Publications
188 results found
Tarleton E, Balint DS, Gong J, et al., 2015, A discrete dislocation plasticity study of the micro-cantilever size effect, Acta Materialia, Vol: 88, Pages: 271-282, ISSN: 1359-6454
Micro-cantilevers are increasingly used to extract elastic and plastic material properties through controlled bending using a nanoindenter. Focused Ion Beam milling can be used to produce small scale single crystal cantilevers with cross-sectional dimensions on the order of microns, and electron backscatter diffraction (EBSD) allows cantilevers to be milled from a grain with a desired crystal orientation. Micro-cantilever bending experiments suggest that sufficiently smaller cantilevers are stronger, which is generally believed to be related to the effect of the neutral axis on the evolution of the dislocation structure. A planar model of discrete dislocation plasticity was used to simulate end-loaded cantilevers to interpret the behaviour observed in experiments. The model allowed correlation of the initial dislocation source density and resulting slip band spacing to the experimental load displacement curve. There are similarities between the predictions of this model and those of earlier discrete dislocation plasticity models of pure bending. However, there are notable differences, including a strong source density dependence of the size effect that cannot be explained by geometrically necessary dislocation (GND) arguments, and the effect of the cantilever stress distribution on the locations of soft pile-ups. The planar model was used to identify zero resolved shear stress isolines, rather than the neutral axis, as controlling the soft pile-up location, and source spacing as limiting the slip band spacing in the observed size effect; strengthening was much greater in the source-limited regime. The effect of sample dimensions and dislocation source density were investigated and compared to small scale mechanical tests conducted on titanium and zirconium. The calculations predict a scaling exponent for the dependence of stress on size if size is normalised by the average source spacing and a term representing the size-independent flow stress is included, whereas
Balint DS, Bordas SPA, 2015, Advances in Applied Mechanics PREFACE, Publisher: ELSEVIER ACADEMIC PRESS INC
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- Citations: 1
El Fakir O, Wang L, Balint D, et al., 2014, Numerical study of the solution heat treatment, forming, and in-die quenching (HFQ) process on AA5754, International Journal of Machine Tools and Manufacture, Vol: 87, Pages: 39-48, ISSN: 0890-6955
An FE model of the solution heat treatment, forming and in-die quenching (HFQ) process was developed. Good correlation with a deviation of less than 5% was achieved between the thickness distribution of the simulated and experimentally formed parts, verifying the model. Subsequently, the model was able to provide a more detailed understanding of the HFQ process, and was used to study the effects of forming temperature and speed on the thickness distribution of the HFQ formed part. It was found that a higher forming speed is beneficial for HFQ forming, as it led to less thinning and improved thickness homogeneity.
Gurrutxaga Lerma B, Balint DS, Dini D, et al., 2014, Dynamic Discrete Dislocation Plasticity, Advances in Applied Mechanics, Publisher: Elsevier, Pages: 93-224, ISBN: 978-0-12-800130-1
This chapter concerns with dynamic discrete dislocation plasticity (D3P), a two- dimensional method of discrete dislocation dynamics aimed at the study of plastic relaxation processes in crystalline materials subjected to weak shock loading. Traditionally, the study of plasticity under weak shock loading and high strain rate has been based on direct experimental measurement of the macroscopic response of the material. Using these data, well-known macroscopic constitutive laws and equations of state have been formulated. However, direct simulation of dislocations as the dynamic agents of plastic relaxation in those circumstances remains a challenge. In discrete dislocation dynamics (DDD) methods, in particular the two-dimensional discrete dislocation plasticity (DDP), the dislocations are modeled as discrete discontinuities in an elastic continuum. However, current DDP and DDD methods are unable to adequately simulate plastic relaxation because they treat dislocation motion quasi- statically, thus neglecting the time-dependent nature of the elastic fields and assuming that they instantaneously acquire the shape and magnitude predicted by elastostatics. This chapter reproduces the findings by Gurrutxaga-Lerma, Balint, Dini, Eakins, and Sutton (2013), who proved that under shock loading, this assumption leads to models that invariably break causality, introducing numerous artifacts that invalidate quasi- static simulation techniques. This chapter posits that these limitations can only be overcome with a fully time-dependent formulation of the elastic fields of dislocations. In this chapter, following the works of Markenscoff and Clifton (1981) and Gurrutxaga- Lerma et al. (2013), a truly dynamic formulation for the creation, annihilation, and nonuniform motion of straight edge dislocations is derived. These solutions extend the DDP framework to a fully elastodynamic formulation that has been called dynamic discrete dislocation plasticity (D3P). This chapter describes t
