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  • 1.
    Alimadadi, Majid
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Lindström, Stefan B.
    Division of Solid Mechanics, Department of Management and Engineering, Linköping University, Linköping, Sweden.
    Kulachenko, Artem
    Department of Solid Mechanics, Royal Institute of Technology (KTH), Stockholm, Sweden.
    Role of microstructures in the compression response of three-dimensional foam-formed wood fiber networks2018In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 14, no 44, p. 8945-8955, article id C7SM02561KArticle in journal (Refereed)
    Abstract [en]

    High-porosity, three-dimensional wood fiber networks made by foam forming present experimentally accessible instances of hierarchically structured, athermal fiber networks. We investigate the large deformation compression behavior of these networks using fiber-resolved finite element analyses to elucidate the role of microstructures in the mechanical response to compression. Three-dimensional network structures are acquired using micro-computed tomography and subsequent skeletonization into a Euclidean graph representation. By using a fitting procedure to the geometrical graph data, we are able to identify nine independent statistical parameters needed for the regeneration of artificial networks with the observed statistics. The compression response of these artificially generated networks and the physical network is then investigated using implicit finite element analysis. A direct comparison of the simulation results from the reconstructed and artificial network reveals remarkable differences already in the elastic region. These can neither be fully explained by density scaling, the size effect nor the boundary conditions. The only factor which provides the consistent explanation of the observed difference is the density and fiber orientation nonuniformities; these contribute to strain-localization so that the network becomes more compliant than expected for statistically uniform microstructures. We also demonstrate that the experimentally manifested strain-stiffening of such networks is due to development of new inter-fiber contacts during compression.

  • 2. Andrić, J.
    et al.
    Fredriksson, S. T.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    A study of a flexible fiber model and its behavior in DNS of turbulent channel flow2013In: Acta Mechanica, ISSN 0001-5970, E-ISSN 1619-6937, Vol. 224, no 10, p. 2359-2374Article in journal (Refereed)
    Abstract [en]

    The dynamics of individual flexible fibers in a turbulent flow field have been analyzed, varying their initial position, density and length. A particle-level fiber model has been integrated into a general-purpose, open source computational fluid dynamics code. The fibers are modeled as chains of cylindrical segments connected by ball and socket joints. The equations of motion of the fibers contain the inertia of the segments, the contributions from hydrodynamic forces and torques, and the connectivity forces at the joints. Direct numerical simulation of the incompressible Navier–-Stokes equations is used to describe the fluid flow in a plane channel, and a one-way coupling is considered between the fibers and the fluid phase. We investigate the translational motion of fibers by considering the mean square displacement of their trajectories. We find that the fiber motion is primarily governed by velocity correlations of the flow fluctuations. In addition, we show that there is a clear tendency of the thread-like fibers to evolve into complex geometrical configurations in a turbulent flow field, in fashion similar to random conformations of polymer strands subjected to thermal fluctuations in a suspension. Finally, we show that fiber inertia has a significant impact on reorientation timescales of fibers suspended in a turbulent flow field.

  • 3. Andrić, J.
    et al.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    A particle-level fiber model, implemented in a general-purpose CFD code2013In: Proc. of Svenska Mekanikdagar, 2013Conference paper (Refereed)
  • 4. Andrić, J.
    et al.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    A particle-level fiber model, implemented in OpenFOAM(R)2013In: Proceedigs of the 8th Int. OpenFOAM(R) Workshop, 2013Conference paper (Refereed)
  • 5. Andrić, J.
    et al.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    A particle-level rigid fiber model for high-Reynolds number flow, implemented in a general-purpose CFD code2013In: Proc. 8th Int. Conf. Multiphase Flow, 2013Conference paper (Refereed)
    Abstract [en]

    A particle-level rigid fiber model has been integrated into a general-purpose, open source computational fluid dynamics code to carry out detailed studies of fiber–flow interactions in realistic flow fields. The fibers are modeled as chains of cylindrical segments, and their translational and rotational degrees of freedom are considered. The equations of motion contain the contributions from hydrodynamic forces and torques, and the segment inertia is taken into account. The model is validated for the rotational motion of isolated fibers in simple shear flow, and the computed period of rotation is in good agreement with the one computed using Jeffery’s equation for a prolate spheroid with an equivalent aspect ratio. The model is applied by suspending a number of fibers in the swirling flow of a conical diffuser, resembling one stage in the dry-forming of pulp mats. The Reynolds-averaged Navier–Stokes equations with an eddy-viscosity turbulence model are employed to describe the fluid motion, and a one-way coupling between the fibers and the fluid phase is included. The dependence of the fiber motion on initial position and density is analyzed.

  • 6. Andrić, J.
    et al.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    Ballistic deflection of fibres in decelerating flow2016In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 85, p. 57-66Article in journal (Refereed)
    Abstract [en]

    We investigate the motion of inertial, rod-like fibres in the decelerating flow of a wedge-shaped channel with non-creeping fibre–flow interactions. We consider the trajectories of isolated fibres to identify the conditions for which these trajectories deflect from the streamlines of the flow as well as a rectilinear path. We carry out analytical and numerical studies under the assumption of an infinite fibre hydrodynamic resistance to transverse flow, and we expand the numerical study by taking into account a finite transverse hydrodynamic resistance. The analytical analysis identifies a longitudinal ballistic number $B_\ell$ and a transverse ballistic number $B_\mathrmt$ as two dimensionless parameters that govern the fibre dynamics. It is found that $B_\ell$ is the product of the Stokes number $At_\ell$ in the longitudinal direction of the fibre and the channel opening angle β. As anticipated, a fibre moves along the streamlines in the viscosity-dominated regime ($B_\ell \ll 1$, $B_\mathrmt \ll 1$), while it moves in a straight line without being rotated in the inertia-dominated regime ($B_\mathrmt \gg 1$). The focus of the present study is on the intermediate regime ($B_\ell \gg 1$, $B_\mathrmt < 1$), in which we identify and analyse a fibre trajectory that significantly deviates from the streamlines of the flow. This behaviour is observed for both infinite and finite resistances to transverse flow, and it is referred to as ballistic deflection. We argue that ballistic deflection may increase the rate of collisions between suspended fibres, and thus potentially affects the rate of fibre aggregation in flowing suspensions. An order of magnitude estimate of the ballistic numbers identifies dry-forming of pulp mats, which includes an air–wood fibre flowing suspension, to operate in the regime of ballistic deflection.

