- Email: [email protected]
Residual stress diffractometer KOWARI at the Australian research reactor OPAL: Status of the project
ARTICLE IN PRESS
Physica B 385–386 (2006) 1040–1042 www.elsevier.com/locate/physb
Residual stress diffractometer KOWARI at the Australian research reactor OPAL: Status of the project Alain Brule, Oliver Kirstein Bragg Institute, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia
Abstract Neutron scattering using diffraction techniques is now recognized as the most precise and reliable method of mapping sub-surface residual stresses in materials or even components, which are not only of academic but also of industrial-economic relevance. The great potential of neutrons in the field of residual stresses was recognized by ANSTO and its external Beam Instrument Advisory Group for the new research reactor OPAL. The recommendation was to build the dedicated strain scanner KOWARI among the first suite of instruments available to users. We give an update on the overall project and present the current status of the diffractometer. It is anticipated that the instrument will be commissioned in mid 2006 and available to users at the end of the OPAL project. Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. Keywords: Neutron diffraction; Strain scanning; Material science; Instrumentation
1. Introduction
2. Principle
Neutron scattering using diffraction techniques are now recognized as the most precise and reliable method of mapping residual stresses in materials and industrial devices. Contrary but complementary to X-ray or synchrotron radiation the use of neutrons allow to investigate properties of materials due to the greater penetration depth which is in the order of mm for high energy synchrotron Xrays but cm for thermal neutrons in case of e.g. steel. Thus whereas the first of both techniques is intrinsically better suited for investigating surface properties the latter one can efficiently be used to study phenomena relatively deep below the surface. The great potential of using neutrons in the field of residuals stresses has been recognized by the Australian Nuclear Science and Technology Organisation which is currently building the new Australian replacement research reactor (RRR) that was recently named OPAL [1]. Among the first suite of eight instruments to be built will be an instrument designed for measuring residual-stresses. In the following sections we will describe the layout and current status of the diffractometer.
The technique of deriving strains by neutron diffraction is based on Bragg’s law
Corresponding author. Tel.: +61 2 9717 9472; fax: +61 2 9717 3606.
E-mail address: [email protected] (A. Brule).
l ¼ 2 d hkl sin Yhkl
(1)
to determine the lattice spacing of planes d hkl labeled by Miller indices ðh k lÞ from the diffraction angle 2 Yhkl with a known wavelength l. At a steady state the obtained peak, superimposed on a (uniform) background, can be well described by Gaussian distribution and the peak position can be obtained by fitting such a Gaussian to the data obtaining a certain peak width at half maximum (FWHM). The elastic strain ehkl may be calculated using the logarithmic derivative of Eq. (1) according to ehkl ¼
dY Yhkl Y0hkl ¼ , tan Y tan Y
(2)
where the index ‘0’ refers to an appropriate stress free sample. The measured strains can then be described in terms of elements of a strain tensor ij in a Cartesian axis system set up to define directions in the sample. Finally, the elements of the elastic strain tensor sij may be calculated from the elements of the elastic strain tensor ij with the aid
0921-4526/$ - see front matter Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.333
ARTICLE IN PRESS A. Brule, O. Kirstein / Physica B 385–386 (2006) 1040–1042
1041
s ¼ C
(3)
and C containing plane specific constants E hkl and nhkl relating the macroscopic stress to the strain ij as measured with the ðh k lÞ reflection. Strain maps are obtained by scanning the sample which is typically mounted on a XYZtranslation table which may include one or even more rotation stages to orient the sample and measure different strain components to get the whole strain tensor. A cubic gauge volume, usually defined by a pair of slits, is therefore advantageous since it allows to measure stresses in the same volume element even though the sample may have been reoriented. A cubic gauge volume is usually achieved by positioning the detector perpendicular to the incident beam. Typically strain scanners illuminate volumes down to the order of 1mm3 . The precision of determining the peak position of the diffracted beam in the detector is 1 typically 100 of a degree and an instrumental resolution of dd dY 4 103 (4) d tan Y corresponds to an error in the determined strain e of de 4 105 ¼ 40 mstrain. The error in the lattice strain of course results in an error of the macroscopic stress s. An upper limit for the error of the stresses ds can be estimated by e.g. assuming an uniaxial loading of high strength steel whose yield strength can be as high as 800 MPa. Taking a standard Young’s modulus of E ¼ 200 GPa and Poisson ratio of n ¼ 0:3 Hooke’s law would give lattice strains of about 4000 and 1200 mstrain along the respective principal axis. If strain gradients in the sample can be assumed to be small a 2D detector can efficiently be used to integrate in the vertical plane, minimising the experimental time. For the above-mentioned example the resulting error in the stress can be estimated to be in the order of ds 15 MPa. 3. Components 3.1. Hardware The diffractometer will be positioned as the first instrument downstream the thermal guide TG3 inside the neutron guide hall of OPAL. The monochromator shielding was designed by ANSTO. The accessible monochromator take-off angle range can be continuously varied between 40 and 120 by pneumatically activated wedges that open the shielding. The monochromator that is currently being designed by AZ Systems, France [2] will be a double focusing Si monochromator. Focusing will be achieved by elastically bending the perfect Si crystal slabs see e.g. Ref. [3]. The (4 0 0) plane of the Si crystals are parallel to the crystal surface, thus the (4 0 0) reflection can be used in symmetric setting, Fig. 1 shows a calculated peak profile. Asymmetric reflections from the h1 1 0i zone
Neutron intensity in a.u.
