| NDT.net - August 2002, Vol. 7 No.08 |
3nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Nov 14-16, 2001, Seville Spain
Thermal and sub-thermal neutrons are invaluable tools for investigations in fundamental, engineering and medical science. Most neutron applications are non-destructive, as neutrons penetrate deeply into most materials. In the engineering field, neutrons are nowadays mostly employed in residual stress analysis. At the High Flux Reactor (HFR) in Petten, the Netherlands, neutron assisted experimental techniques such as neutron diffraction, neutron radiography and small angle neutron scattering (SANS) are used for residual stress, micro-structural and defects analyses in nuclear structural components.
HFR investigations relevant to structural integrity include the following: Evaluation of the effect of thermal and irradiation exposure to the evolution of microstructure, defects and internal stress in aged components and material specimens. Optimization process parameters for dissimilar metal welds. Optimization of current practice in repair welds. Evaluation of the performance of PWHT processes. Reconstitution of residual stress fields in oversize welded structural components through neutron diffraction testing of sub-parts, strain evolution measurement during cutting process, and numerical modelling. Assessing the influence of processes such as bending, rolling or shock quenching, on microstructure, defects and residual stress. Numerical methods are also used in order to scale up experimental data obtained on downscaled components, develop sophisticated models for accurate determination of multi-pass welding residual stresses and finally, analyze the impact of defects on structural integrity.
In this paper we briefly review the HFR horizontal neutron beam facilities currently employed for NDT relevant to structural integrity. This is followed by an account of current and future HFR pre-normative research activities based on these facilities. The role of the HFR in European and international standardization activities is treated separately and includes an account of the conclusions reached within two major projects relevant to the development of a standard code of practice for residual stress analysis based on neutron diffraction. Neutron diffraction based stress data are compared with X-ray and synchrotron diffraction results for Aluminium and Waspalloy friction welds and found to be in excellent agreement. Finally the performance of neutron diffraction on the analysis of residual stresses in a bimetallic weld is presented.
The High Flux Reactor (HFR) of the European Commission is a multi-purpose research reactor operating at a thermal power of 45 MW [1]. There are twelve horizontal beam tubes (HB) installed, which are instrumented for the performance of engineering, medical and fundamental research [2]. Fig. 1 shows the arrangement of these beam tube facilities around the reactor pool. A detailed description of these facilities is given in [3].
Fig 1: Horizontal neutron beam tubes at the HFR.
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Ongoing activities concentrate on the engineering and medical field [4]. In the engineering field at the moment the focus is on measurements of residual stresses. Nevertheless, micro-structural analyses are occasionally performed in parallel. In the frame work of a recently announced network on neutron techniques standardization, NET, which aims at harmonization of engineering applications of neutrons across Europe, several HFR facilities suitable for engineering studies are under upgrading or reactivation. This relates to the combined powder and stress testing, SANS and neutron radiography facilities. The HB4 instrument, dedicated to stress analysis, has been operating at a nearly 100% exploitation rate. In this section the technical features of the facilities mentioned are described.
Residual stress diffractometer/Large Component Neutron Diffraction Facility (LCNDF)
Residual stresses can be measured by neutron diffraction at beam tubes HB4 and HB5. HB4 is equipped with a PG double monochromator, which gives a lot of flexibility to this facility. The wavelength can be varied from 0.17 to 0.65nm, yielding a maximum flux at the sample position of about 1010n/m2s, and offering the possibility to use second order neutrons for stress measurements which enables the experimentalist to increase the instrument resolution significantly. Gauge volumes can be as small as 3mm3 or as big as 2cm3. The sample table can handle specimens or ancillary equipment up to 1000kg.
Combined powder and stress diffractometer
HB5 is equipped with a single copper monochromator rendering a fixed wavelength of 0.257nm and a flux comparable to the maximum flux at HB4. The gauge volume can be set from 1mm3 to 0.6cm3. This facility can also be operated in a different mode for structural analyses (powder mode). It has recently been upgraded by installing a position sensitive neutron detector (ORDELA type) and a new instrument control system. This enhances the analysis capacity mainly for residual stresses. Nevertheless the new detector also allows for accelerated powder studies. Ancillary equipment is available for investigations at high (up to 1600K) and low (less than 1K) temperatures.
Small Angle Neutron Scattering Facility (SANS)
The small angle neutron scattering facility (SANS) at beam tube HB3b is equipped with a pyrolytic graphite (PG) double monochromator rendering a wavelength of 0.475 nm at 2 = 90°. A Be filter at 45K suppresses higher order contamination. The maximum flux at the sample is about 1010n/m2s. The detector tank allows for adjustment of the secondary flight path from 1m to 4.25m. All components of the flight path can be evacuated in order to reduce attenuation through air scattering. Measurements can be conducted at elevated temperatures and in a magnetic field. Structural details with sizes ranging from 1 to 100nm can be analysed with the set-up. Structures/structural defects analyses performed using this facility include precipitates in steel, micro-bubbles in materials for fusion reactors, pores in ceramics etc [5,6].
