Browsing by Author "Fitzpatrick, M. E."

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    The effect of weld residual stresses and their re-distribution with crack growth during fatigue under constant amplitude loading
    (Elsevier, 2010-04) Liljedahl, C. D. M.; Zanellato, O.; Fitzpatrick, M. E.; Lin, J.; Edwards, L.
    In this work the evolution of the residual stresses in a MIG-welded 2024-T3 aluminium alloy M(T) specimen during in situ fatigue crack growth at constant load amplitude has been measured with neutron diffraction. The plastic relaxation and plasticity-induced residual stresses associated with the fatigue loading were found to be small compared with the stresses arising due to elastic re-distribution of the initial residual stress field. The elastic re-distribution was modelled with a finite element simulation and a good correlation between the experimentally-determined and the modelled stresses was found. A significant mean stress effect on the fatigue crack growth rate was seen and this was also accurately predicted using the measured initial residual stresses.
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    An Integrated Approach to the Determination and Consequences of Residual Stress on the Fatigue Performance of Welded Aircraft Structures.
    (2006-02-01T00:00:00Z) Edwards, L.; Fitzpatrick, M. E.; Irving, Phil E.; Sinclair, I.; Zhang, X.; Yapp, David
    Although residual stress in welded structures and components has long been known to have an effect on their fatigue performance, access to reliable, spatially accurate residual stress field data has been limited. Recent advances in neutron and synchrotron X-ray diffraction allow a far more detailed picture of weld residual stress fields to be obtained that permits the development and use of predictive models that can be used for accurate design against fatigue in aircraft structures. This paper describes a fully integrated study of the three- dimensional residual stress distribution accompanying state-of-the-art fusion welds in 2024-T4 aluminum alloy, and how it is affected by subsequent machining and service loading. A particular feature of this work has been the development of techniques allowing the nondestructive evaluation of the residual stress field in the full range of specimens used to provide the design data required for welded aircraft structures and the integration of this information into all aspects of damage tolerant design.
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    Life extension techniques for aircraft structures-Extending durability and promoting damage tolerance through bonded crack retarders
    (2011-12-31T00:00:00Z) Irving, Phil E.; Zhang, Xiang; Doucet, J.; Figueroa-Gordon, Douglas J.; Boscolo, M.; Heinimann, M.; Shepherd, G.; Fitzpatrick, M. E.; Liljedahl, D.; Jerzy, Lomorowski
    This paper explores the viability of the bonded crack retarder concept as a device for life extension of damage tolerant aircraft structures. Fatigue crack growth behaviour in metallic substrates with bonded straps has been determined. SENT and M(T) test coupons and large scale skin-stringer panels were tested at constant and variable amplitude loads. The strap materials were glass fibre polymer composites, GLARE, AA7085 and Ti-6Al-4V. Comprehensive measurements were made of residual stress fields in coupons and panels. A finite element model to predict retardation effects was developed. Compared to the test result, predicted crack growth life had an error range of -29% to 61%. Mechanisms and failure modes in the bonded strap reinforced structures have been identified. The strap locally reduces substrate stresses and bridges the crack faces, inhibiting crack opening and reducing crack growth rates. In the absence of residual stress, global stiffness ratio accounts for effects of both strap modulus and strap cross section area. In elevated temperature cure adhesives, retardation performance was best in aluminium and GLARE strap materials, which have the closest thermal expansion coefficient to the substrate. Strap materials of high stiffness and dissimilar thermal expansion coefficient such as titanium had poor retardation characteristics.
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    Problems in using a comb sample as a stress-free reference for the determination of welding residual stress by diffraction
    (Elsevier Science B.V., Amsterdam., 2010-12-31T00:00:00Z) Ganguly, Supriyo; Edwards, L.; Fitzpatrick, M. E.
    Precise measurement of a stress-free reference lattice parameter is vital in the determination of residual stress by diffraction techniques using θ/2θ-based analyses. For the evaluation of the residual strain profile in fusion-welded material, it is particularly important to correct the measured strain point-by-point by a stress-free reference for each spatial location across the weld. This is to take into account the compositional and microstructural variation across the weld caused by the thermal cycle of welding, as local changes in solute content of the parent alloy cause changes in the stress-free lattice parameter. Although ideally such measurements should be obtained from small cubes machined from the weld, the use of a comb sample has previously been proposed as a macro-stress-free reference, with the assumption that each finger of the comb is of insufficient dimension to hold a macro-stress field. This paper presents an approach towards analysis of the stress-free reference values using a comb sample extracted from a variable polarity plasma arc (VPPA) welded plate. It is shown that there is inter-granular stress and retained macro-stress within the comb teeth, and an experimental approach to deal with the problem is proposed.
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    Residual stress and texture control in Ti-6Al-4V wire + arc additively manufactured intersections by stress relief and rolling
    (Elsevier, 2018-04-08) Honnige, Jan Roman; Colegrove, Paul A.; Ahmad, B.; Fitzpatrick, M. E.; Ganguly, Supriyo; Lee, T.; Williams, Stewart W.
    Additively manufactured intersections have the theoretical risk to contain hydrostatic tensile residual stresses, which eventually cannot be thermally stress relieved. The stresses in Ti-6Al-4V wire + arc additively manufactured (WAAM) intersections are lower compared to single pass walls and stresses in continuous walls are larger compared to discontinuous walls with otherwise identical geometry. Thermal stress relief was found to virtually eliminate them. Inter-pass rolling can yield the desired grain refinement, without having any noteworthy influence on the development of residual stresses. The strain measurement itself by neutron diffraction is facilitated by the refined microstructure, because the otherwise textured microstructure produces anisotropic peak intensity, not allowing Pawley refinement. Without rolling, the {101¯1} and {101¯3} family of hcp planes are the only ones that diffract consistently in the three principal directions.
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    Variation of mechanical properties in a multi-pass weld measured using digital image correlation
    (2009-06-04T00:00:00Z) Acar, M.; Gungor, S.; Ganguly, Supriyo; Bouchard, P. J.; Fitzpatrick, M. E.
    As part of a programme investigating the structural integrity of welds, the Digital Image Correlation (DIC)technique was used to obtain the full-field strain distribution during tensile testing of cross-weld specimens cutfrom a multi-pass girth welded pipe. The displacement maps were analyzed using Matlab scripts to compute localstress-strain variations from which the local proof stress values were extracted. It has been found that the DICparameters, such as the size of interrogation windows (subsets) and speckle sizes, have significant effects on the displacement values due to the local variations in the mechanical properties within the weld between the passes.The DIC parameters were therefore optimized using monolithic aluminium alloy specimens with stressconcentrations giving similar displacement gradients. The use of a high resolution camera at high magnificationallowed the variations between the welding passes to be observed. The variation of the mechanical properties inthe weld region is correlated with the full field hardness maps of the same region.
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    Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351
    (Elsevier Science B.V., Amsterdam., 2009-06-30T00:00:00Z) Liljedahl, C. D. M.; Brouard, J.; Zanellato, O.; Lin, J.; Tan, M. L.; Ganguly, Supriyo; Irving, Phil E.; Fitzpatrick, M. E.; Zhang, X.; Edwards, L.
    The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.