1、TECHNOLOGY ISOITTA 3 TRENDS ASSESSMENT First edition 200 1 -09- 1 5 Polycrystalline materials - Determination of residual stresses by neutron diffraction Matriaux polycristallins - Dtermination des contraintes rsiduelles par diffraction neutronique Reference number ISOiTTA 3:2001 (E) ISOITTA 32001 (
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5、S02001 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either IS0 at the address below or ISOs member body in the
6、 country of the requester. IS0 copyright office Case postale 56 CH-121 1 Geneva 20 Tel. + 41 22 749 O1 11 Fax + 41 22 749 O9 47 E-mail copyrightiso.ch Web www.iso.ch Printed in Switzerland II 0 IS0 2001 -All rights reserved ISOITTA 3:2001(E) ISOITTA Page FOREWORD . iv EXECUTIVE SUMMARY . v BACKGROUN
7、D . vi REFERENCES ix Participants in VAMAS TWA 20 x Participants in RESTAND . xi Introduction xi1 1 . 2 . 3 . 4 . 5 . 6 . 7 . 8 . 9 . Scope 1 Symbols and abbreviations 2 Calibration procedure . 10 Summary of method . 4 Materials characterisation 16 Recording and measurement procedures . 18 Calculati
8、on of stress . 20 Reliability of results . 25 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 O IS0 2001 -All rights reserved iii ISOITT
9、A 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 IS0 technical committees in association with other international organisations and national government
10、al 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
11、. 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, a
12、nd consolidation of existing views on priorities for standardisation action. IS0 Technology Trend Assessment (ISOTTA) documents are published under a memorandum of understanding concluded between IS0 and VAMAS. They enable the technical innovations and developments emerging from a VAMAS activity to
13、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
14、 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 Neut
15、ron Diffraction) aimed at demonstrating that the techniques developed can be applied to real components. iv O IS0 2001 -All rights reserved ISO/TTA 3:2001(E) EXECUTIVE SUMMARY Neutron diffraction is a relatively new method for determining residual (and applied) stresses in crystalline materials. It
16、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 maki
17、ng 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 involve
18、d 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
19、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 b
20、ackground 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 s
21、pecial 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 of k 0,l mm can be achieved with an accuracy in strain of to give a resolution in residual str
22、ess of f 7 to 20 MPa in most materials of practical interest. O IS0 2001 -All rights reserved V ISOITTA 3:2001 (E) BACKGROUND Neutron diffraction is a technique that can be applied for determining residual (and applied) stresses in crystalline materials l-31. With the method, elastic strain is measu
23、red 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 o
24、f 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 (Ver
25、sailles 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
26、non-destructive residual examine a selection of samples in which residual stresses had been introduced conduct inter-laboratory comparisons to establish reproducibility, assemble the necessary information for preparing a draft standard for making the stress measurements by neutron diffraction, by di
27、fferent techniques, - - - 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
28、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
29、 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 p
30、articipating 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
31、 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 a
32、nd statistical analyses carried out to determine the reliability of the vi O IS0 2001 -All rights reserved ISOITTA 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 polyc
33、hromatic 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.
34、 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
35、 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 MPamm) can be measured close to external surfaces and the ferritic weldment to determine whether reliable results can be obtained t
36、hrough 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 positioni
37、ng 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 re ions of strain gradients has been identified. It is recommended that a minimum size
38、 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 gradi
39、ent, 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 gradients approaching 2000 MPa/mm can be measured with a 1xlxlO mm3 match-stick sampling volume with its axis aligned in the p
40、lane of the plate. In regions away from interfaces and steep gradients a 3x33 mm3 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. 5 From statistical analysis of the data, it has been established in
41、 most cases, that a positional accuracy with a standard deviation of 0,l mm can be achieved provided proper alignment procedures are adopted. It has also been ascertained that strains can be recorded away from surfaces to a tolerance of corresponding to a stress of k 7 to 20 MPa in most materials. C
42、lose 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 mater
43、ial 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
44、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 all
45、owance for a change in stress-free crystal lattice spacing with position. 0 IS0 2001 -All rights reserved vi ISOITTA 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
46、 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 comp
47、osites, 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. Si
48、milarly 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.
49、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. viii 0 IS0 2001 -All rights reserved ISOITTA 3:2001(E) REFERENCES Allen, A, Andreani
