Assoc. Prof. Jerzy Rojek, Ph.D., Dr. Habil., Eng.

Department of Information and Computational Science (ZIiNO)
Division of Computational Methods in Nonlinear Mechanics (PMOMN)
position: professor IPPT
telephone: (+48) 22 826 12 81 ext.: 147
room: 423
e-mail: jrojek
personal site: http://bluebox.ippt.pan.pl/~jrojek/

Doctoral thesis
1993-01-21Numeryczna analiza nieliniowych zagadnień mechaniki konstrukcji złożonych z części odkształcalnych i sztywnych: zastosowanie do analizy kabin ciągników 
supervisor -- Prof. Michał Kleiber, Ph.D., Dr. Habil., Eng., IPPT PAN
493 
Habilitation thesis
2008-10-02Modelowanie i symulacja komputerowa złożonych zagadnień mechaniki nieliniowej metodami elementów skończonych i dyskretnych939
 
Supervision of doctoral theses
1.2016-02-25Nosewicz Szymon  Discrete element modeling of powder metallurgy processes666
 
2.2012-07-29González Carlos Andrés Labra  
(UPC - BarcelonaTech)
Advances in the development of the discrete element method for excavation processes 

Recent publications
1.Labra C., Rojek J., Oñate E., Discrete/Finite Element Modelling of Rock Cutting with a TBM Disc Cutter, Rock Mechanics and Rock Engineering, ISSN: 0723-2632, DOI: 10.1007/s00603-016-1133-7, Vol.50, pp.621-635, 2017
Labra C., Rojek J., Oñate E., Discrete/Finite Element Modelling of Rock Cutting with a TBM Disc Cutter, Rock Mechanics and Rock Engineering, ISSN: 0723-2632, DOI: 10.1007/s00603-016-1133-7, Vol.50, pp.621-635, 2017

Abstract:
This paper presents advanced computer simulation of rock cutting process typical for excavation works in civil engineering. Theoretical formulation of the hybrid discrete/finite element model has been presented. The discrete and finite element methods have been used in different subdomains of a rock sample according to expected material behaviour, the part which is fractured and damaged during cutting is discretized with the discrete elements while the other part is treated as a continuous body and it is modelled using the finite element method. In this way, an optimum model is created, enabling a proper representation of the physical phenomena during cutting and efficient numerical computation. The model has been applied to simulation of the laboratory test of rock cutting with a single TBM (tunnel boring machine) disc cutter. The micromechanical parameters have been determined using the dimensionless relationships between micro- and macroscopic parameters. A number of numerical simulations of the LCM test in the unrelieved and relieved cutting modes have been performed. Numerical results have been compared with available data from in-situ measurements in a real TBM as well as with the theoretical predictions showing quite a good agreement. The numerical model has provided a new insight into the cutting mechanism enabling us to investigate the stress and pressure distribution at the tool–rock interaction. Sensitivity analysis of rock cutting performed for different parameters including disc geometry, cutting velocity, disc penetration and spacing has shown that the presented numerical model is a suitable tool for the design and optimization of rock cutting process.

Keywords:
Rock cutting, Disc cutters, TBM, Numerical model, Discrete/finite element method, Simulation

2.Nosewicz S., Rojek J., Chmielewski M., Pietrzak K., Discrete element modeling and experimental investigation of hot pressing of intermetallic NiAl powder, ADVANCED POWDER TECHNOLOGY, ISSN: 0921-8831, DOI: 10.1016/j.apt.2017.04.012, Vol.28, pp.1745-1759, 2017
Nosewicz S., Rojek J., Chmielewski M., Pietrzak K., Discrete element modeling and experimental investigation of hot pressing of intermetallic NiAl powder, ADVANCED POWDER TECHNOLOGY, ISSN: 0921-8831, DOI: 10.1016/j.apt.2017.04.012, Vol.28, pp.1745-1759, 2017

Abstract:
This paper presents the numerical and experimental analysis of hot pressing of NiAl powder with an emphasis on the best possible representation of its main stages: initial powder compaction and pressure-assisted sintering. The numerical study has been performed within the discrete element framework. In the paper, an original viscoelastic model of hot pressing has been used. In order to ensure that the applied values of material parameters in numerical simulations are appropriate, the reference literature has been reviewed. It produced the relations and equations to estimate the values of all required sintering material parameters of the considered viscoelastic model. Numerical simulations have employed the geometrical model of the initial dense specimen generated by a special algorithm which uses the real grain distribution of powder. The numerical model has been calibrated and validated through simulations of the real process of hot pressing of intermetallic NiAl material. The kinetics of compaction, sintering and cooling stage indicated by the evolution of density, shrinkage and densification rate have been studied. The comparison of numerical and experimental results has shown a good performance of the developed numerical model.

Keywords:
Powder metallurgy; Hot pressing; Sintering; Simulation; Discrete element method; Nickel aluminide

3.Nosewicz S., Rojek J., Chmielewski M., Pietrzak K., Lumelskyj D., Application of the Hertz formulation in the discrete element model of pressure-assisted sintering, GRANULAR MATTER, ISSN: 1434-5021, DOI: 10.1007/s10035-016-0699-9, Vol.19, No.1, pp.16-1-8, 2017
Nosewicz S., Rojek J., Chmielewski M., Pietrzak K., Lumelskyj D., Application of the Hertz formulation in the discrete element model of pressure-assisted sintering, GRANULAR MATTER, ISSN: 1434-5021, DOI: 10.1007/s10035-016-0699-9, Vol.19, No.1, pp.16-1-8, 2017

Abstract:
This paper presents the numerical modelling of initial powder compaction and pressure-assisted sintering performed by original viscoelastic discrete element model. The research is focused on the influence of the type of the model representing an elastic part of interparticle force. Two elastic contact models—linear and nonlinear Hertz model—have been implemented and used to analyse interaction of NiAl powder particles during compaction and sintering process. Numerical models have been validated using own experimental results. Microscopic effects (particle penetration) and macroscopic changes (relative density) have been compared. It has been shown that although both models represent properly macroscopic behaviour of the material at the sintering process, the Hertz model produces the results closer to the real experimental ones during the initial compaction stage. Evaluation of macroscopic quantities enables implementation of the discrete element model in the framework of the multiscale modelling framework which is currently developed for sintering processes.

Keywords:
Powder metallurgy, Sintering, Initial compaction, Elasticity, Discrete element method

4.Chmielewski M., Nosewicz S., Jakubowska D., Lewandowska M., Mizera J., Rojek J., Bazarnik P., The influence of sintering time on the microstructural properties of chromium-rhenium matrix composites, International Journal of Refractory Metals and Hard Materials, ISSN: 0263-4368, DOI: 10.1016/j.ijrmhm.2016.05.017, Vol.59, pp.78-86, 2016
Chmielewski M., Nosewicz S., Jakubowska D., Lewandowska M., Mizera J., Rojek J., Bazarnik P., The influence of sintering time on the microstructural properties of chromium-rhenium matrix composites, International Journal of Refractory Metals and Hard Materials, ISSN: 0263-4368, DOI: 10.1016/j.ijrmhm.2016.05.017, Vol.59, pp.78-86, 2016

Abstract:
This paper comprises the results of studies of the changes in the structure of Cr-Re-Al2O3 metal matrix depending on heat treatment time in sintering temperature. The density of material with the following composition: 95%(75%Cr-25%Al2O3)+5%Re was increased using the technique of sintering under pressure (30MPa) at the temperature of 1450°C. As a result, materials characterized by a high relative density (< 97% of theoretical density) were obtained. Next, they were subjected to structural tests including scanning and transmission electron microscopy as well as X-ray diffraction. Changes in the phase composition, grains size and parameters of crystallographic structure depending on heat treatment time were analysed. It was found that during sintering rhenium is dissolved in the chromium matrix and Cr-Re solid solution is formed. When sintering time is extended to 120 min, the matrix of the composite becomes completely homogenous, which results in an increased strength of the composite.

Keywords:
Metal matrix composites, Rhenium, Hot pressing, Microstructure analysis, XRD

5.Marczewska I., Rojek J., Kačianauskas R., Investigation of the effective elastic parameters in the discrete element model of granular material by the triaxial compression test, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, ISSN: 1644-9665, DOI: 10.1016/j.acme.2015.09.010, Vol.16, pp.64-75, 2016
Marczewska I., Rojek J., Kačianauskas R., Investigation of the effective elastic parameters in the discrete element model of granular material by the triaxial compression test, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, ISSN: 1644-9665, DOI: 10.1016/j.acme.2015.09.010, Vol.16, pp.64-75, 2016

Abstract:
The general objective of the present paper is to improve the understanding of micromechanical mechanisms in granular materials and their representation in numerical models. Results of numerical investigations on micro–macro relationships in the discrete element model of granular material are presented. The macroscopic response is analysed in a series of simulations of the triaxial compression test. The numerical studies are focused on the influence of microscopic parameters on the initial response. The effect of the contact stiffness and friction coefficient on the effective elastic moduli is investigated. Numerical results are compared with the analytical estimations based on the kinematic Voigt's hypothesis as well as with selected numerical results of other authors. The comparisons show that a better agreement between the numerical and analytical results is observed for particle assemblies with higher coordination numbers. Higher coordination numbers are related to more compact specimens and for a given specimen can be associated with low values of the contact stiffness and a higher confining pressure.

Keywords:
Discrete element method, Granular material, Triaxial test, Micro–macro relationship, Voigt hypothesis

6.Jurczak K., Rojek J., Nosewicz S., Lumelskyy D., Bochenek K., Chmielewski M., Pietrzak K., Modelowanie wstępnego prasowania proszków metodą elementów dyskretnych, HUTNIK - WIADOMOŚCI HUTNICZE, ISSN: 1230-3534, DOI: 10.15199/24.2016.1.1, Vol.83, No.1, pp.3-7, 2016
Jurczak K., Rojek J., Nosewicz S., Lumelskyy D., Bochenek K., Chmielewski M., Pietrzak K., Modelowanie wstępnego prasowania proszków metodą elementów dyskretnych, HUTNIK - WIADOMOŚCI HUTNICZE, ISSN: 1230-3534, DOI: 10.15199/24.2016.1.1, Vol.83, No.1, pp.3-7, 2016

Abstract:
W niniejszym artykule zaprezentowano wyniki modelowania zagęszczania proszku stanowiącego wstępny etap procesu prasowania na gorąco. Modelowanie numeryczne zrealizowano metodą elementów dyskretnych z wykorzystaniem kulistych cząstek. Analizę skoncentrowano na badaniu mechanizmów zagęszczania proszku przy ciśnieniu do 50 MPa oraz poszukiwaniu modeli odpowiednich przy zastosowanych warunkach realizacji procesu. Symulacje numeryczne wykonano wykorzystując dwa modele oddziaływania cząstek proszku: sprężysty model Hertza-Mindlina-Deresiewicza oraz plastyczny model Storåkersa. Wyniki numeryczne zostały porównane z wynikami laboratoryjnymi prasowania proszku NiAl. Otrzymano dużą zgodność wyników eksperymentalnych i numerycznych.

This paper presents the results of discrete element simulation of powder compaction which is the initial stage in the hot pressing process. Numerical simulation has been performed by discrete element method with using spherical particles. The research has been focused on densification mechanisms under pressure 50 MPa and models appropriate for these conditions. Numerical simulations have been carried out for two contact models: elastic Hertz-Mindlin-Deresiewicz and plastic - Storåkers. Numerical results and results from laboratory test of the uniaxial pressing of NiAl powder have been compared. The obtained results of numerical simulation and laboratory tests showing a good agreement.

