Nataliya Sakharova

University of Coimbra | UC · Department of Mechanical Engineering

In the recent years, tubular nanostructures have been related with immense advances in various fields of science and technology. Considerable research efforts have been centered on the theoretical prediction and manufacturing of non-carbon nanotubes (NTs), which meet modern requirements for the developing of novel devices and systems. In this conte...

This study investigates the sensitivity of the square cup forming process. It analyses how the uncertainties in the material properties, friction and process conditions affect the results of the square cup, such as equivalent plastic strain, geometry change, thickness reduction, punch force and springback. The cup flange and the die curvature regio...

An identification strategy based on a machine learning approach is proposed to identify the constitutive parameters of metal sheets. The main novelty lies in the use of Gaussian Process Regression with the objective of identifying the constitutive parameters of metal sheets from the biaxial tensile test results on a cruciform specimen. The metamode...

Two-dimensional (2D) nanostructures of aluminum nitride (AlN) and gallium nitride (GaN), called nanosheets, have a graphene-like atomic arrangement and represent novel materials with important upcoming applications in the fields of flexible electronics, optoelectronics, and strain engineering, among others. Knowledge of their mechanical behavior is...

Hexagonal boron nitride (h-BN) nanosheets are attractive materials for various applications that require efficient heat transfer, surface adsorption capability, biocompatibility, and flexibility, such as optoelectronics and power electronics devices, nanoelectromechanical systems, and aerospace industry. Knowledge of the mechanical behavior of boro...

One-dimensional (nanotubes) and two-dimensional (nanosheets) germanium carbide (GeC) and tin carbide (SnC) structures have been predicted and studied only theoretically. Understanding their mechanical behaviour is crucial, considering forthcoming prospects, especially in batteries and fuel cells. Within this framework, the present study aims at the...

Among the compounds formed by an element of the 13th group and nitrogen, boron nitride, also called white graphene, stands out for its high strength and thermal conductivity, transparency to visible light, antimicrobial properties, high resistance to oxidation, and biocompatibility. One-dimensional and two-dimensional boron nitride nanostructures,...

Among the semiconductors formed by a 13th group element and nitrogen, indium nitride (InN) has promising electronic and optical properties, which make it an appropriate material for light-emitting devices and high-speed electronic applications. One-dimensional and two-dimensional InN structures, such as nanotubes and nanosheets, respectively, are e...

Silicon carbide nanotubes (SiCNTs) have generated significant research interest due to their potential use in the fabrication of electronic and optoelectronic nanodevices and biosensors. The exceptional chemical, electrical and thermal properties of SiCNTs are beneficial for their application in high-temperature and harsh-environments. In view of t...

Systematic research efforts have been focused on the development of low-dimensional structures, such as nanotubes (NTs), because of the potential of their use in nanodevices and in applications in nanoelectronics and biomedicine [...]

After a large-scale investigation into carbon nanotubes, significant research efforts have been devoted to discovering and synthesizing other nanotubes formed by chemical elements other than carbon. Among them, non-carbon nanotubes based on compounds of the elements of the 13th group of the periodic table and phosphorus. These inorganic nanotubes h...

Low-dimensional structures, such as nanotubes, have been the focus of research interest for approximately three decades due to their potential for use in numerous applications in engineering and technology. In addition to extensive investigation of carbon nanotubes, those composed of elements other than carbon, the so-called non-carbon nanotubes, h...

The elastic properties of chiral and non-chiral single-walled boron nitride nanotubes in a wide range of their chiral indices and diameters were studied. With this aim, a three-dimensional finite element model was used to assess their rigidities and, subsequently, elastic moduli and Poisson’s ratio. An extensive study was performed to understand th...

The elastic properties of single-walled carbon nanotube heterojunctions were investigated using conventional tensile, bending and torsion tests. A three-dimensional finite element model was built in order to describe the elastic behaviour of cone heterojunctions (armchair–armchair and zigzag–zigzag). This comprehensive systematic study, to evaluate...

The elastic properties of armchair and zigzag multiwalled carbon nanotubes were investigated under tensile, bending, and torsion loading conditions. A simplified finite element model of the multiwalled carbon nanotubes, without taking into account the van der Waals interactions between layers, was used to assess their tensile, bending, and torsiona...

Nowadays, composite materials are playing an increasingly important role in material forming processes because they can combine remarkable physical and mechanical properties with relatively low weight. The main objective of this work is to study the forming behaviour of multi-layer sheets by finite element analysis. The possibility of replacing the...

Depth-sensing indentation (DSI) technique allows easy and reliable determination of two mechanical properties of materials: hardness and Young’s modulus. Most of the studies are focusing on the Vickers, Berkovich, and conical indenter geometries. In case of Knoop indenter, the existing experimental and numerical studies are scarce. The goal of the...

