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  • 1.
    Abdulgadir, Alamin
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. MoRe Research.
    Norgren, Sven
    MoRe Research.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Engstrand, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Performance Simulation of a Composite Filter Material Containing Mechanical Pulp Fibers, Microfibrillated Cellulose, and Cellulose Nanofibers2022In: Proceedings of the International Mechanical Pulping Conference, 2022, p. 160-Conference paper (Other academic)
    Abstract [en]

    Nonwoven fibrous materials with reticular support of an interconnected fiber network and a tortuous airflow pathway have been commonly used in filtration applications. To meet the criteria of filter efficiency and performance, the filter materials are recommended to contain different types of fibers such as mechanical pulp fibers, microfibrillated cellulose, cellulose nanofibers, and other polymer or synthetic fibers with a range of dimensions, i.e., length and diameter. Cellulose fibers in filter media possess irregular and complex structures with hollow or collapsed lumen structures owing to their refinement or pulping method. The development of an appropriate filter media model requires information on actual fiber characteristics. In this study, a simulation method was used to investigate the complex microstructures of filter media. The physical parameters such as fiber wall thickness, diameter, length, cross-section shapes, and curliness were obtained from fiber analyzers and scanning electron microscopy. Based on the experimental findings, GeoDict database comprising different types of common fiber models was constructed. 3-Dimensional fibrous models corresponding to the wet-laid binderless filter material were generated. Using the GeoDict modules, the pore size distributions, average pore sizes, air permeability, pressure drop and initial filter efficiency simulations were performed. The simulation results appear to be in close agreement with the experimental results. The incorporation of cellulose nanofibers resulted in reduced average pore sizes and air permeability of the filter material, thus enhancing the initial filter efficiency. The filter media developed a biobased material derived from pulp fibers for advanced applications such as medical facemask, and air filtration purposes.

  • 2.
    Edvardsson, Sverker
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    Gulliksson, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    The dynamical functional particle method: an approach for boundary value problems2012In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 79, no 2, p. art. no. 021012-Article in journal (Refereed)
    Abstract [en]

    The present work is concerned with new ideas of potential value for solving differential equations. First, a brief introduction to particle methods in mechanics is made by revisiting the vibrating string. The full case of nonlinear motion is studied and the corresponding nonlinear differential equations are derived. It is suggested that the particle origin of these equations is of more general interest than usually considered. A novel possibility to develop particle methods for solving differential equations in a direct way is investigated. The dynamical functional particle method (DFPM) is developed as a solution method for boundary value problems. DFPM is based on the concept of an interaction functional as a dynamical force field acting on quasi particles. The approach is not limited to linear equations. We exemplify by applying DFPM to several linear Schrödinger type of problems as well as a nonlinear case. It is seen that DFPM performs very well in comparison with some standard numerical libraries. In all cases, the convergence rates are exponential in time. © 2012 American Society of Mechanical Engineers.

  • 3.
    Fallahjoybari, Nima
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Engberg, Birgitta A.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Large eddy simulation of turbulent pulp flow in a channel2020In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 285, article id 104409Article in journal (Refereed)
    Abstract [en]

    Large eddy simulation (LES) of turbulent flow of concentrated fiber suspension or pulp is carried out to investigate the flow and turbulence structures in a channel. The simulations are carried out for the turbulent flow of Eucalyptus pulp suspension using OpenFOAM for three fiber concentrations (c = 1.5, 2.0 and 2.5) and six different Reynolds numbers (6 <= Re-s <= 16,600). It is observed that the variation in flow regime is similar in the two lower fiber concentrations while the flow regime is highly affected by fiber concentration for c = 2.5. Visualizations of vortical structures for different Reynolds numbers and fiber concentrations are used to investigate different flow regimes. Variation of apparent viscosity with Reynolds number and fiber concentration is also presented to show its effect on the turbulent properties of fiber suspension flow. It is shown that the deviation of turbulent velocity profile from that of a Newtonian flow increases with an increase in Reynolds number and fiber concentration. Also, the extend of buffer layer increases at higher Re. Using the calculated turbulent velocity profile, the values of constant in logarithmic velocity profile is proposed for fiber suspension. Finally, a discussion is presented on the variation of turbulent intensities and Reynolds stress with Reynolds number and fiber concentration.

