| 1. |
- Suarez, Laura, 1991-
(författare)
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Mechanical Characterization of Heterogeneous Brittle Materials
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Comminution is one of the highest energy-consuming processes in the mining and mineral processing industry by consuming around 2% of the global generated energy with an overall efficiency of 1-3%. Different approaches to the optimization of processes have been developed, but there is still room for improvement. The macro events where energy is mostly spent require numerical methods, so an overall optimization of the system is performed by the analysis and optimization of individual subsystems, such as machines and material to be crushed. The challenge when applying numerical analysis lies on the calibration of the models with mechanical parameters inherent to the constitutive laws and physics of the system. It has been seen that mineral material is exposed to a great variety of time dependent forces within the process. A baseline to understanding the interaction of the material with the machines is the analysis of fracture processes under different loading conditions. This thesis focuses on the mechanical characterization of manganese slag core material for the development, calibration and validationof constitutive models via direct and indirect measurements of the strength and fracture behavior. Diametrial and axial compressive tests under quasi-static and dynamic conditions were used by the hand of optical techniques to obtain information about the evolution of damage. Digital image correlation in 2D and 3D was implemented, considering that it is a method virtually independent ofthe geometry, size, material and deformation rate. Quasi-static tests on both Brazilian disc and unconfined axial compression configurations exposed a mechanical behavior of composite-like material where random failure of the components caused high variability of the elastic parameters. Irreversible damage was perceived globally as non-linearities in load-strain curves, while cyclic loading revealed a degradation of the material affecting the elastic modulus where a weakening of the matrix and dominant behavior of the inclusions on the mechanical response is perceived. Dynamic tests were performed in an in-house built Split Hopkinson Pressure Bar which follows the wave propagation theory in the material generated by the impact of a pressure driven projectile. 2D high speed imaging was performed in order to obtain informationabout the crack initiation and fracture process so that a sampling frequency of 380,000 fps and 663,200 fps was obtained for axially and diametrically loaded samples, respectively. Full-field deformations showed a staggered fracture process were on set failure points vary due to the internal events happening in the material. Localized frictional occurrences and inertial effects acting in the pre-cracked matrix have a strong effect on the global mechanical response and, therefore, a great variability of ultimate compressive and tensile strengths was found. The overall strain/loading rate dependency of the material was perceived as a general increase of the UCT and maximum load compared to quasi-static values. In general, the objective of this work was to study the effect of different loading conditions on the mechanical behavior and material parameters of unprocessed slag for the future development of numerical models of large-scale comminution processes.
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| 2. |
- Bahaloohoreh, Hassan, 1983-
(författare)
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Experiments and simulations on the mechanics of ice and snow
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study, experiments and simulations were conducted to investigate ice and snow. The ice sintering force as a function of temperature, pressing force (contact load), contact duration, and particle size during the primary stage of sintering was formulated using experimental methods along with an approximate, semi-analytic, close-form solution. It was shown that the ice sintering force increases nearly linear with increasing external pressing force but best approximated as a power law for dependency on both contact duration and particle size. Moreover, the exponent of the power law for size dependence is around the value predicted by general sintering theory. The temperature dependence of the sintering force is highly nonlinear and follows the Arrhenius equation. It was observed that at temperatures closer to the melting point, a liquid bridge is observed upon these paration of the contacted ice particles. The ratio of ultimate tensile strength of ice to the axial stress concentration factor in tension is found as an important factor in determining the sintering force, and a value of nearly 1.1 MPa was estimated to best catch the sintering force of ice in different conditions. From the temperature dependency, the activation energy is calculated to be around 41.4 kJ/mol, which is close to the previously reported value. Also, the results for the sintering force suggest that smaller particles are “stickier” than larger particles. Moreover, cavitation and surface cracking is observed during the formation of the ice particles and these can be one of the sources for the variations observed in the measured ice sintering force values.The presence of a capillary bridge in contact between an ice particle and a "smooth" (or rough) Aluminum surface at relative humidity around 50% and temperatures below the melting point was experimentally demonstrated. Experiments were conducted under controlled temperature conditions and the mechanical instability of the bridge upon separation of the ice particle from the Aluminum surface with a constant speed was considered. It was observed that a liquid bridge with a more pronounced volume at temperatures near the melting point is formed. It was showen that the separation distance is proportional to the cube root of the volume of the bridge. The volume of the liquidbridge is used to estimate the thickness of the liquid layer on the ice particle and the estimated value was shown to be within the range reported in the literature. The thickness of the liquid layer decreases from nearly 56 nm at -1.7◦C to 0.2 nm at -12.7◦C. The dependence can be approximated with a power law, proportional to (TM − T)−β, where β < 2.6. We further observe that for a rough surface, the capillary bridge formation in the considered experimental conditions vanishes.The Discrete Element Method (DEM) was employed to simulate the filling behavior of dry snow. Snow as a heterogeneous, hot material which is constituted from spherical ice particles which can form bonds. The bonding behavior of ice particles is important in determining the macroscopic behavior of snow. The bond diameter of ice-ice contacts as a function of time, compressive load, and strain rate is used and a DEM for dry snow was developed and programmed in MATLAB. A beam element with implemented damage model was used in the simulation. The simulated parameters were macroscopic angle of repose, packing density, and surface conditions as a function of temperature and fillingrate. The DEM results were able to verify the existing published experimental data. The simulation results showed that angle of repose of snow decreased with decreasing the temperature, the surface became irregular due to particles rotation and re-arrangement for lower falling speeds of particles, and density increased with depth of deposition.
