R. A. Winholtz University of Missouri, Missouri, USA.
P. J. Withers University of Manchester/UMIST, Manchester, UK.
J. S. Wright ISIS, Rutherford Appleton Laboratory, UK.
T. Youtsos JRC, Petten, Netherlands.


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ISO/TTA 3:2001(E)
Participants in RESTAND
T. Youtsos (Co-ordinator) JRC, Petten, Netherlands.
C. Ohms (Secretary) JRC, Petten, Netherlands.

C. Achmus Volkswagen AG, Wolfsburg, Germany.
K. M. Beinborn Sintec Keramik GmbH, Halblech (Buching), Germany.
N. Bonner Rolls Royce, Derby, UK.
R. Burguete BAE Systems, Bristol, UK.
E. Calvet European Commission, Brussels, Belguim.
M. R. Daymond ISIS, Rutherford Appleton Laboratory, UK.
L. Edwards Open University, Milton Keynes, UK.
A. Hewat ILL, Grenoble, France.
D. J. Hughes University of Salford, Salford, UK.
M. T. Hutchings AEA Technology plc, Abingdon, UK.
M.W. Johnson ISIS, Rutherford Appleton Laboratory, UK.
T. Lorentzen Riso National Laboratory, Roskilde, Denmark.
P. Lukas NPI, Prague, Czech Republic.
G. Mills University of Salford, Salford, UK.
E. Oliver University of Manchester/UMIST, Manchester, UK.
J. Pang University of Manchester/UMIST, Manchester, UK.
R. L. Peng Studsvik NRL, Studsvik, Uppsala University, Sweden.
T. Pirling ILL, Grenoble, France.
A. Pyzalla HMI, Berlin, Germany.
W. Reimers HMI, Berlin, Germany.
P. Schmidt Schunk Kohlenstofftechnik GmbH, Heuchelheim, Germany.
G. A. Webster Imperial College, London, UK.
P. J. Webster University of Salford, Salford, UK.
R. Weiss Schunk Kohlenstofftechnik GmbH, Heuchelheim, Germany
R. C. Wimpory Imperial College, London, UK.
P. Withers University of Manchester/UMIST, Manchester, UK.
J. S. Wright ISIS, Rutherford Appleton Laboratory, UK.





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ISO/TTA 3:2001(E)
Introduction

Neutron diffraction is a non-destructive method for determining residual stresses in crystalline
materials. It can also be used for establishing applied stresses. The procedure can be
employed for determining stresses within the interior of materials and adjacent to surfaces. It
requires test pieces to be transported to a neutron source. Measurements of the lattice spacing
or lattice parameter are obtained which are then converted to strain and stress.







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TECHNOLOGY TRENDS ASSESSMENT ISO/TTA 3:2001(E)

POLYCRYSTALLINE MATERIALS - DETERMINATION OF RESIDUAL
STRESSES BY NEUTRON DIFFRACTION



WARNING - This Technology Trends Assessment does not purport to address all of the
safety concerns, if any, associated with its use. It is the responsibility of the user of the
document to establish appropriate safety and health practices and determine the
applicability of regulatory limitations prior to use.



1. Scope

1.1 This Technology Trends Assessment specifies a test method for determining residual
stresses in polycrystalline materials by neutron diffraction. It may be applied to
homogeneous and inhomogeneous materials and to test pieces containing distinct
phases.

1.2 The principles of the neutron diffraction technique are outlined. Advice is provided
on the crystalline planes on which measurements should be made for different
categories of materials. Guidance is provided about the directions in which the
measurements should be obtained and of the volume of material which should be
examined, in relation to material grain size and the stress state envisaged, when
making measurements.

1.3 Procedures are described for accurately positioning and aligning test pieces in a
neutron beam and for precisely defining the volume of material that is sampled when
individual measurements are being made.

1.4 The precautions needed for calibrating the neutron diffraction facilities are described.
Techniques for obtaining a stress-free reference are presented.

1.5 The methods of determining individual elastic strains 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 (or applied) stress from the strain data and of how to estimate the
uncertainty in the results.



