1.1. BACKGROUND
Nuclear technologies such as fission and fusion reactors, including associated waste storage and disposal, rely on the availability not only of nuclear fuels but also of advanced structural materials. Various techniques have been developed to measure material properties at the microscopic and macroscopic level. Neutron scattering has played an important role in studying the structure and dynamics of condensed matter. The special nature of neutron interaction with matter provides important data which is complementary and supplementary to data gathered through other techniques. The location of hydrogen atoms in the presence of heavy elements, for example, can only be determined by means of neutron diffraction studies. Crystal structures of biological systems, such as amino acids and polypeptides, have been elucidated using single crystal neutron diffraction. The properties of magnetic materials can be studied at the microscopic scale using neutrons and such studies have been useful in both scientific and industrial applications.
The substantial penetration depth and selective absorption of neutrons make them a powerful tool in the non-destructive testing of materials with large and bulky samples. Residual stress formed in a material during manufacturing, welding, utilization or repairs can be investigated by means of neutron diffraction. In fact neutron diffraction is the only non-destructive testing method which can facilitate three dimensional mapping of residual stress in a bulk component. Such studies are important in order to improve the quality of engineering components in production and to optimize design criteria in applications. The technique has applications in nuclear technology such as testing pipes and tubes, weld joints or structures under various conditions representative of those which might be experienced in service. Anisotropies in macroscopic properties such as thermal and electrical conductivities, for instance, of fuel elements, and mechanical properties of materials depend on the textures developed during their preparation or thermal treatment. Such textures can also be studied using neutron diffraction techniques.
The IAEA conducted a Coordinated Research Project (CRP) on the Development and Application of the Techniques of Residual Stress Measurements in Materials (2006-2009). This project relied on the participation of practitioners from highly specialized user facilities from various Member States, including the Czech Republic, Germany, Hungary, India, the Netherlands, Pakistan, Romania, the Russian Federation and South Africa. Various activities were implemented to increase Member State capabilities in the development and applications of residual stress measurements. The project participants initiated and contributed significantly to the preparation of this report, which will certainly be useful for other Member States interested in residual stress measurements using neutron beams.
1.2. PURPOSE AND SCOPE
The CRP objectives, among others, were:
— To optimize neutron beams for residual stress measurement using modern simulation techniques;
— To enhance beam intensity using modern neutron optics;
— To develop and test standardized procedures for the comparison of data from various instruments.
This publication is the main output of the project, and its purpose is to relate guidance on the basic principles, requirements, preparation, design, execution and standardization of residual stress measurements using neutron beams in a single publication. The publication will promote the use of neutron beams in residual stress measurements both at the microscopic and the macroscopic scale; facilitate preparation, design and standardization processes at less well equipped smaller research centres around the world; and offer guidance for young researchers and graduate students new to the field.
1.3. STRUCTURE
This report consists of nine sections including references. Section 2 defines the terms and techniques discussed in the report. Section 3 includes details of experimental techniques, while Section 4 describes associated equipment and instrumentation and their commissioning, calibration and control as well as data acquisition. Section 5 is dedicated to data analysis and interpretation. Section 6 of the report provides a number of selected examples for applications of residual stress measurements and Section 7 discusses future trends for development and use of this powerful technique. In addition, Annex I contains complete information on a number of round robin tests with standard samples and some results reported by the IAEA project participants. Finally, information on a dozen residual stress instruments worldwide and their characteristics is provided in Annex II.

The deep penetration and selective absorption of neutrons make them a powerful tool for the non-destructive testing of large samples of material or large objects. Residual stress that is formed in a material during manufacturing, welding, utilization or repair can be measured by means of neutron diffraction. In fact, neutron diffraction is the only non-destructive testing method which can facilitate three dimensional mapping of residual stress in a bulk component. Stress measurement using neutron beams is a technique that enables this kind of high quality non-destructive investigation, and provides insight into the material strain and stress state deep within engineering components and structures under various conditions representative of those which might be experienced in service. Such studies are of importance to improve the quality of industrial components in production and to optimize design criteria in applications. Anisotropies in macroscopic properties such as thermal and electrical conductivities, for instance of fuel elements, and mechanical properties of materials depend on the textures developed during their preparation or thermal treatment. Such textures also can be studied using neutron diffraction techniques.
There is currently substantial scientific and industrial demand for high quality non-destructive residual stress measurements, and the continuing competitive drive to optimize performance and minimize weight in many applications indicates that this demand will continue to grow. As such, the neutron diffraction technique is an increasingly important tool for mechanical and materials engineering in the search for improved manufacturing processes to reduce stress and distortion. Considering this trend, and in accordance with its purpose of promoting the peaceful use of nuclear applications, in 2006-2009 the IAEA organized a Coordinated Research Project on the Development and Application of the Techniques of Residual Stress Measurements in Materials. This project relied on the participation of practitioners from highly specialized user facilities from various Member States, including the Czech Republic, Germany, Hungary, India, the Netherlands, Pakistan, Romania, the Russian Federation and South Africa.
This report is the culmination of the project, and provides guidance on the basic principles, requirements, preparation, design, execution and standardization of residual stress measurements using neutron beams. It includes details of experimental techniques; associated equipment and instrumentation; their commissioning, calibration and control; and, finally, data acquisition. A separate section is dedicated to data analysis and interpretation. The publication also provides a number of selected examples of applications of residual stress measurements as well as future trends for the development and use of this powerful technique. Finally, complete information on a number of round robin exercises with standard samples, as well as detailed information on a dozen residual stress instruments worldwide, has been included. Ultimately, the report is intended to promote the use of neutron beams in residual stress measurements; facilitate the preparation, design and standardization process at less well equipped smaller research centres around the world; and to serve as a guidance text for young researchers and graduate students new to the field.
The IAEA officer responsible for this publication was D. Ridikas of the Division of Physical and Chemical Sciences.

