Publications
199 results found
Jiang J, Yang J, Zhang T, et al., 2015, On the mechanistic basis of fatigue crack nucleation in Ni superalloy containing inclusions using high resolution electron backscatter diffraction, Acta Materialia, Vol: 97, Pages: 367-379, ISSN: 1873-2453
A series of interrupted three-point bend low-cycle fatigue tests were carried out on a powder metallurgy FHG96 nickel superalloy sample containing non-metallic inclusions. High resolution electron backscatter diffraction (HR-EBSD) was used to characterise the distribution and evolution of geometrically necessary dislocation (GND) density, residual stress and total dislocation density near a non-metallic inclusion. A systematic study of room temperature cyclic deformation is presented in which slip localisation, cyclic hardening, ratcheting and stabilisation occur, through to crack formation and microstructurally-sensitive propagation. Particular focus is brought to bear at the inclusion–matrix interface. Complex inhomogeneous deformation structures were directly observed from the first few loading cycles, and these structures were found not to vary significantly with increasing number of cycles. A clear link was observed between crack nucleation site and microstructurally-sensitive growth path and the spatially-resolved sites of extreme values of residual stress and GND density.
Lan B, lowe M, dunne F, 2015, A generalized spherical harmonic deconvolution to obtain texture of cubic materials from ultrasonic wave speed, Journal of the Mechanics and Physics of Solids, Vol: 83, Pages: 221-242, ISSN: 0022-5096
In this paper, the spherical harmonic convolution approach for HCP materials (Lan et al., 2015) is extended into a generalised form for the principal purpose of bulk texture determination in cubic polycrystals from ultrasonic wave speed measurements. It is demonstrated that the wave speed function of a general single crystal convolves with the polycrystal Orientation Distribution Function (ODF) to make the resultant polycrystal wave speed function such that when the three functions are expressed in harmonic expansions, the coefficients of any one function may be determined from the coefficients of the other two. All three Euler angles are taken into account in the description of the ODF such that the theorem applies for all general crystal systems.The forward problem of predicting polycrystal wave speed with knowledge of single crystal properties and the ODF is solved for all general cases, with validation carried out on cubic textures showing strong sensitivity to texture and excellent quantitative accuracy in predicted wave speed amplitudes. Importantly, it is also revealed by the theorem that the cubic structure is one of only two crystal systems (the other being HCP) whose orientation distributions can be inversely determined from polycrystal wave velocities by virtue of their respective crystal symmetries. Proof of principle is then established by recovering the ODFs of representative cubic textures solely from the wave velocities generated from a computational model using these texture inputs, and excellent accuracies are achieved in the recovered ODF coefficients as well as the resultant pole figures. Hence the methodology is argued to provide a powerful technique for wave propagation studies and bulk texture measurement in cubic polycrystals and beyond.Keywords Texture; Generalised spherical convolution; Ultrasound; Cubic polycrystals
Zhang T, Jiang J, Shollock BA, et al., 2015, Slip localization and fatigue crack nucleation near a non-metallic inclusion in polycrystalline nickel-based superalloy, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 641, Pages: 328-339, ISSN: 1873-4936
Fatigue crack nucleation at a non-metallic agglomerate inclusion has been studied by high spatial resolution Digital Image Correlation (HR-DIC) and high angular resolution Electron Backscatter Diffraction (HR-EBSD). Spatial and temporal characterization and correlation of deformation with underlying microstructures has been performed, with distributions of plastic strain measured from HR-DIC; and residual stress and density of geometrically necessary dislocations (GND) measured from HR-EBSD. Initial residual stress and GND fields, as a consequence of differing thermal expansivities in the metallic and oxide phases, localized around the agglomerate have been quantified using HR-EBSD. The localization of the pre-existing stress and dislocation states appear to lead to early onset of plasticity upon subsequent mechanical loading. Heterogeneous distributions of plastic strain have been observed in the course of the fatigue test by HR-DIC. Crack nucleation via agglomerate/nickel interface decohesion and particle fracture has been demonstrated and this is correlated with the elevation in strain and dislocation density. The measurements of residual stress, strain, and dislocation density provide key information for the mechanisms of fatigue cracking and the development of damage nucleation criteria in these material systems.
Britton TB, Dunne FP, Wilkinson AJ, 2015, On The Mechanistic Basis of Deformation at the Microscale in Hexagonal Close Packed Metals, Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 471, ISSN: 1471-2946
This is an overview of micromechanical deformation mechanisms in hexagonal close-packed metals. We start with an in-depth discussion of single-crystal behaviour concerning crystallographic slip, plastic anisotropy and deformation twinning. We move on to discuss some complexities involved in polycrystalline deformation and modelling approaches, focusing on rate effects in titanium alloys that are thought to play a significant role in dwell fatigue. We finish our review with a brief commentary on current understanding and state-of-the-art techniques, and outline some key areas where further study is recommended.
