Sasa Kovacevic | University of Oxford (original) (raw)

Papers by Sasa Kovacevic

Corrosion Science, 2024

An electro-chemo-mechanical phase-field formulation is developed to simulate pitting and stress c... more An electro-chemo-mechanical phase-field formulation is developed to simulate pitting and stress corrosion in polycrystalline materials. The formulation incorporates dependencies of mechanical properties and corrosion potential on crystallographic orientation. The model considers the formation and charging dynamics of an electric double layer through a new general boundary condition for the solution potential. The potential of the model is demonstrated by simulating corrosion in polycrystalline materials with various grain morphology distributions. The results show that incorporating the underlying microstructure yields more extensive defects, faster defect kinetics, and irregular pit and crack shapes relative to a microstructurally-insensitive homogeneous material scenario.

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Materials & Design, 2023

A multiscale computational framework combining the first-principles calculations and the CALPHAD ... more A multiscale computational framework combining the first-principles calculations and the CALPHAD approach with the phase-field method is presented to simulate the microstructure evolution in multicomponent steel alloys. We demonstrate the potential of the framework by predicting the microstructural evolution in elastically periodic arrays of the Mo-V binary subsystem. The framework utilizes the first-principles calculations using special quasi-random structures. Hitherto unavailable thermodynamic and material properties of the alloy are obtained by employing the first-principles calculations and the CALPHAD approach and fed into the phase-field model to predict the microstructure evolution at different temperatures within the miscibility gap region. In addition to the temperature and cooling rates, the model incorporates the role of mechanical fields in decomposition kinetics in the Mo-V binary alloy system. Regimes for temperatures and cooling rates at which spinodal decomposition occurs are identified. Applying external loading leads to directional phase separation in the Mo-V binary system. The elastic inhomogeneity in terms of material properties between the two phases initiates the directional alignment while eigenstrains and applied external loading control the degree of alignment. The framework developed is general and extendable to higher multicomponent subsystems in steel alloys.

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ce/papers, 2021

In this paper, an in-depth nonlinear finite element analysis is carried out on longitudinally uns... more In this paper, an in-depth nonlinear finite element analysis is carried out on longitudinally unstiffened and stiffened welded I-steel plate girders subjected to patch loading. The most realistic load case is during the incremental launching of multi-span steel and composite bridges over temporary or permanent supports. Bridge plate girders are usually reinforced by longitudinal and transverse stiffeners in order to prevent web panel buckling and to increase the bending and shear strength. The current European design standard EN 1993-1-5 also requires a stability control check for concentrated transverse forces (patch loading) since it can be a decisive design criterion. By using longitudinal stiffeners for flexural and shear resistance, the patch loading carrying capacity can be increased as well. Previous experimental studies showed that the EN1993-1-5 patch loading resistance model significantly underestimates the ultimate strength of longitudinally stiffened plate girders. The paper is primarily dealing with numerical research of the post-buckling behavior and ultimate strength of longitudinally stiffened I-steel plate girders reinforced by one longitudinal stiffener. The main objective of the present work is to investigate the influence of different geometrical parameters, including patch load length, web panel aspect ratio, and initial geometrical imperfections, on the ultimate strength of longitudinally stiffened plate girders. We showed in this study that for all the considered initial geometrical imperfections, the patch load resistance increases as the patch load length is increased. The lowest ultimate strength of longitudinally stiffened plate girders was determined for initial geometrical imperfections that resembled the deformed shape at collapse (collapse-affine imperfections). The web panel aspect ratio also influenced the patch load resistance and lower ultimate strengths were returned for bigger web panel aspect ratios. Finally, further research recommendations are presented.

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Journal of Constructional Steel Research, 2022

The present paper aims to numerically investigate the influence of patch load length related to v... more The present paper aims to numerically investigate the influence of patch load length related to various geometric parameters on the patch loading resistance of steel plate girders. Geometric parameters such as the web thickness, web panel width, and flange thickness are varied in combination with flat longitudinal stiffeners with varying rigidity in determining ultimate strength for various patch load lengths. Of primary interest in this paper is the strengthening effect, defined as the ratio between the ultimate strength of longitudinally stiffened and unstiffened steel plate girders.

The strengthening effect for different patch load lengths depends on the rigidity of the longitudinal stiffener, web slenderness, web panel aspect ratio, and stiffness of the loaded flange. Small strengthening effects are obtained for relatively small patch load lengths and all parameters considered. A noticeable strengthening effect is observed for longer patch load lengths. The European design standard EN 1993-1-5 returns uniform strengthening effects independent of patch load length. This is a direct consequence of the definition of the effective loaded length in EN 1993-1-5. The effective loaded length is the same for longitudinally unstiffened and stiffened steel plate girders, and thus, there is no enhancing contribution due to longitudinal stiffening. Therefore, new calculation formulae for predicting the collapse load of longitudinally unstiffened and stiffened steel plate girders subjected to patch loading are defined. The design expressions are determined based on the present numerical results and verified by comparison with experimental and computational results available in the literature.