  • 7. Andrić, J.
    et al.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    Description and validation of a flexible fiber model, implemented in a general purpose CFD code2013In: Proc. 8th Int. Conf. Multiphase Flow, 2013Conference paper (Refereed)
    Abstract [en]

    A flexible fiber model has been implemented in a general purpose open-source Computational Fluid Dynamics code. The fibers are modeled as chains of cylindrical segments, and all the degrees of freedom necessary to realistically reproduce the dynamics of real fibers, are taken into account. Each segment is tracked individually and their equations of motion account for the hydrodynamic forces and torques from the interaction with the fluid, the elastic bending and twisting torques, and the connectivity forces and moments that ensure the fiber integrity. The segment inertia is taken into account and a one-way coupling with the fluid phase is considered. The model is applied to simulate the rotational motion of an isolated fiber in a low segment Reynolds number shear flow. In the case of a stiff fiber, the computed period of rotation is in good agreement with the one computed using Jeffery’s equation for an equivalent spheroid aspect ratio. A qualitative comparison is made with experimental data for flexible fibers. Further, a generic test case is described and used to validate the energy conservation and the response time of the fiber model concept. These results show that the implemented model can reproduce the known dynamical behavior of rigid and flexible fibers successfully.

  • 8. Andrić, J.
    et al.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    Numerical investigation of fiber flocculation in the air flow of an asymmetrical diffuser2014In: Proc. ASME 12th Int. Conf. on nanochannels, microchannels, and minichannels, ASME , 2014Conference paper (Refereed)
  • 9. Andrić, J.
    et al.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    Particle-level simulations of flocculation in a fiber suspension flowing through a diffuser2016In: Thermal Science, ISSN 0354-9836, E-ISSN 2334-7163, Vol. 20, no suppl. 6Article in journal (Refereed)
    Abstract [en]

    We investigate flocculation in dilute suspensions of rigid, straight fibers in a decelerating flow field of a diffuser. We carry out numerical studies using a particle-level simulation technique that takes into account the fiber inertia and the non-creeping fiber–flow interactions. The fluid flow is governed by the Reynolds-averaged Navier–Stokes equations with the standard k–omega eddy-viscosity turbulence model. A one-way coupling between the fibers and the flow is considered with a stochastic model for the fiber dispersion due to turbulence. The fibers interact through short-range attractive forces that cause them to aggregate into flocs when fiber–fiber collisions occur. We show that ballistic deflection of fibers greatly increases the flocculation in the diffuser. The inlet fiber kinematics and the fiber inertia are the main parameters that affect fiber flocculation in the pre-diffuser region.

  • 10. Andrić, J.
    et al.
    Lindström, S. B.
    Sasic, S.
    Nilsson, H.
    Rheological properties of dilute suspensions of rigid and flexible fibers2014In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 212, p. 36-46Article in journal (Refereed)
    Abstract [en]

    Particle-level simulations are used to study the rheology of monodispersed suspensions of rigid and flexible fibers in a creeping, simple shear flow of a Newtonian fluid. We also investigate the influence of different equilibrium shapes (straight and curved) of the fibers on the behavior of the suspension. A parametric study of the impacts of fiber flexural rigidity and morphology on rheology quantifies the effects of these realistic fiber features on the experimentally accessible rheological properties. A fiber is modeled as a chain of rigid cylindrical segments, interacting through a two-way coupling with the fluid described by the incompressible three-dimensional Navier–Stokes equations. The initial fiber configuration is in the flow–gradient plane. We show that, when the shear rate is increased, straight flexible fibers undergo a buckling transition, leading to the development of finite first and second normal stress differences and a reduction of the viscosity. These effects, triggered by shape fluctuations, are dissimilar to the effects induced by the curvature of stiff, curved fibers, for which the viscosity increases with the curvature of the fiber. An analysis of the orbital drift of fibers initially oriented at an angle to the flow–gradient plane provides an estimate for the time-scale within which the prediction of the rheological behavior is valid. The information obtained in this work can be used in the experimental characterization of fiber morphology and mechanics through rheology.

  • 11. Azeez, A.
    et al.
    Eriksson, R.
    Calmunger, M.
    Lindström, S. B.
    Simonsson, K.
    Low cycle fatigue modelling of a steam turbine rotor steel2019In: Procedia Structural Integrity, 2019, Vol. 23, p. 149-154Conference paper (Refereed)
    Abstract [en]

    Materials in steam turbine rotors are subjected to cyclic loads at high temperature, causing cracks to initiate and grow. To allow for more flexible operation, accurate fatigue models for life prediction must not be overly conservative. In this study, fully reversed low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done isothermally, within temperature range of room temperature to 600 $°$C, under strain control with 0.8-1.2 % total strain range. Some tests included hold time to calibrate the short-time creep behaviour of the material. Different fatigue life models were constructed. The life curve in terms of stress amplitude was found unusable at 600 $°$C, while the life curve in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 $°$C. To construct better life model, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges. This allowed the formulation of life curves based on plasticity or creep domination, which showed creep domination at 600 $°$C, while at 500 $°$C, creep only dominates for higher strain range.

  • 12. Bakker, H. E.
    et al.
    Lindström, S. B.
    Sprakel, J.
    Geometry- and rate-dependent adhesive failure of micropatterned surfaces2012In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 6Article in journal (Refereed)
    Abstract [en]

    The dynamic nature of adhesive interface failure remains poorly understood, especially when the contact between the two surfaces is localized in microscopic points of adhesion. Here, we explore the dynamic failure of adhesive interfaces composed of a large number of micron-sized pillars against glass. Surprisingly, we find a large influence of the microcontact geometry; ordered arrays of these pillars exhibit significantly stronger adhesive properties than equivalent surfaces in which the pillars are disordered. This can be understood with a simple geometric argument that accounts for the number of adhesive bonds that needs to be broken simultaneously to propagate the crack front. Moreover, the adhesive strength in both cases depends largely on the velocity with which the surfaces are separated. This rate dependence is explained on the basis of a semi-phenomenological model that describes macroscopic failure as a consequence of microscopic bond-rupture events. Our results suggest that the dynamics of adhesive failure, in the limit explored here, is predominantly stress-driven and highly sensitive to local geometry effects.