Keywords:
metoda elementów dyskretnych, prasowanie proszków, materiały intermetaliczne, discrete element method, powder compaction, intermetallics

7.Rojek J., Nosewicz S., Jurczak K., Chmielewski M., Bochenek K., Pietrzak K., Discrete element simulation of powder compaction in cold uniaxial pressing with low pressure, Computational Particle Mechanics, ISSN: 2196-4378, DOI: 10.1007/s40571-015-0093-0, Vol.3, pp.513-524, 2016
Rojek J., Nosewicz S., Jurczak K., Chmielewski M., Bochenek K., Pietrzak K., Discrete element simulation of powder compaction in cold uniaxial pressing with low pressure, Computational Particle Mechanics, ISSN: 2196-4378, DOI: 10.1007/s40571-015-0093-0, Vol.3, pp.513-524, 2016

Abstract:
This paper presents numerical studies of powder compaction in cold uniaxial pressing. The powder compaction in this work is considered as an initial stage of a hot pressing process so it is realized with relatively low pressure (up to 50 MPa). Hence the attention has been focused on the densification mechanisms at this range of pressure and models suitable for these conditions. The discrete element method employing spherical particles has been used in the numerical studies. Numerical simulations have been performed for two different contact models—the elastic Hertz–Mindlin–Deresiewicz model and the plastic Storåkers model. Numerical results have been compared with the results of laboratory tests of the die compaction of the NiAl powder. Comparisons have shown that the discrete element method is capable to represent properly the densification mechanisms by the particle rearrangement and particle deformation.

Keywords:
Discrete element method, Simulation, Powder compaction, Cold uniaxial pressing

8.Rojek J., Zohdi T., Editorial foreword to the Special Issue of Journal of Computational Particle Mechanics “Particle modeling of powder-based processes in advanced manufacturing", Computational Particle Mechanics, ISSN: 2196-4378, DOI: 10.1007/s40571-016-0139-y, Vol.3, pp.435-436, 2016
9.Zabulionis D., Kačianauskas R., Rimša V., Rojek J., Pilkavičius S., Spring Method for modelling of particulate solid composed of spherical particles and weak matrix, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, ISSN: 1644-9665, DOI: 10.1016/j.acme.2015.06.006, Vol.15, No.4, pp.775-785, 2015
Zabulionis D., Kačianauskas R., Rimša V., Rojek J., Pilkavičius S., Spring Method for modelling of particulate solid composed of spherical particles and weak matrix, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, ISSN: 1644-9665, DOI: 10.1016/j.acme.2015.06.006, Vol.15, No.4, pp.775-785, 2015

Abstract:
In the present paper, a possibility of an approximation of elastic particulate composite with a network of elastic springs that undertake only axial forces is considered. It is assumed that the springs are equivalent to two hemispheres interacting through a weaker interface member. In a frame of the suggested approach, the description of the composite is limited to translational degrees of freedom, therefore, only a normal interaction between the spheres was considered. The methodology for calculation of the axial stiffness of the elastic springs and obtained solutions of the stiffness in explicit form are the main novelty of the article. A comparison of the stiffnesses of the springs obtained by the proposed methodology and by the three dimensional Finite Element Method (FEM) has shown a good agreement between them in a wide range of the ratio of the modulus of elasticity of the particles and matrix at four different distances between surfaces of the particles. A possibility of the approximation of particulate composite by springs was tested and discussed in details by comparing results of a mechanical response of a sample (under three different loading cases) modelled as a three dimensional solid and as a system comprised of the springs. The solutions were obtained by the FEM.

Keywords:
Spring network model, Lattice model, Particulate composites, Heterogeneous materials, Finite element modelling

10.Chmielewski M., Nosewicz S., Rojek J., Pietrzak K., Mackiewicz S., Romelczyk B., A study of densification and microstructure evolution during hot pressing of NiAl/Al2O3 composite, Advanced Composite Materials, ISSN: 0924-3046, DOI: 10.1080/09243046.2013.879408, Vol.24, No.1, pp.57-66, 2015
Chmielewski M., Nosewicz S., Rojek J., Pietrzak K., Mackiewicz S., Romelczyk B., A study of densification and microstructure evolution during hot pressing of NiAl/Al2O3 composite, Advanced Composite Materials, ISSN: 0924-3046, DOI: 10.1080/09243046.2013.879408, Vol.24, No.1, pp.57-66, 2015

Abstract:
Evolution of the density and the microstructure during hot pressing of NiAl/Al2O3 composite has been investigated in the present paper. In particular, the effect of the process parameters, viz. compacting pressure, sintering temperature and sintering time, on the evolution of the density of the intermetallic–ceramic composite has been studied. Evolution of the density has been related to microstructure changing. Porosity, pore structures and grains rearrangement have been analysed in microscopic observations.

Keywords:
hot pressing, sintering, intermetallic–ceramic composite, density evolution, microstructure

11.Rojek J., Nosewicz S., Pietrzak K., Chmielewski M., Evaluation of macroscopic stresses in discrete element models of sintering processes, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.15, No.1, pp.219-255, 2015
Rojek J., Nosewicz S., Pietrzak K., Chmielewski M., Evaluation of macroscopic stresses in discrete element models of sintering processes, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.15, No.1, pp.219-255, 2015

Abstract:
This paper presents investigation of macroscopic stresses in powder metallurgy process modelled with the discrete element method. The discrete element model belongs to the class of micromechanical models. In the DEM model the material is represented by an assembly of particles interacting by contact forces and the method is formulated in terms of forces and displacements. In order to evaluate macroscopic stresses a special upscaling procedure is necessary. The paper presents basic formulation of the discrete element method with special attention for the contact interaction models for powder compaction and sintering. A method to evaluate macroscopic stresses based on the two level averaging is presented. The discrete element model of sintering is verified using own experimental results. Macroscopic stresses are calculated for the whole process including loading, heating, sintering, cooling and unloading. It has been found out that the macroscopic stresses are consistent with changing process parameters. The procedure is suitable for multiscale modelling of sintering.

Keywords:
sintering, modeling, discrete element method, macroscopic stresses

12.Lumelskyj D., Rojek J., Tkocz M., Numerical simulations of nakazima formability tests with prediction of failure, ROMANIAN JOURNAL OF TECHNICAL SCIENCES - APPLIED MECHANICS, ISSN: 0035-4074, Vol.60, No.3, pp.184-194, 2015
Lumelskyj D., Rojek J., Tkocz M., Numerical simulations of nakazima formability tests with prediction of failure, ROMANIAN JOURNAL OF TECHNICAL SCIENCES - APPLIED MECHANICS, ISSN: 0035-4074, Vol.60, No.3, pp.184-194, 2015

Abstract:
This paper presents results of numerical simulations of the Nakazima test with determination of formability without using the forming limit curve. The onset of localized necking has been determined using the criterion based on analysis of the major principal strain and its first and second time derivatives in the most strained zone. The strain localization has been determined by the maximum of strain acceleration which corresponds to the inflection point of the strain velocity versus time. The limit strains have been determined for different specimens undergoing deformation at different strain paths covering a whole range of the strain paths typical for sheet forming processes. This has allowed us to construct the numerical FLC. The numerical FLC has been compared with the experimental one. It has been shown that the numerical FLC predicts higher formability limits but the differences are not large so the method can be used as a potential alternative tool to determine formability in standard finite element simulations of sheet forming processes.

Keywords:
sheet forming, formability, forming limit curve, numerical simulation

13.Nosewicz S., Rojek J., Mackiewicz S., Chmielewski M., Pietrzak K., Romelczyk B., The influence of hot pressing conditions on mechanical properties of nickel aluminide/alumina composite, Journal of Composite Materials, ISSN: 0021-9983, DOI: 10.1177/0021998313511652, Vol.48, No.29, pp.3577-3589, 2014
Nosewicz S., Rojek J., Mackiewicz S., Chmielewski M., Pietrzak K., Romelczyk B., The influence of hot pressing conditions on mechanical properties of nickel aluminide/alumina composite, Journal of Composite Materials, ISSN: 0021-9983, DOI: 10.1177/0021998313511652, Vol.48, No.29, pp.3577-3589, 2014

Abstract:
The influence of hot pressing conditions on mechanical properties of nickel aluminide/alumina composite has been investigated in the present paper. In particular, effect of the process parameters, viz. compacting pressure, sintering temperature and sintering time on the evolution of density, elastic constants and tensile strength properties of the intermetallic-ceramic composite has been studied. Elastic constants, the Young's modulus and Poisson's ratio, have been evaluated using an ultrasonic testing method, and the tensile strength has been determined by a Brazilian-type splitting test. Microscopic observations of microstructure evolution complemented the experimental procedure. Experimental results have been confronted with theoretical models showing a good agreement between the data compared.

Keywords:
Hot pressing, sintering, intermetallic-ceramic composite, elastic properties, Brazilian test, tensile strength, ultrasonic method

14.Chmielewski M., Nosewicz S., Pietrzak K., Rojek J., Strojny-Nędza A., Mackiewicz S., Dutkiewicz J., Sintering Behavior and Mechanical Properties of NiAl, Al2O3, and NiAl-Al2O3 Composites, Journal of Materials Engineering and Performance, ISSN: 1059-9495, DOI: 10.1007/s11665-014-1189-z, Vol.23, No.11, pp.3875-3886, 2014
Chmielewski M., Nosewicz S., Pietrzak K., Rojek J., Strojny-Nędza A., Mackiewicz S., Dutkiewicz J., Sintering Behavior and Mechanical Properties of NiAl, Al2O3, and NiAl-Al2O3 Composites, Journal of Materials Engineering and Performance, ISSN: 1059-9495, DOI: 10.1007/s11665-014-1189-z, Vol.23, No.11, pp.3875-3886, 2014

Abstract:
It is commonly known that the properties of sintered materials are strongly related to technological conditions of the densification process. This paper shows the sintering behavior of a NiAl-Al2O3 composite, and its individual components sintered separately. Each kind of material was processed via the powder metallurgy route (hot pressing). The progress of sintering at different stages of the process was tested. Changes in the microstructure were examined using scanning and transmission electron microscopy. Metal-ceramics interface was clean and no additional phases were detected. Correlation between the microstructure, density, and mechanical properties of the sintered materials was analyzed. The values of elastic constants of NiAl/Al2O3 were close to intermetallic ones due to the volume content of the NiAl phase particularly at low densities, where small alumina particles had no impact on the composite’s stiffness. The influence of the external pressure of 30 MPa seemed crucial for obtaining satisfactory stiffness for three kinds of the studied materials which were characterized by a high dense microstructure with a low number of isolated spherical pores.

Keywords:
ceramics, composites, electron, intermetallic, metallic matrix, microscopy, powder metallurgy, sintering, structural

15.Kowalczyk P., Rojek J., Stocki R., Bednarek T., Tauzowski P., Lasota R., Lumelskyy D., Wawrzyk K., NUMPRESS − integrated computer system for analysis and optimization of industrial sheet metal forming processes, HUTNIK - WIADOMOŚCI HUTNICZE, ISSN: 1230-3534, Vol.81, No.1, pp.56-63, 2014
Kowalczyk P., Rojek J., Stocki R., Bednarek T., Tauzowski P., Lasota R., Lumelskyy D., Wawrzyk K., NUMPRESS − integrated computer system for analysis and optimization of industrial sheet metal forming processes, HUTNIK - WIADOMOŚCI HUTNICZE, ISSN: 1230-3534, Vol.81, No.1, pp.56-63, 2014

Abstract:
The NUMPRESS System has been developed in IPPT PAN as a result of a project financially supported by European Regional Development Fund (within the Innovative Economy Programme) and is dedicated to small and middle enterprises dealing with sheet metal forming. The program consists of (i) an analytical module for analysis of forming processes with the finite element method, (ii) an optimization module controlling execution of the analytical module and performing optimization with respect to selected process parameters, in both deterministic and robust formulation, (iii) a reliability analysis module controlling execution of the analytical module to assess how random distribution of design parameters affects forming results, and (iv) a graphical user interface enabling communication between modules and easy definition of design parameters and optimization criteria. The analytical module consists of two independent programs up to the user's choice: NUMPRESS-Flow, a faster and less accurate program for implicit quasi-static analysis of rigid-viscoplastic shells (based on the flow approach) and NUMPRESS-Explicit, a program for explicit dynamical analysis of elastic-plastic and elastic-viscoplastic shells. Both programs are interfaced to a well-known commercial graphical pre- and postprocessor GiD. Fundamentals of formulations employed in the system and numerical examples are presented in the paper.