  • 4.
    Fallahjoybari, Nima
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Engberg, Birgitta A.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Berg, Jan-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    CFD Simulation Of Pulp Flow In Rotating And Non-Rotating Grooves2022In: Proceedings of the International Mechanical Pulping Conference, 2022, p. 24-Conference paper (Other academic)
    Abstract [en]

    The present study deals with the numerical simulation of softwood pulp flow in the rotating and non-rotating grooves in an aim to investigate the fluid flow and forces acting on a representative surface mounted in the groove. The viscosity of softwood pulp in different consistencies is available from the experimental measurements reported in the literature providing the opportunity to examine the effects of fiber consistency on the velocity and pressure distribution within the groove. The simulations are carried out in OpenFOAM for different values of gap thickness and angular velocity from which the pressure coefficient and shear forces values are obtained. It is found that the pressure increases at the stagnation point located at the gap entrance in the non-rotating groove due to tangential motion of the upper wall which induces the helical motion of the pulp flow in the groove’s cavity. However, such an effect is not observed in the rotating cavity close to the groove inlet. Meanwhile, by moving further along the channel length toward the outlet the helical motion is enhanced and an increase in the pressure is observed at the stagnation point. The shear forces over the representative surface are found to be independent of representative surface’s location and it is in the same level in the rotating and non-rotating grooves. In addition to the numerical simulations, an analytical discussion is also presented to provide a deeper understanding of pressure coefficient and shear forces variations with different parameters in the rotating and non-rotating grooves.

  • 5.
    Fallahjoybari, Nima
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Engberg, Birgitta
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Berg, Jan-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Investigation of pulp flow helicity in rotating and non-rotating grooves2021In: Proceedings of the Institution of mechanical engineers. Part E, journal of process mechanical engineering, ISSN 0954-4089, E-ISSN 2041-3009, Vol. 235, no 6, p. 2045-2058Article in journal (Refereed)
    Abstract [en]

    Numerical simulation of pulp flow in rotating and non-rotating grooves is carried out to investigate the effect of pulp rheological properties and groove geometry on the rotational motion of the pulp flow. The eucalyptus pulp suspension is considered as a working fluid in the present study whose apparent viscosity correlation is available from the experimental measurements reported in the literature. The simulations are carried out with OpenFoam for different values of pulp material, fiber concentrations, and groove cross-section. Helicity is introduced to measure the turnover rate of pulp flow in the groove due to the importance of such motion on the final properties of the pulp flow. A measurement of helicity magnitude and its distribution along the groove revealed that a change in the pulp material would significantly affect the flow structures within the groove. Further investigation on the effects of fiber concentration, c, showed that this parameter does not have a significant effect on the averaged helicity magnitude for c = 2.0 and 2.5, whereas the helicity distribution over the groove cross-section changes clearly for c = 1.5. The results showed that the helicity level is negligible for almost half of the cavity cross-section in the non-rotating groove simulations, which can be considered as a shortcoming of the original geometry of the groove. Therefore, a smaller cross-section for the groove is considered through which an enhancement in the helicity magnitude is observed. 

  • 6.
    Fallahjoybari, Nima
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Engberg, Birgitta
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Berg, Jan-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Lundstrom, T. Staffan
    Luleå Univ Technol, Div Fluid Mech, S-97187 Luleå, Sweden..
    An investigation of forces on a representative surface in a pulp flow through rotating and non-rotating grooves2023In: Journal of the Brazilian Society of Mechanical Sciences and Engineering, ISSN 1678-5878, E-ISSN 1806-3691, Vol. 45, no 5, article id 280Article in journal (Refereed)
    Abstract [en]

    Softwood pulp flow in rotating and non-rotating grooves is numerically simulated in the present study to investigate the fluid flow and the forces acting on a representative surface mounted in the groove. The viscosity of softwood pulp with various consistencies is available from the measurements reported in the literature providing the opportunity to examine the effects of fiber consistency on the velocity and pressure distribution within the groove. The simulations are carried out in OpenFOAM for different values of gap thickness, angular velocity and radial positions from which the pressure coefficient and shear forces values are obtained. It is found that the shear forces within the gap increase linearly with the angular velocity for all fiber consistencies investigated and in both grooves. Also, this behavior can be successfully predicted by modeling the gap flow as a Couette flow in a two-dimensional channel. Meanwhile, a more detailed analysis of the flow kinetic energy close to the stagnation point using Bernoulli's principle is carried out to provide a better understanding of the pressure coefficient variation with angular velocity in the non-rotating groove. A comparison of pressure coefficients obtained numerically with those calculated by considering the compression effects revealed that the comparison effects are dominating in the pulp flow within the groove.

