Imperial College London

DrDanielBalint

Faculty of EngineeringDepartment of Mechanical Engineering

Reader in Solid Mechanics
 
 
 
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Contact

 

+44 (0)20 7594 7084d.balint Website

 
 
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Location

 

519City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

152 results found

Xu Y, Balint D, Dini D, 2019, A new hardness formula incorporating the effect of source density on indentation response: a discrete dislocation plasticity analysis, Surface and Coatings Technology, Vol: 374, Pages: 763-773, ISSN: 0257-8972

Planar discrete dislocation plasticity (DDP) calculations that simulate thin single crystal films bonded to a rigid substrate indented by a rigid wedge are performed for different values of film thickness and dislocation source density. As in prior studies, an indentation size effect (ISE) is observed when indentation depth is sufficiently small relative to the film thickness. Thedependence of the ISE on dislocation source density is quantified in this study, and a modified form of the scaling law for the dependence of hardness on indentation depth, first derived by Nix and Gao, is proposed, which is valid over the entire range of indentation depths and correlates the length scale parameter with the average dislocation source spacing. Nanoindentation experimental data from the literature are fitted using this formula, which further verifies the proposed scaling of indentation pressure on dislocation source density.

Journal article

Ebrahimi M, Balint D, Sutton A, Dini Det al., 2019, A discrete crack dynamics model of toughening in brittle polycrystalline material by crack deflection, Engineering Fracture Mechanics, Vol: 214, Pages: 95-111, ISSN: 0013-7944

This paper focuses on the study of the effect of the interfacial strength of grain boundaries and elliptical inclusions on crack path deflection. The method is developed to channel a crack into a toughening configuration (arrays of elliptical holes and inclusions are considered) in order to obtain the optimised microstructure required to enhance fracture toughness through different mechanisms. The proposed technique is shown to reproduce experimental crack propagation paths in various configurations and is capable of capturing the effect of that variation of the GB and the inclusion interfacial strength; it provides a powerful tool to understand the interplay between microstructural features and improve materials performance.

Journal article

Zheng Z, Prastiti NG, Balint DS, Dunne FPEet al., 2019, The dislocation configurational energy density in discrete dislocation plasticity, Journal of the Mechanics and Physics of Solids, ISSN: 0022-5096

Dislocation configurational energy is the term assigned to describe the elastically-stored energy associated with the interaction of dislocations and their structures. It is the energy which is over and above that from the summation of the dislocation line energies when considered isolated and non-interacting. It is therefore different to the free energy and the stored energy. This paper presents a formulation for its determination utilising discrete dislocation plasticity. The total geometrically necessary (GND) and statistically stored dislocation density mean free distance allows the configurational energy density to be determined, thus providing a length scale over which the configurational energy is stored. This quantity is assessed in polycrystals undergoing fatigue loading showing that clear microstructural locations, often associated with high GND density, become established at which the progressive, cyclic, increasing configurational energy occurs. A higher length scale crystal plasticity stored energy density has recently been introduced which attempts to capture local dislocation configurational energy density as an indicator of fatigue crack nucleation and growth. The former is compared and assessed against the dislocation configurational energy density in this paper.

Journal article

Wood J, Gauvin C, Young C, Taylor A, Balint D, Charalambides Met al., Reconstruction of historical temperature and relative humidity cycles within Knole House, Kent, Journal of Cultural Heritage, ISSN: 1296-2074