  • 13. Benselfelt, T.
    et al.
    Nordenström, M.
    Lindström, S. B.
    Wågberg, L.
    Explaining the exceptional wet integrity of transparent cellulose nanofibril films in the presence of multivalent ions—suitable substrates for biointerfaces2019In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 6, no 13Article in journal (Refereed)
    Abstract [en]

    Cellulose nanofibrils (CNFs) assemble into water-resilient materials in the presence of multivalent counter-ions. The essential mechanisms behind these assemblies are ion–ion correlation and specific ion effects. A network model shows that the interfibril attraction indirectly influences the wet modulus by a fourth power relationship to the solidity of the network ($E_w \propto \varphi^4$). Ions that induce both ion–ion correlation and specific ion effects significantly reduce the swelling of the films, and due to the nonlinear relationship dramatically increase the wet modulus. Herein, this network model is used to explain the elastoplastic behavior of wet films of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized, carboxymethylated, and phosphorylated CNFs in the presence of different counter-ions. The main findings are that the aspect ratio of the CNFs influences the ductility of the assemblies, that the bivalency of phosphorylate ligands probably limits the formation of interfibril complexes with divalent ions, and that a higher charge density increases the friction between fibrils by increasing the short-range attraction from ion–ion correlation and specific ion effects. These findings can be used to rationally design CNF materials for a variety of applications where wet strength, ductility, and transparency are important, such as biomaterials or substrates for bioelectronics.

  • 14.
    Benselfelt, Tobias
    et al.
    Department of Fibre and Polymer Technology School of Engineering Sciences in Chemistry Biotechnology and Health KTH Royal Institute of Technology Stockholm 100 44 Sweden.
    Shakya, Jyoti
    Department of Fibre and Polymer Technology School of Engineering Sciences in Chemistry Biotechnology and Health KTH Royal Institute of Technology Stockholm 100 44 Sweden.
    Rothemund, Philipp
    Robotic Materials Department Max Planck Institute for Intelligent Systems 70569 Stuttgart Germany.
    Lindström, Stefan B
    Department of Management and Engineering Division of Solid Mechanics Linköping University Linköping 58183 Sweden.
    Piper, Andrew
    Department of Fibre and Polymer Technology School of Engineering Sciences in Chemistry Biotechnology and Health KTH Royal Institute of Technology Stockholm 100 44 Sweden.
    Winkler, Thomas E.
    Institute of Microtechnology &amp; Center of Pharmaceutical Engineering Technische Universität Braunschweig 38106 Braunschweig Germany.
    Hajian, Alireza
    Department of Fibre and Polymer Technology School of Engineering Sciences in Chemistry Biotechnology and Health KTH Royal Institute of Technology Stockholm 100 44 Sweden.
    Wågberg, Lars
    Department of Fibre and Polymer Technology School of Engineering Sciences in Chemistry Biotechnology and Health KTH Royal Institute of Technology Stockholm 100 44 Sweden.
    Keplinger, Christoph
    Robotic Materials Department Max Planck Institute for Intelligent Systems 70569 Stuttgart Germany;Paul M. Rady Department of Mechanical Engineering University of Colorado Boulder CO 80309 USA;Materials Science and Engineering Program University of Colorado Boulder CO 80309 USA.
    Hamedi, Mahiar Max
    Department of Fibre and Polymer Technology School of Engineering Sciences in Chemistry Biotechnology and Health KTH Royal Institute of Technology Stockholm 100 44 Sweden.
    Electrochemically Controlled Hydrogels with Electrotunable Permeability and Uniaxial Actuation2023In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 35, no 45, article id 2303255Article in journal (Refereed)
    Abstract [en]

    The unique properties of hydrogels enable the design of life-like soft intelligent systems. However, stimuli-responsive hydrogels still suffer from limited actuation control. Direct electronic control of electronically conductive hydrogels can solve this challenge and allow direct integration with modern electronic systems. An electrochemically controlled nanowire composite hydrogel with high in-plane conductivity that stimulates a uniaxial electrochemical osmotic expansion is demonstrated. This materials system allows precisely controlled shape-morphing at only −1 V, where capacitive charging of the hydrogel bulk leads to a large uniaxial expansion of up to 300%, caused by the ingress of ≈700 water molecules per electron–ion pair. The material retains its state when turned off, which is ideal for electrotunable membranes as the inherent coupling between the expansion and mesoporosity enables electronic control of permeability for adaptive separation, fractionation, and distribution. Used as electrochemical osmotic hydrogel actuators, they achieve an electroactive pressure of up to 0.7 MPa (1.4 MPa vs dry) and a work density of ≈150 kJ m−3 (2 MJ m−3 vs dry). This new materials system paves the way to integrate actuation, sensing, and controlled permeation into advanced soft intelligent systems.

  • 15. Carrick, C.
    et al.
    Lindström, S. B.
    Larsson, P. T.
    Wågberg, L.
    Lightweight, highly compressible, noncrystalline cellulose capsules2014In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 30, no 26, p. 7635-7644Article in journal (Refereed)
    Abstract [en]

    We demonstrate how to prepare extraordinarily deformable, gas-filled, spherical capsules from non-modified cellulose. These capsules have a low nominal density, ranging from 7.6 to 14.2 kg/m$^3$, and can be deformed elastically to 70% deformation at 50% relative humidity. No compressive strain-at-break could be detected for the se dry cellulose capsules, since they did not rupture even when compressed into a disc with pockets of highly compressed air. A quantitative constitutive model for the large deformation compression of these capsules is derived, including their high-frequency mechanical response and their low-frequency force relaxation, where the latter is governed by the gas barrier properties of the dry capsule. Mechanical testing corroborated these models with good accuracy. Force relaxation measurements at a constant compression rendered an estimate for the gas permeability of air through the capsule wall, calculated to 0.4 mLμm/m$^2$ days kPa at 50% relative humidity. These properties taken together open up a large application area for the capsules, and they could most likely be used for applications in compressible lightweight materials and also constitute excellent model materials for adsorption and adhesion studies.

  • 16. Cortes Ruiz, Maria F.
    et al.
    Brusentsev, Yury
    Lindström, Stefan B
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Mathematics, and Science Education (2023-).
    Xu, Chunlin
    Wågberg, Lars
    Shape-recovering nanocellulose networks: Preparation, characterization and modeling2023In: Carbohydrate Polymers, ISSN 0144-8617, E-ISSN 1879-1344, Vol. 315, article id 120950Article in journal (Refereed)
    Abstract [en]

    Development of strong cellulose nanofibril (CNF) networks for advanced applications, such as in the biomedical field, is of high importance owing to the biocompatible nature and plant-based origin of cellulose nanofibrils. Nevertheless, lack of mechanical strength and complex synthesis methods hinder the application of these materials in areas where both toughness and manufacturing simplicity are required. In this work, we introduce a facile method for the synthesis of a low solid content (< 2 wt%), covalently crosslinked CNF hydrogel where Poly (N-isopropylacrylamide) (NIPAM) chains are utilized as crosslinks between the nanofibrils. The resulting networks have the capability to fully recover the shape in which they were formed after various drying and rewetting cycles. Characterization of the hydrogel and its constitutive components was performed using X-ray scattering, rheological investigations and uniaxial testing in compression. Influence of covalent crosslinks was compared with networks crosslinked by the addition of CaCl2. Among other things the results show that the mechanical properties of the hydrogels can be tuned by controlling the ionic strength of the surrounding medium. Finally, a mathematical model was developed based on the experimental results, which describes and predicts to a decent degree the large-deformation, elastoplastic behavior, and fracture of these networks.

