A complete reassessment of standard residual stress uncertainty analyses using neutron diffraction emphasizing the influence of grain size

The determination of residual stress in engineering materials with large grains is a challenge when it comes to using diffraction techniques. Not only are the accuracies of the residual stresses themselves important but also the accurate evaluation of their uncertainties. An austenitic steel three-pass slot weld (NeT- TG4) with varying grain size high-lights the potential problems with the data evaluation when grain size is not taken into account whilst measuring strain. Neutron diffraction results are compared with each other (with combinations of slit and radial oscillating collimator beam defining optics) and with high energy synchrotron radiation results with a spiral slit gauge volume defining system. The impact of the grain size on the determination of residual stresses and their associated uncertainties when using diffraction techniques in engineering components is emphasized and discussed. A simple model to estimate the extra random uncertainty contribution due to the so-called grain size statistics is applied and verified. The benefit of continuous or stepwise oscillation to increase the number of detected grains on the detector is discussed together with how to optimize the time of a measurement. From the data obtained, best practice guidelines will be suggested on dealing with large grains when determining strain and stress with neutron diffraction

A Step Towards a Complete Uncertainty Analysis of Residual Stress Determination Using Neutron Diffraction

Up to now the tendency in residual stress determination (using neutron diffraction) has been to assess the uncertainties in terms of the propagation of 'fitting uncertainty' of the Bragg peaks only. There are many other sources of uncertainty, some more obvious than others, that should be taken into account or at least be considered in terms of their impact when interpreting the data. These cover not just the instrument calibration and characteristics and the technique itself but also the properties of the sample. A discussion of how best to combine the uncertainty of all contributing factors will be made. These factors (on the sample side) will include variations in chemical composition, grain size related problems, surfaces/interfaces cutting through the sampling volume, texture variations within the sampling volume, presence of intergranular strains (plastic anisotropy) etc. The knowledge of appropriate elastic constants, for example, and their uncertainty is necessary for a more reliable stress determination. One should also be aware of the more subtle influences on the elastic constants such as texture or chemical variation. This should be a step in the right direction for a ¿unified uncertainty analysis¿ covering all possible aspects of uncertainty in residual stress determination using neutron diffraction.JRC.DG.F.4-Safety of future nuclear reactor

Residual Stress Measurement by Neutron Diffraction in a Single Bead on Plate Weld - Influence of Instrument and Measurement Settings on the Scatter of the Results

Residual stress measurements on a single bead weld on a steel plate had been performed at the High Flux Reactor of the Joint Research Centre in 2003. For these measurements a relatively old diffractometer had been used. The results obtained were characterized by significant scatter of the data, and the measurement quality suffered from the short movement ranges of the specimen positioning table as well. In 2008, a second, nominally identical, specimen from the same activity was investigated on a second diffractometer, which allowed the repetition of the residual stress measurements using different measurement settings. The present paper compares the old and the new measurement results, with a view to assessing the impact of the instrumental settings on their quality. It has been found that the overall stress distributions from the 2003 and the 2008 measurements were in fact very similar. Nevertheless, the new settings used, such as in-situ specimen rocking, extended measurement duration, increased density of measurement positions and spatial resolution among others, have been found to have considerable impact on the real and on the apparent scatter of the experimental results showing that the presented methodologies can be used for improvement of neutron diffraction measurements.JRC.DG.F.4-Safety of future nuclear reactor

Residual Stress Analyses by Neutron Diffraction in Irradiated Double-V Butt Welded Steel Plates

Neutron irradiation is known to have a considerable impact on the mechanical characteristics and the behaviour of materials and components. The distribution of residual stresses is one of the properties affected by irradiation. However, because of the difficulties in performing measurements in radioactive components, not many experimental data have to date been collected. At the High Flux Reactor (HFR) of the European Commission¿s Joint Research Centre (JRC), a facility has been developed for residual stress measurements in steel specimens subjected to long-term irradiation. The objective of this development was to establish the neutron radiation induced changes in the residual stresses around welds in test pieces representative of the core shroud of boiling water reactors. Residual Stress measurements on such double-V butt welds in stainless steel plates after irradiation exposure have been performed by neutron diffraction using this facility. The comparison with measurements in non-irradiated companion specimens showed that irradiation changed the distribution of residual stresses. The results suggested that the impact of irradiation varied with the distance of the test location from the specimen surface. No conclusions could be drawn from the measurements as far as the importance of irradiation duration is concerned.JRC.F.3-High Flux and Future Reactor

