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  • 1.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Multi-axial fatigue initiation at inclusions and subsequent crack growth in a bainitic high strength roller bearing steel at uniaxial experiments2012In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 41, p. 130-139Article in journal (Refereed)
    Abstract [en]

    The behaviour of inclusion initiated fatigue was studied for a high strength bearing steel with a bainite micro-structure. The analysis included experiments and numerical simulations. It was realized that the stress-state was multi-axial in the matrix material that met the inclusion also for a uniaxial far field stress. Fatigue initiation risk at the interface between the inclusion and matrix material was therefore predicted with the Findley multi-axial critical plane criterion. The fatigue parameters were determined from independent experiments on smooth specimens with tensile surface stress gradients. Crack growth from the inclusion to final rupture was modelled as a penny shaped crack with closure compensated effective material parameters. The growth simulations suggested that the majority of the fatigue life was consumed as fatigue crack initiation at the non-metallic inclusion.

  • 2.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Wåtz, Veronica
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fatigue crack initiation and growth at holes in a high strength bainitic roller bearing steel when loaded with non-proportional shear and compressive cycles2011In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 33, no 9, p. 1244-1256Article in journal (Refereed)
    Abstract [en]

    Fatigue initiation from an artificial defect was investigated for a bainitic high strength roller bearing steel. Thin walled pipe specimens with small holes were subjected to multi-axial and non-proportional load cycles. The experimental fatigue crack positions around the hole were predicted with the Findley critical plane criterion. The criterion also ranked the severeness of three load sequences with respect to fatigue risk. Crack growth simulations and crack life measurements with strain gauges confirmed the ranking between the load sequences. Three uni-axial fatigue series with stress gradients were used to determine surface endurance data for the Findley criterion.

  • 3.
    Larsson, Per Lennart
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Elastic-plastic contact between hard metal particles2016In: Key Engineering Materials, 2016, p. 86-99Conference paper (Refereed)
    Abstract [en]

    In the present study contact between elastic-plastic dissimilar spherical particles are investigated. The investigation is based on analytical and numerical methods and in the latter case in particular the finite element method. The results presented are pertinent to force-displacement relations at contact when elastic and plastic deformations are of equal magnitude. Especially, hard metal particles are considered with a typical application area being analysis of powder compaction.

  • 4.
    Larsson, Per-Lennart
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A numerical study of the mechanical behavior at contact between particles of dissimilar elastic-ideally plastic materials2015In: Journal of Physics and Chemistry of Solids, ISSN 0022-3697, E-ISSN 1879-2553, Vol. 77, p. 92-100Article in journal (Refereed)
    Abstract [en]

    In the present study contact between elastic-ideally plastic dissimilar spheres are investigated in detail. The investigation is based on numerical methods and in particular the finite element method. The numerical results presented are discussed with respect to correlation of global contact properties as well as the behavior of local field variables such as contact pressure distribution and the evolution of the effective plastic strain. Large deformation effects are accounted for and discussed in detail. The constitutive behavior is described by classical Mises plasticity. It is shown that correlation of the dissimilar contact problem can be accurately achieved based on the Johnson contact parameter with the representative stress chosen as the yield stress of the softer material.

  • 5.
    Larsson, Per-Lennart
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Contact between elastic-plastic hardmetal powder particles2015In: Proceedings Euro PM 2015: International Power Metallurgy Congress and Exhibition, European Powder Metallurgy Association , 2015Conference paper (Refereed)
    Abstract [en]

    In the present study contacts between elastic-plastic dissimilar spherical particles are investigated. The investigation is based on analytical and numerical methods and in the latter case in particular the finite element method. The results presented are pertinent to force-displacement relations at contact when elastic and plastic deformations are of equal magnitude. Especially, hard metal particles are considered with a typical application area being analysis of powder compaction.

  • 6.
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Micromechanics of Powder Compaction2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Compaction of powders followed by sintering is a convenient manufacturing method for products of complex shape and components of materials that are difficult to produce using conventional metallurgy. During the compaction and the handling of the unsintered compact, defects can develop which could remain in the final sintered product. Modeling is an option to predict these issues and in this thesis micromechanical modeling of the compaction and the final components is discussed. Such models provide a more physical description than a macroscopic model, and specifically, the Discrete Element Method (DEM) is utilized.

    An initial study of the efect of particle size distribution, performed with DEM, was presented in Paper A. The study showed that this effect is small and is thus neglected in the other DEM studies in this thesis. The study also showed that good agreement with experimental data can be obtained if friction effects is correctly accounted for.