It is essential for the preservation of cultural heritage that the effects of climate change are investigated. With this in mind, the daily temperature and relative humidity (RH) cycles within the Brown Gallery at Knole House, Kent, have been reconstructed for the period 1605 – 2015 enabling the study of low-cycle environmental fatigue on a set of 17th century panel paintings. By establishing a relationship between the temperature in the Brown Gallery and the Hadley Centre Central England Temperature (HadCET) dataset over a sixteen year period (2000 – 2015), it is possible to use the full HadCET dataset to obtain the daily minimum and maximum temperatures in the Brown Gallery for the period 1878 – 2015. Using a Fourier series to fit the periodic data it is then possible to extrapolate back to 1605. Furthermore, correction factors derived using the HadCET average daily temperature in the period 1772 – 1877 and average monthly temperature in the period 1659 – 1771 are applied to the temperature data to increase the model accuracy. The daily minimum and maximum RH for the period 1605 – 2015 are obtained using the Brown Gallery maximum and minimum temperatures respectively, and assuming that the daily dew point temperature at Knole is calculated by subtracting a monthly-dependent constant from the daily minimum temperature at Knole, thus enabling the calculation of the daily actual water vapour pressure of air. Changes in RH are a result of the daily temperature cycle changing the saturation vapour pressure of air in the gallery. This data is valuable as it enables a study of the effects of low-cycle fatigue on the 17th century panel paintings housed in the Brown Gallery at Knole House, Kent due to these temperature and relative humidity cycles. Furthermore, the method presented offers a technique that can be utilised to replicate the internal environment for any unheated monument building so that the effects of past and future temper

Journal article

Li Y, Shao Z, Rong Q, Shi Z, Balint D, Sun X, Meng L, Lin Jet al., 2019, Development of similarity-based scaling criteria for creep age forming of large/extra-large panels, The International Journal of Advanced Manufacturing Technology, Vol: 101, Pages: 1537-1551, ISSN: 0268-3768

A scaling method is developed for the creep age forming (CAF) process to downscale manufacturing of large/extra-large panels to lab-scale experimental trials for industrial application. Similarity theory is applied to identify both the geometrical and physical (non-geometrical) similarities between large-size prototypes and scaled-down models in all process stages of CAF, including loading, stress-relaxation and unloading (springback). A constitutive model is incorporated into the theory in order to identify the similarity in the highly non-linear stress-relaxation behaviour for aluminium alloy plates during CAF, and to obtain the effective scaling criteria for the CAFed plates after springback. The method was demonstrated by scaling down CAF manufacturing of both singly curved and doubly curved large plates under both proportional and non-proportional geometrical scaling conditions. The analytical results of the scaling method and numerical results obtained by CAF FE modelling were found to be in good agreement. Scaling diagrams linking the key deformation (springback) and structural (flexural rigidity) variables to scaling ratios under both proportional and non-proportional conditions were generated, and the developed scaling diagrams have been validated by corresponding CAF experiments. The scaling method developed in this study provides guidance on the design of scaled-down CAF experimental trials and will be used in the practical CAF process of large/extra-large panels.

Journal article

Cimbaro L, Sutton A, Balint D, Paxton A, Hardy Met al., 2019, Embrittlement of an elasto-plastic medium by an inclusion, International Journal of Fracture, Vol: 216, Pages: 87-100, ISSN: 0376-9429

A mathematical model for the embrittlement of a long elastic-plastic crack by a relatively small, misfitting inclusion is presented. The model makes direct contact with the Dugdale–Bilby–Cottrell–Swinden model as a limiting case. The particular case of an oxide inclusion with a triangular cross-section at the tip of an intergranular crack in the Ni-based superalloy RR1000 at 650∘C is considered. The positive misfit of the intrusion provides an additional tensile load on the crack tip and on the plastic zone, raising the local stress intensity factor kI and the crack tip opening displacement Δu above those when the inclusion is replaced by a dislocation-free zone of the same length. It is shown that for a given misfit strain and inclusion shape, the enhancement of kI and Δu is controlled by a dimensionless parameter ω=(σ/σ1)c/(2l)−−−−−√ where σ is the applied stress, σ1 is the yield stress, c is the crack length and l is the length of the inclusion. The anti-shielding effect of the intrusion is significant only when ω≲6. As a result of the anti-shielding effect of the intrusion, the stress singularity at the crack tip always exceeds the compressive normal stress that exists within the thickest part of the intrusion when it is isolated. It is also shown that the gradient of the hydrostatic stress within the intrusion subjected to different applied stresses drives the oxygen diffusion and, hence, assists the oxidation at the grain boundary. The fracture toughness is considerably greater than that of a bulk sample of the oxide particle, which we attribute to the plastic zone.

