Papers by Author: Lyndon Edwards

Abstract: The safe operation of both thermal and nuclear power plant is increasingly dependent upon structural integrity assessment of pressure vessels and piping. Furthermore, structural failures most commonly occur at welds so the accurate design and remnant life assessment of welded plant is critical. The residual stress distribution assumed in defect assessments often has a deciding influence on the analysis outcome, and in the absence of accurate and reliable knowledge of the weld residual stresses, the design codes and procedures use assumptions that yield very conservative assessments that can severely limit the economic life of some plant. However, recent advances in both the modeling and measurement of residual stresses in welded structures and components open up the possibility of characterising weld residual stresses in operating plant using state-of–the–art fully validated Finite Element simulations. This paper describes research undertaken to predict residual stresses in stainless steel welds in order to provide validated reliable, accurate Structural Integrity assessment of nuclear power plant components

Abstract: This paper summarizes the results of a neutron diffraction study of a single weld bead on a rectangular austenitic stainless steel plate. The measurement was carried out at SALSA, the engineering strain scanner at the ILL, Grenoble, France. The work has been carried out under the European NET project, and is a round robin exercise of residual stress simulation and validation benchmark in a stainless steel bead-on-plate (BOP) weldment. A monochromatic beam of wavelength 1.494 Å was used and the lattice spacing of {311} crystallographic plane was measured. The principal strain measured in the plate was corrected by measuring small cube sample of 3×3×3 mm3 as stress free reference. The measured strain was then used to calculate the principal stress distribution. Finally, the measured strain was compared with the strain measured in a similar specimen in a pulsed neutron source by the time-of-flight (TOF) technique.

Abstract: This paper reports results of an in-situ compression experiment carried out on a hot rolled Zircaloy-4 plate at ENGIN-X, ISIS. The experiment was aimed at characterizing the plastic anisotropy of the alloy, which can give rise to high intergranular stresses in the polycrystal. As expected from the crystal anisotropy, the various lattice reflections had very different behaviours. In the compression directions, the basal <0002> reflections appeared to bear much more load than the other planes. The resulting intergranular elastic strains could therefore reach up to 5000 microstrain after 10% total deformation, and were responsible for high type II residual stresses after unloading. Considering the macroscopic behaviour, the normal direction had higher mechanical properties than the other two processing directions. The strong texture measured from EBSD measurements suggest that the crystal anisotropy has been brought to a macroscopic level. The experiment also evidenced a significant change in texture for compression along the rolling direction which indicates twinning activation.

Abstract: Current analytical techniques for simulating the generation of residual stresses within welded structures make approximations and simplifications relating to the mechanical properties of the weld metal. This is due to the paucity of knowledge on the anisotropic nature of weld metal. Consequently, the study of elastic and plastic anisotropy, within metals has been a subject of considerable interest to both experimentalists and modellers alike. This study investigates the elastic and plastic response of crystal lattice planes within single-pass and multi-pass austenitic steel weld metal using time-of -flight (TOF) neutron diffraction. As this material is often used for the fabrication of high temperature, pressure vessel components, it is important to understand the evolution of lattice strains within each set of planes during elastic and plastic loading. Neutron diffraction measurements were carried out for both single and multi-pass weld metal subjected to in-situ uni-axial tensile loading. The elastic response of individual reflections was recorded by diffraction measurements. All measurements were carried out at the ENGIN-X facility at ISIS, UK. The objective of the measurements was to determine the Young's modulus and characterise the elastic and plastic anisotropy of the weld metal. Measurements indicate a significant variation between the Young's modulus for the two materials, with values close to 210 GPa for the singlepass weld metal and 90-130 GPa for the multi-pass welds. These results are explained in terms of the texture of the welds.

Abstract: There have been many theoretical studies to predict the stress fields around the tip of a growing fatigue crack. However, until recently the highly-localized, small scale nature of the stresses has meant that direct measurement has not been possible. With the current generation of synchrotron X-ray sources, sub-millimetre sampling dimensions are now possible, and it has become possible to evaluate directly the stresses at the tip of a fatigue crack and to see how the stresses evolve as the result of an overload, for example. In this paper we present results of synchrotron X-ray diffraction analysis of the stress fields around a fatigue crack in aluminium alloy 5091 (Al-Mg-Li-C-O); this is a dispersion-strengthened alloy with a fine grain size, which makes it ideal for such experiments. Compact tension (CT) specimens were prepared with constant amplitude fatigue loading. The energy dispersive X-ray diffraction (EDXRD) technique was used for measuring strains around the crack tip along the mid thickness of the specimen under in-situ loading. The measurement was carried out at the ESRF (European Synchrotron Radiation Facility), Grenoble, France on the ID15A beam line. The experimental crack tip stresses have been compared with the analytical fracture mechanics solution.

