Papers by Author: Carsten Ohms

Abstract: Bi-metallic piping welds are frequently used in light water nuclear reactors to connect ferritic steel pressure vessel nozzles to austenitic stainless steel primary cooling piping systems. An important aspect for the integrity of such welds is the presence of residual stresses. Measurement of these residual stresses presents a considerable challenge because of the component size and because of the material heterogeneity in the weld regions. The specimen investigated here was a thin slice cut from a full-scale bi-metallic piping weld mock-up. A similar mock-up had previously been investigated by neutron diffraction within a European research project called ADIMEW. However, at that time, due to the wall thickness of the pipe, stress and spatial resolution of the measurements were severely restricted. One aim of the present investigations by high energy synchrotron radiation and neutrons used on this thin slice was to determine whether such measurements would render a valid representation of the axial strains and stresses in the uncut large-scale structure. The advantage of the small specimen was, apart from the easier manipulation, the fact that measurement times facilitated a high density of measurements across large parts of the test piece in a reasonable time. Furthermore, the recording of complete diffraction patterns within the accessible diffraction angle range by synchrotron X-ray diffraction permitted mapping the texture variations. The strain and stress results obtained are presented and compared for the neutron and synchrotron X-ray diffraction measurements. A strong variation of the texture pole orientations is observed in the weld regions which could be attributed to individual weld torch passes. The effect of specimen rocking on the scatter of the diffraction data in the butt weld region is assessed during the neutron diffraction measurements.

Abstract: A new method has been designed to automatically adapt the geometry of the fusion zone of a weld according to the temperature calculations when the thermal welding heat source parameters are known. In the material definition in a Finite Element code for welding stress calculations, the fusion zone material has different properties than the base material since, among others, the temperature at which the material is stress free is the melting temperature instead of room temperature. In this work, the fusion zone has been determined by postprocessing the thermal model results. The calculated dimensions of the fusion zone have been used as an input for a user subroutine which automatically creates a Finite Element mesh that separates fusion zone elements and base material elements according to the physical reality. It has been shown that the method allows a more accurate stress prediction in the fusion zone and the heat affected zone close to the fusion line, where residual stresses can be important. Neutron diffraction measurements have been used as a validation of the model.

Abstract: An 18 mm thick three-pass slot weld specimen in austenitic stainless steel, manufactured for the purpose of benchmarking Finite Element weld residual stress simulation codes, is currently undergoing extensive non-destructive characterization within a research network. First results from the non-destructive full three dimensional spatially resolved macro-strain mapping in this specimen are presented here. Focussed high-energy synchrotron radiation together with the spiral slit technique was used to obtain depth-resolved information about the variation of lattice parameters. The results show a strong concentration of tensile strain, transverse to the weld at mid-depth of the weld. The maximum longitudinal strains were observed beneath the weld. Furthermore significant weld start- and stop-effects were observed.

Abstract: 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.

Abstract: 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.

Abstract: 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 longterm 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 [1] 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. On the basis of currently measured data we could not draw definitive conclusion about the influence of neutron dose and irradiation duration upon the stress modification.

Abstract: In recent years the use of a special Bayesian approach on averaging ‘round-robin’ residual stress data has been implemented. This averaging approach is useful in that it copes with the situation where systematic errors have occurred in one or more of the measurements and thus diminishes the influence of these particular ‘wrong value’ outlier data points. The analyses not only take into account the measurand value, but also the uncertainties associated with each measurand. It should deal with data that may contain individual members with uncertainties larger than the stated error and assumes that the quoted error bar is only a lower bound on the uncertainty. This work shows what could happen when there is a ‘strong mismatch’ in uncertainties when averaging over a limited amount of data. It has been observed that in a case where there are few data points (for example 5 or less), a strong bias can occur towards data points with a relatively small quoted uncertainty compared to other data points with larger quoted uncertainties. A ‘mismatch’ in uncertainty quotation can arise when averaging very good data with poorer data or when averaging with data obtained from other measurement techniques. This effect is demonstrated in this work by using fictitious data and also based on the example of real measurement data obtained by neutron diffraction.

Abstract: As part of the European project “high and ultrahigh temperature heat exchangers” (HITHEX) the prediction and experimental assessment of the lifetime behaviour, characterisation and qualification of particular CMC materials, including carbon fibre reinforced carbonsiliconcarbides (C/C-SiC), has been executed. Part of the programme of the HITHEX project was the measurement of the strain development within the C/C-SiC tubular specimens from room to high temperature, the results of which are presented here. Residual strains have been determined in several specimens by neutron diffraction at the High Flux Reactor (HFR) of the Joint Research Centre in Petten, The Netherlands. At the HFR two facilities are available for residual strain investigations. Both instruments were utilised in the investigations. The first facility at beam tube HB5, the combined stress and powder diffractometer, employs a constant neutron wavelength of 0.257 nm, and the second facility at HB4, the Large Component Neutron diffraction facility, LCNDF, has a flexible wavelength. The installation of a vacuum furnace has enabled the residual strain measurement of specimens at high temperature on HB4. The furnace had to fulfil three main criteria for the investigation of these specimens; high-temperature, good neutron penetration and negligible oxidation of the specimens. The ceramic specimens, which have outer and inner diameters of 50 and 40 mm, respectively, and a length of 100 mm have been measured to temperatures of up to 1450°C. Measurements were carried out in two directions on the SiC phase of several specimens, i.e. in the radial and tangential (hoop) directions. The implications of these results with respect to the structural integrity assessment of these components at high temperatures are discussed.