Journal article

Dzepina B, Balint D, Dini D, 2019, A phase field model of pressure-assisted sintering, Journal of the European Ceramic Society, Vol: 39, Pages: 173-182, ISSN: 0955-2219

The incorporation of an efficient contact mechanics algorithm into a phase field sintering model is presented. Contact stresses on the surface of arbitrarily shaped interacting bodies are evaluated and built into the model as an elastic strain energy field. Energy relaxation through deformation is achieved by diffusive fluxes along stress gradients and rigid body motion of the deforming particles maintain contact between the particles. The proposed model is suitable for diffusion deformation mechanisms occurring at stresses below the yield strength of a defect-free material; this includes Nabarro-Herring creep, Coble creep and pressure-solution. The effect of applied pressure on the high pressure-high temperature (HPHT) liquid phase sintering of diamond particles was investigated. Changes in neck size, particle coordination and contact flattening were observed. Densification rates due to the externally applied loads were found to be in good agreement with a new theory which implicitly incorporates the effect of applied external pressure.

Journal article

Khosla G, Balint D, Farrugia D, Davies CMet al., 2019, Toughness measurements of a Cr martensitic high alloy steel susceptible to clinking, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, Vol: 233, Pages: 63-72, ISSN: 1464-4207

'Clinking’ is an audible fracture that occurs during the cool down and reheating of as-cast high alloy materials. When this process occurs, audible fracture can be heard and observed as large transverse cracks that propagate through large slabs. This causes high material losses and major disruption to processing operations. Given the fracture is known to be brittle, this research is aimed at developing a way to predict the onset of clinking through the application of fracture mechanics. Linear elastic and elastic–plastic fracture mechanics were both used to assess the fracture behaviour. The stress state during cool down and reheating was estimated through finite element analysis using a three-dimensional finite element model. Tensile tests were conducted to obtain the stress–strain characteristics to be used in the fracture analysis. Charpy tests were completed to assess the relative toughness dependent on temperature across the temperature range for which the high alloy steel is susceptible to clinking. Four C(T) specimens were tested at a room temperature. Despite showing little ductile crack propagation on the fracture surface, the fractured samples did not meet the Linear Elastic Fracture Mechanics (LEFM) validity criterion but did meet the Jcvalidity criterion. This allows a minimum Jcvalue of 118 N/mm to be attributed to the onset of unstable fracture. Conversion into a KJcgives 164MP√m, which gives a minimum critical crack length of 138 mm for the onset of brittle fracture. Charpy tests showed a pronounced increase in the energy for fracture between 20 ℃ and 300 ℃ which is in line with practical observations, where the onset of clinking is reduced with a higher reheat temperature.

Journal article

Waheed S, Zheng Z, Balint D, Dunne Fet al., 2019, Microstructural effects on strain rate and dwell sensitivity in dual-phase titanium alloys, Acta Materialia, Vol: 162, Pages: 136-148, ISSN: 1359-6454

In this study, stress relaxation tests are performed to determine and compare the strain rate sensitivity of different titanium alloy microstructures using discrete dislocation plasticity (DDP) and crystal plasticity finite element (CPFE) simulations. The anisotropic α and β phase properties of alloy Ti-6242 are explicitly included in both the thermally-activated DDP and CPFE models together with direct dislocation penetration across material-interfaces in the DDP model. Equiaxed pure α, colony, Widmanstatten and basketweave microstructures are simulated together with an analysis of the effect of α grain size and dislocation penetration on rate sensitivity. It is demonstrated that alloy morphology and texture significantly influence microstructural material rate sensitivity in agreement with experimental evidence in the literature, whereas dislocation penetration is found not to be as significant as previously considered for small deformations. The mechanistic cause of these effects is argued to be changes in dislocation mean free path and the total propensity for plastic slip in the specimen. Comparing DDP results with corresponding CPFE simulations, it is shown that discrete aspects of slip and hardening mechanisms have to be accounted for to capture experimentally observed rate sensitivity. Finally, the dwell sensitivity in a polycrystalline dual-phase titanium alloy specimen is shown to be strongly dependent on its microstructure.