of Hooke’s law Fe (211) sample 400
δΘ/tanΘ = 2'10-3 FWHM = 0.11°
Si (400) symmetric 2ΘM = 75.35°
Slitwidth = 2 mm Slitheight = 10mm
200
0 -46
-45.5
-45 ΘS in deg
-44.5
-44
Fig. 1. Expected profile for a Fe(2 1 1) diffraction peak with the Si monochromator in symmetric reflection setting.
such as (4 2 2) can be used too and the combination of the above-mentioned angular range with the appropriate reflection allows to compromise between resolution and intensity. The monochromator stages of all the powder instruments, i.e. Kowari, Wombat and Echidna, are standardized and will be equipped with kinematical mounts to allow to swap monochromators between instruments easily without realignment, if necessary. Therefore it is possible to use a e.g. Ge monochromator if the need should arise. In order to collect as many neutrons as possible, in particular if strain variations in the vertical direction are assumed to be small or negligible, the instrument will be equipped with a 2D position sensitive detector, which was purchased from DENEX, Germany [4]. The detector is 30 30 cm2 a multi-wire gas detector with a delay time encoding. The gas chamber is filled with a partial 3He pressure of 2.5 bar and a partial CF4 pressure of 2 bar. This combination results in a neutron detection efficiency of about 73% at a wavelength of 2 A˚ and a spatial resolution of o2 mmFWHM . The data acquisition electronics is based on a commercially available system provided by FastcomTech [5] and will be standardized among Kowari and the reflectometer Platypus [6]. The majority of samples that will be investigated are potentially large and/or heavy. Therefore, it is necessary to have a sample table that is capable of orienting heavy objects with high accuracy keeping in mind that strain measurements need to be done at different sample orientations at (ideally) the same location in the sample. In addition, a large clearance under the beam and a large travel along the vertical axis is desirable to allow handling of large objects. The sample table is being designed by MTF Transfer Technology GmbH, Germany [7] and incorporates a telescopic system. Besides high accuracy in positioning samples ðp10 mmÞ the table will be capable of moving masses up to 1000 kg. The geometry was optimized and any deformations or deflections minimized by applying finite element analysis resulting in a cylindrical geometry, as schematically shown in Fig. 2. In the beginning the table will provide xyzo motions but it has been designed to allow for any other
ARTICLE IN PRESS 1042
A. Brule, O. Kirstein / Physica B 385–386 (2006) 1040–1042
tion will be available, too. Data and results will be provided to the user in a format that can be used by any other standard software. In the long term it is planned to incorporate the SSCANS software environment [9] that is e.g. used at ENGIN-X at ISIS in order to plan and control the experiment. Fig. 2. Schematic view of the sample table—detector table setup of Kowari.
positioning ancillary such as Eulerian cradles or robotic arms. Parameters of the table are:
travel along x and y (in the plane): 250 mm; travel along z (vertical): 600 mm; angular range o: 0–370 ; maximum load: p1000 kg; distance monochr.-table: variable up to 3.5 m; distance detector-table: variable up to 1.5 m.