  • 17. Eskilsson, O.
    et al.
    Lindström, S. B.
    Sepulveda, B.
    Shahjamali, M. M.
    Güell-Grau, P.
    Sivlér, P.
    Skog, M.
    Aronsson, C.
    Björk, E. M.
    Nyberg, N.
    Khalaf, H.
    Bengtsson, T.
    James, J.
    Ericson, M. B.
    Martinsson, E.
    Selegård, R.
    Aili, D.
    Self-assembly of mechanoplasmonic bacterial cellulose–metal nanoparticle composites2020In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 30Article in journal (Refereed)
    Abstract [en]

    Nanocomposites of metal nanoparticles (NPs) and bacterial nanocellulose (BC) enable fabrication of softand biocompatible materials for optical, catalytic, electronic and biomedical applications. Current BC-NP nanocomposites are typically preparedbyin situ synthesis of the NPs or electrostatic adsorption of surface functionalized NPs, whichlimitspossibilities to control and tune NP size, shape, concentration and surface chemistryand influence the properties and performance of the materials. Here we describe a self-assembly strategy for fabrication of complex and well-defined BC–NP composites using colloidal gold and silver NPs ofdifferent sizes, shapes and concentrations. The self-assembly processresults innanocompositeswith distinct biophysical and optical properties.In addition to antibacterial materials and materials with excellent senor performance, materials with unique mechanoplasmonic properties are developed. The homogenous incorporation of plasmonic gold NPs in the BC enablesextensivemodulation of the optical properties by mechanical stimuli. Compressiongives rise to near-field coupling between adsorbed NPs, resultingin tunable spectral variations and enhanced broadband absorptionthat amplifyboth non-linear optical and thermoplasmonic effectsand enablesnovel biosensing strategies.

  • 18. Fall, A. B.
    et al.
    Lindström, S. B.
    Sprakel, J.
    Wågberg, L.
    A physical cross-linking process of cellulose nanofibril gels with shear-controlled fibril orientation2013In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 9, no 6, p. 1852-1863Article in journal (Refereed)
    Abstract [en]

    Cellulose nanofibrils constitute the smallest fibrous components of wood, with a width of approximately 4\,nm and a length in the micrometer range. They consist of aligned linear cellulose chains with crystallinity exceeding 60\,%, rendering stiff, high-aspect-ratio rods. These properties are advantageous in the reinforcement components of composites. Cross-linked networks of fibrils can be used as templates into which a polymer enters. In the semi-concentrated regime (i.e. slightly above the overlap concentration), carboxy methylated fibrils dispersed in water have been physically cross-linked to form a volume-spanning network (a gel) by reducing the pH or adding salt, which diminishes the electrostatic repulsion between fibrils. By applying shear during or after this gelation process, we can orient the fibrils in a preferred direction within the gel, for the purpose of fully utilizing the high stiffness and strength of the fibrils as reinforcement components. Using these gels as templates enables precise control of the spatial distribution and orientation of the dispersed phase of the composites, optimizing the potentially very large reinforcement capacity of the nanofibrils.

  • 19. Fall, A. B.
    et al.
    Lindström, S. B.
    Sundman, O.
    Ödberg, L.
    Wågberg, L.
    Colloidal stability of aqueous nanofibrillated cellulose dispersions2011In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 27, no 18, p. 11332-11338Article in journal (Refereed)
    Abstract [en]

    Cellulose nanofibrils constitute an attractive raw material for carbon neutral, biodegradable, nanostructured materials. In a aqueous suspensions, these nanofibrils are stabilized by electrostatic repulsion arising from deprotonated carboxyl groups at the fibril surface. In the present work, a new model is developed for colloidal stability by considering the deprotonation and electrostatic screening. This model predicts the fibril-fibril interaction potential in a given pH and ionic strength environment. Experiments support the model predictions that aggregation is induced by decreasing pH, thus reducing the surface charge, or by increasing salt concentration. It is shown that the primary aggregation mechanism for salt addition is the surface charge reduction through specific interactions of counter-ions with the deprotonated carboxyl groups, while the screening effect of the salt is of secondary importance.

  • 20. Fall, A.
    et al.
    Lindström, S. B.
    Sprakel, J.
    Löfroth, J. -E
    Wågberg, L.
    Shear-stiffening cellulose nanofiber gels with tuneable mechanical characteristics2011In: Proc. of the 241st National Meeting and Exposition of the American-Chemical-Society, 2011, Vol. 241Conference paper (Refereed)
  • 21. Gibaud, T.
    et al.
    Perge, C.
    Lindström, S. B.
    Taberlet, N.
    Manneville, S.
    Multiple yielding processes in a colloidal gel under large amplitude oscillatory stress2016In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 12, p. 1701-1712Article in journal (Refereed)
    Abstract [en]

    Fatigue refers to the changes in material properties caused by repeatedly applied loads. It has been widely studied for, e.g., construction materials, but much less has been done on soft materials. Here, we characterize the fatigue dynamics of a colloidal gel. Fatigue is induced by large amplitude oscillatory stress (LAOStress), and the local displacements of the gel are measured through high-frequency ultrasonic imaging. We show that fatigue eventually leads to rupture and fluidization. We evidence four successive steps associated with these dynamics: (i) the gel first remains solid, (ii) it then slides against the walls, (iii) the bulk of the sample becomes heterogeneous and displays solid–fluid coexistence, and (iv) it is finally fully fluidized. It is possible to homogeneously scale the duration of each step with respect to the stress oscillation amplitude $σ_0$. The data are compatible with both exponential and power-law scalings with $σ_0$, which hints at two possible interpretations in terms of delayed yielding in terms activated processes or of the Basquin law. Surprisingly, we find that the model parameters behave nonmonotonically as we change the oscillation frequency and/or the gel concentration.