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ISO/TTA 3:2001(E)
2. Symbols and abbreviations
2.1 Symbols

Symbol Definition Units

a,b,c Lattice parameter nm
The value of the lengths of the sides of a unit cell
a , b , c Strain-free lattice parameter nm
0 0 0
B Background -
The value of counts that constitutes the height of the
background on a neutron detector
d Lattice spacing nm
The perpendicular distance between adjacent parallel lattice
planes (crystallographic planes), also called d-spacing
d Strain-free lattice spacing nm
0
E Elastic modulus GPa
E Diffraction elastic modulus GPa
hkl
hkl, hkil Miller indices of crystallographic plane -
H Peak height nm
This is the height of the Bragg peak above that of the
background
I Integrated neutron intensity above background -
-1
k , k Incident and diffracted neutron wave-vectors nm
i d
L Neutron attenuation length mm
N Number of measurements -
N Total number of neutrons counted -
n
-1
Q Scattering vector (k – k) nm
d i
t Time-of-flight of neutrons from source to detector µs
u, u Standard and combined standard uncertainty -
c
u , u Uncertainty in d and d , respectively nm
d d0 0
u , u Uncertainty in λ and θ, respectively nm
λ θ
s Measured standard deviation in strain -
ε
-1
S , S Elastic compliance constants MPa
1 2
w Slit width mm
x,y,z Coordinate axes (relevant to sample) -
Y Measurand, the quantity being measured. -
∆a, ∆b, ∆c Change in lattice parameter nm
∆d Change in lattice spacing nm
∆t Change in time of flight of neutrons from source to detector µs
∆θ Change in Bragg angle degrees, rads
∆λ Change in wavelength of neutrons nm
ε Strain -
ε Components of strain tensor -
ij
λ Wavelength of neutrons nm
ν Poisson’s ratio -


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ISO/TTA 3:2001(E)
Ω Angular rotation about reference point degrees
The angular motion of the goniometer of the diffraction
instrument in the scattering plane
σ Stress MPa
σ Components of stress tensor MPa
ij
σ Yield stress MPa
Y
θ Bragg angle degrees, rads
θ Strain-free Bragg angle degrees, rads
0
θ, φ, ψ, ω Angular rotations degrees, rads

Subscripts

a,c Relevant to lattice parameter
hkl, hkil Relevant to crystallographic plane
xx, yy, zz Relative to Cartesian co-ordinate axis
φ, ψ Relevant to strain axis
0 Strain-free reference
ref Reference value

Reference point
Location in space at which all measurements are made. This will normally correspond
with a point on the axis of rotation of the diffractometer.

2.2 Abbreviations

DED Detector to reference point distance mm
DEC Diffraction elastic constant GPa
DSD Detector slit to reference point distance mm
The distance from the centre of the exit slit
(or equivalent optic) to the reference point.
DSH Detector slit height mm
The height of the exit slit (or equivalent optic).
DSW Detector slit width mm
The width of the exit slit (or equivalent optic).
FWHM Full width at half maximum
degrees, µs, nm
The width of the diffraction peak at half the
maximum height above the background
ISD Incident slit to reference point distance mm
ISH Incident slit height mm
ISW Incident slit width mm
PSD Position sensitive detector -
TOF Time-of-flight
µs
The time-of-flight of neutrons from source to

detection

3
IGV Instrumental gauge volume
mm
3
NGV Nominal gauge volume mm
3
SGV Sampled gauge volume
mm
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3 Summary of method

3.1 Outline of Principle – Bragg’s law

Neutron diffraction can be used to measure components of strain directly from
changes in crystal lattice spacing. When illuminated by radiation of wavelength
similar to interplanar spacings, crystalline materials diffract this radiation as
distinctive Bragg peaks. The angle at which any given peak occurs can be calculated
using Bragg’s law of diffraction,

2d sinθ = λ (1)

hkl hkl

where λ is the wavelength of the radiation, d is the lattice plane spacing responsible
hkl
for the Bragg peak and θ is the Bragg angle of this diffraction peak. The peak will
hkl
be observed at an angle of 2θ from the incident beam, as shown schematically in
hkl
Figure 1.


3.2 Neutron sources

Neutrons can be generated by fission or spallation: the former is predominantly
employed in reactor and the latter in spallation sources. In both cases the neutrons
produced are moderated to bring their energies to the thermal (u 100meV) range. At
reactor sources, a continuous monochromatic beam of neutrons is usually produced by
using a crystal monochromator to select a given neutron wavelength from a “white”
polychromatic neutron beam by Bragg diffraction. At spallation sources, the neutron
beam usually consists of a series of short pulses containing a spread of wavelengths
that arrive at the sample over a very small period of time (of order 20 ms). The energy
(and th
...