Keywords:
sheet metal forming, finite element method, deterministic and robust design optimization, reliability analysis

16.Rojek J., Discrete element thermomechanical modelling of rock cutting with valuation of tool wear, Computational Particle Mechanics, ISSN: 2196-4378, DOI: 10.1007/s40571-014-0008-5, Vol.1, No.1, pp.71-84, 2014
Rojek J., Discrete element thermomechanical modelling of rock cutting with valuation of tool wear, Computational Particle Mechanics, ISSN: 2196-4378, DOI: 10.1007/s40571-014-0008-5, Vol.1, No.1, pp.71-84, 2014

Abstract:
The paper presents a thermomechanical discrete element model of rock cutting process. The thermomechanical formulation of the discrete element method considers mechanical and thermal phenomena and their reciprocal influence. The thermal model developed for transient heat conduction problems takes into account conductive heat transfer at the contact between particles and convection on the free surface. The thermal and mechanical problems are coupled by consideration of: (1) heat generated due to friction which is calculated in the mechanical problem and passed to the thermal solution, (2) influence of thermal expansion on mechanical interaction between particles. Estimation of temperature dependent wear has been included into the contact model. The coupled problem is solved using the staggered scheme.The thermomechanical algorithm has been implemented in a discrete element program and applied to simulation of rock cutting with single pick of a dredge cutter head. Numerical results confirm good performance of the developed algorithm.

Keywords:
Discrete element method, Thermomechanical, Coupling, Rock cutting, Wear

17.Oñate E., Celigueta M.A., Latorre S., Casas G., Rossi R., Rojek J., Lagrangian analysis of multiscale particulate flows with the particle finite element method, Computational Particle Mechanics, ISSN: 2196-4378, DOI: 10.1007/s40571-014-0012-9, Vol.1, pp.85-102, 2014
Oñate E., Celigueta M.A., Latorre S., Casas G., Rossi R., Rojek J., Lagrangian analysis of multiscale particulate flows with the particle finite element method, Computational Particle Mechanics, ISSN: 2196-4378, DOI: 10.1007/s40571-014-0012-9, Vol.1, pp.85-102, 2014

Abstract:
We present a Lagrangian numerical technique for the analysis of flows incorporating physical particles of different sizes. The numerical approach is based on the particle finite element method (PFEM) which blends concepts from particle-based techniques and the FEM. The basis of the Lagrangian formulation for particulate flows and the procedure for modelling the motion of small and large particles that are submerged in the fluid are described in detail. The numerical technique for analysis of this type of multiscale particulate flows using a stabilized mixed velocity-pressure formulation and the PFEM is also presented. Examples of application of the PFEM to several particulate flows problems are given.

Keywords:
Lagrangian analysis, Multiscale particulate flows, Particle finite element method

18.Nosewicz S., Rojek J., Pietrzak K., Chmielewski M., Viscoelastic discrete element model of powder sintering, POWDER TECHNOLOGY, ISSN: 0032-5910, DOI: 10.1016/j.powtec.2013.05.020, Vol.246, pp.157-168, 2013
Nosewicz S., Rojek J., Pietrzak K., Chmielewski M., Viscoelastic discrete element model of powder sintering, POWDER TECHNOLOGY, ISSN: 0032-5910, DOI: 10.1016/j.powtec.2013.05.020, Vol.246, pp.157-168, 2013

Abstract:
This paper presents an original viscoelastic model of powder sintering developed within the discrete element framework. The viscous model used by other authors has been enriched by adding a spring connected in series to the viscous rheological element. In this way elastic and viscous effects in the particle interaction during sintering are treated using the Maxwell viscoelasticity. The new numerical model has been verified through simulation of simple problems of free sintering and sintering under pressure. Sintering processes have been treated as isothermic. In order to accelerate the analysis an algorithmic mass scaling has been used allowing to use larger time steps in the explicit time integration scheme. The results obtained using the new model are consistent with the standard viscous model. At the same time, a much better efficiency of the new model in comparison to the standard viscous one has been found because the critical time steps required by the viscoelastic model are much larger than those required by the viscous model. The new model has been applied to the simulation of real process of sintering of NiAl powder. The kinetics of sintering indicated by the evolution of density has been studied. The comparison of numerical and experimental results has shown a good performance of the developed numerical model.

Keywords:
Powder sintering, Simulation, Discrete element method, Viscoelastic model

19.Rojek J., Karlis G.F., Malinowski L.J., Beer G., Setting up virgin stress conditions in discrete element models, COMPUTERS AND GEOTECHNICS, ISSN: 0266-352X, DOI: 10.1016/j.compgeo.2012.07.009, Vol.48, pp.228-248, 2013
Rojek J., Karlis G.F., Malinowski L.J., Beer G., Setting up virgin stress conditions in discrete element models, COMPUTERS AND GEOTECHNICS, ISSN: 0266-352X, DOI: 10.1016/j.compgeo.2012.07.009, Vol.48, pp.228-248, 2013

Abstract:
In the present work, a methodology for setting up virgin stress conditions in discrete element models is proposed. The developed algorithm is applicable to discrete or coupled discrete/continuum modeling of underground excavation employing the discrete element method (DEM). Since the DEM works with contact forces rather than stresses there is a need for the conversion of pre-excavation stresses to contact forces for the DEM model. Different possibilities of setting up virgin stress conditions in the DEM model are reviewed and critically assessed. Finally, a new method to obtain a discrete element model with contact forces equivalent to given macroscopic virgin stresses is proposed. The test examples presented show that good results may be obtained regardless of the shape of the DEM domain.

Keywords:
Discrete element method, Initial stress conditions, Virgin stresses, Discrete/continuum modeling

20.Rojek J., Lumelskyy D., Pęcherski R., Grosman F., Tkocz M., Chorzępa W., Forming limit curves for complex strain paths, ARCHIVES OF METALLURGY AND MATERIALS, ISSN: 1733-3490, DOI: 10.2478/amm-2013-0042, Vol.58, pp.587-593, 2013
Rojek J., Lumelskyy D., Pęcherski R., Grosman F., Tkocz M., Chorzępa W., Forming limit curves for complex strain paths, ARCHIVES OF METALLURGY AND MATERIALS, ISSN: 1733-3490, DOI: 10.2478/amm-2013-0042, Vol.58, pp.587-593, 2013

Abstract:
This paper presents results of experimental studies of forming limit curves (FLC) for sheet forming under complex strain paths. The Nakazima-type formability tests have been performed for the as-received steel blank and for the blank pre-strained by13%. Prestraining leads to abrupt change of strain path in the blank deformation influencing the forming limit curve. The experimental FLC of the pre-strained blank has been compared with the FLC constructed by transformation of the as-received FLC. Quite a good agreement has been found out. The concept of strain-path independent FLCs in polar coordinates has been verified. Two types of the polar diagrams have been considered, the first one with the strain-path angle and effective plastic strain as the polar coordinates, and the second one originally proposed in this work in which the thickness strain has been used instead of the effective plastic strain as one of the polar coordinates. The second transformation based on our own concept has given a better agreement between the transformed FLCs, which allows us to propose this type of polar diagrams as a new strain-path in dependent criterion to predict sheet failure in forming processes.

Keywords:
sheet forming, formability, forming limit curve, complex strain-path

21.Lumelskyy D., Marczewska I., Rojek J., Pęcherski R., Grosman F., Tkocz M., Effect of friction on failure location in sheet metal formability tests, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.13, No.1, pp.43-48, 2013
Lumelskyy D., Marczewska I., Rojek J., Pęcherski R., Grosman F., Tkocz M., Effect of friction on failure location in sheet metal formability tests, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.13, No.1, pp.43-48, 2013

Abstract:
This paper presents numerical investigations of the influence of friction on sheet deformation in Nakazima type formability tests. Numerical simulations have been performed using the authors’ own explicit dynamic finite element program. Numerical results have been compared with experimental data. Location of fracture was of major interest in this work. The studies confirmed that the fracture location near the center of the specimen as required by the standards can be obtained for low values of the friction coefficient. Numerical simulation combined with the inverse analysis has been used to estimate a real value of the friction coefficient in the Nakazima formability test.

Keywords:
formability test, explicit FE method, friction, fracture location

22.Rojek J., Nosewicz S., Pietrzak K., Chmielewski M., Simulation of Powder Sintering Using a Discrete Element Model, ACTA MECHANICA ET AUTOMATICA, ISSN: 1898-4088, DOI: 10.2478/ama-2013-0030, Vol.7, pp.175-179, 2013
Rojek J., Nosewicz S., Pietrzak K., Chmielewski M., Simulation of Powder Sintering Using a Discrete Element Model, ACTA MECHANICA ET AUTOMATICA, ISSN: 1898-4088, DOI: 10.2478/ama-2013-0030, Vol.7, pp.175-179, 2013

Abstract:
This paper presents numerical simulation of powder sintering. The numerical model introduced in this work employs the discrete element method which assumes that material can be modelled by a large assembly of discrete elements (particles) of spherical shape interacting among one another. Modelling of sintering requires introduction of the cohesive interaction among particles representing interparticle sintering forces. Numerical studies of sintering have been combined with experimental studies which provided data for calibration and validation of the model. In the laboratory tests evolution of microstructure and density during sintering have been studied. Comparison of numerical and experimental results shows a good performance of the numerical model developed

Keywords:
Powder Sintering, Simulation, Discrete Element Method

23.Nosewicz S., Rojek J., Numeryczne modelowanie naprężeń rezydualnych w spiekanych materiałach kompozytowych, PRZEGLĄD MECHANICZNY, ISSN: 0033-2259, Vol.10, pp.30-34, 2013
Nosewicz S., Rojek J., Numeryczne modelowanie naprężeń rezydualnych w spiekanych materiałach kompozytowych, PRZEGLĄD MECHANICZNY, ISSN: 0033-2259, Vol.10, pp.30-34, 2013

Abstract:
Sintering process is one of the major method of manufacture technology of composite materials with intermetallic matrix reinforced by ceramic particles. In the final stage of sintering, during cooling of material, the microcracks may occur due to appearance of significant residual stress at the grain boundaries, which leads to progressive degradation of the material. This paper presents numerical modeling of micro- and macroscopic stress during and after sintering process composite materials. The original thermo-viscoelastic model of discrete elements have been performed. Numerical simulations have been carried out on the example of the NiAl-Al2O3 composite. The obtained results confirm correct and efficient performance of the proposed numerical model.