  • 22. Hajian, A.
    et al.
    Lindström, S. B.
    Pettersson, T.
    Hamedi, M. M.
    Wågberg, L.
    Understanding the dispersive action of nanocellulose for carbon nanomaterials2017In: Nano Letters, ISSN 1530-6984, E-ISSN 1530-6992, Vol. 17, p. 1439-1447Article in journal (Refereed)
    Abstract [en]

    This work aims at understanding the excellent ability of nanocelluloses to disperse carbon nanomaterials (CNs) in aqueous media to form long-term stable colloidal dispersions without the need for chemical functionalization of the CNs or the use of surfactant. These dispersions are useful for composites with high CN content when seeking water-based, efficient and green pathways for their preparation. To establish a comprehensive understanding of such dispersion mechanism, colloidal characterization of the dispersions has been combined with surface adhesion measurements using colloidal probe atomic force microscopy (AFM) in aqueous media. AFM results based on model surfaces of graphene and nanocellulose further suggest that there is an association between the nanocellulose and the CN. This association is caused by fluctuations of the counterions on the surface of the nanocellulose inducing dipoles in the sp2 carbon lattice surface of the CNs. Furthermore, the charges on the nanocellulose will induce an electrostatic stabilization of the nanocellulose–CN complexes that prevents aggregation. Based on this understanding, nanocelluloses with high surface charge density was used to disperse and stabilize carbon nanotubes (CNTs) and reduced graphene oxide in water and further increase in the dispersion limit of CNTs could be obtained. The dispersion limit reached the value of 75 wt% CNTs and resulted in high electrical conductivity (515 S/cm) and high modulus (14 GPa) of the CNT composit e nanopapers.

  • 23.
    Holmvall, Martin
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Drolet, Francois
    Uesaka, Tetsu
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Lindström, Stefan
    Micro-fluidics in printing nip - Liquid transfer on random fibre network surface2011In: Progress in Paper Physics Seminar, Graz, Austria: Verlag der Technischen Universität Graz , 2011Conference paper (Other academic)
  • 24.
    Holmvall, Martin
    et al.
    SCA R&D Ctr, SE-85121 Sundsvall, Sweden.
    Lindström, Stefan B
    Royal Inst Technol, Dept Fibre & Polymer Technol, S-10044 Stockholm, Sweden.
    Uesaka, Tetsu
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Simulation of two-phase flow with moving immersed boundaries2011In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, E-ISSN 1097-0363, Vol. 67, no 12, p. 2062-2080Article in journal (Refereed)
    Abstract [en]

    A two-dimensional model for immiscible binary fluid flow including moving immersed objects is presented. The fluid motion is described by the incompressible Navier-Stokes equation coupled with a phase-field model based on van der Waals’ free energy density and the Cahn-Hilliard equation. The immersed boundary method has been utilised to handle moving immersed objects and the phase-field boundary conditions have been adapted accordingly. Numerical stability and execution time was significantly improved by the use of a new boundary condition which implements minimisation of the free energy in a direct way. Convergence toward the analytical solution was demonstrated for equilibrium contact angle, the Lucas-Washburn theory and Stefan’s problem. The proposed model may be used for two-phase flow problems with moving boundaries of complex geometry, such as the penetration of fluid into a deformable, porous medium.

  • 25.
    Holmvall, Martin
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Uesaka, Tetsu
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Drolet, Francois
    FPInnovations-Paprican, 570 boul. St-Jean, Pointe-Claire, Québec, Canada.
    Lindström, Stefan B
    Transfer of a microfluid to a stochastic fibre network2011In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 27, no 7, p. 937-946Article in journal (Refereed)
    Abstract [en]

    The transfer of a microscopic fluid droplet from a flat surface to a deformable stochastic fibre network is investigated. Fibre networks are generated with different levels of surface roughness, and a two-dimensional, two-phase fluid-structure model is used to simulate the fluid transfer. In simulations, the Navier-Stokes equations and the Cahn-Hilliard phase-field equations are coupled to explicitly include contact line dynamics and free surface dynamics. The compressing fibre network is modelled as moving immersed boundaries. The simulations show that the amount of transferred fluid is approximately proportional to the contact area between the fluid and the fibre network. However, areas where the fluid bridges and never actually makes contact with the substrate must be subtracted.

  • 26. Hämäläinen, J.
    et al.
    Lindström, S. B.
    Hämäläinen, T.
    Niskanen, H.
    Papermaking fiber suspension flow simulations at multiple scales2011In: Journal of Engineering Mathematics, ISSN 0022-0833, E-ISSN 1573-2703, Vol. 71, no 1, p. 55-79Article in journal (Refereed)
    Abstract [en]

    Papermaking flows are extremely challenging for modeling and simulation, if one accepts their full complexity. A wide range of particles, including fibers, fiber fragments (fines) and fillers (non-organic particles), flow and interact with each other in a nondilute suspension, a complex geometry and at a high flow rate. Different simulation approaches are reviewed from particle-level simulations, through mesoscale simulations to the full flow geometry of the papermaking line. Their application to papermaking and potential to provide fundamental understanding as well as direct process-optimization support are discussed.

  • 27.
    Kapidzic, Z.
    et al.
    Linköpings Universitet.
    Lindström, S. B.
    Linköpings Universitet.
    Lundgren, J.
    Saab AB.
    Incremental fatigue damage model: Application to plane problems with non-proportional loading2022In: ICAS PROCEEDINGS 33th Congress of the International Council of the Aeronautical Sciences Stockholm, Sweden, international council of the aeronautical sciences , 2022Conference paper (Refereed)
    Abstract [en]

    Modern airframe aluminum structures are increasingly manufactured in large and complex integral parts. Suchparts are often exposed to in–plane, non–proportional loading and contain stress–raisers with complex geometry. Conventional methods, based on the stress concentration factor and cycle counting, are unsuitable forassessment of this type of problem. We present a fatigue damage model, based on the concept of movingendurance surface and incremental damage evolution. The fatigue damage is entirely dependent on the localstress history and the notch effect is accounted for by introduction of the relative stress gradient. Also, wepresent an automated and efficient implementation of the model, for the purpose of computing fatigue damageat stress raisers in plane problems with non–proportional loading. We demonstrate the implementation by anexample of a fatigue calculation in an aircraft frame.