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Acta Biomaterialia, 2023

A phase-field model is developed to simulate the corrosion of Mg alloys in body fluids. The model... more A phase-field model is developed to simulate the corrosion of Mg alloys in body fluids. The model incorporates both Mg dissolution and the transport of Mg ions in solution, naturally predicting the transition from activation-controlled to diffusion-controlled bio-corrosion. In addition to uniform corrosion, the presented framework captures pitting corrosion and accounts for the synergistic effect of aggressive environments and mechanical loading in accelerating corrosion kinetics. The model applies to arbitrary 2D and 3D geometries with no special treatment for the evolution of the corrosion front, which is described using a diffuse interface approach. Experiments are conducted to validate the model and a good agreement is attained against in vitro measurements on Mg wires. The potential of the model to capture mechano-chemical effects during corrosion is demonstrated in case studies considering Mg wires in tension and bioabsorbable coronary Mg stents subjected to mechanical loading. The proposed methodology can be used to assess the in vitro and in vivo service life of Mg-based biomedical devices and optimize the design taking into account the effect of mechanical deformation on the corrosion rate. The model has the potential to advocate further development of Mg alloys as a biodegradable implant material for biomedical applications.

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Nature Communications, 2023

Shape distortion in sintering results from nonhomogeneous temperature activating a long-range mas... more Shape distortion in sintering results from
nonhomogeneous temperature activating a
long-range mass transport

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International Journal of Solids and Structures, 2022

The lattice continuum formulation for diffusional creep is implemented into the computational fra... more The lattice continuum formulation for diffusional creep is implemented into the computational framework based on the finite difference method for space and the predictor-corrector algorithm for time discretization, to solve the coupled elasticity-diffusion problem with moving boundaries. The numerical scheme is implemented and tested considering 3D periodic structures without grain boundary sliding. Of primary interest in this paper are the stress non-uniformities resulting from nonuniform composition eigenstrains. We consider two regimes: the one where the rate limiting process is bulk diffusion of vacancies, and the one where the rate is controlled by vacancy nucleation/annihilation at grain boundaries (nucleation-controlled creep). We found that the stress concentration factor for diffusion-controlled creep is independent of the applied stress and grain size. No stress concentrations are present for the nucleation-controlled creep case. Stress and grain size dependence of minimum strain creep rates are determined by the present model for a variety of applied stresses, grain sizes, different driving processes (diffusion and nucleation-controlled creep) and compared with the classical theory for diffusional creep. We found that steady-state creep rates varied linearly with applied stresses for both diffusion and nucleation-controlled creep. The numerical results show good correspondence to analytical predictions for idealized diffusional Nabarro-Herring creep. Significantly lower steadystate strain rates were computed for nucleation-controlled creep. These results demonstrate the ability of the present model to reproduce the stress and grain size dependence of the steady-state strain creep rates.

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Journal of Constructional Steel Research, 2019

An experimental research considering the behavior and ultimate strength of longitudinally stiffen... more An experimental research considering the behavior and ultimate strength of longitudinally stiffened plate girders subjected to localized edge loading in the plane of the web, a load case usually referred to as patch loading, is presented. Based on a literature review, the limitations of previous investigations are listed, which proved the need for additional experimental research. Part I of this research includes the main experimental test results, originally given in (Markovic, 2003 [1]), while in Part II the finite element modeling, verification of numerical models as well as results of an extensive parametric study will be presented. The driving force for performing this experimental campaign was to investigate the influence of patch load length on the ultimate capacity of longitudinally stiffened girders since this parameter has not been systematically studied. The influence of patch load length is thoroughly analyzed and results concerning the influence of longitudinal stiffeners are elaborated.

The experiments are described in detail and the main conclusions of the experimental investigation are presented. It may be concluded that there is a significant influence of the length of patch load on the behavior and ultimate strength of the girders. The combined influence of increased patch load length and longitudinal stiffener can significantly increase the patch load resistance of plate girders. It can be concluded that the ultimate load of stiffened girders follows the ultimate strength of unstiffened ones under small patch load lengths. When a specific applied load length is reached, an appreciable strengthening effect can be obtained.

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Thin-Walled Structures, 2020

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Acta Materialia, 2020

Diffusion bonding of ceramics with a metallic interlayer can give a variety of very complex joint... more Diffusion bonding of ceramics with a metallic interlayer can give a variety of very complex joint microstruc-tures, which are highly influenced by ceramic compositions, the material and thickness of the interlayer, bonding temperature as well as time at the peak bonding temperature. Experiments with a diffusion bonding of ZrC using a Ti interlayer clearly show that under a certain bonding condition, a seamless joint with the total dissolution of the interlayer can be obtained. They also indicate the existence of the critical interlayer thickness, below which the seamless homogeneous joint domain is obtained, and above which the joint does not homogenize. The key process leading to these outcomes is the diffusion of carbon from ZrC into Ti, which, when the critical carbon concentration is reached, initiates the phase transformation of bcc Ti to TiC, while the binary Zr/Ti diffusion is then driven by entropy and results in a seamless Zr(Ti)C joint. We first show that the dependence of ZrC/Ti interfacial energy on the carbon concentration jump across the interface is the main thermodynamic driving force of the diffusion of carbon from ZrC to the Ti interlayer. Then, we show that the characteristic length (critical thickness of the interlayer) arises as the ratio of this driving force (energy/area) and the bulk energy densities, which oppose the carbon diffusion. Finally, we develop a diffuse interface (phase-field) model to simulate the process. The novelty in the phase-field model is the introduction of a dependence of the interfacial energy on the carbon concentrations on the two sides of the interface. The critical thickness of the interlayer is estimated employing both models and good agreement with experimental findings is obtained.