Neutron-radiography facility
The neutron radiography facility installed at HB8 is equipped to facilitate inspection of radioactive samples (mainly fuel rods) as well as non-radioactive materials and structures. It allows for selection of thermal or sub-thermal neutron beams and variation of beam cross section and collimation. The areas of application for this facility range from integrity analysis in nuclear and non-nuclear samples via testing of adhesion or bonding of coatings to detection of pores, cracks and precipitations in components [2,3].
The Institute for Energy of the Joint Research centre of the European Commission has recently included in its institutional research programme a new project named NET- NETWORK ON NEUTRON TECHNIQUES STANDARDISATION. Based on this activity, a synonymous European Network is about to be launched with the participation of neutron research facilities, universities, research centres and nuclear energy production vendors and operators from the enlarged EU. The mission of the NET network is twofold, i.e.,
EU Policy issues addressed include nuclear safety, energy and sustainability and enlargement. The new network will capitalize on HFR facilities development and know-how acquired in the context of neutron research work performed within the following past and current shared cost and institutional projects:
VORSAC: Variation of residual stresses in aged components; thick structural nuclear weld, stainless steel grade 316, 66 mm wall thickness, artificial ageing 2 hours at 750°C, pre- and post-heat treatment measurements. BIMET: Structural integrity of bi-metallic welds; down-scaled model of bimetallic (ferritic to austenitic) nuclear weld, 25 mm wall thickness, three-dimensional stress mapping. ADIMEW: Similar to BIMET, but up to scale mock-up of bimetallic nuclear weld, 51 mm wall thickness, 3 dimensional stress mapping. INTERWELD, stress measurements in irradiated stainless steel (grades 304 and 347 foreseen) welds, downscale components for facilitating irradiation at HFR, Hot Cell for tests to be developed. ENPOWER, stress measurements in repair welds, various welding geometries to be tested. RESTAND & VAMAS TWA 20, residual stress mapping of fusion and friction stir welds in steel and aluminium alloys. UHTHE & HITHEX, residual stress analysis in fibre reinforced ceramic matrix composites.
The NET Objectives are:
The role of IE is threefold, i.e., Network Operating Agent & Network Reference Laboratory, Convenor of the joint CEN & ISO group of experts CEN/TC 138/AHG7 working on drafting of an International "Technical Specification" on "standard test method for residual stress determination by neutron diffraction", and finally Scientific Network partner. In fact, IE is currently providing reference laboratory services in the area of mapping of residual stresses in thick welded structural components relevant to nuclear energy production safety
Following are the activities that are proposed to be included in the Network work-programme:
1. Structural integrity assessment of nuclear components based on NDT and numerical modelling techniques. Neutron assisted experimental techniques will be applied for residual stress, micro-structure and defects analyses in nuclear structural components comprising
FEM based numerical methods will be also used in order to scale up experimental data obtained on downscaled components, develop sophisticated models for accurate determination of multi-pass welding residual stresses, and analyse the impact of defects on structural integrity.
2. Harmonization and standardization of neutron assisted test methods include
3. Training of young scientists in neutron methods in support of
A main objective of a recently completed European research project, RESTAND, was to develop industrial confidence in the application of the neutron diffraction technique for residual stress measurement and its principal deliverable was a relevant draft code of practice. As no such standard is yet available, and on the basis of this draft standard document, the European Standards Committee on Non-Destructive Testing (CEN TC 138) has established a new Ad hoc Work Group (AHG7). The objective of this group is the development of an International Technical Specification on "standard test method for residual stress determination by neutron diffraction". The 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.
Based on carefully drafted experimental protocols, measurements have been made, within RESTAND, 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 [7,8]. The data have established that strains can be recorded away from surfaces to a tolerance of ± 10-4 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.
Five complementary methods have been extensively used to study residual stresses in a large range of material specimens and industrial components:
Based on the above techniques a number of specimens with industrial interest were investigated and a few comparative results are shown below. Fig. 2 compares residual stress data in a waspalloy linear friction weld specimen based on neutron and X-ray diffraction [8]. The neutron data have been "extrapolated" to the specimen surface for the comparison to be possible. Fig. 3 shows plotted strain data based on synchrotron and neutron diffraction analyses [9]. The comparison is striking with both data sets revealing the same sharp strain gradient.
Fig 2: Comparison between X-ray diffraction and corrected average neutron diffraction results for residual stress in a waspalloy linear friction weld.
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Fig 3: Comparison between neutron and synchrotron diffraction data for residual strain in the weld longitudinal direction of an Al alloy friction stir welded specimen.