Journal article

Hussein MI, Bordas SPA, Balint DS, 2019, Preface

Book

Wood JD, Gauvin C, Young CRT, Taylor AC, Balint DS, Charalambides MNet al., 2018, Cracking in paintings due to relative humidity cycles, 22nd European Conference on Fracture (ECF) - Loading and Environmental Effects on Structural Integrity, Publisher: Elsevier B.V., Pages: 379-384, ISSN: 2452-3216

A numerical study is performed using the finite element method to consider the effects of low-cycle fatigue, specifically induced through relative humidity cycles on paintings. It has been identified that there are two major crack types in paintings, these being (i) an interfacial crack (delamination) between paint and support and (ii) a through-thickness (channel) crack in the paint layer itself, arresting on the interface. Therefore a 2D plane strain model for each type of crack has been created, which both consist of an alkyd paint modelled using a visco-hyperelastic material model and a primed canvas which is assumed to behave in a linear elastic manner. To account for fatigue damage in both models, cohesive elements located along the interface or through the film thickness respectively, are used and the traction-separation law has been modified to incorporate a fatigue damage parameter. It is possible to expose the models to the same relative humidity cycles, which would typically be seen in museums, enabling the prediction of time to first crack and which crack type is more readily grown in the painting.

Conference paper

Mercer C, Lee J, Balint DS, 2018, An investigation of the mechanical behavior of three-dimensional low expansion lattice structures fabricated via laser printing, COMPOSITE STRUCTURES, Vol: 206, Pages: 80-94, ISSN: 0263-8223

Journal article

Tajabadi-Ebrahimi M, Dini D, Balint DS, Sutton AP, Ozbayraktar Set al., 2018, Discrete crack dynamics: a planar model of crack propagation and crack-inclusion interactions in brittle materials, International Journal of Solids and Structures, Vol: 152-153, Pages: 12-27, ISSN: 0020-7683

The Multipole Method (MPM) is used to simulate the many-body self-consistentproblem of interacting elliptical micro-cracks and inclusions in single crystals. Acriterion is employed to determine the crack propagation path based on the stressdistribution; the evolution of individual micro-cracks and their interactions withexisting cracks and inclusions is then predicted using what we coin the DiscreteCrack Dynamics (DCD) method. DCD is fast (semi-analytical) and particularlysuitable for the simulation of evolving low-speed crack networks in brittle orquasi-brittle materials. The method is validated against finite element analysispredictions and previously published experimental data.

Journal article

Verschueren J, Gurrutxaga-Lerma B, Balint D, Sutton A, Dini Det al., 2018, Instabilities of high speed dislocations, Physical Review Letters, Vol: 121, ISSN: 0031-9007

Despite numerous theoretical models and simulation results, a clear physical picture of dislocations traveling at velocities comparable to the speed of sound in the medium remains elusive. Using two complementary atomistic methods to model uniformly moving screw dislocations, lattice dynamics and molecular dynamics, the existence of mechanical instabilities in the system is shown. These instabilities are found at material-dependent velocities far below the speed of sound. We show that these are the onset of an atomistic kinematic generation mechanism, which ultimately results in an avalanche of further dislocations. This homogeneous nucleation mechanism, observed but never fully explained before, is relevant in moderate and high strain rate phenomena including adiabatic shear banding, dynamic fracture, and shock loading. In principle, these mechanical instabilities do not prevent supersonic motion of dislocations.