  • 28. Kulachenko, A.
    et al.
    Danoyelle, T.
    Galland, S.
    Lindström, S. B.
    Elastic properties of cellulose nanopaper2012In: Cellulose, ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, no 3, p. 793-807Article in journal (Refereed)
    Abstract [en]

    Nanopaper is a transparent film made of network-forming nanocellulose fibrils. These fibrils are several micrometers long with a diameter of 4–50\,nm. The reported elastic modulus of nanopaper often falls short of even conservative theoretical predictions based on the modulus of crystalline cellulose, although such predictions usually perform very well for macroscopic fiber materials, such as paper or fiber composites. We investigate this inconsistency and suggest explanations by identifying the critical factors affecting the stiffness of the nanopaper: The effect of drying constraints cannot solely explain the relatively low elastic modulus of nanopaper. Among the factors that showed the most influence is the presence of noncrystalline regions along the length of the nanofibrils, initial strains within the network structure and the three-dimensional structure of individual bonds.

  • 29. Kulachenko, A.
    et al.
    Lindström, S. B.
    Uesaka, T.
    Strength of wet fiber networks: Size scaling2009In: Proc. Papermaking Research Symposium, 2009Conference paper (Refereed)
    Abstract [en]

    In this work we investigate the strength scaling of wet fiber networks with the help of particle level network simulations. The objective is to identify the factors that control wet strength distributions and the way it changes with the size. This question is relevant for wet strength runnability. The simulations show that if the mean values of network properties are kept constant by process conditions, the disordered structure of the network produces only small scatters in strength for a sufficiently big network. The strength distribution at small scales does not follow weakest-link scaling with neither change of length nor width, presumably, due to the fact that "damage" clusters are too big compared to the size of considered networks.

  • 30. Kulachenko, A.
    et al.
    Uesaka, T.
    Lindström, S. B.
    Reinventing mechanics of fiber networks2008In: Progress in Paper Physics Seminar, 2008, p. 185-187Conference paper (Refereed)
  • 31.
    Kulachenko, Artem
    et al.
    KCL Science and Consulting.
    Lindström, Stefan B
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Uesaka, Tetsu
    FPInnovations.
    Strength of wet fiber networks-Strength scaling2009In: Papermaking Research Symposium 2009, Kuopio: University of Kuopio , 2009, p. 35-Conference paper (Other academic)
  • 32.
    Kulachenko, Artem
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Uesaka, Tetsu
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Lindström, Stefan B
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Reinventing mechanics of fibre network2008In: Progress in Paper Physics Seminar, Helsinki: Helsinki University Press, 2008, p. 185-193Conference paper (Other academic)
  • 33. Lavrykov, S.
    et al.
    Lindström, S. B.
    Singh, K. M.
    Ramarao, B. V.
    3D network simulations of paper structure2012In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 2, p. 256-263Article in journal (Refereed)
    Abstract [en]

    The structure of paper influences its properties and simulations of it are necessary to understand the impact of fiber and papermaking conditions on the sheet properties. We show a method to develop a representative structure of paper by merging different simulation techniques for the forming section and the pressing operation. The simulation follows the bending and drape of fibers over one another in the final structure and allows estimation of sheet properties without recourse to arbitrary bending rules or experimental measurements of density and/or RBA. Fibers are first modeled as jointed beams following the fluid mechanics in the forming section. The sheet structure obtained from this is representative of the wet sheet from the couch. The pressing simulation discretizes fibers into a number of solid elements around the lumen. Bonding between fibers is simulated using spring elements. The resulting fiber network was analyzed to determine its elastic modulus and deformation under small strains. The influence of fiber dimensions, namely fiber lengths, widths and thicknesses as well as bond stiffnesses on the elasticity of the network are studied. A brief account of inclusion of fines, represented by individual cubical elements is also shown.

  • 34. Lilja, M.
    et al.
    Karlsson, J.
    Jonsson, A.
    Hilding, D.
    Lindström, S. B.
    Leidermark, D.
    Schmidt, P.
    Incremental damage model for fatigue life assessment in complete machinery simulation2020In: 16th International LS-DYNA(R) Users Conference, 2020Conference paper (Refereed)
  • 35. Lindström, S.
    Lectures on Engineering Mechanics: Statics och Dynamics2019Book (Other academic)
  • 36. Lindström, S.
    Matematisk ordbok för högskolan: engelsk-svensk, svensk-engelsk2019Book (Other academic)
    Abstract [en]

    Matematisk ordbok med syftet att underlätta för svenskspråkiga högskole- och universitetsstudenter som läser matematikkurser med engelsk kurslitteratur på grundniv\aa. Urvalet av uppslagsord består av matematiska termer, termer som får utökad betydelse i en matematisk kontext samt mindre vanliga ord, som sällan förekommer i vardagligt språkbruk, men ofta i vetenskaplig text. Boken är fritt tillgänglig online.

  • 37. Lindström, S. B.
    Continuous-time, high-cycle fatigue model for nonproportional stress with validation for 7075-T6 aluminum alloy2020In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 140Article in journal (Refereed)
    Abstract [en]

    The continuous-time, high-cycle fatigue model of Ottosen et al. (2008) is modified by introducing a quadratic, instead of linear, endurance surface. This quadratic endurance surface induces an elliptical Haigh diagram, consistent with experimental fatigue data for ductile materials. With this modification we fit the model to uniaxial fatigue data, and predict the fatigue behavior for nonproportional, biaxial stress for \mboxAA7075-T6 in fully reversed shear with a superimposed, static tension/compression. However, the fatigue life is sometimes overestimated for a combination out-of-phase shear and tensile stress fluctuations.

  • 38. Lindström, S. B.
    Modeling and simulation of paper structure development2008Other (Other academic)
    Abstract [en]