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Journal of Constructional Steel Research, 2019

The companion paper Part I [1] deals with the experimental campaign of longitudinally stiffened p... more The companion paper Part I [1] deals with the experimental campaign of longitudinally stiffened plate girders subjected exclusively to patch loading. The current paper focuses on the numerical research and parametric study of the influence of patch load length and initial geometrical imperfections on the ultimate strength of longitudinally stiffened plate girders. In order to assess the patch load resistance, a geometrically and materially nonlinear finite element analysis has been performed. For a better verification with the experimental results, the finite element model includes the experimentally measured initial geometrical imperfections and material properties based on laboratory tests. The verification of the numerical model has been obtained through the comparison of the numerically and experimentally attained results for the ultimate loads and elastoplastic behavior of the girders. It has been shown that the numerical and experimental results are in perfect agreement, which enabled a fruitful background for parametric analysis, in which different initial geometrical imperfections have been used to ameliorate understandings about their influence on the ultimate strength under different patch load lengths. Conclusively, it may be stated that initial geometrical imperfections can play a decisive role, especially for longitudinally stiffened girders. Initial geometrical imperfections of stiffened girders that correspond to deformed shape at the collapse (collapse-affine imperfections), especially in the zone where the load is applied, will give the most unfavorable ultimate strengths. For the considered geometry in the present paper, the third buckling mode of longitudinally stiffened girders corresponds to the deformed shape and the lowest ultimate strengths are obtained.

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Engineering Structures, 2018

A novel rotation-free isogeometric formulation of in-plane dynamic analysis of an arbitrarily cur... more A novel rotation-free isogeometric formulation of in-plane dynamic analysis of an arbitrarily curved Bernoulli-Euler beam in the convective frame of reference is presented. The driving force behind the present study has been the development of the NURBS-based element which enables an elegant framework of in-plane vibrations of arbitrarily curved Bernoulli-Euler beams, being a function only of the global Cartesian coordinates. Due to the fact that no additional simplifications are made, besides those related to the classic Bernoulli-Euler hypothesis and small strain theory, the formulation is particularly applicable for problems regarding the behavior of strongly curved beams.

An excellent agreement of the results is accomplished and efficiency for academic and practical use are shown. The influence of the product of the maximum curvature and the thickness of the beam on the accuracy of the solution is specially treated and debated. The effects of the hpk-refinements are thoroughly checked and a highly nonlinear convergence behavior under the h-refinement is noticed. The well-known fact that models with the highest interelement continuities return superior accuracy per degree of freedom is substantiated by an in-depth numerical analysis of order of convergence. Furthermore, the accuracy of the developed model is analyzed utilizing normalized numerical discrete spectrums. It is remarked that the accuracy per degree of freedom degrades with the complexity of reference geometry of the beam.

Keywords: Isogeometric analysis, Linear dynamics, Arbitrarily curved in-plane beam, Bernoulli-Euler beam, Rotation-free model, Order of convergence

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Computer Methods in Applied Mechanics and Engineering, 2018

The present study elucidates linear static analysis for plane beam structures using the isogeomet... more The present study elucidates linear static analysis for plane beam structures using the isogeometric approach. A novel methodology for rotation-free analysis of an arbitrarily curved Bernoulli–Euler beam in the convective frame of reference is derived in detail. The full degeneration of a 3D continuum beam to a 1D line has been presented and a fully applicable isogeometric finite element has been obtained. The driving force behind developing the present research has been the derivation of the NURBS-based isogeometric analysis which will enable an elegant formulation of the plane Bernoulli–Euler beams, being a function only of the global rectangular Cartesian coordinates. The verification and accuracy of the research are obtained via a thorough comparison between theory, finite element analyses and relevant examples from literature. An excellent agreement of results is achieved and usefulness for academic and practical purposes alike are proved. The effects of the hpk-refinements are illuminated and it is observed that the convergences for the most variables and refinement techniques are not monotonic. A special attention is paid to the influence of the product of maximum curvature and thickness of beam on the accuracy of the solution. The limits of applicability of the present approach are defined for a few specific types of analyses. The derived formulation is geometrically exact and appropriate for the analysis of strongly curved Bernoulli–Euler beams.

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Engineering Structures, 2013

ABSTRACT The harmonic compound finite strip method has been applied to linear transient vibration... more ABSTRACT The harmonic compound finite strip method has been applied to linear transient vibration analysis of stiffened plates. In this method, eigenfunctions of Bernoulli-Euler beam have been used as the displacement interpolation functions in longitudinal direction, while finite element shape functions have been used for it in transverse direction. The Kirchhoff–Love thin plate theory has been used and the equation of motion of structure is derived from Lagrange’s equation of motion. The governing equations have been solved by the mode superposition where step-by-step procedure has been used for the solution of modal equation. The stiffener has been modeled so that it may lie anywhere within the plate strip which helps to increase the flexibility in mesh generation. The formulation is applicable for rectangular plates stiffened with longitudinal and transverse beams and supported on columns. The proposed method is validated through several examples. The strips with free end give erroneous results for non-zero Poisson’s ratio.