The gauge volume will be defined by a pair of slits on the incident and scattered side of the neutron beam. The slits will be motorized horizontally allowing to vary the width of the gauge volume continuously between 0.5 and 10 mm whereas the height will be adjusted manually by using appropriate Cd masks. Sample and slit alignment will be done automatically by using lasers and high-resolution cameras with appropriate software.
4. Summary Following the optimization of instrumental parameters the construction of the residual stress diffractometer commenced in 2003. The installation of the hardware began in August 2005 and should be finished with the delivery of the double focusing monochromator in 2006. The instrument was designed to allow handling and positioning of potentially large objects with high precision. The neutron flux was estimated to be around 2 107 n=cm2 =s [10], which will be comparable to other reactor-based strain-scanners such as SALSA at the ILL or STRESS-SPEC at the FRM-II. Standardization among the powder instruments not only with regards to the hardware but the software too should encourage the engineering community and materials scientists to use not only the strain scanner but also the high-intensity/highresolution powder diffractometers that are available at OPAL. All three instruments are anticipated to be ready for experiments in January 2007. References
3.2. Software The instrument will be controlled via the Gumtree graphical user interface [8], which is of similar ‘‘look and feel’’ on all the instruments. Within Gumtree it is possible to e.g. program batch files that automatically position the sample and instrument specific software is currently being developed that will provide the user with information about the sample positions, as well as the fitted position, FWHM and intensity of the diffraction peaks and the background and more sophisticated analysis tools such as deconvoluting peak profiles with the instrumental resolu-
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
hhttp://home.ansto.gov.au/ansto/RRR/i. hhttp://www.ill.fr/dif/iucr/AZ-Systemes.htmli. P. Mikula, et al., Physica B 283 (2000) 289. DENEX Detektoren fu¨r Neutronen und Ro¨ntgenstrahlung GmbH, D-21339 Lu¨neburg, Germany, [email protected]. hhttp://www.fastcomtec.com/i. hhttp://www.ansto.gov.au/ansto/bragg/2005/reflectom/instrument_refl.html/i. hhttp://www.mtf-tech.com/i. hhttp://www.ansto.gov.au/ansto/bragg/2005/comp/gui.html/i; P. Hathaway, et al., Presented at this conference. J.A. James, et al., Appl. Phys. A, submitted for publication. O. Kirstein, J. Neutr. Res. 11 (4) (2003) 283.
Physica B 385–386 (2006) 1040–1042 www.elsevier.com/locate/physb
Residual stress diffractometer KOWARI at the Australian research reactor OPAL: Status of the project Alain Brule, Oliver Kirstein Bragg Institute, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia
Abstract Neutron scattering using diffraction techniques is now recognized as the most precise and reliable method of mapping sub-surface residual stresses in materials or even components, which are not only of academic but also of industrial-economic relevance. The great potential of neutrons in the field of residual stresses was recognized by ANSTO and its external Beam Instrument Advisory Group for the new research reactor OPAL. The recommendation was to build the dedicated strain scanner KOWARI among the first suite of instruments available to users. We give an update on the overall project and present the current status of the diffractometer. It is anticipated that the instrument will be commissioned in mid 2006 and available to users at the end of the OPAL project. Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. Keywords: Neutron diffraction; Strain scanning; Material science; Instrumentation
1. Introduction
2. Principle
Neutron scattering using diffraction techniques are now recognized as the most precise and reliable method of mapping residual stresses in materials and industrial devices. Contrary but complementary to X-ray or synchrotron radiation the use of neutrons allow to investigate properties of materials due to the greater penetration depth which is in the order of mm for high energy synchrotron Xrays but cm for thermal neutrons in case of e.g. steel. Thus whereas the first of both techniques is intrinsically better suited for investigating surface properties the latter one can efficiently be used to study phenomena relatively deep below the surface. The great potential of using neutrons in the field of residuals stresses has been recognized by the Australian Nuclear Science and Technology Organisation which is currently building the new Australian replacement research reactor (RRR) that was recently named OPAL [1]. Among the first suite of eight instruments to be built will be an instrument designed for measuring residual-stresses. In the following sections we will describe the layout and current status of the diffractometer.
The technique of deriving strains by neutron diffraction is based on Bragg’s law
Corresponding author. Tel.: +61 2 9717 9472; fax: +61 2 9717 3606.