Keywords:
sintering, composite, residual stresses, discrete element method

24.Nosewicz S., Rojek J., Numeryczne modelowanie naprężeń występujących w trakcie oraz po procesie metalurgii proszków materiałów kompozytowych, PRACE NAUKOWE POLITECHNIKI WARSZAWSKIEJ, SERIA: MECHANIKA, ISSN: 0137-2335, Vol.253, pp.13-18, 2013
Nosewicz S., Rojek J., Numeryczne modelowanie naprężeń występujących w trakcie oraz po procesie metalurgii proszków materiałów kompozytowych, PRACE NAUKOWE POLITECHNIKI WARSZAWSKIEJ, SERIA: MECHANIKA, ISSN: 0137-2335, Vol.253, pp.13-18, 2013

Abstract:
W technologii metalurgii proszków spiekanie, wraz z chłodzeniem, jest jednym z kluczowych etapów wytwarzania materiałów kompozytowych na osnowie metalicznej, podczas którego może dochodzić do pękania materiału na skutek występujących na granicach faz naprężeń rezydualnych. Prezentowana praca przedstawia wyniki modelowania numerycznego naprężeń mikro- oraz makroskopowych występujących w trakcie oraz po procesie metalurgii proszków materiałów kompozytowych. Do analizy procesów metalurgii proszków został użyty oryginalny termo-lepkosprężysty model elementów dyskretnych. Symulacje numeryczne zostały przeprowadzone na przykładzie kompozytu NiAl-Al2O3. Uzyskane wyniki potwierdzają poprawne oraz efektywne działanie zaproponowanego modelu numerycznego.

Keywords:
metoda elementów dyskretnych, symulacje numeryczne, spiekanie, metalurgia proszków, naprężenia rezydualne

25.Lumelskyy D., Marczewska I., Rojek J., Pęcherski R., Grosman F., Tkocz M., Wpływ tarcia na lokalizację pękania w próbach tłoczności Nakazimy, PRACE NAUKOWE POLITECHNIKI WARSZAWSKIEJ, SERIA: MECHANIKA, ISSN: 0137-2335, Vol.253, pp.57-62, 2013
Lumelskyy D., Marczewska I., Rojek J., Pęcherski R., Grosman F., Tkocz M., Wpływ tarcia na lokalizację pękania w próbach tłoczności Nakazimy, PRACE NAUKOWE POLITECHNIKI WARSZAWSKIEJ, SERIA: MECHANIKA, ISSN: 0137-2335, Vol.253, pp.57-62, 2013

Abstract:
W artykule przedstawiono analizę numeryczną wpływu tarcia na rozkład odkształceń uzyskanych w próbach tłoczności przeprowadzonych metodą Nakazimy. Symulacje numeryczne zostały przeprowadzone w autorskim programie opartym na metodzie elementów skończonych z jawnym całkowaniem ruchu względem czasu. Wyniki numeryczne porównano z danymi eksperymentalnymi. Główną uwagę zwrócono na lokalizację miejsca pęknięcia. Badania potwierdziły, że miejsce pęknięcia w pobliżu środka próbki, zgodnie z wymaganiami norm, można uzyskać przy małych wartościach współczynnika tarcia. Symulacja numeryczna, w połączeniu z analizą odwrotną, została wykorzystana do oszacowania rzeczywistej wartości współczynnika tarcia w przeprowadzonych próbach tłoczności metodą Nakazimy.

Keywords:
próba tłoczności Nakazimy, wpływ tarcia, lokalizacja pękania, symulacje numeryczne MES, analiza odwrotna

26.Rojek J., Labra C., Su O., Oñate E., Comparative study of different discrete element models and evaluation of equivalent micromechanical parameters, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, ISSN: 0020-7683, DOI: 10.1016/j.ijsolstr.2012.02.032, Vol.49, pp.1497-1517, 2012
Rojek J., Labra C., Su O., Oñate E., Comparative study of different discrete element models and evaluation of equivalent micromechanical parameters, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, ISSN: 0020-7683, DOI: 10.1016/j.ijsolstr.2012.02.032, Vol.49, pp.1497-1517, 2012

Abstract:
Comparative studies of different discrete element models of a rock-type material are presented. The discrete element formulation employs spherical particles with the cohesive interaction model combining linear elastic behaviour with brittle failure. Numerical studies consisted in simulation of the uniaxial compression test. Two cylindrical specimens with particle size distributions yielding different degree of heterogeneity have been used. Macroscopic response produced by different discrete element models has been compared. The main difference between the compared models consists in the evaluation of micromechanical constitutive parameters. Two approaches are compared. In the first approach, the contact stiffness and strength parameters depend on the local particle size, while in the second approach, global uniform contact parameters are assumed for all the contacting pairs in function of average geometric measures characterizing the particle assembly. The size dependent contact parameters are calculated as functions of geometric parameters characterizing each contacting particle pair. As geometric scaling parameters, the arithmetic and harmonic means, as well as the minimum of the radii of two contacting particles are considered. Two different models with size dependent contact parameters are formulated. The performance of these models is compared with that of the discrete element model with global uniform contact parameters. Equivalence between the models with size dependent and uniform contact parameters has been checked. In search of this equivalence, different methods of evaluation of global uniform parameters have been studied. The contact stiffness has been evaluated in terms of the average radius of the particle assembly or in terms of the averages of the arithmetic and harmonic means of the contact pair radii, the geometric parameters used in the evaluation of the contact stiffness in the size-dependent models. The uniform contact strengths have been determined as functions of the averages of radii squares, squares of arithmetic radii means or squares of minimum radii of the contacting pairs.

For the more homogenous specimen, the models with local size dependent parameters and models with global uniform parameters give similar response. The models with uniform parameters evaluated according to the averages of the geometric parameters used in the evaluation of local parameters ensure better agreement with the respective models with size-dependent parameters than the models with uniform parameters evaluated according to the particle radii. Simulations using the more heterogenous specimen reveal differences between the considered models. There are significant differences in stress–strain curves as well as in the failure pattern. The models with local size-dependent parameters are more sensitive to the change of heterogeneity than the model with global uniform parameters.

Keywords:
Discrete element method, Brittle, Compression, Contact, Heterogeneity, Modelling

27.Zabulionis D., Kačianauskas R., Markauskas D., Rojek J., Investigation of nonlinear tangential contact behaviour of a spherical particle under varying loading, BULLETIN OF THE POLISH ACADEMY OF SCIENCES: TECHNICAL SCIENCES, ISSN: 0239-7528, Vol.60, No.2, pp.265-278, 2012
28.Rojek J., Hyrcza-Michalska M., Bokota A., Piekarska W., Determination of mechanical properties of the weld zone in tailor-welded blanks, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, ISSN: 1644-9665, DOI: 10.1016/j.acme.2012.04.004, Vol.12, pp.156-162, 2012
29.Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Numerical simulation of formability tests of pre-deformed steel blanks, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, ISSN: 1644-9665, DOI: 10.1016/j.acme.2012.04.010, Vol.12, No.2, pp.133-141, 2012
Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Numerical simulation of formability tests of pre-deformed steel blanks, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, ISSN: 1644-9665, DOI: 10.1016/j.acme.2012.04.010, Vol.12, No.2, pp.133-141, 2012

Abstract:
This paper presents the results of numerical simulations of the formability tests carried out for a pre-stretched 1 mm thick DC04 steel sheet. Simulation consisted of the subsequent stages as follows: uniaxial stretching of the sheet, unloading and stress relaxation, cutting specimens out of the pre-stretched sheet and bulging the blank with a hemispherical punch. Numerical modeling has been verified by comparison of the simulation results with the experimental ones. Good concordance of the results indicates correct performance of the numerical model and possibility to use it in further theoretical studies.

Keywords:
Sheet forming, Formability, Forming limit diagram, Pre-stretching, Numerical simulation

30.Nosewicz Sz., Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Modelowanie procesu spiekania materiałów dwufazowych metodą elementów dyskretnych, RUDY I METALE NIEŻELAZNE, ISSN: 0035-9696, Vol.57, No.9, pp.599-603, 2012
Nosewicz Sz., Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Modelowanie procesu spiekania materiałów dwufazowych metodą elementów dyskretnych, RUDY I METALE NIEŻELAZNE, ISSN: 0035-9696, Vol.57, No.9, pp.599-603, 2012

Abstract:
W niniejszym artykule zostały przedstawione nowe wyniki modelowania procesu spiekania metodą elementów dyskretnych. W sformułowaniu teoretycznym dla części sprężystej zastosowano model kontaktu Hertza w celu lepszego odwzorowania oddziaływania elementów kulistych w trakcie prasowania. Sformułowanie i implementację modelu rozszerzono na przypadek spiekania materiałów dwufazowych. Na podstawie badań literaturowych wyznaczono parametry materiałowe procesu, które zostały następnie zweryfikowane za pomocą wyników eksperymentalnych. Wyniki numeryczne ewolucji gęstości próbki porównano z wynikami doświadczalnymi otrzymując dużą zgodność.

Keywords:
materiały dwufazowe, metalurgia proszków, spiekanie, metoda elementów dyskretnych

31.Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Numeryczna symulacja doświadczalnych prób tłoczności wstępnie wyprężonych blach ze stali DC04, HUTNIK - WIADOMOŚCI HUTNICZE, ISSN: 1230-3534, Vol.79, No.1, pp.14-18, 2012
Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Numeryczna symulacja doświadczalnych prób tłoczności wstępnie wyprężonych blach ze stali DC04, HUTNIK - WIADOMOŚCI HUTNICZE, ISSN: 1230-3534, Vol.79, No.1, pp.14-18, 2012

Abstract:
W pracy przedstawiono wyniki symulacji numerycznej próby tłoczności wstępnie wyprężonej blachy ze stali DC04 o grubości 1 mm. Symulacja obejmowała kolejno następujące etapy: wyprężanie blachy, odciążenie, wycięcie wykrojki z blachy wyprężonej oraz próbę wybrzuszania półkulistym stemplem. Wyniki numeryczne porównano z wynikami doświadczalnymi. Uzyskana zgodność wyników wskazuje na prawidłowe działanie modelu numerycznego oraz możliwość wykorzystania go do dalszych badań teoretycznych.

Keywords:
blacha wstępnie wyprężona, próby tłoczności, symulacja numeryczna

32.Rojek J., Oñate E., Labra C., Kargl H., Discrete element simulation of rock cutting, INTERNATIONAL JOURNAL OF ROCK MECHANICS AND MINING SCIENCES, ISSN: 1365-1609, Vol.48, pp.996-1010, 2011
33.Lumelskyy D., Rojek J., Numerical Accuracy and Efficiency of Algorithms for Springback Calculation, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.11, No.2, pp.387-393, 2011
34.Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Nosewicz S., Discrete Element Simulation of Powder Sintering, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.11, No.1, pp.68-73, 2011
Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Nosewicz S., Discrete Element Simulation of Powder Sintering, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.11, No.1, pp.68-73, 2011

Abstract:
This paper presents numerical modelling of powder sintering. The numerical model introduced in this work employs the discrete element method which assumes that material can be modelled by a large assembly of discrete elements (particles) of spherical shape interacting among one another. Modelling of sintering requires introduction of the cohesive interaction among particles representing inter-particle sintering forces. Numerical studies of sintering have been supplemented with experimental studies which provided data for calibration and validation of the model. In the laboratory tests evolution of microstructure and density during sintering have been studied. Comparison of numerical and experimental results shows a good performance of the numerical model developed.