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A B S T R A C T The present study sheds light on geometric nonlinear static analysis of prismatic... more A B S T R A C T The present study sheds light on geometric nonlinear static analysis of prismatic shells using the semi-analytical finite strip method. A new computational model, which includes a fully nonlinear compound strip with a longitudinal and transverse stiffener, has been presented. Furthermore, strips with non-uniform characteristics in the longitudinal direction have been used in nonlinear analysis. This has, to the best knowledge of authors, never been reported. Also, this paper describes the design and implementation of eighteen ideal boundary conditions using three different longitudinal and six well-known transverse displacement interpolation functions. The results of the presented study were obtained using an open-source software and multipurpose software Abaqus. Moreover, the accuracy of the applied computational approach has been verified by comparison with results from the literature. An excellent agreement of displacement fields is achieved for large deflection analyses of plates with a hole and stiffeners as well as for shells with a stepped thickness in the longitudinal direction. Additionally, results from post-buckling analyses of thin-walled structures, a snap-through and snap-back behavior of shallow shells, are matched. The work presented here has profound implications for future studies of the finite strip deployment.

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2Si-B-3C-N ceramic was successfully vacuum brazed to Nb. Brazing was executed at 850 °C for 10 mi... more 2Si-B-3C-N ceramic was successfully vacuum brazed to Nb. Brazing was executed at 850 °C for 10 min using Ag-Cu-Ti/Mo/Ag-Cu-Ti composite interlayer. Effects of the Mo interlayer thickness within the composite interlayer on the microstructure of the joint and its shear strength were investigated. In joints brazed without Mo and with 50 μm Mo interlayer a continuous through cracks propagated within the ceramic during cooling. However, no cracks were found in joints when Mo interlayers thicker than 100 μm were used. The improvement of the shear strength with an increase in the thickness of Mo interlayer within the composite interlayer was achieved. To explain the effects of the Mo layer thickness, detailed FEM simulations of the cooling process were performed. The analysis of the residual stress is consistent with the observed joint strengthening and crack propagation.

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Conference Presentations by Sasa Kovacevic

UK Association for Computational Mechanics Conference, 2024

Magnesium alloys exhibit great potential for use as implants that gradually and entirely dissolve... more Magnesium alloys exhibit great potential for use as implants that gradually and entirely dissolve in the human body after healing. Rapid corrosion in body fluids and the susceptibility to pitting corrosion and stress corrosion cracking are the main reasons limiting the widespread use of Mg alloys as a biodegradable material. The concurrence of mechanical loading and a corrosive environment dramatically accelerates the corrosion rate and promotes crack propagation, leading to the sudden failure of implants. A phase-field model is developed to simulate the corrosion of bioabsorbable metals in biological fluids. The model incorporates both Mg dissolution and the transport of Mg ions in solution. The framework captures pitting corrosion and incorporates the role of mechanical fields in enhancing the corrosion of biodegradable metals. The model is validated against in vitro corrosion data on Mg alloys immersed in simulated body fluid. The potential of the model to capture mechano-chemical effects is demonstrated by considering bioabsorbable coronary stents subjected to mechanical loading. The results show that pitting severely compromises the structural integrity of the stent and the application of mechanical loading initiates a pit-to-crack transition and crack propagations, leading to premature fracture after a short time in solution. This work extends phase-field modeling to biomaterial degradation and provides a novel mechanistic tool for assessing the service life of bioabsorbable medical devices.

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International Conference "Contemporary Achievements in Civil Engineering 2015", Subotica, Serbia, 2015

Free vibrations of rectangular plates with uniform thickness and with rectangular cutouts are inv... more Free vibrations of rectangular plates with uniform thickness and with rectangular cutouts are investigated. Plates are modeled using a finite strip method where the displacement field is approximated with a product of trigonometric and polynomial functions. In order to analyze the influence of polynomial order, two types of strips are utilized: LO2 and HO3. Strips are divided into cells in longitudinal direction, and the zero stiffness is given for cells which represent cutouts. The presented approach is coded using Wolfram Mathematica. Numerical tests contain comparison of results and convergence properties for LO2 and HO3 strips. Obtained results are in good agreement with data available in the literature.

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Proceedings of the Fourth South-East European Conference on Computational Mechanics, Kragujevac, Serbia, 2017

Elastic post-buckling analysis of imperfect rectangular plates subjected to a uniform compression... more Elastic post-buckling analysis of imperfect rectangular plates subjected to a uniform compression stress along one direction is presented. The semi-analytical finite strip method is improved by generalization of its nonlinear formulation through inclusion of various types of strips and boundary conditions. The equations of balance are derived via incremental formulation of virtual work, while the Newton-Raphson and arc-length methods are implemented into a solver. Geometric initial imperfections are modeled as a scaled buckling mode of a structure. The presented theory is verified via comparison of results with the ones from literature and Abaqus. The semi-analytical finite strip method is ideally suited for this type of analysis due to good convergence properties.

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Corrosion Science, 2024

An electro-chemo-mechanical phase-field formulation is developed to simulate pitting and stress c... more An electro-chemo-mechanical phase-field formulation is developed to simulate pitting and stress corrosion in polycrystalline materials. The formulation incorporates dependencies of mechanical properties and corrosion potential on crystallographic orientation. The model considers the formation and charging dynamics of an electric double layer through a new general boundary condition for the solution potential. The potential of the model is demonstrated by simulating corrosion in polycrystalline materials with various grain morphology distributions. The results show that incorporating the underlying microstructure yields more extensive defects, faster defect kinetics, and irregular pit and crack shapes relative to a microstructurally-insensitive homogeneous material scenario.