  • 7.
    Ferritsius, Olof
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Ferritsius, Rita
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Rundlöf, Mats
    Capisco.
    Engberg, Birgitta A.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Opportunities and challenges in describing the heterogeneity of fibres2022In: Proceedings of the International Mechanical Pulping Conference, 2022, p. 28-33Conference paper (Other academic)
    Abstract [en]

    Particles in mechanical pulp are a heterogeneous popu-lation, but commonly described using averages based on wide and skewed distributions. It was found that these aver-ages may lead to erroneous conclusions regarding the char-acter of the material and also how the material has been de-veloped along the process. This study is based on measure-ments of individual particle dimensions (length, curl, and ex-ternal fibrillation) in mill operation of CTMP and TMP as detected in an optical analyser.

  • 8.
    Haller, Stefan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Cheng, Peng
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Bertilsson, Kent
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Multi-phase winding with in-conductor direct cooling capability for a 48V traction drive design2020In: Proceedings - 2020 International Conference on Electrical Machines, ICEM 2020, IEEE, 2020, p. 2118-2124Conference paper (Refereed)
    Abstract [en]

    Traction drive applications demand high power density motors with a good stator cooling design. We propose a novel multi-phase winding for a 48 V traction drive design having identical preformed hollow copper conductors with in-conductor direct cooling capability. This paper studies the cooling performance of an individual conductor phase using either EGW50/50 or water as coolant. Analytical calculations and experiments are conducted on a straight conductor of the same length using 20°C water as coolant. The results are then cross verified with those from the FEM simulations to validate the simulation setup. Then a final simulation is conducted at a current of 700A and a current density of 49.5 A/mm 2 on the preformed conductor using 65 CEGW50/50 as coolant at a pressure of 140kPa. The results highlight the exceptional performance of the cooling design which enables a power dissipation of 71OW at a maximum conductor temperature rise of only 56. 9 C. 

  • 9.
    Lindström, Stefan B
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Amjad, R.
    Gåhlin, E.
    Andersson, L.
    Kaarto, M.
    Liubytska, Kateryna
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-). Kharkiv Polytechnic Institute.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Berg, Jan-Erik
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Engberg, Birgitta A.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Nilsson, Fritjof
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-). KTH.
    Pulp Particle Classification Based on Optical Fiber Analysis and Machine Learning Techniques2024In: Fibers, ISSN 2079-6439, Vol. 12, no 1, article id 2Article in journal (Refereed)
    Abstract [en]

    In the pulp and paper industry, pulp testing is typically a labor-intensive process performed on hand-made laboratory sheets. Online quality control by automated image analysis and machine learning (ML) could provide a consistent, fast and cost-efficient alternative. In this study, four different supervised ML techniques—Lasso regression, support vector machine (SVM), feed-forward neural networks (FFNN), and recurrent neural networks (RNN)—were applied to fiber data obtained from fiber suspension micrographs analyzed by two separate image analysis software. With the built-in software of a commercial fiber analyzer optimized for speed, the maximum accuracy of 81% was achieved using the FFNN algorithm with Yeo–Johnson preprocessing. With an in-house algorithm adapted for ML by an extended set of particle attributes, a maximum accuracy of 96% was achieved with Lasso regression. A parameter capturing the average intensity of the particle in the micrograph, only available from the latter software, has a particularly strong predictive capability. The high accuracy and sensitivity of the ML results indicate that such a strategy could be very useful for quality control of fiber dispersions. 

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  • 10.
    Lindström, Stefan B
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Ferritsius, Rita
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Ferritsius, Olof
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Engberg, Birgitta A.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Multivariate lognormal mixture for pulp particle characterization2024In: Cellulose, ISSN 0969-0239, E-ISSN 1572-882X, Vol. 31, no 3, p. 1843-1854Article in journal (Refereed)
    Abstract [en]

    We present a method for pulp particle characterization based on a truncated lognormal mixture (TLM) model, as motivated by size statistics of organisms. We use an optical fiber analyzer to measure the length–width distribution of kraft-cooked roundwood or sawmill sources, of chemi-thermomechanical pulp (CTMP) samples from roundwood or sawmill sources, and the same CTMP samples after kraft post-processing. Our results show that bimodal TLMs capture salient features of the investigated pulp particle distributions, by decomposition into a large-particle and a small-particle fraction. However, we find that fibers from sawmill sources, which have not undergone mechanical treatment, cannot be described by TLM, likely due to non-random sampling. Within the confines of our dataset, the TLM characterization predicts laboratory sheet properties more effectively than conventional averaging methods for pulp particle size distributions. The TLM characterization is intended as a tool for controlling the pulp production process towards higher product quality, uniformity, and energy efficiency, pending further mill trials for validation. 