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| 3. |
- Hammarberg, Samuel, 1988-
(författare)
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A Study on Sandwich Structures : Development, Mechanical Characterization and Numerical Modeling
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Legislative demands force the automotive industry to reduce greenhouse gas (GHG) emissions. At the same time, crashworthiness must not be compromised. A ve-hicle’s GHG emissions, such as carbon dioxide, is dependent on its fuel consump-tion. Lowering the vehicle weight, reducing fuel consumption, will therefor reduce emissions. Thus, high performance lightweight materials and structures are on demand. Several methods for achieving high-performance lightweight components are available. One of the most successful approaches has been replacing mild steels with press-hardened steels, e.g. ultra high strength steels (UHSS). In the press-hardening process, a low-alloyed boron steel blank is austenitized followed by simultaneously forming and cooling. By controlling cooling rates, a martensitic microstructure can be obtained, resulting in components with superior properties compared to mild steels. Other methods of achieving lightweight components in-clude the usage of sandwich structures where stiff skins are bonded to a low-density core. In the present thesis, several types of sandwich structures are studied both numerically and experimentally. A UHSS sandwich with a bidirectionlly corru-gated core, intended for stiffness application, is manufactured and evaluated in three-point bending. Finite element models are utilized to recreate the three-point bend test. A large amount of finite elements are required for precise discretization of the core. The number of finite elements are reduced by replacing the sandwich with an homogeneous, equivalent model with input data obtained from analyzing representative volume elements (RVEs) of the core, subjected to periodic and ho-mogeneous boundary conditions. Good agreement is found between experiments and finite element models. A UHSS sandwich with a partly perforated core is evaluated numerically for energy absorption applications. Several hole configu-rations for the core are evaluated with respect to specific energy absorption. A fracture criterion is utilized for the sandwich skins. Computational time is re-duced by homogenization of the core using a stress-resultant based constitutive model. It is found that the sandwich concept allows for an increase in specific energy absorption and that the computational time can be reduced while still be-ing able to predict energy absorption. An experimental methodology is developed for mechanical characterization of micro-sandwich materials. Tools are developed for loading the micro-sandwich in out-of-plane tension and shear, where digital image correlation is used for measuring displacements fields and fracture of the micro-sandwich core. Statistical methods are adopted for analyzing the variation in the mechanical properties of the micro-sandwich from which statistical means may be obtained. The experimental data is used as input for constitutive models, simulating the micro-sandwich material subjected to peeling, using a T-peel test. The numerical models are validated against experiments, found to agree within one standard deviation, suggesting that the experimental methodology produces robust data.The present work has thus presented methods, further increasing the usability of UHSS with regard to lightweighting, and explored how such components may be simulated numerically with adequate accuracy and reasonable computation time. Furthermore, the present thesis contributes by presenting methods for character-izing micro-sandwich materials, including statistical methods for analyzing scatter in mechanical properties, and how such sandwich materials may be modeled, tak-ing elasto-plasticity and damage into account. These results opens up possibilities for further development and optimization of lightweight constructions.
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| 4. |
- Hammarberg, Samuel, 1988-, et al.
(författare)
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Calibration of orthotropic plasticity- and damage models for micro-sandwich materials
- 2022
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Ingår i: SN Applied Sciences. - : Springer Nature. - 2523-3963 .- 2523-3971. ; 4:6
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Tidskriftsartikel (refereegranskat)abstract
- Sandwich structures are commonly used to increase bending-stiffness without significantly increasing weight. In particular, micro-sandwich materials have been developed with the automotive industry in mind, being thin and formable. In the present work, it is investigated if micro-sandwich materials may be modeled using commercially available material models, accounting for both elasto-plasticity and fracture. A methodology for calibration of both the constitutive- and the damage model of micro-sandwich materials is presented. To validate the models, an experimental T-peel test is performed on the micro-sandwich material and compared with the numerical models. The models are found to be in agreement with the experimental data, being able to recreate the force response as well as the fracture of the micro-sandwich core.