The NeT Task Group 4 residual stress measurement and analysis round robin on a three-pass slot-welded plate specimen

Accurate prediction and measurement of residual stresses in welds is an important part of assuring their short and long-term structural performance in high value, safety critical engineering components and structures. However, both measurements and predictions of weld residual stresses often exhibit high levels of variability that are not widely appreciated. The mission of the European Network on Neutron Techniques Standardization for Structural Integrity (NeT) is to develop experimental and numerical techniques and standards for the reliable characterisation of residual stresses in structural welds. The NeT Task Group 4 project examined residual stresses in a three-pass slot-welded plate specimen fabricated from AISI 316L(N) austenitic stainless steel plate. Several nominally identical specimens were fabricated under closely controlled conditions, with detailed records kept of the manufacturing history, weld process parameters, transient temperatures during welding, and the resulting geometric distortions. Comprehensive stress-strain material property characterisation was then undertaken, extending to the isothermal cyclic tests necessary to calibrate the mixed isotropic-kinematic material hardening models required for accurate weld residual stress prediction. Parallel residual stress measurement and simulation round robins were performed by a large number of participants from around the world. Residual stresses were measured using neutron and high energy synchrotron diffraction, surface X-ray diffraction, surface and deep hole drilling, the contour method, and ultrasonics. Neutron diffraction measurements were made at eight different instruments. The diffraction measurements database alone is large enough to generate reliable mean profiles, to identify clear outliers, and to establish that there is no statistically significant difference in the residual stress field in the specimens used for the non-destructive measurements. NeT Task Group 4 gives a unique insight into the real-world variability of diffraction-based residual stress measurements, and forms a reliable foundation against which to benchmark other measurement methods. NeT Task Group 4 is also a unique test bed for the development and validation of weld residual stress simulation techniques in austenitic stainless steel. Its combination of extensive materials characterisation, accurately characterized welding temperature transients, and reliable residual stress and distortion measurements is currently unrivalled. About thirty finite element simulations were submitted to the network over the course of the project, giving insights into the required accuracy of welding thermal solutions, the mechanical solution accuracy achievable using optimized material constitutive models, and the level of acceptable error in finite element residual stress simulation results for use in structural integrity assessments of high integrity engineering components.JRC.G.I.4-Nuclear Reactor Safety and Emergency Preparednes

Combination of Stimulation and Experiment in Designing Repair Weld Strategies - A Feasibility Study

Numerical simulations, based on an off-the-self commercial finite element (FE) code and experimental tests using the neutron diffraction (ND) technique, are combined in an attempt to evaluate post-weld heat treatment (PWHT) of a letterbox-type repair weld, in respect of its effect on the residual stress field. 21/4CrMo steel plates with an 18-pass repair weld were heat treated at various temperature levels and for different durations. Due to the prohibitive cost of a complete residual stress mapping, ND tests were performed only at selected specimen locations. In this sense, FE simulation acts as a supplement to ND, since it predicts the complete residual stress field. Uncoupled quasi-static thermoelasticity in conjunction with an element activation/deactivation technique, simulating deposition of new weld material, are combined in a 3D FE analysis. Grouping of the 18 weld beads in lumps, following a sensitivity analysis, reduces computational costs to feasible levels, whereas a creep strain hardening law is used to simulate stress relaxation during PWHT. Computed residual stresses are compared to ND measurements for verification purposes. Comparison of heat treated specimen measurements to heat treated and untreated specimen predictions illustrates that PWHT has a strong effect on the residual stress field, achieving significant stress relaxation.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen

Neutron diffraction stress determination in W-laminates for structural divertor applications