  • 11. Liubytska, K
    et al.
    Engberg, Birgitta A.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Lundberg, Bengt
    Uppsala Universitet.
    Elastic wave’s tail reconstruction in a split-Hopkinson bar2023In: Book of Abstracts Euromech Colloquium 634, 2023Conference paper (Other academic)
    Abstract [en]

    The split Hopkinson pressure bar (SHPB) is a widely used method for analyzing the strength-strain characteristics of wood materials [1]. Experiments show that, as wood is a relatively soft material and the experimental setup is limited in size, the entire wave is not fully recorded before it is mixed with waves reflected from the ends. To be able to analyze how much energy is dissipated in a deformation process, it is required that the whole wave be recorded. In the present investigation, the pressure tail was reconstructed theoretically in the incident bar using the data from the transmission bar, which should allow for a reduction in the error in the energy of the waves.

    When a deformation wave propagates along bars, part of its energy dissipates into the environment. In this study, a modification of the SHPB was proposed to calculate and analyze the amount of energy the system loses, not due to the sample. Formulas for energy and momentum equilibrium were used [2]. The influence of the length of the striker and the level of input energy were also analyzed.

    In the presented experiment, all energy tails were completely recorded. This allowed for the theoretical reconstruction of the tail and a comparison with the recorded one. The pressure tail in the transmitted wave was manually shortened and replaced with a theoretical tail. The theoretical tail was created by performing an exponential curve fit with points from the tail up to the point where it had been shortened. The results show that only about 10% of the tail needs to be registered to accurately reconstruct it. When a registered wave is replaced with a reconstructed one, the resulting error in the total wave's energy value is no greater than 0.3% for a 500mm striker and 1.5% for a 250mm striker.

  • 12.
    Persson, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    On dynamic crack growth in discontinuous materials2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis work numerical procedures are developed for modeling dynamic fracture of discontinuous materials, primarily materials composed of a load-bearing network. The models are based on the Newtonian equations of motion, and does not require neither stiffness matrices nor remeshing as cracks form and grow. They are applied to a variety of cases and some general conclusions are drawn. The work also includes an experimental study of dynamic crack growth in solid foam. The aims are to deepen the understanding of dynamic fracture by answering some relevant questions, e.g. What are the major sources of dissipation of potential energy in dynamic fracture? What are the major differences between the dynamic fracture in discontinuous network materials as compared to continuous materials? Is there any situation when it would be possible to utilize a homogenization scheme to model network materials as continuous? The numerical models are compared with experimental results to validate their ability to capture the relevant behavior, with good results. The only two plausible dissipation mechanisms are energy spent creating new surfaces, and stress waves, where the first dominates the behavior of slow cracks and the later dominates fast cracks. In the numerical experiments highly connected random fiber networks, i.e. structures with short distance between connections, behaves phenomenologically like a continuous material whilst with fewer connections the behavior deviates from it. This leads to the conclusion that random fiber networks with a high connectivity may be treated as a continuum, with appropriately scaled material parameters. Another type of network structures is the ordered networks, such as honeycombs and semi-ordered such as foams which can be viewed as e.g. perturbed honeycomb grids. The numerical results indicate that the fracture behavior is different for regular honeycombs versus perturbed honeycombs, and the behavior of the perturbed honeycomb corresponds well with experimental results of PVC foam.

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  • 13.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Engberg, Birgitta A.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Granfeldt, Thomas
    Valmet AB.
    Feeding of double disc refiners - modeling of particle flow2018In: IMPC 2018, Trondheim, Norway, 2018Conference paper (Refereed)
    Abstract [en]

    Improvements to the energy efficiency of double disk refiners are hampered by the feeding rate in tothe machine. This study aims to evaluate parameters towards a complete model, which will help tobring clarity to the limiting factors in feeding rate. A combined computational fluid dynamic anddiscrete element model is used to evaluate the flowrate of a hopper discharge unaided, and aided by agas flow with varying density. The results of the study shows that it is essential to capture the exactgeometry of the double disk feeding in relation to the woodchips in the complete model. The resultsalso shows that while it is essential to include the gas phase in the full model, the model is not sensitiveto small variance in the density of the gas.