Gurrutxaga-Lerma B, Balint DS, Dini D, et al., 2014, Dynamic Discrete Dislocation Plasticity, Advances in Applied Mechanics, Vol: 47, ISSN: 0065-2156
This chapter concerns with dynamic discrete dislocation plasticity (D3P), a two- dimensional method of discrete dislocation dynamics aimed at the study of plastic relaxation processes in crystalline materials subjected to weak shock loading. Traditionally, the study of plasticity under weak shock loading and high strain rate has been based on direct experimental measurement of the macroscopic response of the material. Using these data, well-known macroscopic constitutive laws and equations of state have been formulated. However, direct simulation of dislocations as the dynamic agents of plastic relaxation in those circumstances remains a challenge. In discrete dislocation dynamics (DDD) methods, in particular the two-dimensional discrete dislocation plasticity (DDP), the dislocations are modeled as discrete discontinuities in an elastic continuum. However, current DDP and DDD methods are unable to adequately simulate plastic relaxation because they treat dislocation motion quasi- statically, thus neglecting the time-dependent nature of the elastic elds and assuming that they instantaneously acquire the shape and magnitude predicted by elastostatics. This chapter reproduces the ndings by Gurrutxaga-Lerma, Balint, Dini, Eakins, and Sutton (2013), who proved that under shock loading, this assumption leads to models that invariably break causality, introducing numerous artifacts that invalidate quasi- static simulation techniques. This chapter posits that these limitations can only be overcome with a fully time-dependent formulation of the elastic elds of dislocations. In this chapter, following the works of Markensco and Clifton (1981) and Gurrutxaga- Lerma et al. (2013), a truly dynamic formulation for the creation, annihilation, and nonuniform motion of straight edge dislocations is derived. These solutions extend the DDP framework to a fully elastodynamic formulation that has been called dynamic discrete dislocation plasticity (D3P). This chapter describes the s
Betts C, Balint D, Lee J, et al., 2014, In situ microtensile testing and X-ray microtomography-based finite element modelling of open-cell metal foam struts and sandwich panels, JOURNAL OF STRAIN ANALYSIS FOR ENGINEERING DESIGN, Vol: 49, Pages: 592-606, ISSN: 0309-3247
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- Citations: 9
Politis DJ, Lin J, Dean TA, et al., 2014, An investigation into the forging of Bi-metal gears, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, Vol: 214, Pages: 2248-2260, ISSN: 0924-0136
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- Citations: 33
Li N, Lin J, Dean TA, et al., 2014, Materials Modelling for Selective Heating and Press Hardening of Boron Steel Panels with Graded Microstructures, Procedia Engineering, Vol: 81, Pages: 1675-1681, ISSN: 1877-7058
Kardoulaki E, Lin J, Balint D, et al., 2014, Investigation of the effects of thermal gradients present in Gleeble high-temperature tensile tests on the strain state for free cutting steel, JOURNAL OF STRAIN ANALYSIS FOR ENGINEERING DESIGN, Vol: 49, Pages: 521-532, ISSN: 0309-3247
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- Citations: 21
Li N, Lin J, Dean TA, et al., 2014, Concept Validation for Selective Heating and Press Hardening of Automotive Safety Components with Tailored Properties, Key Engineering Materials, Vol: 622-623, Pages: 1124-1131, ISSN: 1013-9826
A new strategy termed selective heating and press hardening, for hot stamping of boron steel parts with tailored properties is proposed in this paper. Feasibility studies were carried out through a specially designed experimental programme. The main aim was to validate the strategy and demonstrate its potential for structural optimisation. In the work, a lab-scale demonstrator part was designed, and relevant manufacturing and property-assessment processes were defined. A heating technique and selective-heating rigs were designed to enable certain microstructural distributions in blanks to be obtained. A hot stamping tool set was designed for forming and quenching the parts. Demonstrator parts of full martensite phase, full initial phase, and differentially graded microstructures have been formed with high dimensional quality. Hardness testing and three point bending tests were conducted to assess the microstructure distribution and load bearing performance of the as-formed parts, respectively. The feasibility of the concept has been validated by the testing results.