Neutron diffraction measurements have been carried out to develop a non-destructive experimental tool for characterizing the crystallographic structure and the internal stress field in W foil laminates for structural divertor applications in future fusion reactors. The model sample selected for this study had been prepared by brazing, at 1085°C, 13 W foils with 12 Cu foils. A complete strain distribution measurement through the brazed multilayered specimen and determination of the corresponding stresses has been obtained, assuming zero stress in the through-thickness direction. The average stress determined from the technique across the specimen (over both ‘phases’ of W and Cu) is close to zero at −17±32 MPa, in accordance with the expectations.JRC.F.4-Innovative Technologies for Nuclear Reactor Safet

NET TG1 - Residual Stress Assessment by Neutron Diffraction and Finite Element Modeling on a Single Bead Weld on a Steel Plate

In the context of the efforts of Task Group 1 (TG1) of the European Network on Neutron Techniques Standardization for Structural Integrity (NET), the Joint Research Centre (JRC) participated in the experimental round robin campaign for residual stress analysis on a single weld bead on a steel plate. In parallel, the University of Patras (UP), in collaboration with the JRC, contributed to the corresponding numerical analysis round robin exercise. Neutron diffraction measurements were performed on a specimen, designated as A12, using the residual stress diffractometer at beam tube HB5 at the High Flux Reactor (HFR) in Petten, The Netherlands. Several line scans of strains and stresses were performed in accordance with an experimental protocol devised for this exercise and their results are presented in this paper. Two scans were made along the weld longitudinal direction beneath the upper surface of the plate, three were made in the weld transverse direction, and three through the thickness of the plate. The measured residual stresses are presented in detail. The measurements confirm that the stress distribution around this single weld bead on a plate is intrinsically 3-dimensional. The procedure followed by UP in the numerical assessment of the problem is presented in detail. The numerical results are presented in direct comparison to the JRC measurement data.JRC.F.4-Safety of future nuclear reactor

Statistical Analysis of Residual Stress Determinations Using Neutron Diffraction

Within Task Group 1 (TG1) of the European Network on Neutron Techniques standardization for Structural Integrity (NeT), residual stress determinations by neutron diffraction were performed on a single weld bead on a plate (BoP) specimen by several laboratories. By following a pre-defined protocol, residual stress distributions have been estimated in the plate. Each residual strain/stress instrument at each institute have their own characteristics and their own methods by which the data is analyzed, however, the final estimate of the stress distributions are expected to be ¿the same¿ to within a quoted uncertainty. This implies that the quoted uncertainty for each stress determination should be realistic. At specific locations in the BoP specimen a modified Bayesian approach was used to obtain an average stress. This special approach is less affected by outliers, although care must be taken as it has been observed that in the case where there are few data points in the average, a strong bias can occur towards data points with a relatively small quoted uncertainty. In the data analyses used here, a ¿fairer average¿ less influenced by such bias is implemented. Presented here is an overview of the main features that influence the stress and associated uncertainty determination, such as reference values, choice of moduli values, grain size effects and positioning etc.JRC.F.4-Safety of future nuclear reactor

Minimizing and Characterizing Uncertainties in Neutron Strain Measurements with Special Attention to Grain Size Effects

The accurate determination of strain during measurement using neutron diffraction depends on many factors. The statistical uncertainty of the diffraction data is not always the most important contributor to the total uncertainty in the measured strain. Other contributors, such as sample positioning, size and shape of the sampling (gauge) volume and the size and distribution of grains within the sampling volume, often play an important role as well. Grain size issues have been the least studied and their impact is often ignored even though the potential uncertainty contribution can be large. Certain methods such as oscillating the sample during measurement can help in reducing the magnitude of the grain size effect and hence also that of the related uncertainty contribution. A thorough characterization of uncertainties due to grain size effects however, in terms of absolute values that should be added to the uncertainties propagated from the Bragg peak fits, have not yet been implemented. This paper will present an improved method to characterize and estimate absolute uncertainty values due to grain size effects.JRC.G.I.4-Nuclear Reactor Safety and Emergency Preparednes