Journal article

Khosla G, Balint D, Farrugia D, Hole M, Davies CMet al., 2018, Analysis of an as-cast high Si slab to elucidate fundamental causes of the fracture mechanism: Clinking, 22nd European Conference on Fracture (ECF) - Loading and Environmental Effects on Structural Integrity, Publisher: ELSEVIER SCIENCE BV, Pages: 1447-1452, ISSN: 2452-3216

Conference paper

Zhang B, Mohammed IK, Wang Y, Balint DSet al., 2018, On the use of HCP and FCC RVE structures in the simulation of powder compaction, JOURNAL OF STRAIN ANALYSIS FOR ENGINEERING DESIGN, Vol: 53, Pages: 338-352, ISSN: 0309-3247

Journal article

Zheng Z, Waheed S, Balint D, Dunne Fet al., 2018, Slip transfer across phase boundaries in dual phase titanium alloys and the effect on strain rate sensitivity, International Journal of Plasticity, Vol: 104, Pages: 23-38, ISSN: 0749-6419

Dislocation transmission through α/β phase boundaries in titanium alloys is studied using integrated crystal plasticity (CP) and discrete dislocation plasticity (DDP) modelling techniques, combined with experimental micro-pillar compression test results. Direct dislocation transmission together with the nucleation of new dislocations ahead of a pile-up at an α/β interface, termed indirect slip transfer, are both assessed and their role in controlling microstructure-dependent strain rate sensitivity considered. A critical shear stress criterion for direct slip transfer across an α/β interface in Ti-6242 has been established by capturing the local slip penetration through the phase boundary using CP and DDP comparisons with experimental two phase micro-pillar compression. The competition between direct and indirect slip transfer has been investigated using a single Frank-Read source DDP model. Direct slip transfer is found to occur only under specific conditions which have been quantified. The strain rate sensitivity of dual phase titanium alloys is demonstrated to depend on average pile-up size which is significantly influenced by α/β morphology.

Journal article

Waheed S, Hao R, Zheng Z, Wheeler J, Michler J, Balint D, Giuliani Fet al., 2018, Temperature-dependent plastic hysteresis in highly confined polycrystalline Nb films, Modelling and Simulation in Materials Science and Engineering, Vol: 26, ISSN: 0965-0393

In this study, the effect of temperature on the cyclic deformation behaviour of a confined polycrystalline Nb film is investigated. Micropillars encapsulating a thin niobium interlayer are deformed under cyclic axial compression at different test temperatures. A distinct plastic hysteresis is observed for samples tested at elevated temperatures, whereas negligible plastic hysteresis is observed for samples tested at room temperature. These results are interpreted using planar discrete dislocation plasticity incorporating slip transmission across grain boundaries. The effect of temperature-dependent grain boundary energy and dislocation mobility on dislocation penetration and, consequently, the size of plastic hysteresis is simulated to correlate with the experimental results. It is found that the decrease in grain boundary energy barrier caused by the increase in temperature does not lead to any appreciable change in the cyclic response. However, dislocation mobility significantly affects the size of plastic hysteresis, with high mobilities leading to a larger hysteresis. Therefore, it is postulated that the experimental observations are predominantly caused by an increase in dislocation mobility as the temperature is increased above the critical temperature of body-centred cubic niobium.

Journal article

Bordas SPA, Balint DS, Hussein MI, 2018, ADVANCES IN APPLIED MECHANICS Advances in Crystals and Elastic Metamaterials, Part 1 PREFACE, ADVANCES IN CRYSTALS AND ELASTIC METAMATERIALS, PT 1, Editors: Bordas, Balint, Hussein, Publisher: ELSEVIER ACADEMIC PRESS INC, Pages: IX-XI, ISBN: 978-0-12-815100-6

Book chapter

Politis DJ, Politis NJ, Lin J, Dean TA, Balint DSet al., 2018, An analysis of the tooth stress distribution of forged bi-metallic gears, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART C-JOURNAL OF MECHANICAL ENGINEERING SCIENCE, Vol: 232, Pages: 124-139, ISSN: 0954-4062