Zhu HX, Zhang P, Balint D, et al., 2014, The effects of regularity on the geometrical properties of Voronoi tessellations, PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS, Vol: 406, Pages: 42-58, ISSN: 0378-4371
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- Citations: 23
Lin J, Mohamed M, Balint D, et al., 2014, The development of continuum damage mechanics-based theories for predicting forming limit diagrams for hot stamping applications, INTERNATIONAL JOURNAL OF DAMAGE MECHANICS, Vol: 23, Pages: 684-701, ISSN: 1056-7895
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- Citations: 65
Kardoulaki E, Lin J, Balint D, et al., 2014, Triaxiality effect on material damage evolution in hot rolling, Pages: 1041-1048, ISSN: 1013-9826
High temperature tensile tests have been conducted on double-notched bars (DNBs) of varying notch acuity ratios (a/R), achieving a range of triaxiality, to evaluate the effect of stress state on edge cracking in hot rolling of free cutting steel. The conditions of the tensile tests are selected to physically simulate hot rolling condition of as-cast material to wrought structure. All experiments were conducted using a Gleeble 3800 thermo-mechanical testing system. The aim here is to develop a qualitative relationship between the activated damage mechanisms with respect to different stress states (arising from the sample geometry). Double-notch bars have been selected for this investigation as they enable the optical examination of the pre-failure mechanisms, captured in the unbroken notched area. The damage features have been observed at the notch areas and different damage mechanisms have been identified for different stress state deforming at different temperatures.
El Fakir O, Das S, Stone I, et al., 2014, Solution heat treatment, forming and in-die quenching of a commercial sheet magnesium alloy into a complex-shaped component: Experimentation and FE simulation, Key Engineering Materials, Vol: 622-623, Pages: 596-602, ISSN: 1013-9826
Interest in lightweight materials, particularly magnesium alloys, has increased significantly with rising efficiency requirements in the automotive sector. Magnesium is the lightest available structural metal, with a density approximately 35% lower than that of aluminium. The potential is great for magnesium to become a primary material used in future low carbon vehicle structures; however, there are significant obstacles, namely low ductility and formability, particularly at room temperature. The aim of this work is to present the feasibility of using the solution Heat treatment, Forming, and in-die Quenching (HFQ) process to produce complex shapes from a sheet magnesium alloy, and to use the results to verify a simulation of the process developed using commercial FE software. Uniaxial tensile tests were initially conducted to establish the optimum parameters for forming the part. Stamping trials were then carried out using these parameters, and a simulation set up modelling the forming operation. It was shown that the HFQ process could be used to form a successful component from this alloy, and that a good match was achieved between the results of the forming experiments and the simulation.
Bai Q, Lin J, Tian G, et al., 2014, A novel forming process for powder metallurgy of superalloys, Pages: 833-839, ISSN: 1013-9826
Powder metallurgy (PM) of nickel-based superalloys has been used for a wide range of products owing to their excellent special properties in processing and applications. Typical processes for high performance PM superalloys include hot isostatic pressing, hot extrusion and hot isothermal forging. Hot isostatic pressing is normally conducted at a high temperature, by using a low pressure for a long time in a closed vessel, resulting in high cost and low product efficiency. In this paper a novel forming process, i.e. direct powder forging for powder metallurgy of superalloys has been proposed. In this process, the encapsulated and vacuumed powder is heated up to the forming temperature and forged directly to the final shape, by using a high forming load for a very short time. Direct powder forging is a low-cost and energy-saving process compared to conventional PM processes, and in addition, press machines of conventional forging can be used for direct powder forming process. In direct powder forging it is important to control the relative density of the deformed part since the existence of voids could reduce the mechanical strength and fatigue life. In this paper, feasibility tests of direct powder forging are presented. Microstructure, relative density and hardness of the formed specimen were studied.
El Fakir O, Wang L, Balint D, et al., 2014, Predicting effect of temperature, strain rate and strain path changes on forming limit of lightweight sheet metal alloys, 11TH INTERNATIONAL CONFERENCE ON TECHNOLOGY OF PLASTICITY, ICTP 2014, Vol: 81, Pages: 736-741, ISSN: 1877-7058
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- Citations: 18
Betts C, Balint D, Lin J, 2014, In-situ micro-tensile testing and X-ray micro-tomography based FE modeling of open-cell metal foam struts and sandwich panels, 8th International Conference on Porous Metals and Metallic Foams, Publisher: ELSEVIER SCIENCE BV, Pages: 197-202, ISSN: 2211-8128
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- Citations: 3
Millar TM, Williams JG, Balint DS, 2014, The cohesive zone model applied to blunt cracks, 20th European Conference on Fracture (ECF), Publisher: ELSEVIER SCIENCE BV, Pages: 313-317, ISSN: 2211-8128
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- Citations: 3
Zhu H, Balint D, Guo Z, 2014, Theoretical Analysis and Computational Simulation of Advanced Structured Materials, ADVANCES IN CONDENSED MATTER PHYSICS, Vol: 2014, Pages: 1-2, ISSN: 1687-8108
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- Citations: 47