Comprehensive studies on the modelling and numerical simulation of the mechanical behaviour under tension, bending and torsion of single-walled carbon nanotubes and their heterojunctions are performed. It is proposed to deduce the mechanical properties of the carbon nanotubes heterojunctions from the knowledge of the mechanical properties of the si...

High-tech miniaturization is a strategic area to empower new scientific challenges for which carbon nanotubes are ideal candidates with outstanding electronic, optical and mechanical properties. Carbon nanotubes and their heterojunctions are efficient components for reinforcement of composites, for constructing micro- and nanodevices, and for desig...

The mechanical behaviour of non-chiral multi-walled carbon nanotubes under tensile and bending loading conditions was investigated. For this purpose, a simplified finite element model of armchair and zigzag multi-walled carbon nanotubes, which does not take into account the van der Waals forces acting between layers, was tested in order to evaluate...

This article is a review regarding recently developed inverse strategies coupled with finite element simulations for the identification of the parameters of constitutive laws that describe the plastic behaviour of metal sheets. It highlights that the identification procedure is dictated by the loading conditions, the geometry of the sample, the typ...

The mechanical behaviour of non-chiral and chiral single-walled carbon nanotubes under torsional loading is studied. For this purpose, three-dimensional finite element modelling is used in order to evaluate the torsional rigidity and shear modulus. It is shown that the evolution of torsional rigidity as a function of nanotube diameter can be descri...

The study of the mechanical behaviour of single-walled carbon nanotube heterojunctions has been carried out, implementing nanoscale continuum approach. A three-dimensional finite element model is used in order to evaluate the elastic behaviour of cone heterojunctions. It is shown that the bending rigidity of heterojunctions is sensitive to bending...

The increasing use of thin films in various fields of industry have created the necessity to develop methodologies for evaluating their mechanical properties. The hardness test is a widely used technique for determining the mechanical properties of bulk materials and thin films. The test equipment for depth-sensing indentation (DSI) extended the ap...

An inverse analysis methodology for determining the parameters of the kinematic law of sheet metals is proposed. The sensitivity of the load versus displacement curves, obtained by reverse shear tests of rectangular and notched specimens, to the kinematic law parameters are studied following a forward analysis, based on finite element simulations....

The mechanical behaviour of non-chiral and chiral single-walled carbon nanotubes under tensile and bending loading conditions is investigated. For this purpose, three-dimensional finite element modelling is used in order to evaluate the tensile and bending rigidities and, subsequently, the Young's moduli. It is shown that the evolution of rigidity,...

A threedimensional finite element model is used in order to evaluate the tensile and bending rigidities and, subsequently, Young’s modulus of nonchiral and chiral singlewalled carbon nanotubes containing vacancy defects. It is shown that the Young’s modulus of singlewalled carbon nanotubes with vacancies is sensitive to the presence of vacancy defe...

In the present study a three-dimensional finite element model was used in order to evaluate the tensile and bending rigidities and, subsequently, Young's moduli of non-chiral and chiral single-walled carbon nanotubes. A comprehensive study on the influence of the nanotube wall thickness on the Young's modulus results was also carried out using data...

Carbon nanotubes (CNTs) have attracted great research interest, because of their extraordinary mechanical, optical, thermal properties. Due to the restriction of CNTs manufacturing, it is almost impossible the production of CNTs with perfect structure. It should be notes that the quality of the CNTs used in the experimental studies has a significan...

The main difficulty in the mechanical characterisation of thin films using depth-sensing indentation is the determination of the relative substrate and film contributions to the measured properties of the film/substrate composite. In this study, a three-dimensional numerical simulation of the Vickers hardness test is used to study the influence of...

The numerical simulation of sheet metal forming processes needs the accurate identification of the material parameters, for a given constitutive model. This identification can follow different methodologies and different sets of experimental data can be used, which lead to distinct sets of material parameters. In order to accurately compare the res...

This work proposes an inverse analysis methodology for determining the constitutive models parameters, used to describe the plastic behaviour of metal sheets, from results of a single experimental biaxial tensile test on a flat cruciform sample. The proposed methodology is based on a direct analysis of finite element simulation results carried out...

The mechanical behaviour of heterogeneous specimens under uniaxial tensile test is studied using finite element analysis. The difference between mechanical properties of adjacent regions in the heterogeneous specimen creates constraints which alter the strain path relatively to pure tension. A methodology for determining the local stress-strain cur...

The numerical simulation of the biaxial bulge test with circular die was performed with help of the in-house code DD3IMP, in order to characterize the plastic behaviour of sheet metals. The methodology for determining the hardening law used in the experimental determination, with recourse to the membrane theory, and the errors associated with such...

Dissimilar friction stir welds of aluminum alloys AA5086 in annealed and AA6061 in T6 temper conditions were investigated in terms of residual stress, grain structure and precipitation distribution in different zones of the welded joints. Optical metallography and transmission electron microscopy were used to characterize microstructures of differe...