  • 14.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Fallahjoybari, Nima
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Engberg, Birgitta A.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    Granfeldt, Thomas
    Valmet.
    Feeding of double disc refiners – rotor design evaluation2022In: Proceedings of the International Mechanical Pulping Conference, 2022, p. 69-Conference paper (Other academic)
    Abstract [en]

    This work presents a hypothesis of how steam flow effect the chip flow in the Double disc (DD) refiner and test it with a numerical simulation. DD refiners are often considered one of the most energy efficient refiner models. However, feeding chips into these machines is not as easy as feeding single disc refiners due to the rotating geometries. It is our belief that to increase energy efficiency in refining we need to increase also the production rates. The authors have previously noticed that in a standard DD rotor, steam flowed both in the same direction as the flow of woodchips and in the opposite direction. It is our hypothesis that backwards flowing steam in and in close proximity to the critical transition from the non-rotating geometry to the rotating geometry is negative for the material flow. To evaluate the hypothesis a new rotor was designed to eliminate the backwards flow. The authors have previously presented a two way coupled multiphase model with steam flow modeled with Computational Fluid Dynamics and wood chips modeled as groups of connected spherical particles with Discrete Element Method with a momentum exchange model. This model was utilized to model the flow of steam and woodchips in a DD under normal operational parameters, with the conventional rotor and with the new rotor. The throughput of wood chips was evaluated and normalized with regards to the chip flow to the refiner. The flow was considerable more stable in the new rotor, the throughput was close to 100 % for the observed time window, and the steam flow was more uniform. The results of the simulation supports the hypothesis. The next step in the research would be to test the new rotor in full scale operation.

  • 15.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nyström, Staffan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
    The contact stress distribution in a double tapered bolt joint2021In: Journal of Testing and Evaluation, ISSN 0090-3973, E-ISSN 1945-7553, Vol. 49, no 4, p. 3028-3034Article in journal (Refereed)
    Abstract [en]

    The mechanical problem that occurs when a thin elastic layer (e.g., a part of a composite panel) is inserted between two tapered elastic parts and bolted together is studied. The problem considered is of relevance for situations when it is of value to get a rough estimate of the contact pressure in the elastic layer and, by extension, the contact friction. The intermediate elastic layer (i.e., panel) is assumed to behave like a Winkler foundation, and engineering beam theory is assumed to describe the behavior of the tapered outer parts. The coupling between the transverse stresses in the panel and the longitudinal stress due to an applied external tensile load in the plane of the panel is ignored. By using the influence functions for the tapered parts, one obtains an integral equation involving the compression of the panel as the only unknown function. The integral equation is solved numerically by standard procedures. An experiment was also performed in a way that differs slightly from what is covered by the more general theoretical model. This is due to practical reasons connected to the experiment. It was found that the theoretical model gave results in fair agreement with the experimental results. 

  • 16.
    Persson, Johan
    et al.
    Uppsala universitet.
    Helgason, Benedikt
    Institute for Biomechanics, Zürich, Switzerland.
    Engqvist, Håkan
    Uppsala universitet.
    Ferguson, Stephen J.
    Institute for Biomechanics, Zürich, Switzerland.
    Persson, Cecilia
    Uppsala universitet.
    Stiffness and strength of cranioplastic implant systems in comparison to cranial bone2018In: Journal of Cranio-Maxillofacial Surgery, ISSN 1010-5182, E-ISSN 1878-4119, Vol. 46, no 3, p. 418-423Article in journal (Refereed)
    Abstract [en]

    Purpose: The aim of this study was to evaluate skull replacement options after decompressive craniectomy by systematically investigating which combination of geometrical properties and material selection would result in a mechanical response comparable in stiffness to that of native skull bone and a strength as high or higher than the same.

    Materials and methods: The study was conducted using a Finite Element Model of the top part of a human skull. Native skull bone, autografts and commercial implants made of PEEK, solid titanium, two titanium meshes and a titanium-ceramic composite were modeled under a set load to evaluate deformation and maximum stress.

    Results: The computational result showed a large variation of the strength and effective stiffness of the autografts and implants. The stiffness of native bone varied by a factor of 20 and the strength by a factor of eight. The implants span the entire span of the native skull, both in stiffness and strength.

    Conclusion: All the investigated implant materials had a potential for having the same effective stiffness as the native skull bone. All the materials also had the potential to be as strong as the native bone. To match inherent properties, the best choice of material and thickness is thus patient specific, depending on the quality of the patient’s native bone.