Keywords:
powder sintering, powder metallurgy, simulation, discrete element method

35.Lumelskyy D., Rojek J., Dokładność i efektywność numeryczna algorytmów obliczania sprężynowania powrotnego, PRACE NAUKOWE POLITECHNIKI WARSZAWSKIEJ, SERIA: MECHANIKA, ISSN: 0137-2335, Vol.238, pp.45-50, 2011
36.Nosewicz S., Rojek J., Modelowanie spiekania proszków metodą elementów dyskretnych, PRACE NAUKOWE POLITECHNIKI WARSZAWSKIEJ, SERIA: MECHANIKA, ISSN: 0137-2335, Vol.238, pp.7-12, 2011
37.Rojek J., Hyrcza-Michalska M., Bokota A., Piekarska W., Metody wyznaczania właściwości mechanicznych złączy w spawanych laserowo wsadach do tłoczenia, PRZEGLĄD MECHANICZNY, ISSN: 0033-2259, Vol.LXIX, No.11, pp.13-20, 2010
38.Hyrcza-Michalska M., Rojek J., Fruitos O., Numerical simulation of car body elements pressing applying tailor welded blanks – practical verification of results, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, ISSN: 1644-9665, Vol.10, No.4, pp.31-44, 2010
39.Rojek J., Labra C., Oñate E., Discrete element simulation of rock cutting with evaluation of tool wear, Discrete Element Methods. Simulations of Discontinua: Theory and Applications, pp.388-393, 2010
40.Labra C., Rojek J., Oñate E., Adaptive discrete/finite element coupling for rock cutting processes simulations, Discrete Element Methods. Simulations of Discontinua: Theory and Applications, pp.428-433, 2010
41.Rojek J., Hyrcza-Michalska M., Bokota A., Piekarska W., Doświadczalne i numeryczne wyznaczanie właściwości mechanicznych złącz wsadów do tłoczenia spawanych techniką laserową, Polska Metalurgia w latach 2006-2010, pp.582-589, 2010
42.Hyrcza-Michalska M., Rojek J., Laserowe spawanie wsadów do tłoczenia. Wybrane problemy wytwarzania i symulacja numeryczna, MECHANIK, ISSN: 0025-6552, Vol.4, pp.274-281, 2009
Hyrcza-Michalska M., Rojek J., Laserowe spawanie wsadów do tłoczenia. Wybrane problemy wytwarzania i symulacja numeryczna, MECHANIK, ISSN: 0025-6552, Vol.4, pp.274-281, 2009

Abstract:
Wsady spawane laserowo (WSL) stanowią wysoko zaawansowane technologicznie wsady przeznaczone do kształtowania w procesach tłoczenia. Wsady tego rodzaju powstają poprzez połączenie pasów blach składowych techniką spawania laserowego. Elementy tłoczone, wykonane z tego rodzaju wsadów, znajdują zastosowanie głównie w konstrukcji pojazdów. Wytwarzanie z WSL wymaga opanowania technik spawania laserowego blach cienkich różnego gatunku i grubości, o różnym rodzaju pokrycia powierzchni. Uzyskanie spoiny laserowej o zaprojektowanych właściwościach mechanicznych wymaga rozwiązania wielu problemów związanych z inicjacją wiązki i stabilizacją doczołowo łączonych pasów blach lub rur szczelinowych. Natomiast do właściwej oceny podatności do kształtowania wytworzonych WSL wymagana jest analiza plastycznego płynięcia blach składowych oraz WSL z nich wykonanych, w próbach podstawowych i technologicznych oceny tłoczności. Potrzeba wytwarzania szerokiego asortymentu wytłoczek uzasadnia zastosowanie symulacji numerycznych do prognozowania wyników tłoczenia WSL i stanowi nowoczesne narzędzie ułatwiające pracę projektanta i technologa. Kompleksowa analiza rozkładów odkształceń lokalnych wybranych wytłoczek laboratoryjnych oraz analiza wyników symulacji MES procesów tłoczenia wytłoczek laboratoryjnych pozwoliły na opracowanie wytycznych tłoczenia WSL.

43.Rojek J., Hyrcza-Michalska M., Bokota A., Piekarska W., Determination of mechanical properties of the weld zone of tailor-welded blanks, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.9, pp.153-158, 2009
44.Labra C., Rojek J., Oñate E., Zarate F., Advances in discrete element modelling of underground excavations, ACTA GEOTECHNICA, ISSN: 1861-1125, DOI: 10.1007/s11440-008-0071-2, Vol.3, No.4, pp.317-322, 2008
Labra C., Rojek J., Oñate E., Zarate F., Advances in discrete element modelling of underground excavations, ACTA GEOTECHNICA, ISSN: 1861-1125, DOI: 10.1007/s11440-008-0071-2, Vol.3, No.4, pp.317-322, 2008

Abstract:
The paper presents advances in the discrete element modelling of underground excavation processes extending modelling possibilities as well as increasing computational efficiency. Efficient numerical models have been obtained using techniques of parallel computing and coupling the discrete element method with finite element method. The discrete element algorithm has been applied to simulation of different excavation processes, using different tools, TBMs and roadheaders. Numerical examples of tunnelling process are included in the paper, showing results in the form of rock failure, damage in the material, cutting forces and tool wear. Efficiency of the code for solving large scale geomechanical problems is also shown.

Keywords:
Coupling, Discrete element method, Finite element method, Parallel computation, Tunnelling

45.Rojek J., Discrete element modelling of rock cutting, COMPUTER METHODS IN MATERIALS SCIENCE / INFORMATYKA W TECHNOLOGII MATERIAŁÓW, ISSN: 1641-8581, Vol.7, No.2, pp.224-230, 2007
46.Oñate E., Rojek J., Chiumenti M., Idelsohn S.R., Del Pin F., Aubry R., Advances in stabilized finite element and particle methods for bulk forming processes, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2004.10.018, Vol.195, pp.6750-6777, 2006
Oñate E., Rojek J., Chiumenti M., Idelsohn S.R., Del Pin F., Aubry R., Advances in stabilized finite element and particle methods for bulk forming processes, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2004.10.018, Vol.195, pp.6750-6777, 2006

Abstract:
The paper describes some recent developments in finite element and particle methods for analysis of a wide range of bulk forming processes. The developments include new stabilized linear triangles and tetrahedra using finite calculus and a new procedure combining particle methods and finite element methods. Applications of the new numerical methods to casting, forging and other bulk metal forming problems and mixing processes are shown.

Keywords:
Bulk forming processes, Stabilized finite element method, Particle method, Particle finite element method, Mixing processes

47.Rojek J., Oñate E., Taylor R.L., CBS-based stabilization in explicit solid dynamics, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, ISSN: 0029-5981, DOI: 10.1002/nme.1689, Vol.66, pp.1547-1568, 2006
48.Marczewska I., Bednarek T., Marczewski A., Sosnowski W., Jakubczak H., Rojek J., Practical fatigue analysis of hydraulic cylinders and some design recommendations, INTERNATIONAL JOURNAL OF FATIGUE, ISSN: 0142-1123, Vol.28, pp.1739-1751, 2006
49.Jakubczak H., Rojek J., Roquet P., Assessment of realistic fatigue data for hydraulic cylinders, PROBLEMY MASZYN ROBOCZYCH / ENGINEERING MACHINES PROBLEMS, ISSN: 1232-9304, Vol.27, pp.63-73, 2006
50.Rojek J., Zarate F., Agelet de Saracibar C., Gilbourne C., Verdot P., Discrete element modelling and simulation of sand mould manufacture for the lost foam process, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, ISSN: 0029-5981, DOI: 10.1002/nme.1221, Vol.62, pp.1421-1441, 2005

List of recent monographs
1.
287
Labra C.A., Oñate E., Rojek J., Advances in the Development of the Discrete Element Method for Excavation Processes, Monograph CIMNE Nº-132, pp.1-203, 2012
2.
103
Rojek J., Modelowanie i symulacja komputerowa złożonych zagadnień mechaniki nieliniowej metodami elementów skończonych i dyskretnych, IPPT Reports on Fundamental Technological Research, 4, pp.1-341, 2007
List of chapters in recent monographs
1.
38
Rojek J., Oñate E., Labra C., Kargl H., Particle-Based Methods, Series: Computational Methods in Applied Sciences, rozdział: Discrete Element Modelling of Rock Cutting, Springer, 25, pp.247-267, 2011
2.
204
Rojek J., Oñate E., Labra C., Kargl H., Akerman J., Technology innovation in underground construction, rozdział: Optimizing rock cutting through computer simulation, CRC Press, Beer G. (Ed.), pp.299-314, 2009

Conference papers
1.Rojek J., Nosewicz S., Maździarz M., Kowalczyk P., Wawrzyk K., Lumelskyj D., Modeling of a Sintering Process at Various Scales, Procedia Engineering, ISSN: 1877-7058, DOI: 10.1016/j.proeng.2017.02.210, Vol.177, pp.263-270, 2017
Rojek J., Nosewicz S., Maździarz M., Kowalczyk P., Wawrzyk K., Lumelskyj D., Modeling of a Sintering Process at Various Scales, Procedia Engineering, ISSN: 1877-7058, DOI: 10.1016/j.proeng.2017.02.210, Vol.177, pp.263-270, 2017

Abstract:
This paper presents modeling of a sintering process at various scales. Sintering is a powder metallurgy process consisting in consolidation of powder materials at elevated temperature but below the melting point. Sintering models at the atomistic, microscopic and macroscopic scales have been presented. Sintering is a process governed by diffusion therefore the atomistic modeling using the molecular dynamics has been focused on investigation of the diffusion process. The micromechanical model has been developed within the framework of the discrete element method. It allows us to consider microstructure and its changes during sintering. The macroscopic model is based on the continuum phenomenological approach. It combines elastic, thermal and viscous creep deformation. The methodology to determine macroscopic quantities: stress, strains and constitutive viscous properties from the discrete element simulations has been presented. Possibilities of the developed models have been demonstrated by applying them to simulation of sintering of the intermetallic NiAl powder. Own experimental results have been used to calibrate and validate numerical models.

Keywords:
sintering, modeling, discrete element method, diffusion, molecular dynamics, macroscopic model

2.Marijnissen M.J., Rojek J., Particle-fluid interaction inside a beater mill, JOURNAL OF PHYSICS: CONFERENCE SERIES, ISSN: 1742-6588, DOI: 10.1088/1742-6596/760/1/012017, Vol.760, pp.1-8, 2016
Marijnissen M.J., Rojek J., Particle-fluid interaction inside a beater mill, JOURNAL OF PHYSICS: CONFERENCE SERIES, ISSN: 1742-6588, DOI: 10.1088/1742-6596/760/1/012017, Vol.760, pp.1-8, 2016

Abstract:
In this work a trajectory study of copper ore particles through a fan mill was performed with the use of a commercial CFD code, ANSYS Fluent, coupled with DEM (Discrete Element Method). Particles of different sizes were analysed. Results highlight ore behaviour, fluid flow conditions and mark places requiring geometrical improvements.