    A numerical tool has been developed for particle-level simulations of fiber suspension flows, particularly forming of the fiber network structure of paper sheets in the paper machine. The model considers inert fibers of various equilibrium shapes, and finite stiffness, interacting with each other through normal, frictional, and lubrication forces, and with the surrounding fluid medium through hydrodynamic forces. Fiber–fluid interactions in the non-creeping flow regime are taken into account, and the two-way coupling between the solids and the fluid phases is included by enforcing momentum conservation between phases. The incompressible three-dimensional Navier-Stokes equations are employed to model the motion of the fluid medium. The validity of the model has been tested by comparing simulation results with experimental data from the literature. It was demonstrated that the model predicts well the motion of isolated fibers in shear flow over a wide range of fiber flexibilities. It was also shown that the model predicts details of the orientation distribution of multiple, straight, rigid fibers in a sheared suspension. Furthermore, model predictions of the shear viscosity and first normal stress difference were in fair agreement with experimental data found in the literature. Since the model is based solely on first principles physics, quantitative predictions could be made without any parameter fitting. Based on these validations, a series of simulations have been performed to investigate the basic mechanisms responsible for the development of the stress tensor components for monodispersed, non-Brownian fibers suspended in a Newtonian fluid in shear flow. The effects of fiber aspect ratio, concentration, and inter-particle friction, as well as the tendency of fiber agglomeration, were examined in the non-concentrated regimes. For the case of well dispersed suspensions, semi-empirical relationships were found between the aforementioned fiber suspension properties, and the steady state apparent shear viscosity, and the first/second normal stress differences. Finally, simulations have been conducted for the development of paper structures in the forming section of the paper machine. The conditions used for the simulations were retrieved from pilot-scale forming trial data in the literature, and from real pulp fiber analyses. Dewatering was simulated by moving two forming fabrics toward each other through a fiber suspension. Effects of the jet-to-wire speed difference on the fiber orientation anisotropy, the mass density distribution, and three-dimensionality of the fiber network, were investigated. Simulation results showed that the model captures well the essential features of the forming effects on these paper structure parameters, and also posed new questions on the conventional wisdom of the forming mechanics.

  • 39. Lindström, S. B.
    et al.
    Andrić, J.
    Sasic, S.
    Nilsson, H.
    Deviatoric stresses in suspensions of flexible or curved fibers2016In: Proceedings of th 9th International Conference on Multiphase Flow, 2016Conference paper (Refereed)
  • 40. Lindström, S. B.
    et al.
    Cervin, N. T.
    Wågberg, L.
    Thermally activated capillary intrusion of water into cellulose fiber-based materials2011In: Proceedings of the Fundamental and Applied Pulp & Paper Modeling Symposium 2011  PAPTAC, PAPTAC , 2011, p. 13-26Conference paper (Other academic)
    Abstract [en]

    The imbibition of water into cellulose fiber-based materials is studied with focus on the regime dominated by contact line dynamics rather than hydrodynamic drag of the bulk porous structure. Capillary rise is studied under the influence of gravity for different paper grades with a wide range of porosities. It is found that the capillary rise is logarithmic in time in the limit of long time-scales. This behavior is in excellent agreement with the Molecular Kinetics Theory (MKT) for contact line dynamics. For high-porosity paper grades, this previously neglected logarithmic regime starts already at about $5$\,cm of capillary intrusion, underscoring the critical importance of the contact line dynamics to the performance of cellulosic absorbents.

  • 41. Lindström, S. B.
    et al.
    Holmberg, L. J.
    Automated examination in Statics and Dynamics through individualized home assignments2017In: Svenska Mekanikdagar, 2017Conference paper (Refereed)
  • 42. Lindström, S. B.
    et al.
    Johansson, L.
    Karlsson, N. R.
    Metastable states and activated dynamics in thin-film adhesion to patterned surfaces2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 89Article in journal (Refereed)
    Abstract [en]

    We consider adhesion due to London–van der Waals attraction between a thin film and a patterned surface with nanometer asperities. Depending on the surface topography and the stiffness of the film, three regimes of adhesion are identified: complete contact adhesion, partial contact adhesion, and glassy adhesion. For complete contact adhesion, the film conforms to the undulations of the surface, whereas for partial contact and glassy adhesion, the adhesive interface breaks down into microscopic areas of contact. When a film in the glassy regime is peeled off the surface, metastable states develop at which the crack front becomes arrested, analogously to the frustrated motion of the three-phase contact line across a heterogeneous surface. For this glassy regime, we use transition state theory to model the thermally-activated progression of the crack front. This theoretical treatment suggests that the rate of the adhesive failure increases exponentially with the applied force.

  • 43. Lindström, S. B.
    et al.
    Karabulut, E.
    Kulachenko, A.
    Sehaqui, H.
    Wågberg, L.
    Mechanosorptive creep in nanocellulose materials2012In: Cellulose, ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, no 3, p. 809-819Article in journal (Refereed)
    Abstract [en]

    The creep behavior of nanocellulose films and aerogels are studied in a dynamic moisture environment, which is crucial to their performance in packaging applications. For these materials, the creep rate under cyclic humidity conditions exceeds any constant humidity creep rate within the cycling range, a phenomenon known as mechanosorptive creep. By varying the sample thickness and relative humidity ramp rate, it is shown that mechanosorptive creep is not significantly affected by the through-thickness moisture gradient. It is also shown that cellulose nanofibril aerogels with high porosity display the same accelerated creep as films. Microstructures larger than the fibril diameter thus appear to be of secondary importance to mechanosorptive creep in nanocellulose materials, suggesting that the governing mechanism is found between the length-scales of the fibril diameter and molecular scales.

  • 44. Lindström, S. B.
    et al.
    Kodger, T. E.
    Sprakel, J.
    Weitz, D. A.
    Structures, stresses, and fluctuations in the delayed failure of colloidal gels2012In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 8, no 13, p. 3657-3664Article in journal (Refereed)
    Abstract [en]

    Sample-spanning networks of aggregated colloidal particles have a finite stiffness and deform elastically when subjected to a small shear stress. After some period of creep, these gels ultimately suffer catastrophic failure. This delayed yielding is governed by the association and dissociation dynamics of interparticle bonds and the strand structure of the gel. We derive a model which connects the kinetics of the colloids to the erosion of the strand structure and ultimately to macroscopic failure. Importantly, this model relates time-to-failure of the gel to an applied static stress. Model predictions are in quantitative agreement with experiments. It is predicted that the strand structure, characterized by its mesh size and strand coarseness, has a significant impact on delay time. Decreasing the mesh size or increasing the strand thickness makes colloidal gels more resilient to delayed yielding. The quench and flow history of gels modify their microstructures. Our experiments show that a slow quenching increases the delay time due to the coarsening of the strands; by contrast, preshear reduces the delay time, which we explain by the increased mesh size as a result of shear-induced fracture of strands.

