Publications
199 results found
Yu X, Xu Y, Morales-Espejel G, et al., 2024, On the importance of Crystal Plasticity Finite Element discretisation for the identification of crack initiation in RCF using energy-based criteria, Computational Materials Science, Vol: 232, ISSN: 0927-0256
Material microstructure plays a key role in crack initiation under rolling contact fatigue. When studying microstructure with crystal plasticity finite element method (CPFE), mesh sensitivity study is of great importance, as the surface-near region is under high uniaxial stresses. In this paper, a new structured mesh strategy is purposed and compared with the classical unstructured mesh strategy. Modelling tests on a bi-grain and a polycrystal model show the calculation of geometrically necessary dislocation (GND) density, recently proposed as a suitable fatigue damage indicator, is highly dependent on mesh morphology, when GND hotspots tend to appear near distorted elements even in homogeneous materials. With uniform mesh size and shape, structured mesh elements can provide physically more acceptable GND calculations, which is particularly important in loading scenarios with complex stresses, such as rolling contact fatigue. Computational efficiency is also improved compared to unstructured models because a smaller number of elements are required in a structured mesh model and pre-processing of the mesh is not required.
Piglione A, Bellamy T, Yu J, et al., 2024, Dislocation Arrangements and Cyclic Microplasticity Surrounding Stress Concentration in a Ni-Based Single-Crystal Superalloy, Advanced Engineering Materials, Vol: 26, ISSN: 1438-1656
Local cyclic plasticity near stress concentrations governs the fatigue crack initiation in cyclicly loaded Ni-based single-crystal superalloys, but has not been well studied and understood. The first of its kind transmission electron microscopy (TEM)-based site-specific study of plasticity in the crack initiation region in a notched single-crystal superalloy subjected to fatigue testing at 800 °C, coupling it with microstructure-based crystal plasticity modeling, is presented. Detailed TEM examinations show that local plasticity near the notch significantly differs from bulk plasticity, featuring high dislocation densities and distinctive arrangements of dislocation pairs within γ’ precipitates. It further shows that the increased local stresses alone are responsible for the increase in dislocation density and extensive γ’ shearing, but not solely for the distinctive arrangement of dislocation pairs seen in the notch vicinity, thus highlighting the considerable role played by the local variations in loading rates and stress state surrounding the notch. The results of this work provide new fundamental insights into the deformation micromechanisms leading to fatigue crack initiation in single-crystal superalloys.
Liu Y, Thomas R, Hardie CD, et al., 2023, Exploring the hydride-slip interaction in zirconium alloys, Acta Materialia, Vol: 261, ISSN: 1359-6454
Hydrogen pick-up and hydride precipitation can lead to embrittlement and fracture strength reduction of nuclear fuel cladding tubes made of Zircaloy. Plastic deformation of hydride packets and its interaction with local plasticity in the zirconium matrix is a key linkage of microstructure feature to structural integrity of hydrided polycrystalline bulk Zircaloy. This work focuses on explicit representation of hydride packets from high spatial resolution electron backscatter diffraction onto a crystal plasticity finite element model for capturing and understanding slip localisation near hydride-matrix phase boundaries, based on the extracted material property of hydrides. The mechanisms behind slip evolution including slip nucleation, slip transfer, and slip inhibition are studied by combined high-resolution digital image correlation and crystal plasticity results. Through assessing various slip transfer parameters, new slip transfer criterion is proposed for α/δ phase boundaries. Prior to slip transfer criterion, local micromechanical quantities, specifically shear stress and stored energy density, are necessary to drive and provide pathway for subsequent slip transfer at α/δ phase boundaries.