E-mail address: [email protected] (A. Brule).
l ¼ 2 d hkl sin Yhkl
(1)
to determine the lattice spacing of planes d hkl labeled by Miller indices ðh k lÞ from the diffraction angle 2 Yhkl with a known wavelength l. At a steady state the obtained peak, superimposed on a (uniform) background, can be well described by Gaussian distribution and the peak position can be obtained by fitting such a Gaussian to the data obtaining a certain peak width at half maximum (FWHM). The elastic strain ehkl may be calculated using the logarithmic derivative of Eq. (1) according to ehkl ¼
dY Yhkl Y0hkl ¼ , tan Y tan Y
(2)
where the index ‘0’ refers to an appropriate stress free sample. The measured strains can then be described in terms of elements of a strain tensor ij in a Cartesian axis system set up to define directions in the sample. Finally, the elements of the elastic strain tensor sij may be calculated from the elements of the elastic strain tensor ij with the aid
0921-4526/$ - see front matter Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.333
ARTICLE IN PRESS A. Brule, O. Kirstein / Physica B 385–386 (2006) 1040–1042
1041
s ¼ C
(3)
and C containing plane specific constants E hkl and nhkl relating the macroscopic stress to the strain ij as measured with the ðh k lÞ reflection. Strain maps are obtained by scanning the sample which is typically mounted on a XYZtranslation table which may include one or even more rotation stages to orient the sample and measure different strain components to get the whole strain tensor. A cubic gauge volume, usually defined by a pair of slits, is therefore advantageous since it allows to measure stresses in the same volume element even though the sample may have been reoriented. A cubic gauge volume is usually achieved by positioning the detector perpendicular to the incident beam. Typically strain scanners illuminate volumes down to the order of 1mm3 . The precision of determining the peak position of the diffracted beam in the detector is 1 typically 100 of a degree and an instrumental resolution of dd dY 4 103 (4) d tan Y corresponds to an error in the determined strain e of de 4 105 ¼ 40 mstrain. The error in the lattice strain of course results in an error of the macroscopic stress s. An upper limit for the error of the stresses ds can be estimated by e.g. assuming an uniaxial loading of high strength steel whose yield strength can be as high as 800 MPa. Taking a standard Young’s modulus of E ¼ 200 GPa and Poisson ratio of n ¼ 0:3 Hooke’s law would give lattice strains of about 4000 and 1200 mstrain along the respective principal axis. If strain gradients in the sample can be assumed to be small a 2D detector can efficiently be used to integrate in the vertical plane, minimising the experimental time. For the above-mentioned example the resulting error in the stress can be estimated to be in the order of ds 15 MPa. 3. Components 3.1. Hardware The diffractometer will be positioned as the first instrument downstream the thermal guide TG3 inside the neutron guide hall of OPAL. The monochromator shielding was designed by ANSTO. The accessible monochromator take-off angle range can be continuously varied between 40 and 120 by pneumatically activated wedges that open the shielding. The monochromator that is currently being designed by AZ Systems, France [2] will be a double focusing Si monochromator. Focusing will be achieved by elastically bending the perfect Si crystal slabs see e.g. Ref. [3]. The (4 0 0) plane of the Si crystals are parallel to the crystal surface, thus the (4 0 0) reflection can be used in symmetric setting, Fig. 1 shows a calculated peak profile. Asymmetric reflections from the h1 1 0i zone
Neutron intensity in a.u.
of Hooke’s law Fe (211) sample 400
δΘ/tanΘ = 2'10-3 FWHM = 0.11°
Si (400) symmetric 2ΘM = 75.35°
Slitwidth = 2 mm Slitheight = 10mm
200
0 -46
-45.5
-45 ΘS in deg
-44.5
-44
Fig. 1. Expected profile for a Fe(2 1 1) diffraction peak with the Si monochromator in symmetric reflection setting.