Afshan S, Balint D, Farrugia D, et al., 2013, A new experimental method for identifying the conditions necessary for diffusion bonding in free cutting steels, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 586, Pages: 25-30, ISSN: 0921-5093
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- Citations: 4
Kaye M, Puncreobutr C, Lee PD, et al., 2013, A new parameter for modelling three-dimensional damage evolution validated by synchrotron tomography, ACTA MATERIALIA, Vol: 61, Pages: 7616-7623, ISSN: 1359-6454
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- Citations: 17
Tantideeravit S, Charalambides MN, Balint DS, et al., 2013, Prediction of delamination in multilayer artist paints under low amplitude fatigue loading, ENGINEERING FRACTURE MECHANICS, Vol: 112, Pages: 41-57, ISSN: 0013-7944
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- Citations: 12
Betts C, Balint D, Lin J, 2013, The effect of morphological imperfections on damage in 3D FE analysis of open-cell metal foam core sandwich panels, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, Vol: 75, Pages: 377-387, ISSN: 0020-7403
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- Citations: 1
Gurrutxaga-Lerma B, Balint D, Dini D, et al., 2013, A dynamic discrete dislocation plasticity method for the simulation of plastic relaxation under shock loading, Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 469, ISSN: 1364-5021
In this article, it is demonstrated that current methods of modelling plasticity as the collective motion of discrete dislocations, such as two-dimensional discrete dislocation plasticity (DDP), are unsuitable for the simulation of very high strain rate processes (106 s−1 or more) such as plastic relaxation during shock loading. Current DDP models treat dislocations quasi-statically, ignoring the time-dependent nature of the elastic fields of dislocations. It is shown that this assumption introduces unphysical artefacts into the system when simulating plasticity resulting from shock loading. This deficiency can be overcome only by formulating a fully time-dependent elastodynamic description of the elastic fields of discrete dislocations. Building on the work of Markenscoff & Clifton, the fundamental time-dependent solutions for the injection and non-uniform motion of straight edge dislocations are presented. The numerical implementation of these solutions for a single moving dislocation and for two annihilating dislocations in an infinite plane are presented. The application of these solutions in a two-dimensional model of time-dependent plasticity during shock loading is outlined here and will be presented in detail elsewhere.
Karimpour M, Balint DS, Rzepiejewska-Malyska KA, et al., 2013, An inverse method for extracting the mechanical properties of the constituent materials of a multilayer from nanoindentation data, COMPUTATIONAL MATERIALS SCIENCE, Vol: 68, Pages: 384-390, ISSN: 0927-0256
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- Citations: 11
Betts C, Balint D, Lin J, 2013, An Investigation of the Mechanical Properties of Open Cell Aluminium Foam Struts: Microtensile Testing and Modelling, Advanced Structured Materials, Pages: 53-63
A microtensile test procedure has been developed to directly determine the mechanical properties of individual metal foam struts. The results reveal that the measured strut properties display a considerable reduction in elastic stiffness compared to the typical value of 70 GPa for aluminium alloys. A realistic finite element modelling procedure of the as-tested struts has been established, using X-ray micro-tomography scans of the undeformed struts, to assess the reasons for this reduction in stiffness. The material model in the FE simulations was established using a damage model that comprises of a set of continuum mechanics-based viscoplastic damage constitutive equations. The equations were calibrated with the microtensile test data and implemented into ABAQUS through the user defined subroutine VUMAT. The prime factor in the recorded reduction in stiffness was found to be slippage between the grips and the strut during testing.
Afshan S, Balint D, Lin J, et al., 2013, Micromechanical Modelling of Void Healing, Advanced Structured Materials, Pages: 1-8
Predicting effective consolidation or level of remnant porosity for a range of steel grade (function of solidification regime), billet size, pass schedule/roll design and thermo-mechanical conditions has always been an important issue for steel producers as it will affect the mechanical properties of final products (strength, ductility, etc.). It is known that partial or complete recovery of strength in such porous materials can be obtained by pore closure and diffusive healing processes at elevated temperature. This study investigates the elimination of porosity through two stages of void closure and healing. An Abaqus/UMAT has been developed for the analysis of the material porosity elimination process including two stages of void closure and healing. The model uses the Gurson-Tvergaard (GT) model under compression to predict the void closure. The closure model parameters were calibrated by an optimisation technique using a representative volume element concept. Then a healing model based on a combination of diffusion bonding, creep and plasticity was implemented as a UMAT subroutine and finally the whole behaviour of the material was controlled using a status check method developed in this work.
Xu Y, Balint DS, Dini D, 2013, Multi-scale modeling of indentation and contact fatigue: A coupled CPFE/DD approach, Pages: 3416-3419
Bai Q, Lin J, Dean TA, et al., 2013, Modelling of dominant softening mechanisms for Ti-6Al-4V in steady state hot forming conditions, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 559, Pages: 352-358, ISSN: 0921-5093
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- Citations: 75
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