Papers by Author: Philip J. Withers

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Authors: Howard J. Stone, Harshad K.D.H. Bhadeshia, Philip J. Withers
Abstract: The level of residual stresses generated in fusion welds has been a major area of interest for many years. For steels, a major influence on the final state of stress is through martensitic transformation. This is because the martensitic transformation is accompanied by significant shear and volume strains. One way to mitigate the development of residual stress is by controlling the onset of the transformation such that the associated strain is able to compensate for thermal contraction all the way down to ambient temperatures. In the past it has only been possible to follow the evolution of the phase transformation during cooling of the weld metal using indirect methods such as dilatometry and differential scanning calorimetry. This paper describes the first work in which the phases present are characterized directly during the cooling of reheated weld metal at conditions typical of those encountered during welding by installing a thermomechanical simulator on a synchrotron diffraction beam line at ESRF.
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Authors: T. James Marrow, David Gonzalez, Mohsin Aswad, Joao Quinta Da Fonseca, Philip J. Withers
Abstract: A 3D model for intergranular thermal stresses in coarse polycrystalline alumina has been derived using Diffraction Contrast Tomography. Larger tensile thermal strains develop when the (0001) pole of adjacent grains lies closer to the grain boundary normal. This agrees with observations of cracked boundaries, obtained through digital image correlation of in-situ observations in fine alumina.
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Authors: M. Karadge, Benedict M.B. Grant, Giovanni Bruno, Javier R. Santisteban, Philip J. Withers, Michael Preuss
Abstract: In this study we present a direct comparison between residual strain measurements carried out on the same inertia friction weld using ENGIN-X at ISIS, UK and the new strain scanner SALSA at ILL, France. ENGIN-X is a time of flight (TOF) instrument, which receives neutrons from a neutron spallation source, while the SALSA Strain-Imager, a high resolution diffractometer, is based at a research reactor source with a continuous neutron flux and is operated with a constant wavelength. The purpose of this study was to demonstrate a confidence in crosscomparing future strain measurements to be performed at ENGIN-X and SALSA. Measurements were carried out on medium size inertia friction welded nickel superalloy test-piece, which show no significant crystallographic texture across the weld line. The results demonstrate that, even though residual stresses determined on SALSA only rely on a single peak analysis (in this case the (111) reflection), the results show excellent agreement with the measurements carried out on ENGIN-X, where strain is determined from multi-peak Rietveld analysis.
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Authors: Fabrice Pierron, Sam A. McDonald, D. Hollis, Philip J. Withers, A. Alderson
Abstract: This paper deals with the use of X-ray tomography and Digital Volume Correlation in order to study the mechanical behaviour of a low density polymeric auxetic foam. First, the metrological performances of the procedure are assessed using rigid body translation. Then, tensile test results are analyzed for two load steps and comments on the deformation process of the foam are given with a specific view to Poisson’s effect.
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Authors: Wei Li, Michael Preuss, Philip J. Withers, Dragos Axinte, Paul Andrews
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Authors: Wei Li, Philip J. Withers, Michael Preuss, Judith Shackleton, Paul Andrews
Abstract: This paper investigates the variation of residual stress with depth and radial location in a nickel base superalloy, RR1000, introduced by face finish turning. X-ray diffraction stress measurement has revealed that the hoop stress at the surface becomes less tensile towards the centre of the face, whilst the level of radial sub-surface compression increases. The unstrained lattice spacing d0 and the diffraction peak width (FWHM) were used to make inferences regarding the thermal excursion and the plastic work, respectively. It was found the increase in the compressive stress from the outer towards the inner radius was associated with an increase in thermal excursion.
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Authors: Koichi Akita, Masatoshi Kuroda, Philip J. Withers
Abstract: Residual stress induced by laser single pulse irradiation was analyzed using a dynamic finite element code, ABAQUS/Explicit. The effects of the magnitude and length of a surface pressure pulse having a circular top-hat shape on the final residual stress in Ti-6Al-4V were investigated. A high peak pressure and/or a long pulse duration was effective in generating large compressive residual stress deep beneath the surface. However, large tensile residual stress features occurred near the centre and edge of the laser spot on the surface for high pressure and/or long pulse durations due to a radial focusing effect. Use of shorter pulse durations avoided this. The peak pressure (3GPa) required to induce a surface compressive residual stress across the whole area of the spot was slightly higher than the threshold pressure needed to plastically deform the surface.
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Authors: E.C. Oliver, Mark R. Daymond, Philip J. Withers
Abstract: The influence of texture and anisotropy on the generation of intergranular stresses in clock-rolled zirconium is investigated using neutron diffraction and elastoplastic self-consistent modelling. Comparison between experimental data and model calculations indicates that the operation mainly of prismatic and basal slip explains the trends in intergranular stress evolution during in-plane tensile and through-thickness compressive deformation, whilst twinning plays a significant role during in-plane compression.
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Authors: Pablo Lopez-Crespo, A. Shterenlikht, Eann A Patterson, J.R. Yates, Philip J. Withers
Abstract: A novel methodology based on a combination of experimental and analytical methods is used for monitoring the stress intensity factor in fatigue cracks subjected to constant amplitude loads. Full-field displacement information is fitted, following a multi-point over-deterministic approach, to an analytical model. This is developed from Muskhelishvili’s complex formulation. The methodology allowed accurate monitoring of the stress intensity factor during three fatigue cycles when small-scale yielding conditions were achieved. Moreover for larger loads where important plastic deformation occurs around the crack tip, Dugdale’s correction accounted for the differences between theoretical and calculated stress intensity factors. Accordingly the tool provides an indirect approach for measuring crack tip plasticity. Due to the fact that image correlation is relatively simple to use and is a non-contacting technique, the approach pioneered in this work seems ideal for monitoring fatigue cracks in industrial applications.
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Authors: David G. Richards, Philip B. Prangnell, Philip J. Withers, Stewart W. Williams, Andrew Wescott, E.C. Oliver
Abstract: Although Friction Stir Welding (FSW) avoids many of the problems encountered when fusion welding high strength Al-alloys, it can still result in substantial residual stresses that have a detrimental impact on service life. An FE model has been developed to investigate the effectives of the mechanical tensioning technique for controlling residual stresses in FSWs. The model purely considered the heat input and the mechanical effects of the tool were ignored. Variables, such as tensioning level, heat input, and plate geometry, have been studied. Good general agreement was found between modelling results and residual stress measurements, justifying the assumption that the stress development is dominated by the thermal field. The results showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~ 50% of the room temperature yield stress, tensile were replaced by compressive residual stresses within the weld.
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