such as (4 2 2) can be used too and the combination of the above-mentioned angular range with the appropriate reflection allows to compromise between resolution and intensity. The monochromator stages of all the powder instruments, i.e. Kowari, Wombat and Echidna, are standardized and will be equipped with kinematical mounts to allow to swap monochromators between instruments easily without realignment, if necessary. Therefore it is possible to use a e.g. Ge monochromator if the need should arise. In order to collect as many neutrons as possible, in particular if strain variations in the vertical direction are assumed to be small or negligible, the instrument will be equipped with a 2D position sensitive detector, which was purchased from DENEX, Germany [4]. The detector is 30 30 cm2 a multi-wire gas detector with a delay time encoding. The gas chamber is filled with a partial 3He pressure of 2.5 bar and a partial CF4 pressure of 2 bar. This combination results in a neutron detection efficiency of about 73% at a wavelength of 2 A˚ and a spatial resolution of o2 mmFWHM . The data acquisition electronics is based on a commercially available system provided by FastcomTech [5] and will be standardized among Kowari and the reflectometer Platypus [6]. The majority of samples that will be investigated are potentially large and/or heavy. Therefore, it is necessary to have a sample table that is capable of orienting heavy objects with high accuracy keeping in mind that strain measurements need to be done at different sample orientations at (ideally) the same location in the sample. In addition, a large clearance under the beam and a large travel along the vertical axis is desirable to allow handling of large objects. The sample table is being designed by MTF Transfer Technology GmbH, Germany [7] and incorporates a telescopic system. Besides high accuracy in positioning samples ðp10 mmÞ the table will be capable of moving masses up to 1000 kg. The geometry was optimized and any deformations or deflections minimized by applying finite element analysis resulting in a cylindrical geometry, as schematically shown in Fig. 2. In the beginning the table will provide xyzo motions but it has been designed to allow for any other
ARTICLE IN PRESS 1042
A. Brule, O. Kirstein / Physica B 385–386 (2006) 1040–1042
tion will be available, too. Data and results will be provided to the user in a format that can be used by any other standard software. In the long term it is planned to incorporate the SSCANS software environment [9] that is e.g. used at ENGIN-X at ISIS in order to plan and control the experiment. Fig. 2. Schematic view of the sample table—detector table setup of Kowari.
positioning ancillary such as Eulerian cradles or robotic arms. Parameters of the table are:
travel along x and y (in the plane): 250 mm; travel along z (vertical): 600 mm; angular range o: 0–370 ; maximum load: p1000 kg; distance monochr.-table: variable up to 3.5 m; distance detector-table: variable up to 1.5 m.
The gauge volume will be defined by a pair of slits on the incident and scattered side of the neutron beam. The slits will be motorized horizontally allowing to vary the width of the gauge volume continuously between 0.5 and 10 mm whereas the height will be adjusted manually by using appropriate Cd masks. Sample and slit alignment will be done automatically by using lasers and high-resolution cameras with appropriate software.
4. Summary Following the optimization of instrumental parameters the construction of the residual stress diffractometer commenced in 2003. The installation of the hardware began in August 2005 and should be finished with the delivery of the double focusing monochromator in 2006. The instrument was designed to allow handling and positioning of potentially large objects with high precision. The neutron flux was estimated to be around 2 107 n=cm2 =s [10], which will be comparable to other reactor-based strain-scanners such as SALSA at the ILL or STRESS-SPEC at the FRM-II. Standardization among the powder instruments not only with regards to the hardware but the software too should encourage the engineering community and materials scientists to use not only the strain scanner but also the high-intensity/highresolution powder diffractometers that are available at OPAL. All three instruments are anticipated to be ready for experiments in January 2007. References
3.2. Software The instrument will be controlled via the Gumtree graphical user interface [8], which is of similar ‘‘look and feel’’ on all the instruments. Within Gumtree it is possible to e.g. program batch files that automatically position the sample and instrument specific software is currently being developed that will provide the user with information about the sample positions, as well as the fitted position, FWHM and intensity of the diffraction peaks and the background and more sophisticated analysis tools such as deconvoluting peak profiles with the instrumental resolu-
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
hhttp://home.ansto.gov.au/ansto/RRR/i. hhttp://www.ill.fr/dif/iucr/AZ-Systemes.htmli. P. Mikula, et al., Physica B 283 (2000) 289. DENEX Detektoren fu¨r Neutronen und Ro¨ntgenstrahlung GmbH, D-21339 Lu¨neburg, Germany, [email protected]. hhttp://www.fastcomtec.com/i. hhttp://www.ansto.gov.au/ansto/bragg/2005/reflectom/instrument_refl.html/i. hhttp://www.mtf-tech.com/i. hhttp://www.ansto.gov.au/ansto/bragg/2005/comp/gui.html/i; P. Hathaway, et al., Presented at this conference. J.A. James, et al., Appl. Phys. A, submitted for publication. O. Kirstein, J. Neutr. Res. 11 (4) (2003) 283.