  • 17.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Isaksson, Per
    Applied Mechanics, Ångström Laboratory, Uppsala University, Box 534, SE-75121 Uppsala, Sweden.
    A mechanical particle model for analyzing rapid deformations and fracture in 3D fiber materials with ability to handle length effects2014In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 51, no 11-12, p. 2244-2251Article in journal (Refereed)
    Abstract [en]

    A mechanical model for analyses of rapid deformation and fracture in three-dimensional fiber materials is derived. Large deformations and fractures are handled in a computationally efficient and robust way. The model is truly dynamic and computational time and memory demand scales linearly to the number of structural components, which make the model well suited for parallel computing. The specific advantages, compared to traditional continuous grid-based methods, are summarized as: (1) Nucleated cracks have no idealized continuous surfaces. (2) Specific macroscopic crack growth or path criteria are not needed. (3) The model explicitly considers failure processes at fiber scale and the influence on structural integrity is seamlessly considered. (4) No time consuming adaptive re-meshing is needed. The model is applied to simulate and analyze crack growth in random fiber networks with varying density of fibers. The results obtained in fracture zone analyses show that for sufficiently sparse networks, it is not possible to make predictions based on continuous material assumptions on a macroscopic scale. The limit lies near the connectivity l(c)/L = 0.1, where is the ratio between the average fiber segment length and the total fiber length. At ratios l(c)/L < 0.1 the network become denser and at the limit l(c)/L -> 0, a continuous continuum is approached on the macroscopic level. (C) 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.

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  • 18.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    A particle-based method for mechanical analyses of planar fiber-based materials2013In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 93, no 11, p. 1216-1234Article in journal (Refereed)
    Abstract [en]

    A new discrete element model to deal with rapid deformation and fracture of flat fibrous materials is derived. The method is based on classical mechanical theories and is a combination of traditional particle dynamics and nonlinear engineering beam theory. It is assumed that a fiber can be seen as a beam that is represented by discrete particles, which are moving according to Newton's laws of motion. Damage is dealt with by fracture of fiber-segments and fiberfiber bonds when the potential energy of a segment or bond exceeds the critical fracture energy. This allows fractures to evolve as a result of material properties only. To validate the model, four examples are shown and compared with analytical results found in literature. Copyright (c) 2013 John Wiley & Sons, Ltd.

  • 19.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Dynamic mode I crack growth in a notched foam specimen under quasi static loadingManuscript (preprint) (Other academic)
  • 20.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Per, Isaksson
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Modeling rapidly growing cracks in planar materials with a view to micro structural effects2015In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 192, no 2, p. 191-201Article in journal (Refereed)
    Abstract [en]

    Dynamic fracture behavior in both fairly continuous materials and discontinuous cellular materials is analyzed using a hybrid particle model. It is illustrated that the model remarkably well captures the fracture behavior observed in experiments on fast growing cracks reported elsewhere. The material's microstructure is described through the configuration and connectivity of the particles and the model's sensitivity to a perturbation of the particle configuration is judged. In models describing a fairly homogeneous continuous material, the microstructure is represented by particles ordered in rectangular grids, while for models describing a discontinuous cellular material, the microstructure is represented by particles ordered in honeycomb grids having open cells. It is demonstrated that small random perturbations of the grid representing the microstructure results in scatter in the crack growth velocity. In materials with a continuous microstructure, the scatter in the global crack growth velocity is observable, but limited, and may explain the small scattering phenomenon observed in experiments on high-speed cracks in e.g. metals. A random perturbation of the initially ordered rectangular grid does however not change the average macroscopic crack growth velocity estimated from a set of models having different grid perturbations and imply that the microstructural discretization is of limited importance when predicting the global crack behavior in fairly continuous materials. On the other hand, it is shown that a similar perturbation of honeycomb grids, representing a material with a discontinuous cellular microstructure, result in a considerably larger scatter effect and there is also a clear shift towards higher crack growth velocities as the perturbation of the initially ordered grid become larger. Thus, capturing the discontinuous microstructure well is important when analyzing growing cracks in cellular or porous materials such as solid foams or wood.

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  • 21.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Sidén, Johan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Bertilsson, Kent
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Geometrical Optimization of Jetting Duct2020In: 2020 8th International Electrical Engineering Congress (iEECON), IEEE, 2020, p. 1-3Conference paper (Refereed)
    Abstract [en]

    This paper presents and evaluates three new designs of fibre optic cable ducts for jetting. The three designs are longitudinal ridges with square, triangular and semicircular geometries. The effects of the geometries are evaluated with regards to electrostatic force, fluid drag force and jetting length with limited push force. An optimal design is suggested, with triangular longitudinal ridges on the inner surface of the duct.

1 - 21 of 21
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