Erinosho TO, Dunne FPE, 2015, Strain localization and failure in irradiated zircaloy with crystal plasticity, International Journal of Plasticity, Vol: 71, Pages: 170-194, ISSN: 1879-2154
This paper presents a micromechanical and mechanistic study of irradiation-induced crystallographic softening known to accelerate failure in irradiated zircaloys typically used as cladding material in pressure water nuclear reactors. The irradiation is known to lead to an increase in yield strength, and reduced ductility is anticipated to result from the progressive reduction in slip system strength. Extensive studies using transmission electron microscopy (TEM) show the formation of <a> type dislocation channels in irradiated zircaloys anticipated to affect basal and prismatic systems. A crystal plasticity approach is established to incorporate basal and prismatic crystallographic softening, both of which are shown to be required in order to capture independent experimental observations for irradiated zircaloy.Representative irradiated zircaloy textures subjected to cyclic loading regimes were modelled to provide an understanding of the failure processes during in-service conditions. Under both strain and stress-controlled cyclic loading, irradiation softening led to the development of strain localization, and the formation of slip banding and its coalescence. This was found to lead to localized ratcheting and macroscale softening, and in strain-controlled loading, ultimately to plastic shakedown. Stress-controlled cyclic loading, however, especially with non-zero mean applied stress, led to pronounced local and macroscale ratcheting, influenced profoundly by the irradiation softening, and hence finally to ductile failure. It was also observed that local strain hardening due to GND development was small compared to irradiation-induced softening processes, supporting the notion that slip system softening dominates shear band formation.
Kartal ME, Kiwanuka R, Dunne FPE, 2015, Determination of sub-surface stresses at inclusions in single crystal superalloy using HR-EBSD, crystal plasticity and inverse eigenstrain analysis, International Journal of Solids and Structures, Vol: 67-68, Pages: 27-39, ISSN: 1879-2146
The complementary techniques of high-resolution electron backscatter diffraction (HR-EBSD), crystal plasticity finite element modelling and the inverse method of eigenstrain are utilised for evaluating stresses resulting from the mismatch in thermal expansivities of a nickel single crystal containing a carbide particle. The EBSD method is employed to measure the complete residual elastic strain tensor on the free surface of the nickel matrix around a particular carbide particle. With these experimental results, the 3D inverse problem of eigenstrain is reconstructed to determine the complete residual stresses local to the particle at the sub-surface of the sample. A gradient-enhanced crystal plasticity finite element (CPFE) model has been developed for the same sample and loading conditions, and detailed comparisons of the eigenstrain and CPFE predicted stresses at the sub-surface of the sample are presented. In addition, free-surface residual elastic strains measured by HR-EBSD and predicted by the CPFE model are compared. Free-surface results show very good agreement, but some differences are apparent for sub-surface results. The eigenstrain technique relies on the assumption of uniform plastic strain in the direction normal to the free surface, and the CPFE approach provides an assessment of this assumption. The free-surface effects of 3D particle depth are also assessed.
Sweeney CA, O'Brien B, Dunne FPE, et al., 2015, Micro-scale testing and micromechanical modelling for high cycle fatigue of CoCr stent material, Journal of the Mechanical Behavior of Biomedical Materials, Vol: 46, Pages: 244-260, ISSN: 1751-6161
This paper presents a framework of experimental testing and crystal plasticity micromechanics for high cycle fatigue (HCF) of micro-scale L605 CoCr stent material. Micro-scale specimens, representative of stent struts, are manufactured via laser micro-machining and electro-polishing from biomedical grade CoCr alloy foil. Crystal plasticity models of the micro-specimens are developed using a length scale-dependent, strain-gradient constitutive model and a phenomenological (power-law) constitutive model, calibrated from monotonic and cyclic plasticity test data. Experimental microstructural characterisation of the grain morphology and precipitate distributions is used as input for the polycrystalline finite element (FE) morphologies. Two microstructure-sensitive fatigue indicator parameters are applied, using local and non-local (grain-averaged) implementations, for the phenomenological and length scale-dependent models, respectively, to predict fatigue crack initiation (FCI) in the HCF experiments.