Paramatmuni C, Dunne FPE, 2023, Effect of stress-states on non-classical twinning in three-point bending of magnesium alloys, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, Vol: 258, ISSN: 0020-7403
Hardie C, Long DJ, Demir E, et al., 2023, A robust and efficient hybrid solver for crystal plasticity, International Journal of Plasticity, Vol: 170, ISSN: 0749-6419
Conventional crystal plasticity (CP) solvers are based on a Newton-Raphson (NR) approach which use an initial guess for the free variables (often stress) to be solved. These solvers are limited by a finite interval of convergence and often fail when the free variable falls outside this interval. Solution failure results in the reduction of the time increment to be solved, thus convergence of the CP solver is a bottleneck which determines the computational cost of the simulation. The numerical stability of the slip law in its inverted form offers a solver that isn't vulnerable to poor pre-conditioning (initial guess) and can be used to progress to a solution from a stable starting point (i.e., from zero slip rate γ˙pk=0 s−1). In this paper, a novel formulation that enables the application of the slip law in its inverted form is introduced; this treats all slip systems as independent by approximating the Jacobian as a diagonal matrix, thus overcomes ill-defined and singular Jacobians associated with previous approaches. This scheme was demonstrated to offer superior robustness and convergence rate for a case with a single slip system, however the convergence rate for extreme cases with several active slip systems was relatively poor. Here, we introduce a novel ‘hybrid scheme’ that first uses the reverse scheme for the first stage of the solution, and then transitions to the forward scheme to complete the solution at a higher convergence rate. Several examples are given for pointwise calculations, followed by CPFEM simulations for FCC copper and HCP Zircaloy-4, which demonstrated solver performance in practise. The performance of simulations using the hybrid scheme was shown to require six to nine times fewer increments compared to the conventional forward scheme solver based on a free variable of stress and initial guess based on a fully elastic increment.
Wan W, Xu Y, Yu X, et al., 2023, Microstructurally-sensitive fatigue crack nucleation in a Zircaloy-4 alloy, Journal of the Mechanics and Physics of Solids, Vol: 180, Pages: 1-15, ISSN: 0022-5096
Microstructurally-sensitive fatigue crack nucleation and subsequently short crack growth were observed at an edge-notch in a textured Zircaloy-4 sample with the c-axis aligned perpendicular to the viewing surface. To understand the competition between the main crack and the secondary crack nucleation at the edge-notch root, a combined experimental and computational investigation was conducted to analyse various micro-scale mechanical quantities, including local strain, local stress, GND density, and stored energy density. A computational crystal plasticity finite element simulation that incorporated grain morphology, crystallographic orientation, and loading conditions was found to have good agreement with experimental results from the three-point bend fatigue test. The results showed that both the main crack and secondary crack nucleation sites had the highest magnitudes of local stored energy density, indicating that this factor was both necessary and sufficient for crack nucleation.
Xu Y, Worsnop F, Dye D, et al., 2023, Slip intermittency and dwell fatigue in titanium alloys: a discrete dislocation plasticity analysis, Journal of the Mechanics and Physics of Solids, Vol: 179, Pages: 1-13, ISSN: 0022-5096
Slip intermittency and stress oscillations in titanium alloy Ti–7Al–O that were observed using in-situ far-field high energy X-ray diffraction microscopy (ff-HEDM) are investigated using a discrete dislocation plasticity (DDP) model. The mechanistic foundation of slip intermittency and stress oscillations are shown to be dislocation escape from obstacles during stress holds, governed by a thermal activation constitutive law. The stress drop events due to <a>-basal slip are larger in magnitude than those along <a>-prism, which is a consequence of their differing rate sensitivities, previously found from micropillar testing. It is suggested that interstitial oxygen suppresses stress oscillations by inhibiting the thermal activation process. Understanding of these mechanisms is of benefit to the design and safety assessment of jet engine titanium alloys subjected to dwell fatigue.
Long DJ, Dunne FPE, 2023, On the mechanistic driving force for short fatigue crack path, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 179, ISSN: 0022-5096
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- Citations: 1
Long DJ, Liu Y, Wan W, 2023, A microstructure-sensitive analytical solution for short fatigue crack growth rate in metallic materials, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, Vol: 253, ISSN: 0020-7403
MacLachlan DW, Karamitros V, Dunne FPE, 2023, Mechanistic modelling of fatigue nucleation and short crack growth in polycrystalline alloys, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 177, ISSN: 0022-5096
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- Citations: 1
Lucarini S, Dunne FPE, Martínez-Pañeda E, 2023, An FFT-based crystal plasticity phase-field model for micromechanical fatigue cracking based on the stored energy density, International Journal of Fatigue, Vol: 172, Pages: 1-11, ISSN: 0142-1123
A novel FFT-based phase-field fracture framework for modelling fatigue crack initiation and propagation at the microscale is presented. A damage driving force is defined based on the stored energy and dislocation density, relating phase-field fracture with microstructural fatigue damage. The formulation is numerically implemented using FFT methods to enable modelling of sufficiently large, representative 3D microstructural regions. The early stages of fatigue cracking are simulated, predicting crack paths, growth rates and sensitivity to relevant microstructural features. Crack propagation through crystallographic planes is shown in single crystals, while the analysis of polycrystalline solids reveals transgranular crack initiation and crystallographic crack growth.
