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Residual stress measurements at the NIST reactor
ELSEVIER
Physica B 241-243 (1998} 1244-1245
Residual stress measurements at the N I S T reactor P.C. Bran&, H.J. Prask a, T. Gnaeupel-Herold b'* National Institute ~?/'Standards and Teehnolog3', Center /br Neutron Research. Gaithersburg, MD 20899, USA bDepartment ~?/Materials and Nuclear Engineering, Universi O, ~?/Mao,land, College Park, MD 20742-2115, USA
Abstract
A neutron-diffraction residual-stress measurement program has been in place for several years at the National Institute of Standards and Technology (NIST). Very recently a new, dedicated, state-of-the-art diffractometer has been installed at the NIST reactor. With it, the residual stress fields in a variety of engineering specimens have been determined. These include slices of railroad rails, weldments in steel plates and plastically bent steel pipes. Features of the new instrument and various experimental results will be discussed. ~ 1998 Elsevier Science B.V. All rights reserved. Kevwords: Residual stresses; Neutron diffraction
1. Introduction
Neutron diffraction is a very powerful method for the non-destructive characterization of strains in a material. The weak attenuation of neutrons by most materials allows penetration depths of the order of a few 10 -.2 m. Specimens of heavy weight and complicated shapes become accessible for investigation of their stress state. The measurement principle is based on Bragg's law
). = 2d sin 0
(1)
* Corresponding author. Present address: National Institute of Standards and Technology, Building 235, Gaithersburg, M D 20899, USA. Fax: 1 301 921 9847; e-mail: gnaeupel@ rrdstrad.nist.gov.
in which the scattering angle 20s depends on the wavelength 2 of the incident neutron beam and on the lattice spacing d. Hence, diffraction is phaseselective so that even composites and multiphase materials can be investigated with respect to the strain state of each phase. Furthermore, the determination of the full-strain tensor can be achieved by choosing the appropriate sample orientation with respect to the incident and diffracted beams. The new residual-stress diffractometer at the N I S T reactor [1] has been fully operational for more than one year and the residual-stress states in a large variety of engineering specimens have been investigated. The design of that diffractometer and the high neutron flux of the 20 M W N I S T reactor enables the user to perform measurements both with high spatial resolution and large penetration depths up to 30 m m (steel) and more. Three monochromators and a take-off angle 20M, continuously
0921-4526/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 8 3 7 - 5
P.C. Brand et al. / Physica B 241 243 (1998) 1244-1245
"i
1245
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1' . . . . .
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im
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.
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,
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"~" 1.5 position sensitive d e t e c t o r Fig. 1. Measurement principle and schematic set up of the BT8 instrument.
adjustable from 15 '~ to 120 ~, allow for the choice of wavelengths appropriate for the optimal definition of the gauge volume at a scattering angle 20s = 90L Fig. 1 shows a sketch of the instrument.
2. Applications The features of the new instrument along with a position-sensitive detector allows the collection of stress values from dense arrays of sampling volumes as required by specimens in which strong stress gradients are expected. These stress scans are interesting because the superposition of localized stress concentrations and external loads under service conditions can be an important lifetime limiting factor. This is illustrated by Fig. 2, which shows a distribution of residual stresses in a slice of railroad rail. The thickness was 6.4 mm. Stress imaging as shown in Fig. 2 allows the direct comparison of calculated results from finiteelement methods with experimental data, which offers the possibility to verify and improve the
(1) a,c""
1.0
>" 0.5 0.0 .... -2
-1 x
Fig. 2. Density plot of the residual stresses in a railroad rail. The stress direction is given in the upper left corner of the graphs. The stresses are given in units of MPa.
underlying models as well as the mechanisms responsible for stress build-up. The capability to investigate the residual-stress state in large depths has been demonstrated by measurements on welded steel plates in which the neutron beam reached pathlengths up to 35 mm. The information depth was 25 mm. Another example is the investigation of the residual-stress state in a plastically bent steel pipe. The wall thickness of the pipe was 7 mm so that a high spatial resolution of 2 × 2 x 2 mm 3 was necessary to obtain a detailed image of the residual-stress state.
References [1] P.C. Brand, H.J. Prask, in: Proc. 4th Int. Conf. on Residual Stresses, Baltimore, MD, 8 10 June, 1994, pp. 23(~234.