This work present a numerical analysis of the error in the strain hardening curve obtained with the circular bulge test, based on the study of the influence of the material mechanical properties on the results of this tests, particularly in the evolution of the radius of curvature with the pole height. The possibility for obtaining a unique curve f...

This work proposes an inverse analysis methodology for determining the constitutive models parameters, used to describe the plastic behaviour of metal sheets, from results of a single experimental biaxial tensile test on a flat cruciform sample. The proposed methodology is based on a direct analysis of finite element simulation results carried out...

This paper presents a study of the indenter geometry influence in the results obtained for the materials Young´s modulus. Three-dimensional numerical simulations of the indentation of materials with different mechanical properties were performed. In the indentation tests, four-sided flat squared and rhombic indenters considering different ratios be...

This paper presents a study of the influence of the mechanical properties of materials on the surface indentation geometry and on the depth-sensing indentation results with a Knoop indenter. Three-dimensional numerical simulations of this indention test were performed for several materials, with different mechanical properties, using the in-house f...

A methodology for determining the local stress-strain curves of heterogeneous specimens is proposed. This methodology resorts to the same type of variables usually obtained in the experimental tensile test with the aid of the digital image correlation technique. This approach was successfully tested for heterogeneous specimens composed by several m...

This paper presents a study of the influence of the mechanical properties of materials on the surface indentation geometry and on the depth-sensing indentation results with a Knoop indenter. Three-dimensional numerical simulations of this indention test were performed for several materials, with different mechanical properties, using the in-house f...

O comportamento mecânico em tracção de provetes heterogéneos foi estudado com recurso à simulação numérica. A diferença entre as propriedades mecânicas de regiões adjacentes no provete heterogéneo cria os constrangimentos, os quais alteram as trajectórias de deformação em relação de tracção pura. Uma metodologia para determinação de curvas locais t...

The present work focuses on developing an inverse analysis methodology for determining the constitutive models parameters used to describe the plastic behaviour of metal sheets. This study proposes a new approach, based on the results of a single experimental biaxial test of a cruciform sample. Firstly, the geometry of the cruciform sample was opti...

Nas últimas décadas foram propostos diversos ensaios experimentais para determinar os parâmetros do modelo constitutivo (superfície limite de elasticidade e lei de encruamento) utilizado para descrever o comportamento plástico de materiais metálicos. Os ensaios propostos caracterizam-se por apresentarem trajectórias de deformação simples, de modo a...

The aim of this investigation was to study the influence of shoulder cavity and welding parameters on torque, defect formation, microstructure and mechanical properties of friction stir welds in very thin sheets of deoxidised copper. Three types of tools were used: a flat shoulder tool and two tools with conical shoulder cavities of 3 and 6° respec...

  The evaluation of residual stresses is an important aspect in many engineering applications, such as surfaces produced by mechanical or thermal treatment processes or even thin films deposited on substrates. Currently, there are several techniques for residual stress measurement. However, its application is limited by problems associated with the...

Os procedimentos de identificação dos parâmetros das leis que regem o comportamento plástico de metais impõem a realização de ensaios mecânicos em que, geralmente, a deformação é homogénea. Uma alternativa a este tipo de identificação dos parâmetros das leis de comportamento, consiste em realizar ensaios em que os estados de tensão e deformação são...

In order to determine the influence of interlayers of ductile metals on the overall mechanical properties of the multilayer coatings, three-dimensional numerical simulations of depth-sensing indentation tests of titanium aluminium nitride (TiAlN) hard coatings with copper, silver, aluminium and titanium ductile interlayers were performed. The contr...

A computational study was undertaken in order to identify the parameters of plastic constitutive models, generally used in numerical simulation of sheet metal forming. This study proposes a new methodology, based on the results of a single experimental biaxial test of a cruciform sample. Firstly, the geometry of the cruciform sample was optimized....

Numerical simulations of indentation tests on samples with and without residual stresses were carried out in order to evaluate the influence of residual stresses on the determination of mechanical properties. Results concerning the presence of tensile and compressive residual stresses on the hardness results are presented. The estimation of residua...

In the present study, numerical simulations of nanohardness tests of titanium aluminium nitride (TiAlN) hard coatings with titanium and aluminium ductile interlayers were performed in order to determine the influence of interlayers of ductile metals on the overall mechanical properties of multilayer coatings. The investigation was designed to deter...

A new methodology for the determination of Young's modulus of thin films, using a single hardness test measurement of film/substrate composites, has been developed. It is based on a recently proposed weight function, the reciprocal of the Gao function. Firstly, results of three-dimensional numerical simulation of the Vickers hardness tests on sever...