Abstract: The aim of this collaborative study was to measure mechanical properties of 14MoV67-3 steel taken from small sections of material machined in-situ from an operating high pressure collector pipe after different operating lifetimes (from 0h to 186 000h) at elevated temperatures (540°C). Conventional methods of measuring mechanical properties of materials, such as the uniaxial tensile test require relatively large test samples. This can create difficulties when the amount of material available for testing is limited. One way of measuring mechanical properties from small quantities of material is using micro tensile test samples. In this work, micro-samples with a total length of 7.22mm were used. Digital Image Correlation method (DIC) was employed for the strain measurements in a uniaxial tensile test. This paper shows that there is measurable difference in the yield, ultimate tensile strength and elongation to failure as a function of the plant operating conditions. This work demonstrates, therefore, a ‘semi-invasive’ method of determining uniaxial stress-strain behaviour from plant components.

Abstract: The aim of this study was to develop a method of extracting local mechanical properties from weld metal by strain mapping using the digital image correlation (DIC) technique. The feasibility of determining local stress-strain behaviour in the weld zone of a 316H stainless steel pipe with a girth weld was investigated by tensile tests of specimens machined from the pipe so that it contained the weld at its centre. The tensile test was recorded using a high resolution digital camera and the DIC technique was used to obtain the complete set of full field displacement maps during the tensile test. The local strain was calculated at every sub-region of 32×32 pixels, which enabled the local stress-strain behaviour for this region to be determined. Results from these tests show the variability of the elastic modulus, yield stress and UTS across the weld. To check the reliability of the technique, a set of micro tensile samples, with gauge length of 3.7mm and crosssectional area of 0.7×0.7 mm2, were machined from the various locations in and around the weld zone. The comparison of stress-strain curves determined from micro-samples to stress-strain curves from the corresponding locations within a larger more conventional tensile specimen shows reasonably good agreement.

Abstract: Plastic anisotropy can affect the strains measured by neutron diffraction. If this is not properly accounted for significant errors can result in the calculated stresses. This paper illustrates addresses this issue using measurements of the residual strain field around a 60 mm long single weld bead deposited on the surface of a 17 mm thick stainless steel plate. Measurements were made on ENGIN-X, the engineering spectrometer at the ISIS facility of the Rutherford Appleton Laboratory (UK). Diffraction spectra from these measurements have been fitted using both single and multi-peak fitting approaches. Both residual strain and stress results have been presented for (111) and (200) single peak fits and compared to a multi-peak fit (Rietveld) analysis. Results from these analyses have revealed significant anisotropy in the response of the individual lattice planes. This effect is most severe in measurements carried out in the normal direction and is shown to be predicted using an elasto-plastic self-consistent model.

Abstract: This paper describes the measurement of longitudinal residual stresses within specially designed 200x180x25mm groove weld specimens. The purpose of these measurements was to compare the residual stress field arising from single and multi-pass weld beads laid down within the constraint of a groove in order to validate finite element simulations of the welding process. Measurements were made over the cross section at the mid-bead length, utilising the relatively new Contour method and neutron diffraction. Results from these measurements indicate a larger peak tensile longitudinal residual stresses within the weld region of the multi-pass weld sample. Good agreement is found between both techniques.

Abstract: We have studied the effect of a post-weld heat treatment on plasma arc welds on Zircaloy 4 plates. The samples consist of two 100 mm long, 50 mm wide, and 6.25 mm thick plates, welded along the rolling (longitudinal) direction. The heat-treatment consisted of a steady increase in temperature from room temperature to 450oC over a period of 4.5 hours; followed by cooling with an equivalent cooling rate. Residual strains and stresses along the longitudinal, transverse and normal directions on an as-welded and a heat-treated specimen were measured by neutron diffraction on the ENGIN-X beamline at the Isis Facility, Rutherford Laboratory, UK. Peak tensile stresses of (105±25) MPa were found in the as-welded specimen, which were reduced to (70±20) MPa after the heat-treatment. Thermal compressive stresses of (-80±20) MPa were found along the normal direction, which were not affected by the heat treatment. The use of a full-pattern Rietveld refinement for the determination of bulk strains in Zircaloy specimens is also discussed.