Journal article

Bordas SPA, Balint DS, Hussein MI, 2018, Preface

Book

Bhowmik A, Britton TB, Lee J, Liu W, Jun T-S, Sernicola G, Karimpour M, Balint D, Giuliani Fet al., 2017, Deformation behaviour of [001] oriented MgO using combined in-situ nano-indentation and micro-Laue diffraction, Acta Materialia, Vol: 145, Pages: 516-531, ISSN: 1359-6454

We report a coupled in-situ micro-Laue diffraction and nano-indentation experiment, with spatial and time resolution, to investigate the deformation mechanisms in [001]-oriented single crystal MgO. Crystal plasticity finite element modelling was applied to aid interpretation of the experimental observations of plasticity. The Laue spots showed both rotation and streaking upon indentation that is typically indicative of both elastic lattice rotation and plastic strain gradients respectively in the material. Multiple facets of streaking of the Laue peaks suggested plastic slip occurring on almost all the {101}-type slip planes oriented 45° to the sample surface with no indication of slip on the 90° {110} planes. Crystal plasticity modelling also supported these experimental observations. Owing to asymmetric slip beneath the indenter, as predicted by modelling results and observed through Laue analysis, sub-grains were found to nucleate with distinct misorientation. With cyclic loading, the mechanical hysteresis behaviour in MgO is revealed through the changing profiles of the Laue reflections, driven by reversal of plastic strain by the stored elastic energy. Crystal plasticity simulations have also shown explicitly that in subsequent loading cycles after first, the secondary slip system unloads completely elastically while some plastic strain of the primary slip reverses. Tracking the Laue peak movement, a higher degree of lattice rotation was seen to occur in the material under the indent, which gradually decreased moving laterally away. With the progress of deformation, the full field elastic strain and rotation gradients were also constructed which showed opposite lattice rotations on either sides of the indent.

Journal article

Shi Z, Wang L, Mohamed M, Balint DS, Lin J, Stanton M, Watson D, Dean TAet al., 2017, A new design of friction test rig and determination of friction coefficient when warm forming an aluminium alloy, International Conference on the Technology of Plasticity, ICTP 2017, Publisher: Elsevier, Pages: 2274-2279, ISSN: 1877-7058

To facilitate reduced fuel consumption and increase environmental friendliness, in recent years, demands for lightweight vehicles have been increasing, and interest in hot or warm forming of sheet aluminium alloys for use in vehicle body structures, has grown. For better understanding and optimisation of the forming processes, knowledge of friction coefficient between tooling and work-piece, at elevated temperature, is critical. However, because of difficulties with measurement at elevated temperature, most studies on friction are limited to room temperature. In this study, a friction rig was designed for isothermal tests at elevated temperature. The test rig enables pure sliding between pins (made of a tool steel) and a metal sheet. The friction behaviour of Forge Ease 278, a water based solid lubricant pre-applied to aluminium alloy AA5754, was investigated, under isothermal warm forming conditions, using the test rig. The effects of testing temperature, sliding speed and applied pressure on the friction coefficient were studied. It was found that Forge Ease produced a low friction coefficient of around 0.05, above room temperature and below 250 °C. The lubricant performance degrades at 350 °C and the friction coefficient increases markedly. Both sliding speed (up to 150 mm s -1 ) and applied pressure (up to 12.8 MPa) had no significant effect on friction coefficient of Forge Ease.

Conference paper

Sernicola G, Giovannini T, Patel P, Kermode J, Balint D, Britton TB, Giuliani Fet al., 2017, In situ stable crack growth at the micron scale, Nature Communications, Vol: 8, ISSN: 2041-1723

Grain boundaries typically dominate fracture toughness, strength, slow crack growth of ceramics. To improve these properties through mechanistically informed grain boundary engineering, precise measurement of the mechanical properties of individual boundaries is essential, although this is rarely achieved due to its complexity. Here we present a new approach to characterise the fracture energy at the lengthscale of individual grain boundaries and demonstrate this capability with measurement of the surface energy of silicon carbide (SiC) single crystals. We perform experiments using an in situscanning electron microscopy based double cantilever beam test, thus enabling viewing and measurement of stable crack growth directly. These experiments correlate well with our density functional theory (DFT) calculations of the surface energy of the same SiC plane. Subsequently, we measure the fracture energy for a bi-crystal of SiC, diffusion bonded with a thin glassy layer. These measurements ultimately promote microstructural engineering of novel and advanced ceramics.