1. INTRODUCTION 1

1.1. Background 1

1.2. Purpose and scope 2

1.3. Structure 2

2. RESIDUAL STRESS 3

2.1. Definition 3

2.2. Residual stress measurement by neutron diffraction 4

      2.2.1. The basic principles 5

2.3. Other factors 7

3. EXPERIMENTAL TECHNIQUES 7

3.1. Neutron sources 7

      3.1.1. Introduction 7

      3.1.2. Steady state nuclear reactors 8

      3.1.3. Pulsed neutron sources 9

      3.1.4. Specific neutron sources 11

      3.1.5. Two types of stress diffractometers 12

3.2. Types of instruments 14

3.3. Standard equipment 16

      3.3.1. Introduction 16

      3.3.2. The neutron source and neutron moderation 17

      3.3.3. Transfer of neutrons from the source to the facility 17

      3.3.4. Beam conditioning: Collimation-monochromatization 19

      3.3.5. Equipment for the definition of the sampled gauge volume 20

      3.3.6. Specimen positioning table 24

      3.3.7. Neutron detectors 26

      3.3.8. Data acquisition 29

3.4. Instrument commissioning and benchmarking 29

      3.4.1. Minimization of radiation background levels 30

      3.4.2. Instrument axes and reference point 30

      3.4.3. Gauge volume 31

      3.4.4. Detectors 32

      3.4.5. Instrumental resolution 32

      3.4.6. Verification of instrument performance 33

      3.4.7. Other relevant comments 33

3.5. Instrument alignment and calibration 34

      3.5.1. Introduction 34

      3.5.2. Calibration 34

      3.5.3. Alignment 35

3.6. Measurement procedures 37

      3.6.1. Introduction 37

      3.6.2. Measurement planning 37

      3.6.3. Specimen handling 38

      3.6.4. Reference specimen 39

      3.6.5. Specimen alignment 40

      3.6.6. Measurement execution — data recording 42

      3.6.7. Grain size and surface effects 42

3.7. Specialized sample environment 44

      3.7.1. Introduction 44

      3.7.2. Bending device 44

      3.7.3. Heating and cooling devices 45

      3.7.4. Uniaxial stress or combined measurements 45

      3.7.5. Activated samples 47

3.8. Advanced instrumentation for residual stress/strain scanning 47

      3.8.1. Bragg diffraction focusing 51

      3.8.2. Correlation technique for stress/strain experiments 53

3.9. Instrument simulation and optimization 56

      3.9.1. Numerical simulations for instrument design 56

      3.9.2. Matrix simulation technique 59

4. INSTRUMENTATION CONTROL AND DATA ACQUISITION 61

4.1. Introduction 61

4.2. Signal transfer from neutron detector to counter 63

4.3. Encoders and motor drivers 63

4.4. Integration into the measurement system 65

5. DATA ANALYSIS 65

5.1. Introduction 65

5.2. Macroscopic strain and stress determination from diffraction data 67

      5.2.1. Residual strain 67

      5.2.2. Generalized Hooke's law 68

      5.2.3. Measured d spacings and full stress tensor determination 68

      5.2.4. Elastic constants with and without texture 70

      5.2.5. Helpful software 71

5.3. Methods for single peak and multipeak analyses 73

      5.3.1. Determination of profile parameters of a single diffraction line 73

      5.3.2. Material and measurement characteristics influencing the parameters of a neutron peak 76

      5.3.3. Determination of crystal microstructure by the Rietveld method 77

      5.3.4. Relation between profile parameters and deformations 79

5.4. Assessment of the uncertainties 80

6. EXAMPLES OF APPLICATIONS 82

6.1. Measurements of stress/strain state induced by a weld deposited pass 82

6.2. Residual stresses in a bimetallic stainless steel zirconium adapter 84

6.3. Measurement of residual stresses in a bimetallic nuclear piping weld 86

7. FUTURE TRENDS IN RESIDUAL STRESS MEASUREMENTS 88

7.1. Instruments at steady state sources 88

      7.1.1. Horizontally focusing monochromator at a large take-off angle 90

      7.1.2. Horizontally focusing monochromator at a small take-off angle 92

      7.1.3. Exploitation of the effect of wavelength dependent attenuation on neutron diffraction stress measurements for large depths in metals 95

      7.1.4. Flux increase through other focusing techniques 97

7.2. Instruments at pulsed neutron sources 101

      7.2.1. Trends in analysis of experimental data measured on a TOF stress diffractometer 105

      7.2.2. Neutron diffraction and other modes of neutron scattering at pulsed sources 106

7.3. The future development of instrument control 107

8. SUMMARY 109

REFERENCES 111

ACRONYMS AND ABBREVIATIONS 119

ANNEX I: ROUND ROBIN TESTS 121

ANNEX II: EXAMPLES OF RESIDUAL STRESS FACILITIES 135

CONTRIBUTORS TO DRAFTING AND REVIEW 157

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