  • 45. Lindström, S. B.
    et al.
    Kulachenko, A.
    Jawerth, L. M.
    Vader, D. A.
    Finite-strain, finite-size mechanics of rigidly cross-linked biopolymer networks2013In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 9, no 30, p. 7302-7313Article in journal (Refereed)
    Abstract [en]

    The network geometries of rigidly cross-linked fibrin and collagen type I networks are imaged using confocal microscopy and characterized statistically. This statistical representation allows for the regeneration of large, three-dimensional biopolymer networks using an inverse method. Finite element analyses with beam networks are then used to investigate the large deformation, nonlinear elastic response of these artificial networks in isotropic stretching and simple shear. For simple shear, we investigate the differential bulk modulus, which displays three regimes: a linear elastic regime dominated by filament bending, a regime of strain-stiffening associated with a transition from filament bending to stretching, and a regime of weaker strain-stiffening at large deformations, governed by filament stretching convolved with the geometrical nonlinearity of the simple shear strain tensor. The differential bulk modulus exhibits a corresponding strain-stiffening, but reaches a distinct plateau at about 5\,% strain under isotropic stretch conditions. The small-strain moduli, the bulk modulus in particular, show a significant size-dependence up to a network size of about 100 mesh sizes. The large-strain differential shear modulus and bulk modulus show very little size-dependence.

  • 46. Lindström, S. B.
    et al.
    Lavrykov, S.
    Singh, K. M.
    Ramarao, B. V.
    Forming of paper sheets: A numerical simulation based on hydrodynamics of fibrous suspensions2012In: Proc. of PaperCon 2012, 2012, p. 1221-1279Conference paper (Refereed)
    Abstract [en]

    A fiber-level simulation of paper forming in a twin-wire roll former is conducted with a furnish typical of fine paper. A mix of bleached kraft softwood and hardwood pulps was chosen, each refined to different levels. The resultant web structure obtained from the couch roll was determined from these numerical experiments. This wet web structure is used as input to a simulation of wet pressing, which produces a consolidated sheet structure. Results regarding how the change in fiber mix (e.g. HW/SW ratio) and changes in pulp refining affect the structure of the web are shown. Important features such as sheet fiber orientations, anisotropy, retention of fibers, fines and fillers and sheet exit moisture are all predicted by the simulation.

  • 47.
    Lindström, S. B.
    et al.
    Linköping University.
    Moverare, J.
    Linköpings universitet.
    Leidermark, D.
    Linköpings universitet.
    Ansell, H.
    Linköpings universitet.
    Kapidzic, Z.
    Linköpings universitet.
    Incremental fatigue damage modeling of 7050-T7 aluminum alloy at stress-raisers2022In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 161, article id 106878Article in journal (Refereed)
    Abstract [en]

    The Ottosen–Stenström–Ristinmaa (OSR) incremental fatigue damage model, based on an moving endurance surface centered around a backstress, is adapted for high-cycle fatigue at stress-raisers in AA7050-T7 specimens. Fatigue experiments are carried out for circumferentially-notched, axisymmetric specimens subjected to constant-amplitude (CA) load. The OSR model parameters are fitted to CA fatigue data, showing fair agreement for one set of model parameters across different stress ratios, stress concentration factors, uniaxial stress and biaxial stress. To demonstrate predictive capability, the fatigue life is integrated for an aircraft load spectrum (TWIST), and compared with experimental fatigue life data for holeplate specimens in the literature.

  • 48.
    Lindström, S. B.
    et al.
    Linköping University.
    Moverare, J.
    Linköpings universitet.
    Leidermark, D.
    Linköpings universitet.
    Ansell, H.
    Linköpings universitet.
    Schmidt, P.
    Linköpings universitet.
    Kapidzic, Z.
    Linköpings universitet.
    Fatigue life assessment of integral structures2022Conference paper (Refereed)
  • 49. Lindström, S. B.
    et al.
    Satha, G.
    Klarbring, A.
    Extension of Murray’s law including nonlinear mechanics of a composite artery wall2015In: Biomechanics and Modeling in Mechanobiology, ISSN 1617-7959, E-ISSN 1617-7940, Vol. 14, no 1, p. 83-91Article in journal (Refereed)
    Abstract [en]

    A goal function approach is used to derive an extension of Murray’s law that includes effects of nonlinear mechanics of the artery wall. The artery is modeled as a thin-walled tube composed of different species of nonlinear elastic materials that deform together. These materials grow and remodel in a process that is governed by a target state defined by a homeostatic radius and a homeostatic material composition. Following Murray’s original idea, this target state is defined by a principle of minimum work. We take this work to include that of pumping and maintaining blood, as well as maintaining the materials of the artery wall. The minimization is performed under a constraint imposed by mechanical equilibrium. We derive a condition for the existence of a cost-optimal homeostatic state. We also conduct parametric studies using this novel theoretical frame to investigate how the cost-optimal radius and composition of the artery wall depend on flow rate, blood pressure and elastin content.

  • 50. Lindström, S. B.
    et al.
    Sprakel, J.
    Thermally-activated delayed failure in heterogeneous solids: An experimental model system2013In: Proc. of Svenska Mekanikdagar, 2013Conference paper (Refereed)
    Abstract [en]

    Many pervasive construction materials, including wood, paper, plastics and concrete, exhibit creep when subjected to stresses below the yield stress. This can lead to catastrophic failure at prolonged, moderate loads. Remarkably, the creep response of these dissimilar materials shares at least two important features: First, the average time-to-failure decreases exponentially with the applied stress. This suggests that a stress-enhanced, thermally-activated process governs the creep. Secondly, the variability of the time-to-failure between samples is huge, which is usually attributed to their stochastic microstructures. Owing to the strong molecular bonds between the constituents of typical construction materials, they creep slowly, and the time-to-failure under typical loading conditions can be very long, which is experimentally challenging. Therefore, we consider an experimental model system for studying the thermally-activated, delayed failure. Weakly attractive colloidal particles in suspension form a sample-spanning network with the low-frequency mechanical response of an elastic solid. This material is a statistically homogeneous, hierarchically structured solid, with two distinct length-scales: that of the particles and that of the filaments. The interaction potential between the particles is controlled in this system, so that the thermally-activated remodeling of the material operates on experimental time-scales. The creep response of the colloidal gel is investigated in simple shear, using a stress-controlled rheometer. For each constant shear stress, the shear strain is measured as a function of time. After an initial elastic deformation, the material deforms slowly at an essentially constant rate. After a time-delay, the material fails abruptly. This delay time decreases exponentially with the applied stress. Moreover, if the gel is made anisotropic by applying a high rate pre-shear, the delay time is reduced, and two regimes appear with different exponential factors. It is hypothesized that the initial creep is governed by a distributed, stochastic failure process, which preserves the statistical homogeneity of the sample. This slow process is followed by avalanching critical crack growth and failure. The time-to-failure is dominated by the initial process, which can be model using mean-field theory, by virtue of the assumed statistical homogeneity. In this model, the stress-enhanced dissociation dynamics of individual particle bonds are related to the stochastic fracture of strands, which, in turn, are govern the delay time of the macroscopic failure.

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