Zhang X, Dunne FPE, 2023, Short crack propagation near coherent twin boundaries in nickel-based superalloy, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 172, ISSN: 0142-1123
Long DJ, Wan W, Dunne FPE, 2023, The influence of microstructure on short fatigue crack growth rates in Zircaloy-4: Crystal plasticity modelling and experiment, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 167, ISSN: 0142-1123
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- Citations: 5
Hardie C, Thomas R, Liu Y, et al., 2022, Simulation of crystal plasticity in irradiated metals: A case study on Zircaloy-4, ACTA MATERIALIA, Vol: 241, ISSN: 1359-6454
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- Citations: 3
Karamitros V, MacLachlan DW, Dunne FPE, 2022, Modelling of short crack growth in single crystal Ni γ- γ' microstructure, ACTA MATERIALIA, Vol: 240, ISSN: 1359-6454
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- Citations: 5
Zhang X, Dunne FPE, 2022, 3D CP-XFEM modelling of short crack propagation interacting with twist/tilt nickel grain boundaries, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 168, ISSN: 0022-5096
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- Citations: 3
Xu Y, Gu T, Xian J, et al., 2022, Multi-scale plasticity homogenization of Sn–3Ag-0.5Cu: From β-Sn micropillars to polycrystals with intermetallics, Materials Science and Engineering: A, Vol: 855, Pages: 1-15, ISSN: 0921-5093
The mechanical properties of β-Sn single crystals have been systematically investigated using a combined methodology of micropillar tests and rate-dependent crystal plasticity modelling. The slip strength and rate sensitivity of several key slip systems within β-Sn single crystals have been determined. Consistency between the numerically predicted and experimentally observed slip traces has been shown for pillars oriented to activate single and double slip. Subsequently, the temperature-dependent, intermetallic-size-governing behaviour of a polycrystal β-Sn-rich alloy SAC305 (96.5Sn–3Ag-0.5Cu wt%) is predicted through a multi-scale homogenization approach, and the predicted temperature- and rate-sensitivity reproduce independent experimental results. The integrated experimental and numerical approaches provide mechanistic understanding and fundamental material properties of microstructure-sensitive behaviour of electronic solders subject to thermomechanical loading, including thermal fatigue.
Xu Y, Xian J, Stoyanov S, et al., 2022, A multi-scale approach to microstructure-sensitive thermal fatigue in solder joints, International Journal of Plasticity, Vol: 155, ISSN: 0749-6419
This paper presents a multi-scale modelling approach to investigate the underpinning mechanisms of microstructure-sensitive damage of single crystal Sn-3Ag-0.5Cu (wt%, SAC305) solder joints of a Ball Grid Array (BGA) board assembly subject to thermal cycling. The multi-scale scheme couples board-scale modelling at the continuum macro-scale and individual solder modelling at the crystal micro-scale. Systematic studies of tin crystal orientation and its role in fatigue damage have been compared to experimental observations. Crystallographic orientation is examined with respect to damage development, providing evidence-based optimal solder microstructural design for in-service thermomechanical fatigue.
