Browsing by Author "Steuwer, A"

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    Evolution of crack-tip stresses during a fatigue overload event.
    (Elsevier, 2010-06) Steuwer, A; Rahman, M; Shterenlikht, A; Fitzpatrick, ME; Edwards, L; Withers, PJ
    The mechanisms responsible for the transient retardation or acceleration of fatigue crack growth subsequent to overloading are a matter of intense debate. Plasticity-induced closure and residual stresses have often been invoked to explain these phenomena, but closure mechanisms are disputed, especially under conditions approximating to generalised plane strain. In this paper we exploit synchrotron radiation to report very high spatial resolution two-dimensional elastic strain and stress maps at maximum and minimum loading measured under plane strain during a normal fatigue cycle, as well as during and after a 100% overload event, in ultra-fine grained AA5091 aluminium alloy. These observations provide direct evidence of the material stress state in the vicinity of the crack-tip in thick samples. Significant compressive residual stresses were found both in front of and behind the crack-tip immediately following the overload event. The effective stress intensity at the crack-tip was determined directly from the local stress field measured deep within the bulk (plane strain) by comparison with linear elastic fracture mechanical theory. This agrees well with that nominally applied at maximum load and 100% overload. After overload, however, the stress fields were not well described by classical K fields due to closure-related residual stresses. Little evidence of overload closure was observed sometime after the overload event, in our case possibly because the overload plastic zone was very small. © 2010, Elsevier Ltd.
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    High-resolution strain mapping through time-of-flight neutron transmission diffraction
    (Trans Tech Publications, 2013-11-21) Tremsin, AS; McPhate, JB; Vallerga, JV; Siegmund, OHW; Kockelmann, W; Paradowska, AM; Zhang, SY; Kelleher, JF; Steuwer, A; Feller, WB
    The spatial resolution of time of flight neutron transmission diffraction was recently improved by the extension of photon/electron counting technology to imaging of thermal and cold neutrons. The development of novel neutron sensitive microchannel plates enables neutron counting with spatial resolution of ~55 um and time-of-flight accuracy of ~1 us, with efficiency as high as 70% for cold and ~40% for thermal neutrons. The combination of such a high resolution detector with a pulsed collimated neuron beam provides the opportunity to obtain a 2-dimensional map of neutron transmission spectra in one measurement. The results of our neuron transmission measurements demonstrate that maps of strains integrated along the beam propagation direction can be obtained with ~100 microstrain accuracy and spatial resolution of ~100 um providing there are sufficient neutron events collected. In this paper we describe the capabilities of the MCP neutron counting detectors and present the experimental results of 2-dimensional strain maps within austenitic steel compact tension (CT) crack samples measured at the ENGIN-X beamline of the ISIS pulsed neutron source. © 2014, Trans Tech Publications.
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    Investigation of reference samples for residual strain measurements in a welded specimen by neutron and synchrotron x-ray diffraction
    (Elsevier, 2006-11-15) Paradowska, AM; Finlayson, TR; Price, JWH; Ibrahim, R; Steuwer, A; Ripley, MI
    In this research, both neutron and synchrotron X-ray diffraction techniques have been used to investigate variations in measured d spacing in two reference samples. The reference “stress-free” samples, a set of cubes and a comb sample, were produced using electro-discharge machining (EDM) to obtain a macro-strain-relieved condition. It is shown that there is a variation in microstructure across the parent metal, heat-affected zone (HAZ) and weld metal. A study of the issues in using the cubes and comb sample to provide reference d0 values for the measurement of residual stress in welding is presented. © 2006 Elsevier B.V.
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    Investigation of strain-free reference for residual strain measurements in a weldments by neutron and synchrotron diffraction
    (Elsevier B. V., 2005-11-27) Paradowska, AM; Finlayson, TR; Price, JWH; Ibrahim, R; Steuwer, A; Ripley, MI
    The measurement of residual strain using diffraction techniques relies on the determination of a change in lattice parameter relative to reference or 'strain free' lattice parameter. Elucidating a relevant reference lattice parameter is the most important part of any diffraction-based, residual strain experiment. In this research, neutron and synchrotron diffraction techniques have been used to investigate the strain distribution in strain reference samples used to establish the residual stresses due to welding. It is shown that there is a variation in measured diffraction angles along the length of the weld, in particular for a parent metal, heat-affected zone (HAZ) and weld metal. The stress-free samples, cuboid and a comb sample, were produced A study of the issues in using the cubes and comb sample, were produced using electro-discharge maching (EDM) to obtain a macro-strain relieved condition and reveal information about microstructural and intergranular strain variations, residual stress in welding is presented. A study is presented of the application of a strain-free cuboid and a comb sample in the flux-cored arc welding (FCAW) process
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    Observations of temperature stability of γ-zirconium hydride by high-resolution neutron powder diffraction
    (Elsevier, 2016-03-15) Wang, Z; Steuwer, A; Liu, N; Maimaitiyili, T; Avdeev, M; Blomqvist, J; Bjerkén, C; Curfs, C; Kimpton, JA; Daniels, JE
    The phase evolution in a zirconium–50 deuterium (Zr–50D, at.%) alloy system during thermal cycling has been investigated using in situ high-resolution neutron powder diffraction. The results showed that the peritectoid reaction α-Zr + δ-ZrD → γ-ZrD previously suggested to occur at high temperatures does not take place in the system. Slow cooling, from high temperatures (≥520 K) to room temperature at a rate of 5 K min−1, promoted the γ-hydride formation rather than fast cooling as reported earlier. In contrast to the observation that the δ-hydride present in the system remained at temperatures up to 740 K, the produced γ phase transformed to δ-hydride in the temperature range of 370 K–559 K, with the transformation completing at approximately 559 K. It is confirmed that the formation of the γ-hydride was reproducible with slow cooling, and a diffusion-controlled sluggish δ-to γ-hydride transformation is suggested to be responsible for the favorable development of γ-hydride during slow cooling. © 2015 Elsevier B.V.