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Materials & Design, 2023

A multiscale computational framework combining the first-principles calculations and the CALPHAD ... more A multiscale computational framework combining the first-principles calculations and the CALPHAD approach with the phase-field method is presented to simulate the microstructure evolution in multicomponent steel alloys. We demonstrate the potential of the framework by predicting the microstructural evolution in elastically periodic arrays of the Mo-V binary subsystem. The framework utilizes the first-principles calculations using special quasi-random structures. Hitherto unavailable thermodynamic and material properties of the alloy are obtained by employing the first-principles calculations and the CALPHAD approach and fed into the phase-field model to predict the microstructure evolution at different temperatures within the miscibility gap region. In addition to the temperature and cooling rates, the model incorporates the role of mechanical fields in decomposition kinetics in the Mo-V binary alloy system. Regimes for temperatures and cooling rates at which spinodal decomposition occurs are identified. Applying external loading leads to directional phase separation in the Mo-V binary system. The elastic inhomogeneity in terms of material properties between the two phases initiates the directional alignment while eigenstrains and applied external loading control the degree of alignment. The framework developed is general and extendable to higher multicomponent subsystems in steel alloys.

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ce/papers, 2021

In this paper, an in-depth nonlinear finite element analysis is carried out on longitudinally uns... more In this paper, an in-depth nonlinear finite element analysis is carried out on longitudinally unstiffened and stiffened welded I-steel plate girders subjected to patch loading. The most realistic load case is during the incremental launching of multi-span steel and composite bridges over temporary or permanent supports. Bridge plate girders are usually reinforced by longitudinal and transverse stiffeners in order to prevent web panel buckling and to increase the bending and shear strength. The current European design standard EN 1993-1-5 also requires a stability control check for concentrated transverse forces (patch loading) since it can be a decisive design criterion. By using longitudinal stiffeners for flexural and shear resistance, the patch loading carrying capacity can be increased as well. Previous experimental studies showed that the EN1993-1-5 patch loading resistance model significantly underestimates the ultimate strength of longitudinally stiffened plate girders. The paper is primarily dealing with numerical research of the post-buckling behavior and ultimate strength of longitudinally stiffened I-steel plate girders reinforced by one longitudinal stiffener. The main objective of the present work is to investigate the influence of different geometrical parameters, including patch load length, web panel aspect ratio, and initial geometrical imperfections, on the ultimate strength of longitudinally stiffened plate girders. We showed in this study that for all the considered initial geometrical imperfections, the patch load resistance increases as the patch load length is increased. The lowest ultimate strength of longitudinally stiffened plate girders was determined for initial geometrical imperfections that resembled the deformed shape at collapse (collapse-affine imperfections). The web panel aspect ratio also influenced the patch load resistance and lower ultimate strengths were returned for bigger web panel aspect ratios. Finally, further research recommendations are presented.

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Journal of Constructional Steel Research, 2022

The present paper aims to numerically investigate the influence of patch load length related to v... more The present paper aims to numerically investigate the influence of patch load length related to various geometric parameters on the patch loading resistance of steel plate girders. Geometric parameters such as the web thickness, web panel width, and flange thickness are varied in combination with flat longitudinal stiffeners with varying rigidity in determining ultimate strength for various patch load lengths. Of primary interest in this paper is the strengthening effect, defined as the ratio between the ultimate strength of longitudinally stiffened and unstiffened steel plate girders.

The strengthening effect for different patch load lengths depends on the rigidity of the longitudinal stiffener, web slenderness, web panel aspect ratio, and stiffness of the loaded flange. Small strengthening effects are obtained for relatively small patch load lengths and all parameters considered. A noticeable strengthening effect is observed for longer patch load lengths. The European design standard EN 1993-1-5 returns uniform strengthening effects independent of patch load length. This is a direct consequence of the definition of the effective loaded length in EN 1993-1-5. The effective loaded length is the same for longitudinally unstiffened and stiffened steel plate girders, and thus, there is no enhancing contribution due to longitudinal stiffening. Therefore, new calculation formulae for predicting the collapse load of longitudinally unstiffened and stiffened steel plate girders subjected to patch loading are defined. The design expressions are determined based on the present numerical results and verified by comparison with experimental and computational results available in the literature.

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Acta Biomaterialia, 2023

A phase-field model is developed to simulate the corrosion of Mg alloys in body fluids. The model... more A phase-field model is developed to simulate the corrosion of Mg alloys in body fluids. The model incorporates both Mg dissolution and the transport of Mg ions in solution, naturally predicting the transition from activation-controlled to diffusion-controlled bio-corrosion. In addition to uniform corrosion, the presented framework captures pitting corrosion and accounts for the synergistic effect of aggressive environments and mechanical loading in accelerating corrosion kinetics. The model applies to arbitrary 2D and 3D geometries with no special treatment for the evolution of the corrosion front, which is described using a diffuse interface approach. Experiments are conducted to validate the model and a good agreement is attained against in vitro measurements on Mg wires. The potential of the model to capture mechano-chemical effects during corrosion is demonstrated in case studies considering Mg wires in tension and bioabsorbable coronary Mg stents subjected to mechanical loading. The proposed methodology can be used to assess the in vitro and in vivo service life of Mg-based biomedical devices and optimize the design taking into account the effect of mechanical deformation on the corrosion rate. The model has the potential to advocate further development of Mg alloys as a biodegradable implant material for biomedical applications.