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Based on extensive strain/stress results obtained by these methods, the following conclusions were drawn:
It has been demonstrated that neutron diffraction is a powerful non-destructive testing method, suitable for the quantitative analysis of residual stress in a large range of engineering applications. Finally, a draft STANDARD document has been prepared.
Welding residual stresses in structural components can significantly compromise their performance and lifetime. Prediction of welding stresses based on numerical modeling has not yet proven to be reliable, while measurement of such stresses based on NDT remains a challenging task. A key issue in applying neutron diffraction to welds is the reliable estimation of the stress-free lattice distance throughout the parent material, heat affected zone and weld pool and in all directions of interest. Based on numerous investigations, it has been recently established that this can be achieved by testing small coupons cut from a companion weld specimen based on the assumption that such coupons are free from macro-stresses. In fact, the feasibility of this approach has been demonstrated in both steel and aluminium welds.
The investigated specimen was a tube of 393 mm length, 168 mm outer diameter and 25 mm wall thickness. The weld has been applied circumferentialy at mid-length. Measurements were performed at various locations within the weld, the buttering layer and the HAZs in three ortho-normal directions, i.e., tube hoop, axial and radial directions. The measurement locations are shown in Fig. 4. In order to facilitate these measurements, windows had to be cut into the component for providing access for the neutron guides. From the thus removed material a slice was cut in order to provide for the reference coupons; these were wire eroded in form of 6 x 6 mm2 columns.
Thus two reference specimens were made, one representing the locations within the buttering layer and the ferritic steel HAZ and the other representing the locations within the weld and the austenitic steel HAZ [10]. Based on the assumption of macro-stress relief in the columns through cutting, these were used for reference measurements at each location in each direction. The most appropriate test procedure was found to be operating at a fixed scattering angle 2, while varying the neutron wavelength, using the adjustable double monochromator, for the various material phases.
Fig 4: Measurement locations within the bimetallic weld.
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First tests revealed a strong (200)-texture within the weld and buttering layer, while in the austenitic and ferritic HAZs the (111)- and (110)-reflections could be used, respectively. The necessity of measuring location dependent references was demonstrated by the obtained data showing that apparent strai ns of more than 1000 µ e could be introduced if this reference parameter variation were ignored. This effect was found to be most significant within the weld.
The measurements were performed using sampling volume of 4x4x5 to 4x4x10 mm3, scattering angle of 76.150° and the following crystallographic reflections: ferrite (110), austenite (200) in weld and buttering, and austenite (111) in HAZ austenite. From the thus derived strains, residual stresses were calculated. The elastic constants related to the various material reflections were taken from the literature [11].
Fig 5: Hoop stresses across the bimetallic weld.
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Fig. 5 shows the residual stresses that were found in the hoop directions. These show compression in the ferritic steel, and tension in the austenitic phase, while in the axial direction, not shown here, high stresses are present closer to the outer and inner specimen surfaces and low stresses near the middle of the piping wall. Radial stresses, also not shown here, remain within ±100 MPa.
Thermal and sub-thermal neutrons are valuable tools for non-destructuive assessment of structural integrity of components. At the High Flux Reactor of the European Commission in Petten, NL, a number of beam tubes is available for serving such purposes. Two diffractometers are set-up for measurement of residual stress; two can be operated in powder mode. SANS and neutron radiography facilities exist as well and are currently being reactivated. A brief description of these facilities has been given here and the latest applications have been mentioned.
Based on the HFR NDE research activities using neutron methods, the JRC has recently included in its institutional research programme a new project named NET- Network on Neutron Techniques Standardisation for Structural Integrity. A synonymous European Network is about to be launched with the participation of neutron research facilities, universities, research centres and nuclear energy production vendors and operators from the enlarged EU. The overall objective of NET is to support progress toward improved performance and safety of European energy production systems through the standardization and harmonization of neutron based NDT methods.
The main objective of a recently completed European research project, RESTAND, co-ordinated by JRC, was to develop industrial confidence in the application of the neutron diffraction technique for residual stress measurement and its principal deliverable was a relevant draft code of practice. As no such standard is yet available, and on the basis of this draft standard document, the European Standards Committee on Non-Destructive Testing (CEN TC 138) has established a new Ad hoc Work Group (AHG7). AHG7 is convened by JRC and based on a decision of ISO/TC 135 it has been recently enlarged to include non-European ISO experts. Thus its objective is the development of an International "Technical Specification" on "standard test method for residual stress determination by neutron diffraction".
With regard to HFR investigations of residual stress based on neutron diffraction, it has been shown that a large number of relevant collaborative European research activities have been either completed or are currently running. These focus on mapping of residual stresses in monolithic and bimetallic aluminium and steel alloy based welded components. The HFR Large Component Neutron Diffraction Facility has played a pivotal role in these investigations as has contributed in the recognition of neutron diffraction as a powerful tool in evaluating internal stresses through thickness in welds.
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