Cao M, Liu Y, Dunne FPE, 2022, A crystal plasticity approach to understand fatigue response with respect to pores in additive manufactured aluminium alloys, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 161, ISSN: 0142-1123
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- Citations: 13
Liu Y, Wan W, Dunne FPE, 2022, Characterisation and modelling of micro- and macroscale creep and strain rate sensitivity in Zircaloy-4, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 840, ISSN: 0921-5093
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- Citations: 3
Bergsmo A, Xu Y, Poole B, et al., 2022, Twin boundary fatigue crack nucleation in a polycrystalline Nickel superalloy containing non-metallic inclusions, Journal of the Mechanics and Physics of Solids, Vol: 160, ISSN: 0022-5096
Non-metallic inclusions and twin boundaries are preferred fatigue crack nucleation locations in polycrystalline Ni-based superalloys. A Heaviside HR-DIC, EBSD and multi-scale modelling strategy (CPFE-DDP) was used to assess experimental observations of fatigue crack nucleation near agglomerated inclusions in RR1000 at elevated temperature. Inclusion fracture and decohesion were observed within the first cycle of loading. Fatigue crack nucleation at the non-metallic inclusion coincided with that in an adjacent coarse grain containing a twin boundary. DDP modelling of the twin boundary showed the establishment of slip activation parallel as well as oblique to the interface, as observed in DIC characterisation. The DDP results showed pile-up driven generation of local GND density at the interface, in turn driving high local stored energy density and fatigue crack nucleation. These conditions were shown to result from the anisotropic elastic constraint at the twin boundary. In addition, the complex stress field arising from the agglomerate drives plasticity near the twin boundary contributing to the necessary conditions for fatigue crack nucleation.
Shen S, Zhan M, Gao P, et al., 2022, Microstructural Effects on Thermal-Mechanical Alleviation of Cold Dwell Fatigue in Titanium Alloys, CRYSTALS, Vol: 12
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- Citations: 3
Karamitros V, MacLachlan DW, Dunne FPE, 2022, Mechanistic fatigue in Ni-based superalloy single crystals: A study of crack paths and growth rates, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 158, ISSN: 0022-5096
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- Citations: 18
Liu Y, El Chamaa S, Wenman MR, et al., 2021, Hydrogen concentration and hydrides in Zircaloy-4 during cyclic thermomechanical loading, Acta Materialia, Vol: 221, Pages: 1-16, ISSN: 1359-6454
Hydride formation in Zircaloy-4 under cyclic thermomechanical loading has been investigated using characterized notched beam samples in four-point beam testing, and microstructurally-representative crystal plasticity modelling of the beam tests which incorporates an atomistically-informed equilibrium-state model for hydrogen concentration. The model provided the locations within the microstructure of high hydrogen content, above that required for saturation, hence predicting the anticipated locations of hydride observations in the experiments. The strain rate sensitivity of this alloy over the temperature range considered led to considerable intragranular slip and corresponding stress redistribution, and cyclic strain ratcheting leading to high hydrostatic stresses and in turn hydrogen concentrations, which explains the locations of experimentally observed hydride formation. The interstitial hydrogen interaction energy as well as the intragranular geometrically necessary dislocation density were shown to be important in controlling the spatial distributions of observed hydrides.
Zhang X, Stinville J-C, Pollock TM, et al., 2021, Crystallography and elastic anisotropy in fatigue crack nucleation at nickel alloy twin boundaries, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 155, ISSN: 0022-5096
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- Citations: 15
Liu Y, Adande S, Britton TB, et al., 2021, Cold dwell fatigue analyses integrating crystal-level strain rate sensitivity and microstructural heterogeneity, International Journal of Fatigue, Vol: 151, Pages: 1-19, ISSN: 0142-1123
Cold dwell fatigue remains an important life-limiting factor in aircraft engine titanium alloys. Microstructure-level creep and stress accumulation during each loading cycle are controlled by strain rate sensitivity. Here, an integrated experimental and computational framework is used to link crystal-level slip properties to microstructure-sensitive cold dwell debit of a forged rotor graded Ti-6Al-4V alloy. Slip strengths and anisotropic strain rate sensitivities are extracted from micro-pillar compression tests for different slip systems, incorporated within α+β microstructurally-faithful polycrystal representations. Dwell and non-dwell cyclic loading in alloy Ti-6Al-4V are investigated for two differing microstructures, and the cycles to failure predicted based solely on the crystal c-axis tensile strength, and the dwell debit quantified. The dwell effect is predicted to diminish to zero below a peak applied stress of about 790 MPa in the alloy studied.