    The most critical issue for accurate results in the DEM simulations is the modeling of normal contact between the powder particles. A unified treatment of this problem for particles of a strain hardening elastic-plastic material is presented in Paper B. Results concerning both the elastic-plastic loading, elastic unloading as well as the adhesive bonding between the particles is included. All results are compared with finite element simulation with good agreement with the proposed model.

    The modeling of industry relevant powders, namely spray dried granules is presented in Paper C. The mechanical behavior of the granules is determined using two types of micromechanical experiments, granule compression tests and nanoindentation testing. The determined material model is used in an FEM simulation of two granules in contact. The resulting force-displacement relationships are exported to a DEM analysis of the compaction of the granules which shows very good agreement with corresponding experimental data.

    The modeling of the tangential forces between two contacting powder particles is studied in Paper D by an extensive parametric study using the finite element method. The outcome are correlated using normalized parameters and the resulting equations provide the tangential contact force as function of the tangential displacement for different materials and friction coefficients.

    Finally, in Paper E, the unloading and fracture of powder compacts, made of the same granules as in Paper C, are studied both experimentally and numerically. A microscopy study showed that fracture of the powder granules might be of importance for the fracture and thus a granule fracture model is presented and implemented in the numerical model. The simulations show that incorporating the fracture of the granules is essential to obtain agreement with the experimental data.

     

  • 7.
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Micromechanics of Powder Compaction and Particle Contact2013Licentiate thesis, comprehensive summary (Other academic)
  • 8.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.). KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Jelagin, Denis
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    A contact model for the normal force between viscoelastic particles in discrete element simulations2019In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 342, p. 985-991Article in journal (Refereed)
    Abstract [en]

    DEM modeling of granular materials composed of viscoelastic particles can provide valuable insights into the mechanical behavior of a wide range of engineering materials. In this paper, a new model for calculating the normal contact force between visoelastic spheres is presented based on contact mechanics that takes the mechanical behavior of the DEM particles into account. The model relies on an application of the viscoelastic correspondence principle to elastic Hertz contact. A viscoelastic relaxation function for the contact is defined and a generalized Maxwell material is used for describing this function. An analytical expression for the increment in contact force given an increment in overlap is derived leading to a computationally efficient model. The proposed model provides the analytical small deformation solution upon loading but provides an approximate solution at unloading. Comparisons are made with FEM simulations of contact between spheres of different sizes of equal and dissimilar materials. An excellent agreement is found between the model and the FEM simulations for almost all cases except at cyclic loading where the characteristic times of the viscoelastic behavior and the loading are similar.

  • 9.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Jelagin, Denis
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Forquin, P. A.
    Computational framework for analysis of contact-induced damage in brittle rocks2019In: International Journal of Solids and StructuresArticle in journal (Refereed)
    Abstract [en]

    This paper presents a numerical approach for predicting damage in rock materials caused by contact loading. The rock material is modelled using a constitutive description that combines pressure dependent plasticity, for capturing shear deformation under high confining pressure, with an anisotropic damage model for capturing mode I cracking in tension. Material parameters for the model are taken from a recently performed investigation on a granite material. The model has been used to simulate two types of contact loading experiments from the literature, cyclic loading and monotonic loading up to fracture. In order to achieve accurate predictions, the model has been extended to account for small loaded volumes which occur at contact loading. The results show that the main damage mechanism at cyclic loading is crack propagation due to Hertzian stresses whereas in the monotonic experiments sub-surface cracks could initiate. All features measured in the contact loading experiments are captured by the model and hence, the modelling framework is judged to be able to capture contact damage if real stone geometries are studied in FEM.

  • 10.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Jelagin, Denis
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Partl, Manfred
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    New discrete element framework for modelling asphalt compaction2019In: International Journal on Road Materials and Pavement Design, ISSN 1468-0629, E-ISSN 2164-7402Article in journal (Refereed)
    Abstract [en]