Keywords:
CFD, DEM, beater mill

3.Rojek J., Graczykowski C., Marijnissen M.J., Szolc T., Marczewska I., Możliwości wykorzystania dyskretnych metod modelowania do symulacji procesów mechanicznego urabiania i przeróbki rud metali, ICNOP 2015, XI Międzynarodowa Konferencja Przeróbki Rud Metali Nieżelaznych, 2015-05-27/05-29, Trzebieszowice (PL), pp.60-67, 2015
Rojek J., Graczykowski C., Marijnissen M.J., Szolc T., Marczewska I., Możliwości wykorzystania dyskretnych metod modelowania do symulacji procesów mechanicznego urabiania i przeróbki rud metali, ICNOP 2015, XI Międzynarodowa Konferencja Przeróbki Rud Metali Nieżelaznych, 2015-05-27/05-29, Trzebieszowice (PL), pp.60-67, 2015

Abstract:
Modelowanie i symulacje numeryczne są obecnie nieodłączną częścią projektowania i optymalizacji różnorodnych procesów technologicznych. Zastosowanie metod numerycznych w projektowaniu procesów mechanicznego urabiania i przeróbki rud metali jest w dalszym ciągu stosunkowo niewielkie. Procesy odspajania kawałków skały od calizny i ich rozdrobnienia z zastosowaniem różnego rodzaju maszyn, wiążą się silnie nieciągłymi zjawiskami zniszczenia materiału i są bardzo trudne do modelowania za pomocą standardowych metod numerycznych, takich jak metoda elementów skończonych, opartych na ciągłym sformułowaniu zagadnienia mechaniki ciała stałego. Duże możliwości w zastosowaniu do tych procesów ma intensywnie rozwijana w ostatnich latach metoda elementów dyskretnych, w której materiał jest reprezentowany przez liczny zbiór ziaren, oddziałujących między sobą poprzez siły kontaktu. Model ten w sposób naturalny uwzględnia materiał rozdrobniony. Uwzględnienie wiązań kohezyjnych między ziarnami oraz możliwości ich zrywania umożliwia modelowanie inicjacji i propagacji pęknięć w materiale. W niniejszej pracy zostaną przedstawione możliwości wykorzystania metody elementów dyskretnych do symulacji urabiania skał za pomocą noży stożkowych i dysków oraz do symulacji zachowania się materiału w młynie. Sprzężenie metody elementów dyskretnych z modelem przepływu płynu umożliwi modelowanie zawiesiny pyłowej rozdrabnianego materiału.

Keywords:
modelowanie dyskretne, mechaniczne urabianie, rudy metali

4.Rojek J., Mikułowski G., Marczewska I., Experimental and numerical studies on magnetorheological fluids, 6WCSCM, 6th World Conference on Structural Control and Monitoring, 2014-07-15/07-17, Barcelona (ES), pp.2152-2159, 2014
Rojek J., Mikułowski G., Marczewska I., Experimental and numerical studies on magnetorheological fluids, 6WCSCM, 6th World Conference on Structural Control and Monitoring, 2014-07-15/07-17, Barcelona (ES), pp.2152-2159, 2014

Abstract:
This paper presents experimental and numerical studies of magnetorheological (MR) fluids. Experimental studies have been focused on the investigation of MR fluid flow in the valve mode. An experimental device operating in the valve mode has been built and used for testing. Numerical investigations h ave included analysis of magnetic field, continuum based analytical modelling of the valve mode as well as micromechanical discrete element simulation of MR fluid. Analytical studies of the MR flow have been carried out using the conventional Buckingham equation with constant yield stress across the valve gap and the modified Buckingham equation with nonlinear yield stress distribution across the channel according to the magnetic field distributionm determined numerically. The analytical results have been compared with experimental data. A better performance of the modified Buckingham model has been observed. A micromechanical model of MR fluids has been developed within the discrete element framework. The DEM model has been verified qualitatively on a test example of forming chains by magnetized particles after application of an external magnetic field.

5.Kačianauskas R., Rojek J., Pilkavičius S., Rimša V., Interaction of particles via solid interface: model and analysis − Particles 2013, Particles 2013, III International Conference on Particle-based Methods, 2013-09-18/09-20, Stuttgart (DE), pp.364-374, 2013
Kačianauskas R., Rojek J., Pilkavičius S., Rimša V., Interaction of particles via solid interface: model and analysis − Particles 2013, Particles 2013, III International Conference on Particle-based Methods, 2013-09-18/09-20, Stuttgart (DE), pp.364-374, 2013

Abstract:
The paper addresses discrete element (DEM) models of the heterogeneous particulate solids where the normal interaction between two deformable spherical particles bonded via weaker solid interface is considered. The integral interaction model aimed for evaluation of the bond stiffness was developed, where analytical expressions of the stiffness parameters reflecting individual contribution of the two particles and of the interface properties are derived. Application of the developed DEM model to particulate solid with many particles is considered. The accuracy and the suitability of this approach are evaluated by considering refined 3D Finite Element analysis.

Keywords:
Heterogeneous Solid, Particles Interaction, DEM, FEM

6.Oñate E., Labra C., Zarate F., Rojek J., Modelling and simulation of the effect of blast loading on structures using an adaptive blending of discrete and finite element methods, 3IWRDD-FORUM, 3rd International Forum on Risk Analysis, Dam Safety, Dam Security, and Critical Infrastructure Management, 2011-10-17/10-21, Valencia (ES), pp.365-372, 2012
Oñate E., Labra C., Zarate F., Rojek J., Modelling and simulation of the effect of blast loading on structures using an adaptive blending of discrete and finite element methods, 3IWRDD-FORUM, 3rd International Forum on Risk Analysis, Dam Safety, Dam Security, and Critical Infrastructure Management, 2011-10-17/10-21, Valencia (ES), pp.365-372, 2012

Abstract:
We present a new computational model for predicting the effect of blast loading on structures. The model is based in the adaptive coupling of the Finite Element Method (FEM) and the Discrete Element Method (DEM) for the accurate reproduction of multifracturing and failure of structures under blast loading. In the paper we briefly describe the basis of the coupled DEM/FEM technology and demonstrate its efficiency in its application to the study of the effect of blast loading on a masonry wall, a masonry tunnel and a double curvature dam.

7.Rojek J., Hyrcza-Michalska M., Bokota A., Piekarska W., Determination of mechanical properties of the Weld Zone in tailor-welded blanks, AutoMetForm, Advanced Metal Forming Processes in Automotive Industry: 3rd International Lower Silesia - Saxony Conference, 2012-05-13/05-16, Wrocław (PL), pp.69-81, 2012
8.Rojek J., Labra C., Oñate E., Discrete Element Simulation of Rock Cutting Processes, 10th International Conference Modern Building Materials, Structures and Techniques, 2010-05-19/05-21, Vilnius (LT), Vol.2, pp.1040-1044, 2010
9.Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Modelowanie spiekania proszków metalicznych metodą elementów dyskretnych, KomPlasTech 2010, XVII Konferencja Informatyka w Technologii Metali, 2010-01-10/01-13, Białka Tatrzańska (PL), pp.1-8, 2010
Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Modelowanie spiekania proszków metalicznych metodą elementów dyskretnych, KomPlasTech 2010, XVII Konferencja Informatyka w Technologii Metali, 2010-01-10/01-13, Białka Tatrzańska (PL), pp.1-8, 2010

Abstract:
W artykule przedstawiono badania doświadczalne oraz modelowanie numeryczne procesu spiekania proszków metalicznych. W części eksperymentalnej pracy badano ewolucję mikrostruktury oraz gęstości spieku w trakcie procesu spiekania. Jako metodę modelowania wybrano metodę elementów dyskretnych, w której zakłada się, że materiał jest reprezentowany przez liczny zbiór elementów dyskretnych (cząstek) o kształcie sferycznym oddziałujących między sobą. Modelowanie spiekania wymaga wprowadzenia oddziaływania kohezyjnego między cząstkami reprezentującego naprężenia powstające między ziarnami w trakcie spiekania. W artykule przedstawiono wstępne wyniki numeryczne pokazujące ewolucję naprężenia w trakcie spiekania między dwoma ziarnami spiekanego proszku.

Keywords:
spiekanie, metalurgia proszków, związki międzymetaliczne, modelowanie, metoda elementów dyskretnych

10.Rojek J., Kargl H., Labra C., Oñate E., 3d simulation of rock cutting in underground excavation, EURO:TUN 2009, 2nd International Conference on Computational Methods in Tunnelling, 2009-09-09/09-11, Bochum (GE), pp.157-162, 2009
11.Labra C., Rojek J., Oñate E., Discrete element simulation of the excavation process using disc cutters, EURO:TUN 2009, 2nd International Conference on Computational Methods in Tunnelling, 2009-09-09/09-11, Bochum (GE), pp.177-182, 2009

Conference abstracts
1.Rojek J., Nosewicz S., Development of a multiscale model of powder sintering, 5th KMM-VIN Industrial Workshop: Multi-scale and multi-physics materials modeling for advanced industries, 2016-01-26/01-27, Madryt (ES), pp.1, 2016
2.Lumelskyj D., Rojek J., Detection of strain localization in numerical simulation of sheet metal forming, AutoMetForm 2016, Advanced Metal Forming Processes in Automotive Industry, The 5th International Lower Silesia - Saxony Conference, 2016-06-28/06-29, Wrocław (PL), pp.37-43, 2016
Lumelskyj D., Rojek J., Detection of strain localization in numerical simulation of sheet metal forming, AutoMetForm 2016, Advanced Metal Forming Processes in Automotive Industry, The 5th International Lower Silesia - Saxony Conference, 2016-06-28/06-29, Wrocław (PL), pp.37-43, 2016

Keywords:
Sheet forming, formability, forming limit curie, numerical simulation

3.Rojek J., Jurczak K., Nosewicz S., Lumelskyj D., Chmielewski M., Contact models for discrete element simulation of the power compaction in a hot pressing process, CMIS 2016, Contact Mechanics International Symposium, 2016-05-11/05-16, Warszawa (PL), pp.28-29, 2016
4.Maździarz M., Rojek J., Nosewicz S., Molecular dynamics/statics simulation of Ni-Al nanoparticles sintering, ECCOMAS 2016, European Congress on Computational Methods in Applied Sciences and Engineering, 2016-06-05/06-10, Hersonissos (GR), pp.1, 2016
Maździarz M., Rojek J., Nosewicz S., Molecular dynamics/statics simulation of Ni-Al nanoparticles sintering, ECCOMAS 2016, European Congress on Computational Methods in Applied Sciences and Engineering, 2016-06-05/06-10, Hersonissos (GR), pp.1, 2016

Keywords:
Sintering, Powder Material, Ni-Al, Molecular Dynamics, Molecular Statics

5.Rojek J., Kowalczyk P., Nosewicz S., Jurczak K., Wawrzyk K., Micro-macro relationships from discrete element simulations of sintering, ECCOMAS 2016, European Congress on Computational Methods in Applied Sciences and Engineering, 2016-06-05/06-10, Hersonissos (GR), pp.1, 2016
Rojek J., Kowalczyk P., Nosewicz S., Jurczak K., Wawrzyk K., Micro-macro relationships from discrete element simulations of sintering, ECCOMAS 2016, European Congress on Computational Methods in Applied Sciences and Engineering, 2016-06-05/06-10, Hersonissos (GR), pp.1, 2016

Keywords:
sintering, discrete element method, multi-scale modeling

6.Marijnissen M.J., Rojek J., Particle-fluid interaction inside a fan mill, EPFDC, 8th European Postgradute Fluid Dynamics Conference, 2016-07-06/07-09, Warszawa (PL), pp.62, 2016
Marijnissen M.J., Rojek J., Particle-fluid interaction inside a fan mill, EPFDC, 8th European Postgradute Fluid Dynamics Conference, 2016-07-06/07-09, Warszawa (PL), pp.62, 2016

Abstract:
Fan mills, although generally used in coal comminution, show potential for the use with other minerals. They achieve high internal fluid velocities due to a spinning flywheel with its axis of revolutions normal to the flow. After hitting the flywheel, the ore is shed upward into a filter, where particles small enough are passed further on towards the next process stage. Particles considered too big are recirculated back onto the flywheel. In this work a trajectory study of copper ore particles through a fan mill was performed with the use of a commercial CFD code, ANSYS Fluent, coupled two-way with DEM (Discrete Element Method). Particles of different sizes were analysed. Results highlight ore behaviour, fluid flow conditions and mark places requiring geometrical improvements.