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| 5. |
- Hammarberg, Samuel, 1988-, et al.
(författare)
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Novel Methodology for Experimental Characterization of Micro-Sandwich Materials
- 2021
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 14:16
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Tidskriftsartikel (refereegranskat)abstract
- Lightweight components are in demand from the automotive industry, due to legislation regulating greenhouse gas emissions, e.g., CO2. Traditionally, lightweighting has been done by replacing mild steels with ultra-high strength steel. The development of micro-sandwich materials has received increasing attention due to their formability and potential for replacing steel sheets in automotive bodies. A fundamental requirement for micro-sandwich materials to gain significant market share within the automotive industry is the possibility to simulate manufacturing of components, e.g., cold forming. Thus, reliable methods for characterizing the mechanical properties of the micro-sandwich materials, and in particular their cores, are necessary. In the present work, a novel method for obtaining the out-of-plane properties of micro-sandwich cores is presented. In particular, the out-of-plane properties, i.e., transverse tension/compression and out-of-plane shear are characterized. Test tools are designed and developed for subjecting micro-sandwich specimens to the desired loading conditions and digital image correlation is used to qualitatively analyze displacement fields and fracture of the core. A variation of the response from the material tests is observed, analyzed using statistical methods, i.e., the Weibull distribution. It is found that the suggested method produces reliable and repeatable results, providing a better understanding of micro-sandwich materials. The results produced in the present work may be used as input data for constitutive models, but also for validation of numerical models.
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| 6. |
- Hammarberg, Samuel, 1988-, et al.
(författare)
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Numerical evaluation of lightweight ultra high strength steel sandwich for energy absorption
- 2020
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Ingår i: SN Applied Sciences. - : Springer. - 2523-3963 .- 2523-3971. ; 2:11
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Tidskriftsartikel (refereegranskat)abstract
- Legislation regarding greenhouse gas emissions forces automotive manufacturers to bring forth new and innovative materials and structures for weight reduction of the body-in-white. The present work evaluates a lightweight ultra high strength steel sandwich concept, with perforated cores, for energy absorption applications. Hat-profile geometries, subjected to crushing, are studied numerically to evaluate specific energy absorption for the sandwich concept and solid hat-profiles of equivalent weight. Precise discretization of the perforated core requires large computational power. In the present work, this is addressed by homogenization, replacing the perforated core with a homogeneous material with equivalent mechanical properties. Input data for the equivalent material is obtained by analyzing a representative volume element, subjected to in-plane loading and out-of-plane bending/twisting using periodic boundary conditions. The homogenized sandwich reduces the number of finite elements and thereby computational time with approximately 95%, while maintaining accuracy with respect to force–displacement response and energy absorption. It is found that specific energy absorption is increased with 8–17%, when comparing solid and sandwich hat profiles of equivalent weight, and that a weight saving of at least 6% is possible for equivalent performance.
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| 7. |
- Hammarberg, Samuel, 1988-, et al.
(författare)
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Ultra high strength steel sandwich for lightweight applications
- 2020
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Ingår i: SN Applied Sciences. - : Springer Nature. - 2523-3963 .- 2523-3971. ; 2:6
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Tidskriftsartikel (refereegranskat)abstract
- Methods for reducing weight of structural elements are important for a sustainable society. Over the recent years ultra high strength steel (UHSS) has been a successful material for designing light and strong components. Sandwich panels are interesting structural components to further explore areas where the benefits of UHSS can be utilized. The specific properties of sandwich panels make them suitable for stiffness applications and various cores have been studied extensively. In the present work, bidirectionally corrugated UHSS cores are studied experimentally and numerically. A UHSS core is manufactured by cold rolling and bonded to the skins by welding. Stiffness is evaluated experimentally in three-point bending. The tests are virtually reproduced using the finite element method. Precise discretization of the core requires large amounts of computational power, prolonging lead times for sandwich component development, which in the present work is addressed by homogenization, using an equivalent material formulation. Input data for the equivalent models is obtained by characterizing representative volume elements of the periodic cores under periodic boundary conditions. The homogenized panel reduces the number of finite elements and thus the computational time while maintaining accuracy. Numerical results are validated and agree well with experimental testing. Important findings from experimental and simulation results show that the suggested panels provide superior specific bending stiffness as compared to solid panels. This work shows that lightweight UHSS sandwiches with excellent stiffness properties can be manufactured and modeled efficiently. The concept of manufacturing a UHSS sandwich panel expands the usability of UHSS to new areas.