    During asphalt mixture compaction, loads in the material are mainly transferred through contact between the stones and the interaction between the stones and the binder. The behaviour of such materials is suitable to model using the Discrete Element Method (DEM). In this study, a new DEM modelling approach has been developed for studying the asphalt compaction process, incorporating contact and damage laws based on granular mechanics. In the simulations, aggregate fracture is handled by a recently developed method of incorporating particle fracture in DEM, based on previously performed fracture experiments on granite specimens. The binder phase is modelled by adding a viscoelastic film around each DEM particle. This surface layer has a thickness that obtains the correct volume of the binder phase and has mechanical properties representative for the binder at different temperatures. The ability of the model to capture the influence of mixture parameters on the compactability and the eventual stone damage during compaction is examined for the cases of compaction flow test and gyratory compaction. Explicitly, the influence of different aggregate gradations, mixture temperatures and binder properties are studied. The results show that the proposed DEM approach is able to capture qualitatively and quantitatively responses in both cases and also provide predictions of aggregate damage. One large benefit with the developed modelling approach is that the influence of different asphalt mixture parameters could be studied without re-calibration of model parameters. Furthermore, based on comparative DEM simulations, it is shown that the proposed approach provides more realistic force distribution networks in the material.

  • 11.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A numerical analysis of cold powder compaction based on micromechanical experiments2013In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 243, p. 71-78Article in journal (Refereed)
    Abstract [en]

    The discrete element method (DEM) is used for predicting the compaction behavior of two types of spray dried cemented carbide granules. The material model of the granules is determined by micromechanical experiments. First, compression tests are performed on single granules giving information of the deformation behavior at relatively small deformations. For larger deformations, nanoindentation tests are performed to give further information of the constitutive behavior indicating a strong hardening behavior at high strains. The material model is implemented in an FE model of two particles in contact and the relation between contact force and indentation depth is exported to a DEM program. The DEM program is used to simulate presently performed uniaxial die compaction experiments where the geometry of the die is taken into account. Excellent agreement is found between the experiments and the numerical predictions in the range where results from DEM simulations are valid.

  • 12.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A unified correlation model describing global properties at elastoplastic sharp indentation contact2016In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 103, p. 252-260Article in journal (Refereed)
    Abstract [en]

    The correlation of sharp indentation testing is analyzed and discussed based on present and previous numerical findings. In particular, correlation of two global indentation properties, hardness and relative contact area, at sharp indentation of classical elastic-plastic materials is discussed in detail. It is shown that the results cannot in general, and in particular at high strain hardening, be correlated accurately using a single parameter, comprising both geometrical and mechanical properties, with the uniaxial stress-strain curve represented by the flow stress at a representative value on the effective plastic strain. Instead, also the level of the plastic hardening must be considered. Based on this finding, a unified model for correlation of global properties is presented for cone indentation of elastoplastic materials.

  • 13.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A Numerical Analysis of Cold Powder Compaction Based on Micromechanical Experiments2013Report (Other academic)
  • 14.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A unified model for the contact behaviour between equal and dissimilar elastic-plastic spherical bodies2016In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 81, p. 23-32Article in journal (Refereed)
    Abstract [en]

    A unified method for calculating the contact force and the contact area between two dissimilar elastic-plastic spheres is presented with the aim of simulating granular materials using particle methods. Explicit equations are presented for the case when the plastic behaviour of the spheres is described with three material parameters. This makes the analysis applicable for a wide range of materials. The model is partly based on dimensionless quantities emerging from the Brinell hardness test. Large deformation of the contact is accounted for in the analysis, which allows for accurate contact relations up to indentation depths relevant for powder compaction. The presented model shows excellent agreement with finite element simulations of two spheres in contact and the results found in literature. An implementation of the contact model in Python is provided together with the online version of this paper.

  • 15.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Effect of particle size distribution at powder compaction2011In: Proceedings of the Euro International Powder Metallurgy Congress and Exhibition, Euro PM 2011, European Powder Metallurgy Association (EPMA) , 2011, Vol. 3, p. 265-270Conference paper (Refereed)
    Abstract [en]

    The Discrete Element Method (DEM) has been used to investigate the effects of particle size distribution during closed die compaction of powders. In the simulations, the effect of considering rotational degrees of freedom in the analysis is studied. Both loading and unloading of the powder compound is investigated and the particle size distribution is assumed to be a truncated normal distribution. The powder particles are assumed rigid plastic in compression and following Hertz elastic contact law during unloading. From a convergence study, it was found that 4000 particles were sufficient to represent the global properties of the powder compact. Among other things, the results show that when the size of the particles does not deviate considerably from the average size, the effect on global compaction properties is basically negligible.