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Nature Communications, 2023

Shape distortion in sintering results from nonhomogeneous temperature activating a long-range mas... more Shape distortion in sintering results from
nonhomogeneous temperature activating a
long-range mass transport

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International Journal of Solids and Structures, 2022

The lattice continuum formulation for diffusional creep is implemented into the computational fra... more The lattice continuum formulation for diffusional creep is implemented into the computational framework based on the finite difference method for space and the predictor-corrector algorithm for time discretization, to solve the coupled elasticity-diffusion problem with moving boundaries. The numerical scheme is implemented and tested considering 3D periodic structures without grain boundary sliding. Of primary interest in this paper are the stress non-uniformities resulting from nonuniform composition eigenstrains. We consider two regimes: the one where the rate limiting process is bulk diffusion of vacancies, and the one where the rate is controlled by vacancy nucleation/annihilation at grain boundaries (nucleation-controlled creep). We found that the stress concentration factor for diffusion-controlled creep is independent of the applied stress and grain size. No stress concentrations are present for the nucleation-controlled creep case. Stress and grain size dependence of minimum strain creep rates are determined by the present model for a variety of applied stresses, grain sizes, different driving processes (diffusion and nucleation-controlled creep) and compared with the classical theory for diffusional creep. We found that steady-state creep rates varied linearly with applied stresses for both diffusion and nucleation-controlled creep. The numerical results show good correspondence to analytical predictions for idealized diffusional Nabarro-Herring creep. Significantly lower steadystate strain rates were computed for nucleation-controlled creep. These results demonstrate the ability of the present model to reproduce the stress and grain size dependence of the steady-state strain creep rates.

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Journal of Constructional Steel Research, 2019

An experimental research considering the behavior and ultimate strength of longitudinally stiffen... more An experimental research considering the behavior and ultimate strength of longitudinally stiffened plate girders subjected to localized edge loading in the plane of the web, a load case usually referred to as patch loading, is presented. Based on a literature review, the limitations of previous investigations are listed, which proved the need for additional experimental research. Part I of this research includes the main experimental test results, originally given in (Markovic, 2003 [1]), while in Part II the finite element modeling, verification of numerical models as well as results of an extensive parametric study will be presented. The driving force for performing this experimental campaign was to investigate the influence of patch load length on the ultimate capacity of longitudinally stiffened girders since this parameter has not been systematically studied. The influence of patch load length is thoroughly analyzed and results concerning the influence of longitudinal stiffeners are elaborated.

The experiments are described in detail and the main conclusions of the experimental investigation are presented. It may be concluded that there is a significant influence of the length of patch load on the behavior and ultimate strength of the girders. The combined influence of increased patch load length and longitudinal stiffener can significantly increase the patch load resistance of plate girders. It can be concluded that the ultimate load of stiffened girders follows the ultimate strength of unstiffened ones under small patch load lengths. When a specific applied load length is reached, an appreciable strengthening effect can be obtained.

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Thin-Walled Structures, 2020

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Acta Materialia, 2020

Diffusion bonding of ceramics with a metallic interlayer can give a variety of very complex joint... more Diffusion bonding of ceramics with a metallic interlayer can give a variety of very complex joint microstruc-tures, which are highly influenced by ceramic compositions, the material and thickness of the interlayer, bonding temperature as well as time at the peak bonding temperature. Experiments with a diffusion bonding of ZrC using a Ti interlayer clearly show that under a certain bonding condition, a seamless joint with the total dissolution of the interlayer can be obtained. They also indicate the existence of the critical interlayer thickness, below which the seamless homogeneous joint domain is obtained, and above which the joint does not homogenize. The key process leading to these outcomes is the diffusion of carbon from ZrC into Ti, which, when the critical carbon concentration is reached, initiates the phase transformation of bcc Ti to TiC, while the binary Zr/Ti diffusion is then driven by entropy and results in a seamless Zr(Ti)C joint. We first show that the dependence of ZrC/Ti interfacial energy on the carbon concentration jump across the interface is the main thermodynamic driving force of the diffusion of carbon from ZrC to the Ti interlayer. Then, we show that the characteristic length (critical thickness of the interlayer) arises as the ratio of this driving force (energy/area) and the bulk energy densities, which oppose the carbon diffusion. Finally, we develop a diffuse interface (phase-field) model to simulate the process. The novelty in the phase-field model is the introduction of a dependence of the interfacial energy on the carbon concentrations on the two sides of the interface. The critical thickness of the interlayer is estimated employing both models and good agreement with experimental findings is obtained.

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Journal of Constructional Steel Research, 2019

The companion paper Part I [1] deals with the experimental campaign of longitudinally stiffened p... more The companion paper Part I [1] deals with the experimental campaign of longitudinally stiffened plate girders subjected exclusively to patch loading. The current paper focuses on the numerical research and parametric study of the influence of patch load length and initial geometrical imperfections on the ultimate strength of longitudinally stiffened plate girders. In order to assess the patch load resistance, a geometrically and materially nonlinear finite element analysis has been performed. For a better verification with the experimental results, the finite element model includes the experimentally measured initial geometrical imperfections and material properties based on laboratory tests. The verification of the numerical model has been obtained through the comparison of the numerically and experimentally attained results for the ultimate loads and elastoplastic behavior of the girders. It has been shown that the numerical and experimental results are in perfect agreement, which enabled a fruitful background for parametric analysis, in which different initial geometrical imperfections have been used to ameliorate understandings about their influence on the ultimate strength under different patch load lengths. Conclusively, it may be stated that initial geometrical imperfections can play a decisive role, especially for longitudinally stiffened girders. Initial geometrical imperfections of stiffened girders that correspond to deformed shape at the collapse (collapse-affine imperfections), especially in the zone where the load is applied, will give the most unfavorable ultimate strengths. For the considered geometry in the present paper, the third buckling mode of longitudinally stiffened girders corresponds to the deformed shape and the lowest ultimate strengths are obtained.