Physica B 241-243 (1998} 1244-1245
Residual stress measurements at the N I S T reactor P.C. Bran&, H.J. Prask a, T. Gnaeupel-Herold b'* National Institute ~?/'Standards and Teehnolog3', Center /br Neutron Research. Gaithersburg, MD 20899, USA bDepartment ~?/Materials and Nuclear Engineering, Universi O, ~?/Mao,land, College Park, MD 20742-2115, USA
Abstract
A neutron-diffraction residual-stress measurement program has been in place for several years at the National Institute of Standards and Technology (NIST). Very recently a new, dedicated, state-of-the-art diffractometer has been installed at the NIST reactor. With it, the residual stress fields in a variety of engineering specimens have been determined. These include slices of railroad rails, weldments in steel plates and plastically bent steel pipes. Features of the new instrument and various experimental results will be discussed. ~ 1998 Elsevier Science B.V. All rights reserved. Kevwords: Residual stresses; Neutron diffraction
1. Introduction
Neutron diffraction is a very powerful method for the non-destructive characterization of strains in a material. The weak attenuation of neutrons by most materials allows penetration depths of the order of a few 10 -.2 m. Specimens of heavy weight and complicated shapes become accessible for investigation of their stress state. The measurement principle is based on Bragg's law
). = 2d sin 0
(1)
* Corresponding author. Present address: National Institute of Standards and Technology, Building 235, Gaithersburg, M D 20899, USA. Fax: 1 301 921 9847; e-mail: gnaeupel@ rrdstrad.nist.gov.
in which the scattering angle 20s depends on the wavelength 2 of the incident neutron beam and on the lattice spacing d. Hence, diffraction is phaseselective so that even composites and multiphase materials can be investigated with respect to the strain state of each phase. Furthermore, the determination of the full-strain tensor can be achieved by choosing the appropriate sample orientation with respect to the incident and diffracted beams. The new residual-stress diffractometer at the N I S T reactor [1] has been fully operational for more than one year and the residual-stress states in a large variety of engineering specimens have been investigated. The design of that diffractometer and the high neutron flux of the 20 M W N I S T reactor enables the user to perform measurements both with high spatial resolution and large penetration depths up to 30 m m (steel) and more. Three monochromators and a take-off angle 20M, continuously
0921-4526/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 8 3 7 - 5
P.C. Brand et al. / Physica B 241 243 (1998) 1244-1245
"i
1245
2.oI.,
1' . . . . .
[
im
"~
_ .~
zoo 30o
o IOO
c- 1.0 >, 0.5
~~,~
0.0
.
-2 diffracted
,
-1
0 x [inches ]
zoo-lo~
mmmm 40o ~o,,
1
2
0 1 [inches ]
2
be
2.0,
aperture
"~" 1.5 position sensitive d e t e c t o r Fig. 1. Measurement principle and schematic set up of the BT8 instrument.
adjustable from 15 '~ to 120 ~, allow for the choice of wavelengths appropriate for the optimal definition of the gauge volume at a scattering angle 20s = 90L Fig. 1 shows a sketch of the instrument.
2. Applications The features of the new instrument along with a position-sensitive detector allows the collection of stress values from dense arrays of sampling volumes as required by specimens in which strong stress gradients are expected. These stress scans are interesting because the superposition of localized stress concentrations and external loads under service conditions can be an important lifetime limiting factor. This is illustrated by Fig. 2, which shows a distribution of residual stresses in a slice of railroad rail. The thickness was 6.4 mm. Stress imaging as shown in Fig. 2 allows the direct comparison of calculated results from finiteelement methods with experimental data, which offers the possibility to verify and improve the
(1) a,c""
1.0
>" 0.5 0.0 .... -2
-1 x
Fig. 2. Density plot of the residual stresses in a railroad rail. The stress direction is given in the upper left corner of the graphs. The stresses are given in units of MPa.
underlying models as well as the mechanisms responsible for stress build-up. The capability to investigate the residual-stress state in large depths has been demonstrated by measurements on welded steel plates in which the neutron beam reached pathlengths up to 35 mm. The information depth was 25 mm. Another example is the investigation of the residual-stress state in a plastically bent steel pipe. The wall thickness of the pipe was 7 mm so that a high spatial resolution of 2 × 2 x 2 mm 3 was necessary to obtain a detailed image of the residual-stress state.
References [1] P.C. Brand, H.J. Prask, in: Proc. 4th Int. Conf. on Residual Stresses, Baltimore, MD, 8 10 June, 1994, pp. 23(~234.