  • 16.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Micromechanical Investigation of the Fracture Behavior of Powder Materials2015In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 286, p. 31article id 11185Article in journal (Refereed)
    Abstract [en]

    Fracture of compacted powder has been studied experimentally and numerically using a micromechanicalapproach. In the experimental investigation, the compacts are crushed in two dierent directions to accountfor general stress states and a microscopy study shows that fracture of the powder granules plays asignicant role in the fracture process. The numerical analysis is based on the Discrete Element Method(DEM) and a novel approach is presented to account for the fracture of the particles in the numericalmodel. The force-displacement relations for two particles in contact, which are needed in DEM, are derivedusing micomechanical experiments together with nite element analyses of the contact problem. The contactmodel accounts for plastic compression, elastic unloading and adhesive bonding together with frictionand tangential bonding. The model shows a very good agreement with the experimental data both for theelastic behavior during unloading and, if failure of the particles is accounted for, the fracture of the compacts.

  • 17.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Micromechanics of green body fracture2014In: Euro PM 2014 Congress and Exhibition, Proceedings, 2014Conference paper (Refereed)
    Abstract [en]

    The typical production route of powder materials is cold compaction of the powder, to a so called green body, followed by sintering which gives the component its full strength. It is of vital importance that the green body is crack-free as defects will remain as weak zones in the final component. For this reason, the mechanical properties of the green body have presently been studied mostly experimentally but also some aspects of numerical modeling will be discussed. The materials studied are two types of spray dried cemented carbide powders which has been uniaxially compacted to different green densities. The compacts have thereafter been crushed in two different directions and a microscope study of the fracture surfaces has been performed in order to investigate the micromechanics of the fracture process. The study shows that both breaking of inter-particle contacts and fracture of the individual particles is important in order to describe the fracture processes. Finally, modeling of the fracture process using the Discrete Element Method (DEM) is discussed.

  • 18.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    On force-displacement relations at contact between elastic-plastic adhesive bodies2013In: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 61, no 5, p. 1185-1201Article in journal (Refereed)
    Abstract [en]

    The loading-unloading of dissimilar adhesive elastic-plastic bodies is studied both analytically and numerically, including elastic-ideal plastic and deformation hardening behavior. The contacting bodies are assumed to be spherical in the region of contact and consequently the presented model is partly based on results pertinent to Brinell indentation. The problem of adhesive unloading is solved in two steps; first the unloading in the absence of adhesion is studied and then an adhesive pressure term is added. The analytical model is derived using fracture mechanics arguments and is based on one parameter, the fracture energy. The model is finally verified with finite element simulations by introducing a cohesive behavior between the modeled spheres. The analytical model shows very good agreement with the FE-simulations both during loading and unloading and also concerning the case of force and displacement at separation.

  • 19.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    On the Appropriate Use of Representative Stress Quantities at Correlation of Spherical Contact Problems2013In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 50, no 2, p. 221-232Article in journal (Refereed)
    Abstract [en]

    Correlation of contact problems is discussed in a detailed manner with focus on spherical contact. The finite element method is used to determine appropriate stress quantities, representative stresses, aiming at a general description of contact quantities such as mean contact pressure, and the size of the contact area. It is shown that the mean contact pressure can be well described by a single master curve, while this is not so for the size of the contact area. The latter feature is explained partly by a pronounced effect from elastic deformation, but is also shown that large deformation effects can have a substantial influence on correlation attempts. The analysis is restricted to classical Mises elastoplasticity, but the results can also serve as a guideline for similar attempts when using more advanced constitutive modeling. An obvious application of the present results concerns material characterization by indentation testing.

  • 20.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    On the effect of particle size distribution in cold powder compaction2012In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 79, no 5, p. 051017-Article in journal (Refereed)
    Abstract [en]

    The effect of particle size distribution in powder compaction has been studied using the discrete element method. Both isostatic compaction and closed die compaction are studied together during the entire loading process. Particle rotation and frictional effects are accounted for in the analysis. The particles are, constitutively described by rigid plasticity, assumed to be spherical with the size of the radii that follows a truncated normal distribution. The results show that size distribution effects are small on global compaction properties like compaction pressure if the size distribution is small. Furthermore, the size distribution had no influence at all on the macroscopic behavior at unloading. To verify the model, comparisons were made on two different sets of experiment found in the literature where the particles were of varying sizes. Good agreement was found both on fundamental properties like the average number of contacts per particle and on more important properties from a practical point of view, like the compaction pressure.

