Papers by Author: Lyndon Edwards

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Authors: Jeffrey Meng Lee Tan, Michael E. Fitzpatrick, Lyndon Edwards
Abstract: Exact closed-form stress intensity factor (SIF) solutions have been developed for a mode- I through-thickness cracks in an infinite plate. Centre-crack problems have been analysed comprehensively in the literature, but the focus has been on the effect of simple loading about the crack centre. In the current work, the formula of Sih-Paris-Erdogan has been extended to consider the SIF difference on the left and right crack tips, under the local influence of general asymmetric and symmetric stress field. Exact SIF magnification factors convenient for computations have been derived that simultaneously circumvent the problem of crack-tip stress singularity. The solutions so obtained are applied to generate the residual SIFs that would act on a crack growing under the influence of the residual stress fields associated with welded plates and cold-worked holes using the measured residual stress profiles.
Authors: Lyndon Edwards, Mike C. Smith, Mark Turski, Michael E. Fitzpatrick, P. John Bouchard
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
Authors: S. Ganguly, Michael E. Fitzpatrick, Lyndon Edwards
Abstract: The residual stress field in a VPPA welded AA2024 coupon sample was measured by neutron diffraction. A similar sample was measured using synchrotron X-rays to determine the inplane strain directions. The macrostrain obtained from both sources compares well in spite of the significant difference in gauge volume and diffraction geometry. The result confirms the repeatability of the weld process and also shows good correspondence between the two diffraction processes.
Authors: M. Kartal, Rafal M. Molak, Mark Turski, S. Gungor, Michael E. Fitzpatrick, Lyndon Edwards
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.
Authors: O. Zanellato, Michael E. Fitzpatrick, Mark R. Daymond, Lyndon Edwards, Mark Turski
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.
Authors: Lyndon Edwards
Abstract: The safe operation of many structures and components is ensured through the operation of damage tolerant design and evaluation. Substantial residual stresses can exist in many systems and it is important that these are incorporated in damage tolerance calculations of fatigue crack growth. Recent improvements in non-destructive measurement techniques and in the application of weight or Green’s functions methods of including residual stress fields into stress intensity factor (SIF) calculations have enabled predictions of the effects of residual stresses on fatigue crack propagation to be made more readily. Two examples from the aerospace industry, structures containing (i) cold expanded holes and (ii) fusion welds are used to show that presently, although final crack growth lives can be accurately predicted, the details of crack growth are not well represented with initial growth typically being underestimated and later growth being over estimated. It is shown that this is most likely to be due to residual stress redistribution. and that this must be built into fatigue life prediction models if accurate damage tolerance based procedures are to be developed for components and systems containing substantial residual stresses.
Authors: M. Rahman, Michael E. Fitzpatrick, Lyndon Edwards, S. Pratihar, Matthew J. Peel, Axel Steuwer, Thomas Buslaps
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.
Authors: Javier R. Santisteban, L. Fernández, H. Corso, R.L. Martínez, L. Boccanera, Lyndon Edwards, Jonathan A. James, Mark Turski
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.
Authors: Ying Zhang, S. Pratihar, Michael E. Fitzpatrick, Lyndon Edwards
Abstract: The contour method, a newly-invented sectioning technique for residual stress measurement, has the potential to measure the cross-sectional residual stress profile of a weld in a simple and time-efficient manner. In this paper we demonstrate the capability of the contour method to measure cross-sectional residual stress profiles, which are compared with neutron diffraction measurements and show excellent agreement. The results provide useful information for safetycritical design of welded components and optimization of welding parameters, and also illustrate the potential of the contour technique as a powerful tool for residual stress evaluation.
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