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| 8. |
- Jonsén, Pär, 1971-, et al.
(författare)
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A Particle Based Modelling Approach for Predicting Charge Dynamics in Tumbling Ball Mills
- 2018
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Ingår i: ABSTRACTS. - : IACM. ; , s. 1385-1385
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Konferensbidrag (refereegranskat)abstract
- Wet grinding of minerals in tumbling mills is a highly important process in the mining industry. During grinding in tumbling mills, lifters submerge into the charge and create motions in the ball charge, the lifters is exposed for impacts and shear loads that will wear down the lifters. Increased loading can accelerate the wear and the lining has to be replaced. Replacing the lining is an expensive and time consuming operation that is preferred to be done within planned maintenance stops. Prediction of the charge motion and wear rate for different grinding operations and linings are therefore desirable to predict the lining life. Modelling of wet grinding in tumbling mills that include pulp fluid and its interaction with both the grinding balls and the mill structure is an interesting challenge and some different approaches have been suggested, see [1-2]. For an effective and successful prediction, the numerical model has to be able to handle the pulp fluid and its simultaneous interactions with both the ball charge and the mill structure, in a computationally efficient approach. In this work, the pulp fluids are modelled with a Lagrange based method called incompressible computational fluid dynamics, (ICFD), which gives the opportunity to model free surface flow. This method gives robustness and stability to the fluid model and is efficient as it gives possibility to use larger time steps than the conventional CFD. The ICFD solver can be coupled to other solvers as in this case the finite element method, (FEM) solver for the mill structure and the discrete element method (DEM) solver for the ball charge. The combined ICFD-DEM-FEM model can predict both charge motion and responses from the mill structure, as well as the pulp liquid flow and pressure. The numerical grinding case presented here is validated against experimentally measured driving torque signatures from an instrumented small-scale batch ball mill, see [3]. This approach opens up the possible to predict the volume of the high-energy zone and optimise lifter design and operating conditions. The ICFD solver improve efficiency and robustness for studying wet grinding in tumbling mill systems and can predict the charge dynamics and the wear distribution in such systems. References[1] Jonsén, P. et al., (2018). Preliminary validation of a new way to model physical interactions between pulp, charge and mill structure in tumbling mills. Minerals Enginering. Accepted for publication[2] Jonsén, P., Stener, J.F., Pålsson, B.I. and Häggblad, H.-Å., (2015). Validation of a model for physical interactions between pulp, charge and mill structure in tumbling mills. Minerals Engineering, Vol. 73, 77–84.[3] Jonsén, P. Stener, J. F. Pålsson, B. I. and Häggblad, H.-Å., (2013). Validation of tumbling mill charge induced torque as predicted by simulations. Minerals and Metallurgical Processing, vol. 30, No. 4, 220-225.
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| 9. |
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| 10. |
- Larsson, Simon, PhD, et al.
(författare)
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A Novel Particle-Based Approach for Modeling a Wet Vertical Stirred Media Mill
- 2021
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Ingår i: Minerals. - : MDPI. - 2075-163X. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Modeling of wet stirred media mill processes is challenging since it requires the simultaneous modeling of the complex multiphysics in the interactions between grinding media, the moving internal agitator elements, and the grinding fluid. In the present study, a multiphysics model of an HIG5 pilot vertical stirred media mill with a nominal power of 7.5 kW is developed. The model is based on a particle-based coupled solver approach, where the grinding fluid is modeled with the particle finite element method (PFEM), the grinding media are modeled with the discrete element method (DEM), and the mill structure is modeled with the finite element method (FEM). The interactions between the different constituents are treated by loose (or weak) two-way couplings between the PFEM, DEM, and FEM models. Both water and a mineral slurry are used as grinding fluids, and they are modeled as Newtonian and non-Newtonian fluids, respectively. In the present work, a novel approach for transferring forces between grinding fluid and grinding media based on the Reynolds number is implemented. This force transfer is realized by specifying the drag coefficient as a function of the Reynolds number. The stirred media mill model is used to predict the mill power consumption, dynamics of both grinding fluid and grinding media, interparticle contacts of the grinding media, and the wear development on the mill structure. The numerical results obtained within the present study show good agreement with experimental measurements.
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