  • 21.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    On the tangential contact behavior at elastic–plastic spherical contact problems2014In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 319, no 1/2, p. 110-117Article in journal (Refereed)
    Abstract [en]

    The problem of tangential contact between an elastic-plastic sphere and a rigid plane is studied analytically and numerically with the specific aim to derive force-displacement relations to be used in numerical simulations of granular materials. The simulations are performed for both ideal-plastic and strain hardening materials with different yield stresses and including large deformation effects in order to draw general conclusions. The results are correlated using normalized quantities pertinent to the correlation of indentation testing experiments leading to a general description of the tangential contact problem. Explicit formulas for the normal and tangential forces are presented as a function of the tangential displacement using data that are easily available from axi-symmetric analyses of spherical contact. The proposed model shows very good agreement when compared with the FE-simulations.

  • 22.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Study of springback of green bodies using micromechanical experiments and the discrete element method2013In: Particle-Based Methods III: Fundamentals and Applications - Proceedings of the 3rd International Conference on Particle-based Methods Fundamentals and Applications, Particles 2013, 2013, p. 590-601Conference paper (Refereed)
    Abstract [en]

    The Discrete Element Method (DEM) is today a commonly used tool to simulate compaction of particulate media. The main issue when using DEM in compaction problems is the description of the contact between two powder particles. If the material properties are known, analytical and semi-analytical methods can be used [1, 2] but for many industrial applications, for instance spray dried granules, the mechanical behaviour is unknown. The compaction behaviour and green properties of a cemented carbide powder is studied in this work and the issue of the contact description is solved by performing experiments on the powder granules. Firstly, compression tests are made on the single granules giving information of the mechanical properties at low strains. To get information at high strains, which are needed in powder compaction simulations, nanoindentation tests are performed. The measured material parameters are used in a FE model of two spheres in contact and the resulting contact law is exported to a DEM program. The DEM program is used to investigate the compaction properties of a powder compact and especially the springback during unloading which is important for predicting the final shape of the product. The results are compared with presently performed experiments and the applicability range of the discrete element simulations will be discussed.

  • 23.
    Olsson, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olander, Anton
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Öberg, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Fatigue of gears in the finite life regime: Experiments and probabilistic modelling2016In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 62, p. 286-286Article in journal (Refereed)
    Abstract [en]

    Fatigue of case hardened gears is investigated experimentally and numerically with focus on the finite fatigue life regime. Pulsating tooth bending fatigue experiments are performed at different load levels on two types of gears of different sizes to determine load–fatigue life relations. The experiments are compared with a probabilistic model for the finite life regime based on weakest-link theory. The stress fields, needed in the evaluations, are obtained by finite element simulations taking residual stresses, both due to case hardening and plastic deformation, into account. The stress history at each element is summarized into two different effective fatigue stress measures; one based on the largest principal stress and the Findley multiaxial fatigue stress. The material parameter needed in the Findley stress is determined by a linear correlation of the parameter with the Vickers hardness of the material using multiaxial fatigue data found in the literature. Both equivalent stress measures show equal behaviour and the probabilistic model shows good agreement with the experimental data in the finite fatigue life regime.

  • 24.
    Staf, Hjalmar
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lindskog, P.
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    On rate-dependence of hardmetal powder pressing of cutting inserts2017In: Powder Metallurgy, ISSN 0032-5899, E-ISSN 1743-2901, Vol. 60, no 1, p. 7-14Article in journal (Refereed)
    Abstract [en]

    The rate-dependence of hardmetal powder pressing in cutting insert production is investigated experimentally and numerically. In the latter case, the finite element method is relied upon using a continuum mechanics approach. In particular, possible rate-dependency due to creep deformation and rate-dependent friction is discussed with the experimental investigation focusing mainly on dimensional changes during sintering but also pressing forces. The results indicate that rate-dependent frictional effects are the dominating feature and accordingly, it can be argued that for the metal powders investigated here, creep deformations do not have to be accounted for in the constitutive description at the timescales relevant for powder pressing and when the shape after sintering is concerned. For the present powder, the apparent frictional effect decreases at higher pressing rates. Additional details of the friction behavior are studied comparing finite element simulations with experiments.

  • 25.
    Staf, Hjalmar
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). Sandvik Coromant AB, Sweden.
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Lindskog, Per
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Determination of the Frictional Behavior at Compaction of Powder Materials Consisting of Spray-Dried Granules2018In: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 27, no 3, p. 1308-1317Article in journal (Refereed)
    Abstract [en]

    The frictional behavior during powder compaction and ejection is studied using an instrumented die with eight radial sensors. The average friction over the total powder pillar is used to determine a local friction coefficient at each sensor. By comparing forces at compaction with forces at ejection, it can be shown that the Coulomb's friction coefficient can be described as a function of normal pressure. Also stick phenomena has been investigated in order to assess its influence on the determination of the local friction coefficient.

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