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Engineering Structures, 2018

A novel rotation-free isogeometric formulation of in-plane dynamic analysis of an arbitrarily cur... more A novel rotation-free isogeometric formulation of in-plane dynamic analysis of an arbitrarily curved Bernoulli-Euler beam in the convective frame of reference is presented. The driving force behind the present study has been the development of the NURBS-based element which enables an elegant framework of in-plane vibrations of arbitrarily curved Bernoulli-Euler beams, being a function only of the global Cartesian coordinates. Due to the fact that no additional simplifications are made, besides those related to the classic Bernoulli-Euler hypothesis and small strain theory, the formulation is particularly applicable for problems regarding the behavior of strongly curved beams.

An excellent agreement of the results is accomplished and efficiency for academic and practical use are shown. The influence of the product of the maximum curvature and the thickness of the beam on the accuracy of the solution is specially treated and debated. The effects of the hpk-refinements are thoroughly checked and a highly nonlinear convergence behavior under the h-refinement is noticed. The well-known fact that models with the highest interelement continuities return superior accuracy per degree of freedom is substantiated by an in-depth numerical analysis of order of convergence. Furthermore, the accuracy of the developed model is analyzed utilizing normalized numerical discrete spectrums. It is remarked that the accuracy per degree of freedom degrades with the complexity of reference geometry of the beam.

Keywords: Isogeometric analysis, Linear dynamics, Arbitrarily curved in-plane beam, Bernoulli-Euler beam, Rotation-free model, Order of convergence

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Computer Methods in Applied Mechanics and Engineering, 2018

The present study elucidates linear static analysis for plane beam structures using the isogeomet... more The present study elucidates linear static analysis for plane beam structures using the isogeometric approach. A novel methodology for rotation-free analysis of an arbitrarily curved Bernoulli–Euler beam in the convective frame of reference is derived in detail. The full degeneration of a 3D continuum beam to a 1D line has been presented and a fully applicable isogeometric finite element has been obtained. The driving force behind developing the present research has been the derivation of the NURBS-based isogeometric analysis which will enable an elegant formulation of the plane Bernoulli–Euler beams, being a function only of the global rectangular Cartesian coordinates. The verification and accuracy of the research are obtained via a thorough comparison between theory, finite element analyses and relevant examples from literature. An excellent agreement of results is achieved and usefulness for academic and practical purposes alike are proved. The effects of the hpk-refinements are illuminated and it is observed that the convergences for the most variables and refinement techniques are not monotonic. A special attention is paid to the influence of the product of maximum curvature and thickness of beam on the accuracy of the solution. The limits of applicability of the present approach are defined for a few specific types of analyses. The derived formulation is geometrically exact and appropriate for the analysis of strongly curved Bernoulli–Euler beams.

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Engineering Structures, 2013

ABSTRACT The harmonic compound finite strip method has been applied to linear transient vibration... more ABSTRACT The harmonic compound finite strip method has been applied to linear transient vibration analysis of stiffened plates. In this method, eigenfunctions of Bernoulli-Euler beam have been used as the displacement interpolation functions in longitudinal direction, while finite element shape functions have been used for it in transverse direction. The Kirchhoff–Love thin plate theory has been used and the equation of motion of structure is derived from Lagrange’s equation of motion. The governing equations have been solved by the mode superposition where step-by-step procedure has been used for the solution of modal equation. The stiffener has been modeled so that it may lie anywhere within the plate strip which helps to increase the flexibility in mesh generation. The formulation is applicable for rectangular plates stiffened with longitudinal and transverse beams and supported on columns. The proposed method is validated through several examples. The strips with free end give erroneous results for non-zero Poisson’s ratio.

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A B S T R A C T The present study sheds light on geometric nonlinear static analysis of prismatic... more A B S T R A C T The present study sheds light on geometric nonlinear static analysis of prismatic shells using the semi-analytical finite strip method. A new computational model, which includes a fully nonlinear compound strip with a longitudinal and transverse stiffener, has been presented. Furthermore, strips with non-uniform characteristics in the longitudinal direction have been used in nonlinear analysis. This has, to the best knowledge of authors, never been reported. Also, this paper describes the design and implementation of eighteen ideal boundary conditions using three different longitudinal and six well-known transverse displacement interpolation functions. The results of the presented study were obtained using an open-source software and multipurpose software Abaqus. Moreover, the accuracy of the applied computational approach has been verified by comparison with results from the literature. An excellent agreement of displacement fields is achieved for large deflection analyses of plates with a hole and stiffeners as well as for shells with a stepped thickness in the longitudinal direction. Additionally, results from post-buckling analyses of thin-walled structures, a snap-through and snap-back behavior of shallow shells, are matched. The work presented here has profound implications for future studies of the finite strip deployment.

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2Si-B-3C-N ceramic was successfully vacuum brazed to Nb. Brazing was executed at 850 °C for 10 mi... more 2Si-B-3C-N ceramic was successfully vacuum brazed to Nb. Brazing was executed at 850 °C for 10 min using Ag-Cu-Ti/Mo/Ag-Cu-Ti composite interlayer. Effects of the Mo interlayer thickness within the composite interlayer on the microstructure of the joint and its shear strength were investigated. In joints brazed without Mo and with 50 μm Mo interlayer a continuous through cracks propagated within the ceramic during cooling. However, no cracks were found in joints when Mo interlayers thicker than 100 μm were used. The improvement of the shear strength with an increase in the thickness of Mo interlayer within the composite interlayer was achieved. To explain the effects of the Mo layer thickness, detailed FEM simulations of the cooling process were performed. The analysis of the residual stress is consistent with the observed joint strengthening and crack propagation.