Journal article

Zheng Z, Balint D, Dunne F, 2017, Mechanistic basis of temperature-dependent dwell fatigue in titanium alloys, Journal of the Mechanics and Physics of Solids, Vol: 107, Pages: 185-203, ISSN: 0022-5096

The temperature-dependent dwell sensitivity of Ti-6242 and Ti-6246 alloys has been assessed over a temperature range from −50∘C to 390 °C  using discrete dislocation plasticity which incorporates both thermal activation of dislocation escape from obstacles and slip transfer across grain boundaries. The worst-case load shedding in Ti-6242 alloy is found to be at or close to 120 °C  under dwell fatigue loading, which diminishes and vanishes at temperatures lower than −50∘C or higher than 230 °C. Load shedding behaviour is predicted to occur in alloy Ti-6246 also but over a range of higher temperatures which are outside those relevant to in-service conditions. The key controlling dislocation mechanism with respect to load shedding in titanium alloys, and its temperature sensitivity, is shown to be the time constant associated with the thermal activation of dislocation escape from obstacles, with respect to the stress dwell time. The mechanistic basis of load shedding and dwell sensitivity in dwell fatigue loading is presented and discussed in the context of experimental observations.

Journal article

Patel M, Waheed S, Wenman MR, Sutton AP, Balint DSet al., 2017, Discrete dislocation plasticity modeling of hydrides in zirconium under thermal cycling, MRS Advances, Vol: 2, Pages: 3353-3358, ISSN: 2059-8521

Understanding the ratcheting effect of hydrogen and hydride accumulation in response to thermal cycling is important in establishing a failure criterion for zirconium alloy nuclear fuel cladding. We propose a simple discrete dislocation plasticity model to study the evolution of the dislocation content that arises as a micro-hydride repeatedly precipitates and dissolves over a series of thermal cycles. With each progressive thermal cycle, we find a steady growth in the residual dislocation density in the vicinity of the hydride nucleation site; this corresponds to a gradual increase in the hydrogen concentration and, consequently, the hydride population. The simulated ratcheting in the dislocation density is consistent with experimental observations concerning the hysteresis in the terminal solid solubility of hydrogen in zirconium, which can be correlated to the plastic relaxation of hydrides.

Journal article

Waheed S, Hao R, Bhowmik A, Balint DS, Giuliani Fet al., 2017, A unifying scaling for the Bauschinger effect in highly confined thin films: a discrete dislocation plasticity study, Modelling and Simulation in Materials Science and Engineering, Vol: 25, ISSN: 0965-0393

In this study, sequential sputter deposition, diffusion bonding and focused ion beam milling are used to fabricate sapphire micropillars encapsulating a thin single crystal niobium film. A distinct Bauschinger effect is observed during the cyclic axial compression of the samples. Plain strain discrete dislocation plasticity is used to interpret the experimental results obtained for the encapsulated film-micropillar geometry. The simulations show that the experimental samples correspond to a saturated source density regime, producing the maximum Bauschinger effect for the chosen mean nucleation strength. Next, the source density and mean nucleation strength are shown to have a coupled effect on the size of the Bauschinger effect, understood in terms of the differing number of pile-ups occurring per source in the film. The coupled effect is found to be represented by the density of dislocations annihilated upon unloading: a consistent linear relationship is observed between the size of the Bauschinger effect and the annihilated dislocation density over the entire source density and nucleation strength parameter space investigated. It is found that different film orientations fulfil the same linear relationship, whereas changing the film thickness causes the slope of the linear trend to vary suggesting a length-scale dependence on reverse plasticity. Finally, all results are found to be unified by a power-law relationship quantifying the Bauschinger effect of the form ${{\rm{\Gamma }}}_{{\rm{B}}}\propto {\rm{\Delta }}{\rho }_{{\rm{ann}}}{l}^{n}$ where it is argued that the number of dislocations undergoing reverse glide in the confined film is represented by ${\rm{\Delta }}{\rho }_{{\rm{ann}}}$, the mean free path of dislocations by l and the effect of hardening processes by the exponent n. The net reverse glide is thus represented by ${\rm{\Delta }}{\rho }_{{\rm{ann}}}{l}^{n}$ which can be used as a measure of the Bauschinger effect.