Wan VVC, MacLachlan DW, Dunne FPE, 2014, A stored energy criterion for fatigue crack nucleation in polycrystals, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 68, Pages: 90-102, ISSN: 0142-1123
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- Citations: 115
Sweeney CA, O'Brien B, Dunne FPE, et al., 2014, Strain-gradient modelling of grain size effects on fatigue of CoCr alloy, ACTA MATERIALIA, Vol: 78, Pages: 341-353, ISSN: 1359-6454
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- Citations: 57
Zhang T, Collins DM, Dunne FPE, et al., 2014, Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading, Acta Materialia, Vol: 80, Pages: 25-38, ISSN: 1873-2453
Electron backscattered diffraction (EBSD) has been employed to study a polycrystalline nickel superalloy containing a complex non-metallic agglomerate under thermal loading. Heterogeneous distributions of elastic strains are observed near the inclusion due to its complex geometry and these have been quantified. Lattice rotations were also related to geometrically necessary dislocation (GND) density (View the MathML source), indicating the development of localized plasticity arising from the mismatch in thermal expansivity between the Ni polycrystal and the inclusion. A crystal plasticity finite-element (CPFE) model which explicitly represents the full detail of the complex microstructure was developed to interpret the experimental measurements, and good quantitative and qualitative agreement has been obtained. However, a limitation of the EBSD technique when investigating polycrystal systems is that full-field, transgranular strain measurement remains difficult due to the necessity to reference a lattice spacing within a grain for strain calculation. An inverse reference shifting methodology has been developed using CPFE modeling to overcome this problem, thereby allowing like-for-like and grain-by-grain strain comparisons to be made. The method, in conjunction with high-resolution EBSD, shows promise for the determination of full-field strains and rotations in polycrystalline materials, and provides key information for fatigue nucleation in these material systems.
Erinosho TO, Dunne FPE, 2014, Lattice strain distributions due to elastic distortions and GND development in polycrystals, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 67, Pages: 62-86, ISSN: 0022-5096
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- Citations: 7
Connor LD, Stone HJ, Collins DM, et al., 2014, The Effect of Cooling Rate from Solution on the Lattice Misfit During Isothermal Aging of a Ni-Base Superalloy, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, Vol: 45A, Pages: 2436-2444, ISSN: 1073-5623
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- Citations: 8
Kartal ME, Cuddihy MA, Dunne FPE, 2014, Effects of crystallographic orientation and grain morphology on crack tip stress state and plasticity, International Journal of Fatigue, Vol: 61, Pages: 46-58, ISSN: 1879-3452
The Sih, Paris and Irwin analytical solution for cracks in anisotropic elastic media has been developed for an hcp Ti single crystal and shown to lead to crack tip normal stresses which are independent of crystal orientation but other stress components which are dependent. Detailed finite element studies confirm that the stress intensity remains independent of crystal orientation but ceases to do so in an edge-cracked bi-crystal.The incorporation of crystallographic slip demonstrates that single-crystal crack tip stresses largely remain independent of crystal orientation but that the plastic zone size and shape depends greatly upon it. Significant differences result in both the magnitude and extent of the plasticity at the crack tip with crystallographic orientation which can be quite different to that predicted using Mises plasticity. For an edge crack terminating in a bi-crystal, the slip fields which result depend upon both crystal mis-orientation and morphology.
Dunne FPE, 2014, Fatigue crack nucleation: Mechanistic modelling across the length scales, Current Opinion in Solid State & Materials Science, Vol: 18, Pages: 170-179, ISSN: 1359-0286
This paper presents an assessment of recent literature on the mechanistic understanding of fatigue crack nucleation and the associated modelling techniques employed. In particular, the important roles of (a) slip localisation and persistent slip band formation, (b) grain boundaries, slip transfer and interfaces, (c) microtexture and twins, and (d) nucleation criteria and microcracks are addressed in the context of the three key modelling techniques of crystal plasticity (CP), discrete dislocation (DD) plasticity and molecular dynamics (MD) where appropriate. In addition, the need for computational fatigue crack nucleation methodologies which incorporate mechanistic understanding is addressed.Key challenges identified include (i) the overall need for multiscale models for fatigue crack nucleation which are continuum-based but mechanistically informed; (ii) full (3D) crystal slip models to capture slip localisation at a DD level; (iii) MD modelling methodologies for slip transfer to inform DD models; and (iv) rigorously validated dislocation structure models at the DD and CP levels.
Lan B, Lowe M, Dunne FPE, 2014, Experimental and computational studies of ultrasound wave propagation in hexagonal close-packed polycrystals for texture detection, Acta Materialia, Vol: 63, Pages: 107-122, ISSN: 1359-6454
Texture in hexagonal close-packed (hcp) polycrystalline metals, often developed during thermomechanical processing, affects ultrasonic wave velocity. In this study, the relationship between bulk texture and ultrasonic wave velocity in aggregates of (predominantly) hcp grains is investigated using theoretical, numerical and experimental methods. A representative volume element methodology is presented, enabling the effects of texture on ultrasonic wave speed to be investigated in two-phase polycrystals, and is employed to examine the ultrasonic response of random polycrystals, textured polycrystals and macro-zones often observed in titanium alloys. Numerical results show that ultrasonic wave speed varies progressively with changing texture, over a range of ∼200 m s−1, within bounds set by the two extreme single-crystal orientations. Experimental ultrasound studies and full electron backscatter diffraction (EBSD) characterization are conducted on unidirectionally rolled and cross-rolled Ti–6Al–4V samples in three orthogonal directions. In addition, the EBSD-determined textures are incorporated within the polycrystal model and predicted ultrasonic velocities compared directly with ultrasonic experiments. Good quantitative agreement is obtained and both the experimental and computed results demonstrate that ultrasonic velocity profiles exist for random, unidirectionally rolled and cross-rolled textures. The combined results indicate the possibility of the development of a methodology for bulk texture determination within Ti polycrystal components using ultrasound.