Paramatmuni C, Guo Y, Withers PJ, et al., 2021, A three-dimensional mechanistic study of the drivers of classical twin nucleation and variant selection in Mg alloys: A mesoscale modelling and experimental study, International Journal of Plasticity, Vol: 143, ISSN: 0749-6419
This work presents a detailed investigation of twin inception to identify site of twinning, variant type selected, and the strain to nucleate it within a full 3D reconstructed microstructure obtained using 3D-EBSD. Microstructurally-faithful 3D crystal plasticity analysis provides quantitative insight to show that stored energy density is a key factor (rather than stress or other criteria) which identifies the experimentally observed twin nucleation site (which supersedes twin inception), and the strain necessary to drive nucleation. 3D (as opposed to 2D surface) analysis has been shown to be essential. The critical energy density for twin nucleation was found to be ∼0.015 Jm-2 in Mg alloy AZ31. Further, at the predicted nucleation site, the experimentally observed twin variant is shown to be driven by the local twin resolved shear stress.
Poole B, Dunne F, 2021, Slip band interactions and GND latent hardening in a galling resistant stainless steel, Materials Science and Engineering: A, Vol: 813, ISSN: 0921-5093
Slip activation, slip band interactions, and GND densities in iron-base, galling resistant alloy Nitronic 60 have been characterised at the grain length scale using small-scale mechanical testing with high resolution digital image correlation and high-angular resolution electron backscatter diffraction. By correlating the two measurement techniques, new insight into slip band interactions, the generation of lattice curvature and the corresponding accumulation of geometrically necessary dislocations (GNDs) is provided. Multiple discrete slip bands are typically active within single grains, resulting in significant slip band interactions. Crossing slip bands were found to generate accumulations of GNDs. Regions where slip bands block other slip bands were associated with the highest GND densities, in excess of three time the densities of crossing slip bands. Representative crystal plasticity modelling investigations have demonstrated that discrete slip blocking events are responsible for locally elevated GND density. This behaviour is rationalised in terms of lattice curvature associated with the differing levels of constraint provided by the crossing or blocking-type behaviours. Ferrite grains are also found to contribute to the generation of GNDs. Together, these two effects provide significant work hardening mechanisms, likely to be key to the development of future iron-base hard facing alloys.
Pan YB, Dunne FPE, MacLachlan DW, 2021, A mechanistic and stochastic approach to fatigue crack nucleation in coarse grain RR1000 using local stored energy, Fatigue and Fracture of Engineering Materials and Structures, Vol: 44, Pages: 505-520, ISSN: 1460-2695
The crystal plasticity finite element (CPFE) method is used in conjunction with a critical local stored energy criterion to predict crack nucleation life for Coarse Grain (CG) nickel superalloy RR1000. Artificial representative microstructures are generated using Dream3D, and through simulation of multiple microstructural instantiations, a distribution of simulated fatigue response is generated. Fatigue of CG RR1000 is studied at 300°C and 700°C and at two R ratios of R = 0.1 and R = −1 giving a range of conditions to test the stored energy method. At higher temperature failure frequently occurs from inclusions, these are represented in the model by adding an inclusion with cohesive zones between inclusion and matrix. The results at 300°C are very good with the one parameter model (the critical stored energy) able to predict the mean, slope and distribution of fatigue data. At 700°C, the results are also good; however, fatigue life at high strain amplitude is overpredicted.
Xu Y, Gu T, Xian J, et al., 2021, Intermetallic size and morphology effects on creep rate of Sn-3Ag-0.5Cu solder, International Journal of Plasticity, Vol: 137, ISSN: 0749-6419
The creep behaviour of directionally solidified SAC305 (96.5Sn-3Ag-0.5Cu wt%) alloy has been investigated with integrated particle matrix composite (PMC) crystal plasticity modelling and quantitative experimental characterisation and test. In this manuscript, the mechanistic basis of creep rate dependence is shown to be influenced by plastic strain gradients, and the associated hardening due to geometrically necessary dislocation (GND) density. These gradients are created due to heterogenous deformation at the Sn phase and intermetallic compound (IMCs) boundaries. The size and distribution of IMCs is important, as finer and well dispersed IMCs leading to higher creep resistance and lower creep rates, and this agrees with experimental observations. This understanding has enabled the creation of a new microstructurally homogenized model which captures this mechanistic link between the GND hardening, the intermetallic size, and the corresponding creep rate. The homogenised model relates creep rates to the microstructure found within the solder alloy as they evolve in service, when ageing and coarsening kinetics are known.
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