Journal article

Gurrutxaga-Lerma B, Shehadeh M, Balint, Dini D, Chen L, Eakinset al., 2017, The effect of temperature on the elastic precursor decay in shock loaded FCC aluminium and BCC iron, International Journal of Plasticity, Vol: 96, Pages: 135-155, ISSN: 1879-2154

This article offers a comprehensive experimental and theoretical study of the causes of thermal hardening in FCC Al and BCC Fe at high strain rates, with the aim to shed light on important mechanisms governing deformation and failures in materials subjected to shocks and impacts at very high strain rates. Experimental evidence regarding the temperature dependence of the dynamic yield point of FCC Al and BCC Fe shock loaded at 107 s−1 is provided. The dynamic yield point of Al increases with temperature in the range 125K–795K; for the same loading and temperate range, the dynamic yield point of BCC Fe remains largely insensitive. A Multiscale Discrete Dislocation Plasticity (MDDP) model of both Fe and Al is developed, leading to good agreement with experiments. The importance of the Peierls barrier in Fe is highlighted, showing it is largely responsible for the temperature insensitivity in BCC metals. The relevance of the mobility of edge components in determining the plastic response of both FCC Al and BCC Fe at different temperatures is discussed, which leads to developing a mechanistic explanation of the underlying mechanisms leading to the experimental behaviour using Dynamic Discrete Dislocation Plasticity (D3P). It is shown that the main contributing factor to temperature evolution of the dynamic yield point is not the mobility of dislocations, but the temperature variation of the shear modulus, the decrease of which is correlated to the experimental behaviour observed for both FCC Al and BCC Fe.

Journal article

Zheng Z, Balint D, Dunne F, 2017, Investigation of slip transfer across HCP grain boundaries with application to cold dwell facet fatigue, Acta Materialia, Vol: 127, Pages: 43-53, ISSN: 1359-6454

This paper addresses the role of grain boundary slip transfer and thermally-activated discretedislocation plasticity in the redistribution of grain boundary stresses during cold dwell fatigue intitanium alloys. Atomistic simulations have been utilised to calculate the grain boundary energies fortitanium with respect to the misorientation angles. The grain boundary energies are utilised within athermally-activated discrete dislocation plasticity model incorporating slip transfer controlled byenergetic and grain boundary geometrical criteria. The model predicts the grain size effect on the flowstrength in Ti alloys. Cold dwell fatigue behaviour in Ti-6242 alloy is investigated and it is shown thatsignificant stress redistribution from soft to hard grains occurs during the stress dwell, which isobserved both for grain boundaries for which slip transfer is permitted and inhibited. However, thegrain boundary slip penetration is shown to lead to significantly higher hard-grain basal stresses nearthe grain boundary after dwell, thus exacerbating the load shedding stress compared to animpenetrable grain boundary. The key property controlling the dwell fatigue response is argued toremain the time constant associated with the thermal activation process for dislocation escape, but theslip penetrability is also important and exacerbates the load shedding. The inclusion of a macrozonedoes not significantly change the conclusions but does potentially lead to the possibility of a largerinitial facet.

Journal article

Bordas SPA, Balint DS, 2017, Preface

Book

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