Erinosho TO, Cocks ACF, Dunne FPE, 2013, Coupled effects of texture, hardening and non-proportionality of strain on ductility in ferritic steel, COMPUTATIONAL MATERIALS SCIENCE, Vol: 80, Pages: 113-122, ISSN: 0927-0256
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- Citations: 14
Erinosho TO, Cocks ACF, Dunne FPE, 2013, Texture, hardening and non-proportionality of strain in BCC polycrystal deformation, INTERNATIONAL JOURNAL OF PLASTICITY, Vol: 50, Pages: 170-192, ISSN: 0749-6419
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- Citations: 28
Sweeney CA, Vorster W, Leen SB, et al., 2013, The role of elastic anisotropy, length scale and crystallographic slip in fatigue crack nucleation, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 61, Pages: 1224-1240, ISSN: 0022-5096
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- Citations: 124
Erinosho TO, Cocks ACF, Dunne FPE, 2013, NON-PROPORTIONALITY OF STRAIN AND COUPLING EFFECTS ON DISLOCATION DISTRIBUTION AND DUCTILITY IN FERRITIC STEEL POLYCRYSTALS, 5th International Conference on Computational Methods for Coupled Problems in Science and Engineering, Publisher: INT CENTER NUMERICAL METHODS ENGINEERING, Pages: 275-286
Dunne FPE, Kiwanuka R, Wilkinson AJ, 2012, Crystal plasticity analysis of micro-deformation, lattice rotation and geometrically necessary dislocation density, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 468, Pages: 2509-2531, ISSN: 1364-5021
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- Citations: 96
Kartal ME, Dunne FPE, Wilkinson AJ, 2012, Determination of the complete microscale residual stress tensor at a subsurface carbide particle in a single-crystal superalloy from free-surface EBSD, ACTA MATERIALIA, Vol: 60, Pages: 5300-5310, ISSN: 1359-6454
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- Citations: 24
Vorster WJJ, Dunne FPE, 2012, Crystal plasticity and multiscale modelling of superalloy creep, PHILOSOPHICAL MAGAZINE, Vol: 92, Pages: 830-848, ISSN: 1478-6435
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- Citations: 3
Collins DM, Heenan RK, Stone HJ, 2011, Characterization of Gamma Prime (γ′) Precipitates in a Polycrystalline Nickel-Base Superalloy Using Small-Angle Neutron Scattering, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, Vol: 42A, Pages: 49-59, ISSN: 1073-5623
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- Citations: 16
Kiwanuka R, Dunne FPE, 2011, EXPERIMENTAL AND CRYSTAL PLASTICITY INVESTIGATIONS OF INITIAL STRESS IN THERMALLY TREATED TITANIUM AND NICKEL ALLOYS, 3rd International Conference on Heterogeneous Material Mechanics (ICHMM 2011), Publisher: DESTECH PUBLICATIONS, INC, Pages: 332-333
McDowell DL, Dunne FPE, 2010, Microstructure-sensitive computational modeling of fatigue crack formation, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 32, Pages: 1521-1542, ISSN: 0142-1123
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- Citations: 382
Britton TB, Liang H, Dunne FPE, et al., 2010, The effect of crystal orientation on the indentation response of commercially pure titanium: experiments and simulations, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 466, Pages: 695-719, ISSN: 1364-5021
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- Citations: 177
Figiel L, Dunne FPE, Buckley CP, 2010, Computational modelling of large deformations in layered-silicate/PET nanocomposites near the glass transition, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, Vol: 18, ISSN: 0965-0393
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- Citations: 12
Bache MR, Dunne FPE, Madrigal C, 2010, Experimental and crystal plasticity studies of deformation and crack nucleation in a titanium alloy, JOURNAL OF STRAIN ANALYSIS FOR ENGINEERING DESIGN, Vol: 45, Pages: 391-399, ISSN: 0309-3247
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- Citations: 29
Tsutsumi S, Toyasada M, Dunne FPE, 2010, Phenomenological cyclic plasticity model for high cycle fatigue, Procedia Engineering, Vol: 2, Pages: 139-146
Liang H, Dunne FPE, 2009, GND accumulation in bi-crystal deformation: Crystal plasticity analysis and comparison with experiments, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, Vol: 51, Pages: 326-333, ISSN: 0020-7403
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- Citations: 23
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