Referem-se aspectos teóricos e experimentais relacionados com o ensaio de nanoindentação, também designado por ensaio dinâmico de dureza, que permite determinar, além da dureza, o módulo de elasticidade de materiais. Abordam-se metodologias de análise inversa, que aplicadas aos resultados deste ensaio permitem estimar, entre outras, as propriedades...

Resumo. De modo a avaliar a influência da presença de tensões residuais na determinação das propriedades mecânicas de materiais maciços, realizaram-se simulações numéricas de ensaios de indentação em provetes com e sem tensões residuais. A análise dos resultados permite constatar que os valores de dureza obtidos para provetes sujeitos a tensões res...

Three-dimensional numerical simulations of nanohardness indentation tests were carried out in order to determine the plastic properties of composites, characterized by the fact that the film is harder than the substrate. A first approach of a model for fitting the numerical results was assumed and its parameters are discussed as a function of the f...

Plastic straining in metal forming usually includes changes of strain path, which are frequently not taken into account in the analysis of forming processes. Moreover, strain path change can significantly affect the mechanical behavior and microstructural evolution of the material. For this reason, a combination of several simple loading test seque...

In the present study, numerical simulations of nanohardness tests of titanium aluminum nitride (TiAlN) hard coatings with titanium and aluminium ductile interlayers were performed in order to determine the influence of interlayers of ductile metals on the overall mechanical properties of multilayer coatings. This allowed to establish the contributi...

Three-dimensional numerical simulations of Berkovich, Vickers and conical indenter hardness tests were carried out to investigate the influence of indenter geometry on indentation test results of bulk and composite film/substrate materials. The strain distributions obtained from the three indenters tested were studied, in order to clarify the diffe...

Multilayer coatings with nanometer scale layer dimensions are attractive to specific engineering requirements, due to their extremely high strength and flexibility, relatively to conventional laminated composites 1. The evaluation of hardness and Young's modulus of multilayered thin films is coupled with difficulties induced by the influence of sub...

Stacking fault energy (SFE) plays an important role in face centred cubic (f.c.c.) metals and alloys in determining the prevailing mechanisms of plastic deformation. Low SFE metals and alloys have a tendency to develop mechanical twinning, besides dislocation slip, during plastic deformations. Deformation behaviour and microstructure evolution unde...

This paper presents an approach to reverse analysis in depth-sensing indentation of composite film/substrate materials, which makes use of numerical simulation. This methodology allows the results of experimental hardness tests, acquired with pyramidal indenter geometry, to be used to determine the Young's modulus of thin film materials. Forward an...

The main difficulty with the characterization of thin coatings using depth-sensing indentation tests is related to the determination of the contributions of the substrate and the film to the measured properties. In this study, three-dimensional numerical simulations of the Vickers hardness test are used in order to examine the influence of the elas...

Comparative three-dimensional numerical simulations of the Vickers, Berkovich and conical hardness tests were carried out in order to establish the indenter geometry influence on the indentation results, such as the load - penetration depth curves and hardness. Also, the indenter geometry shape factor for the Young’s modulus evaluation was estimate...

Depth sensing indentation is a widespread method for mechanical characterization of bulk materials and thin films, by determining hardness under load and the Young's modulus. In this study, reverse analysis of Vickers indentation tests was carried OUI in order 10 determine the plastic properties of thin films deposited on substrate materials, by us...

In this study, coarse-grained copper sheets were subjected to tension–rolling and rolling–tension strain path sequences. In both cases, two different types of strain path change were studied: the tensile and rolling directions were parallel and normal to each other. TEM observations of deformed samples showed the typical dislocation structures for...

The microstructure evolution of copper multicrystalline sheets, undergoing plastic deformation in the sequences of strain paths rolling - tension and tension - rolling, was studied in the present work. For both sequences, two different types of change of strain path were studied, the tensile and rolling directions were parallel and normal to each o...

Depth sensing indentation equipment allows the mechanical properties of thin films to be easily determined, particularly the hardness and Young's modulus. In order to minimize the influence of the substrate on the measured properties, the indentation depth must be limited to a small fraction of the film's thickness. However, for very thin films, th...

The crystallogeometric model of the rigid grain boundaries, which takes into account not only complete coincidences of atoms but also partial ones in a boundary is considered. Such an approximation in consideration of grain-boundaries' structure allows to estimate the effective grain-boundary energy values (a degree of atoms' coincidence in a bound...

The crystallogeometric model of the rigid grain boundaries, which takes into account not only complete coincidences of atoms but also partial ones in a boundary is considered. Such an approximation in consideration of grain boundaries’ structure allows to estimate the effective grain-boundary energy values (a degree of atoms’ coincidence in a bound...

Numerical simulations of indentation tests on samples with and without residual stresses were carried out in order to evaluate the influence of residual stresses on the determination of mechanical properties. Results concerning the presence of tensile and compressive residual stresses on the hardness results are presented. The estimation of residua...