7.Nosewicz S., Jurczak K., Rojek J., Chmielewski M., Pietrzak K., Application of contact interaction of Hertz model to viscoelastic discrete element model of sintering, ISNNM, 14th International Symposium on Novel and Nano Materials, 2016-07-03/07-08, Budapeszt (HU), pp.119, 2016
8.Jurczak K., Rojek J., Nosewicz S., Lumelskyy D., Bochenek K., Chmielewski M., Pietrzak K., Modelowanie wstępnego prasowania proszków metodą elementów dyskretnych, KomPlasTech 2016, XXIII Konferencja Informatyka w Technologii Metali, 2016-01-17/01-20, Wisła (PL), pp.68, 2016
Jurczak K., Rojek J., Nosewicz S., Lumelskyy D., Bochenek K., Chmielewski M., Pietrzak K., Modelowanie wstępnego prasowania proszków metodą elementów dyskretnych, KomPlasTech 2016, XXIII Konferencja Informatyka w Technologii Metali, 2016-01-17/01-20, Wisła (PL), pp.68, 2016

Abstract:
W niniejszym artykule zaprezentowano wyniki modelowania, zagęszczania proszku stanowiącego wstępny etap procesu prasowania na gorąco, metodą elementów dyskretnych opisaną w [1]. Modelowanie numeryczne zrealizowano metodą elementów dyskretnych, z wykorzystaniem kulistych cząstek. Badania skoncentrowano na mechanizmach zagęszczania proszku przy ciśnieniu 50 MPa oraz modelach odpowiednich przy zastosowanych warunkach procesu. Numeryczne symulacje wykonano z wykorzystaniem dwóch modeli: pierwszy - elastyczny Hertz-Mindlin-Deresiewicz, drugi - plastyczny Storakers, opisanych w pracy [2]. Wyniki symulacji numerycznych zostały porównane z wynikami laboratoryjnymi zagęszczania proszku NiAl w matrycy. W rezultacie otrzymano dużą zgodność wyników eksperymentalnych i numerycznych.

Keywords:
metoda elementów dyskretnych, modelowanie, zagęszczanie proszków, prasowanie

9.Rojek J., Nosewicz S., Maździarz M., Kowalczyk P., Wawrzyk K., Modelling of powder sintering at various scales, SolMech 2016, 40th Solid Mechanics Conference, 2016-08-29/09-02, Warszawa (PL), No.P193, pp.1-2, 2016
Rojek J., Nosewicz S., Maździarz M., Kowalczyk P., Wawrzyk K., Modelling of powder sintering at various scales, SolMech 2016, 40th Solid Mechanics Conference, 2016-08-29/09-02, Warszawa (PL), No.P193, pp.1-2, 2016

Keywords:
sintering, multiscale modelling

10.Marijnissen M.J., Rojek J., Particle-fluid interaction inside a beater mill, XXII Fluid Mechanics Conference, 2016-09-11/09-14, Słok k/Bełchatowa (PL), pp.127-128, 2016
Marijnissen M.J., Rojek J., Particle-fluid interaction inside a beater mill, XXII Fluid Mechanics Conference, 2016-09-11/09-14, Słok k/Bełchatowa (PL), pp.127-128, 2016

Abstract:
In this work a trajectory study of copper ore particles through a fan mill was performed with the use of a commercial CFD code, ANSYS Fluent, coupled with DEM (Discrete Element Method). Particles of different sizes were analysed. Results highlight ore behaviour, fluid flow conditions and mark places requiring geometrical improvements.

Keywords:
CFD, DEM, Beater mill

11.Pereira A., Rojek J., Barros G, Beer G., DEM-BEM Coupling in Time Domain, COUPLED PROBLEMS 2015, VI International Conference on Coupled Problems in Science and Engineering, 2015-05-01/05-20, Wenecja (IT), pp.1, 2015
12.Nosewicz S., Rojek J., Pietrzak K., Chmielewski M., Discrete element modelling of hot pressing process, EUROMAT 2015, European Congress and Exhibition on Advanced Materials and Processes, 2015-09-20/09-24, Warszawa (PL), pp.1, 2015
13.Rojek J., Nosewicz S., Jurczak K., Viscoelastic cohesive contact formulation for discrete element model of powder sintering, ICCCM 2015, IV International Conference on Computational Contact Mechanics, 2015-05-27/05-29, Hannover (DE), pp.1-2, 2015
Rojek J., Nosewicz S., Jurczak K., Viscoelastic cohesive contact formulation for discrete element model of powder sintering, ICCCM 2015, IV International Conference on Computational Contact Mechanics, 2015-05-27/05-29, Hannover (DE), pp.1-2, 2015

Keywords:
cohesive contact, discrete element method, viscoelasticity, sintering

14.Rojek J., Marczewska I., Micro-macro constitutive relations for granular material in the elastic range, PARTICLES 2015, IV International Conference on Particle-Based Methods, 2015-09-28/09-30, Barcelona (ES), pp.1, 2015
15.Marczewska I., Rojek J., Kačianauskas R., Investigation of micro-macro relationships of elastic parameters in the discrete element model of granular material, PCM-CMM 2015, 3rd Polish Congress of Mechanics and 21st Computer Methods in Mechanics, 2015-09-08/09-11, Gdańsk (PL), pp.91-92, 2015
Marczewska I., Rojek J., Kačianauskas R., Investigation of micro-macro relationships of elastic parameters in the discrete element model of granular material, PCM-CMM 2015, 3rd Polish Congress of Mechanics and 21st Computer Methods in Mechanics, 2015-09-08/09-11, Gdańsk (PL), pp.91-92, 2015

Abstract:
A general objective of the paper is to improve understanding of micromechanical mechanisms in granular materials and their representation in numerical models. Results of numerical investigation on micro-macro relationship is the discrete element model of granular material are presented. The macroscopic response was analysed in a series of simulations of the triaxial compression test. Numerical studies were focused on the influence of microscopic parameters on the elastic response. The effect of the contact stiffness and the contact stiffness ratio on the effective elastic moduli, the Young’s modulus and Poisson’s ratio, were investigated. Numerical results were compared with the analytical estimations.

Keywords:
Discrete element method, granular material, triaxial test, micro-macro relationship, Voigt hypothesis, elastic moduli

16.Zabulionis D., Kačianauskas R., Rimša V., Rojek J., A model of stiffness of normal interaction of spherical particles embedded in matrix, PCM-CMM 2015, 3rd Polish Congress of Mechanics and 21st Computer Methods in Mechanics, 2015-09-08/09-11, Gdańsk (PL), pp.105-106, 2015
17.Kačianauskas R., Kačinskaja I., Maknickas A., Markauskas D., Rojek J., Simulation of attractive motion of silica microparticles in aerosol under acoustic excitation, PCM-CMM 2015, 3rd Polish Congress of Mechanics and 21st Computer Methods in Mechanics, 2015-09-08/09-11, Gdańsk (PL), pp.563-564, 2015
18.Lumelskyy D., Rojek J., Pęcherski R.B., Grosman F., Tkocz M., Numerical study of an effect of friction in Nakazima formability test, 8th Workshop on Dynamic Behaviour of Materials and Its Applications in Industrial Processes, 2014-06-25/06-27, Warszawa (PL), pp.45-46, 2014
Lumelskyy D., Rojek J., Pęcherski R.B., Grosman F., Tkocz M., Numerical study of an effect of friction in Nakazima formability test, 8th Workshop on Dynamic Behaviour of Materials and Its Applications in Industrial Processes, 2014-06-25/06-27, Warszawa (PL), pp.45-46, 2014

Abstract:
This paper presents numerical investigations of the influence of friction in the contact between sheet and a punch on sheet deformation in Nakazima type formabilitybtests. The Nakazima test [1] is one of the most comonnly used tests to study experimentally formability of metal sheets. It consits in stretching of a sheet specimen by means of a hemispherical punch until fracture occurs.
The aim of this study has been to numerically identify frictional conditions in a selected case of the Nakazima test and study numerically effect of change of friction on strain path and forming limit curve. (FLC). Numerical simulations have been performed assuming the data corresponding to own lboratory tests carried out for the steel grade HC380LA 1.5 mm thick.

Keywords:
formabilty of metal sheets, Nakazima test, numerical simulation, friction effects

19.Kowalczyk P., Rojek J., Stocki R., Bednarek T., Tauzowski P., Lasota R., Lumelskyy D., Wawrzyk K., NUMPRESS – integrated computer system for analysis and optimization of industrial sheet metal forming processes: examples of use, AutoMetForm/SFU 2014, New Materials for Vehicle Components, 2014-11-03/11-05, Freiburg (DE), pp.284-286, 2014
Kowalczyk P., Rojek J., Stocki R., Bednarek T., Tauzowski P., Lasota R., Lumelskyy D., Wawrzyk K., NUMPRESS – integrated computer system for analysis and optimization of industrial sheet metal forming processes: examples of use, AutoMetForm/SFU 2014, New Materials for Vehicle Components, 2014-11-03/11-05, Freiburg (DE), pp.284-286, 2014

Abstract:
This paper presents NUMPRESS System that has been developed in IPPT PAN as a result of a project financially supported by European Regional Development Fund (within the Innovative Economy Programme) and is dedicated to small and middle enterprises dealing with sheet metal forming. It seems undoubted that efficient design of an industrial sheet forming process requires reliable computer simulations and a tool for numerical optimization of the process parameters. It has to be also admitted that. among small and medium enterprises (SME) in this industrial branch, there are many who do not use any such numerical tools in their practice.
Computer simulation of sheet metal forming processes is a very specific branch of computational mechanics. Finite element systems dedicated strictly to this kind of processes are needed and actually present on the market. Commercial systems (like Autoform, PAM-Stamp, Stampack, etc.) are, due to their prices, usually beyond financial ability of SME.

Design of the drawing process and tools, i.e. choice of proper values of several design parameters, require efficient optimization strategy. In this process, random character of at least some of the parameters has to be taken into account. In view of this fact, the traditional, deterministic approach to optimization is insufficient and elements of robust design optimization techniques and reliability analysis have to be included in the formulation of the optimization problem. It has to be admitted that, even if some of the mentioned commercial simulation systems offer numerical optimization modules, not all of them reach beyond the deterministic concept of the optimization process.

Keywords:
sheet metal forming, finite element method, deterministic and robust design optimization, reliability analysis

20.Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Influence of friction on strain distribution in Nakazima formability test of circular specimen, AutoMetForm/SFU 2014, New Materials for Vehicle Components, 2014-11-03/11-05, Freiburg (DE), pp.214-217, 2014
Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Influence of friction on strain distribution in Nakazima formability test of circular specimen, AutoMetForm/SFU 2014, New Materials for Vehicle Components, 2014-11-03/11-05, Freiburg (DE), pp.214-217, 2014

Abstract:
This paper presents experimental and numerical investigations of the influence of friction on sheet deformation in Nakazima type formability tests. Numerical simulations have been performed using the authors own explicit dynamic finite element program. Strain distribution obtained in numerical analyses has been compared with experimental data. Location of fracture was of major interest in this specimen as required by the standards can be obtained for low value of the friction coefficient. With the increase of the friction coefficient the fracture is displaced further from the center.