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UK Association for Computational Mechanics Conference, 2024

Magnesium alloys exhibit great potential for use as implants that gradually and entirely dissolve... more Magnesium alloys exhibit great potential for use as implants that gradually and entirely dissolve in the human body after healing. Rapid corrosion in body fluids and the susceptibility to pitting corrosion and stress corrosion cracking are the main reasons limiting the widespread use of Mg alloys as a biodegradable material. The concurrence of mechanical loading and a corrosive environment dramatically accelerates the corrosion rate and promotes crack propagation, leading to the sudden failure of implants. A phase-field model is developed to simulate the corrosion of bioabsorbable metals in biological fluids. The model incorporates both Mg dissolution and the transport of Mg ions in solution. The framework captures pitting corrosion and incorporates the role of mechanical fields in enhancing the corrosion of biodegradable metals. The model is validated against in vitro corrosion data on Mg alloys immersed in simulated body fluid. The potential of the model to capture mechano-chemical effects is demonstrated by considering bioabsorbable coronary stents subjected to mechanical loading. The results show that pitting severely compromises the structural integrity of the stent and the application of mechanical loading initiates a pit-to-crack transition and crack propagations, leading to premature fracture after a short time in solution. This work extends phase-field modeling to biomaterial degradation and provides a novel mechanistic tool for assessing the service life of bioabsorbable medical devices.

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International Conference "Contemporary Achievements in Civil Engineering 2015", Subotica, Serbia, 2015

Free vibrations of rectangular plates with uniform thickness and with rectangular cutouts are inv... more Free vibrations of rectangular plates with uniform thickness and with rectangular cutouts are investigated. Plates are modeled using a finite strip method where the displacement field is approximated with a product of trigonometric and polynomial functions. In order to analyze the influence of polynomial order, two types of strips are utilized: LO2 and HO3. Strips are divided into cells in longitudinal direction, and the zero stiffness is given for cells which represent cutouts. The presented approach is coded using Wolfram Mathematica. Numerical tests contain comparison of results and convergence properties for LO2 and HO3 strips. Obtained results are in good agreement with data available in the literature.

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Proceedings of the Fourth South-East European Conference on Computational Mechanics, Kragujevac, Serbia, 2017

Elastic post-buckling analysis of imperfect rectangular plates subjected to a uniform compression... more Elastic post-buckling analysis of imperfect rectangular plates subjected to a uniform compression stress along one direction is presented. The semi-analytical finite strip method is improved by generalization of its nonlinear formulation through inclusion of various types of strips and boundary conditions. The equations of balance are derived via incremental formulation of virtual work, while the Newton-Raphson and arc-length methods are implemented into a solver. Geometric initial imperfections are modeled as a scaled buckling mode of a structure. The presented theory is verified via comparison of results with the ones from literature and Abaqus. The semi-analytical finite strip method is ideally suited for this type of analysis due to good convergence properties.

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Proceedings of the Annual Stability Conference, Structural Stability Research Council (SSRC), St. Louis, MO, USA, 2019

An experimental-numerical analysis regarding the ultimate strength of longitudinally stiffened I-... more An experimental-numerical analysis regarding the ultimate strength of longitudinally stiffened I-girders subjected to concentrated loading is presented. The driving force for this research was to investigate the influence of patch load length on ultimate capacity of longitudinally stiffened girders. In order to assess the patch loading resistance of plate girders, a nonlinear finite element analysis has been performed. The numerical results are compared with the experimental tests using different patch load lengths. For a better verification with the experimental results, the finite element model includes the experimentally measured initial geometrical imperfections and material properties based on the laboratory tests. It has been shown that the numerical and experimental results are in perfect agreement which enabled a fruitful background for parametric analysis, in which different initial geometrical imperfections have been used to ameliorate understandings about their influence on the ultimate load under different patch load lengths. Conclusively it may be stated that initial geometrical imperfections can play a decisive role, especially for stiffened girders. Initial geometrical imperfections of stiffened girders that correspond to deformed shapes at collapse (collapse-affine imperfections), will give the most unfavorable ultimate strengths. In the present paper, the third buckling mode of stiffened girders corresponds to the deformed shape and the lowest ultimate strengths are obtained. Future experimental and numerical work will consider the effects of different geometry, material characteristics and aspect ratio of web plate.

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Proceedings of the 10th International Conference on Bridges in Danube Basin, Vienna, Austria, 2019

The paper is dealing with numerical research of the post-buckling behavior and the ultimate stren... more The paper is dealing with numerical research of the post-buckling behavior and the ultimate strength of welded I-shaped plate girders subjected to patch loading. This load case appears during the incremental launching stage of bridges in which many cross-sections are exposed to forces that will not appear during the bridge normal operation. The finite element analysis considering the influence of patch load length and different initial geometrical imperfections on the ultimate load is presented. Results for longitudinally unstiffened and stiffened girders are compared. The combined action of the longitudinal stiffener and larger patch load lengths can significantly increase the ultimate capacity. The most unfavorable ultimate strengths of longitudinally stiffened girders are obtained using initial geometrical imperfections that correspond to the deformed shape at collapse.

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