Keywords:
formability of metal sheets, Nakazima test, numerical simulation, friction effect, failure location

21.Lumelskyy D., Rojek J., Grosman F., Tkocz M., Hyrcza-Michalska M., Wyznaczanie początku lokalizacji odkształcenia w numerycznej symulacji tłoczenia blach, IX Seminarium Naukowe ZINTEGROWANE STUDIA PODSTAW DEFORMACJI PLASTYCZNEJ METALI PLASTMET, 2014-11-25/11-28, Łańcut (PL), pp.7-8, 2014
22.Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Numerical investigation of influence of friction on strain distribution and forming limit curve in Nakazima formability test, SolMech 2014, 39th Solid Mechanics Conference, 2014-09-01/09-05, Zakopane (PL), pp.45-46, 2014
Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Numerical investigation of influence of friction on strain distribution and forming limit curve in Nakazima formability test, SolMech 2014, 39th Solid Mechanics Conference, 2014-09-01/09-05, Zakopane (PL), pp.45-46, 2014

Abstract:
This paper presents numerical investigations of the influence of friction in the contact between sheet and a punch on sheet deformation in Nakazima type formability tests. The Nakazima test [1] is one of the most commonly used tests to study experimentally formability of metal sheets. It consists in stretching of a sheet specimen by means of a hemispherical punch until occurrence of fracture.

Keywords:
formability, Nakazima test, influence of friction, numerical simulations, deformation of metal sheets

23.Nowak Z., Grosman F., Lumelskyy D., Nowak M., Pęcherski R.B., Rojek J., Tkocz M., Metal cup deep drawing processes – numerical simulation and experimental verification, SolMech 2014, 39th Solid Mechanics Conference, 2014-09-01/09-05, Zakopane (PL), pp.47-48, 2014
Nowak Z., Grosman F., Lumelskyy D., Nowak M., Pęcherski R.B., Rojek J., Tkocz M., Metal cup deep drawing processes – numerical simulation and experimental verification, SolMech 2014, 39th Solid Mechanics Conference, 2014-09-01/09-05, Zakopane (PL), pp.47-48, 2014

Abstract:
Finite element method is an efficient numerical tool to analyse problems of the sheet metal forming processes in particular cup drawing and stamping. Proper description of material properties is crucial for accurate analysis. In particular, the anisotropy and asymmetry of elastic range, which is related with strength differential effect (SDE), of considered materials play an important role in finite element simulation. For metal forming analysis with use of traditional models many experimental tests are usually needed to obtain the adequate description of anisotropic behaviour of metal sheets. Therefore, the search for new models, which are based on simplified description of the effects of anisotropy and SDE requiring less experimental tests seems to be justified.
The paper presents the application of a new yield criterion for the transversal isotropy of metal sheets under plane stress conditions. The proposed criterion is based on the study of yield criteria accounting for SDE and anisotropy nade by W. Burzyński [1]. The system of equations describing the sheet metal forming process is solved by the algorithm using the return mapping procedure. Plane stress constraint is incorporated into the Newton-Raphson iteration loop. The proposed algorithm is verified by performing the numerical calculations using shell elements of the commercial FEM sftware ABAQUS/EXPLICIT with own VUMAT subroutine.

Keywords:
metal sheet forming, metal cup deep drawing, FE numerical simulations, strength differential effect, anisotropy and asymmetry of elastic range, transversal isotropy

24.Kowalczyk P., Rojek J., Stocki R., Bednarek T., Tauzowski P., Lasota R., Lumelskyy D., Wawrzyk K., NUMPRESS – integrated computer system for analysis and optimization of industrial sheet metal forming processes: numerical investigation of square cup drawing, SolMech 2014, 39th Solid Mechanics Conference, 2014-09-01/09-05, Zakopane (PL), pp.237-238, 2014
Kowalczyk P., Rojek J., Stocki R., Bednarek T., Tauzowski P., Lasota R., Lumelskyy D., Wawrzyk K., NUMPRESS – integrated computer system for analysis and optimization of industrial sheet metal forming processes: numerical investigation of square cup drawing, SolMech 2014, 39th Solid Mechanics Conference, 2014-09-01/09-05, Zakopane (PL), pp.237-238, 2014

Abstract:
This paper presents basic features of the NUMPRESS system and some examples ofuse. The system has been developed at IPPT PAN as a result of a project financially supported by European Regional Development Fund and is dedicated to small and middle enterprises (SME) dealing with sheet metal forming. The program consists of (i) an analytical finite element method module (ii) an optimization module, (iii) a reliability analysis module, and (iv) a graphical user interface enabling communication between modules. The analytical module consists of two independent programs up to the user’s choice: NUMPRESS-Flow, a faster and less accurate program for implicit quasi-static analysis of rigid-viscoplastic shells (based on the flow approach) and NUMPRESS-Explicit, a program for explicit dynamical analysis of elastic-plastic shells. Both programs are interfaced to a well-known commercial graphical pre-and postprocessor GiD.

Keywords:
sheet metal forming, finite element method, deterministic and robust design optimization, reliability analysis

25.Marczewska I., Rojek J., Kačianauskas R., Micro-macro dependency for elastic constants in a numerical model of granular material, SolMech 2014, 39th Solid Mechanics Conference, 2014-09-01/09-05, Zakopane (PL), pp.175-176, 2014
26.Rojek J., Stocki R., Lasota R., Tauzowski P., Efficient reliability analysis of sheet metal forming processes accounting for forming limit curve uncertainty, CMM 2013, 20th International Conference on Computer Methods in Mechanics, 2013-08-27/08-31, Poznań (PL), No.MS05, pp.35-36, 2013
Rojek J., Stocki R., Lasota R., Tauzowski P., Efficient reliability analysis of sheet metal forming processes accounting for forming limit curve uncertainty, CMM 2013, 20th International Conference on Computer Methods in Mechanics, 2013-08-27/08-31, Poznań (PL), No.MS05, pp.35-36, 2013

Abstract:
The failure probability estimation of FEM simulated sheet metal forming process is a computationally challenging task. The application of efficient gradient-based reliability techniques is very much limited due to the numerical noise introduced by the explicit dynamic algorithm used to perform the sheet stamping analysis and by the nonlinearity of the failure function. To cope with this difficulty, in the current study a two stage metamodel-based adaptive importance sampling method is employed. In order to assess the reliability of sheet metal forming operations the stochastic character of such parameters as friction, blankholding force, blank thickness, strain hardening parameters of the constitutive law as well as parameters defining the forming limit curve (FLC) are considered. Using the numerical example of a square cup deep drawing, the benchmark problem of the Numisheet’93 conference, it is investigated how the assumptions concerning the probabilistic distribution of the FLC location parameter affect the probability of sheet metal fracture.

Keywords:
Reliability, Metal forming

27.Rojek J., Labra C., Marczewska I., Effect of parameter evaluation on failure mode in discrete element models of rock materials, CMM 2013, 20th International Conference on Computer Methods in Mechanics, 2013-08-27/08-31, Poznań (PL), No.MS13, pp.5-6, 2013
Rojek J., Labra C., Marczewska I., Effect of parameter evaluation on failure mode in discrete element models of rock materials, CMM 2013, 20th International Conference on Computer Methods in Mechanics, 2013-08-27/08-31, Poznań (PL), No.MS13, pp.5-6, 2013

Abstract:
Numerical studies of effect of parameter evaluation on a failure mode in discrete element models of rock materials have been performed. The discrete element formulation employs spherical particles with the cohesive interaction model combining linear elastic behaviour with brittle failure. Numerical studies consisted in simulation of the uniaxial compression test using a cylindrical specimen with particle size distributions characterized by high degree of heterogeneity. Two different approaches to evaluation of micromechanical constitutive parameters have been compared. In the first approach, the contact stiffness and strength parameters depend on the local particle size, while in the second approach, global uniform contact parameters are assumed for all the contacting pairs in function of average geometric measures characterizing the particle assembly. Significant differences in the failure pattern have been observed. The uniform constitutive parameters result in localized brittle-like fractures, while a distributed damage typical for a ductile failure is obtained for the model . with local size-dependent parameters.

Keywords:
discrete element method, rock, modelling, failure mode, brittle, compression, contact

28.Rojek J., Lumelskyy D., Marczewska I., Grosman F., Tkocz M., Influence of friction on strain distribution in Nakazima formability tests, ICCCM 2013, III International Conference on Computational Contact Mechanics, 2013-07-10/07-12, Lecce (IT), pp.95-96, 2013
Rojek J., Lumelskyy D., Marczewska I., Grosman F., Tkocz M., Influence of friction on strain distribution in Nakazima formability tests, ICCCM 2013, III International Conference on Computational Contact Mechanics, 2013-07-10/07-12, Lecce (IT), pp.95-96, 2013

Keywords:
Friction, contact, metal sheet forming, Nakazima test, failure location

29.Nosewicz S., Rojek J., Pietrzak K., Chmielewski M., Numerical modeling of stresses in composites manufactured by powder metallurgy, ICMM3, 3rd International Conference on Material Modelling incorporating 13th European Mechanics of Materials Conference, 2013-09-08/09-11, Warszawa (PL), pp.133, 2013
30.Rojek J., Nosewicz S., Pietrzak K., Chmielewski M., Discrete element modelling of powder metallurgy processes, Particles 2013, III International Conference on Particle-based Methods, 2013-09-18/09-20, Stuttgart (DE), pp.1, 2013
31.Rojek J., Nosewicz S., Pietrzak K., Chmielewski M., Simulation of powder sintering using a discrete element method, VII International Symposium on Mechanics of Materials and Structures, 2013-06-03/06-06, Augustów (PL), pp.59-60, 2013
32.Rojek J., Nosewicz Sz., Pietrzak K., Chmielewski M., Kaliński D., Discrete element simulation of powder metallurgy manufacturing process of metal-ceramic composites, ECCOMAX 2012, 6th European Congress on Computational Methods in Applied Sciences and Engineering, 2012-09-10/09-14, Wiedeń (AT), pp.1-2, 2012
33.Rojek J., Lumelskyy D., Grosman F., Tkocz M., Pęcherski R., Chorzępa W., Graniczne krzywe tłoczności przy zmiennych ścieżkach odkształcenia, PLASMET/2012, VIII Seminarium Naukowe Zintegrowane Studia Podstaw Deformacji Plastycznej Metali, 2012-11-20/11-23, Łańcut (PL), pp.x1-x3, 2012
34.Lumelskyy D., Rojek J., Pęcherski R., Grosman F., Tkocz M., Numerical Studies of Formability of Pre-Stretched Steel Sheet, SolMech 2012, 38th Solid Mechanics Conference, 2012-08-27/08-31, Warszawa (PL), pp.172-173, 2012
35.Nosewicz Sz., Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Kačianauskas R., Discrete Element Modelling of Solid State Sintering Process of Metal-Ceramic Composite, SolMech 2012, 38th Solid Mechanics Conference, 2012-08-27/08-31, Warszawa (PL), pp.172-173, 2012
36.Rimša V., Pilkavičius S., Kačianauskas R., Rojek J., Investigation of the Normal Contact Between Two Spherical Particles with Interface Material, SolMech 2012, 38th Solid Mechanics Conference, 2012-08-27/08-31, Warszawa (PL), pp.210-211, 2012
37.Rojek J., Nosewicz S., Pietrzak K., Chmielewski M., Kaliński D., Modelling of powder sintering using the discrete element method, CMM 2011, 19th International Conference on Computer Methods in Mechanics, 2011-05-09/05-12, Warszawa (PL), pp.241-1-2, 2011
38.Karlis G., Malinowski L., Beer G., Rojek J., Iterative coupling of DEM-BEM regions with an overlapping FEM zone, 8th Workshop on Fast Boundary Element Methods in Industrial Applications, 2010-09-30/10-03, Söllerhaus (DE), 2010
39.Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Discrete element modelling of powder sintering, SolMech 2010, 37th Solid Mechanics Conference, 2010-09-06/09-10, Warszawa (PL), pp.64-65, 2010
40.Rojek J., Pietrzak K., Chmielewski M., Kaliński D., Constitutive modelling of sintering processes using the discrete element method, Workshop on Constitutive Modeling in Applications for Industrial Processes, 2010-09-01/09-03, Kraków (PL), pp.1-2, 2010