| 1. |
- Kumar Rajasekharan, Anand, 1990, et al.
(författare)
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Hierarchical and Heterogeneous Bioinspired Composites-Merging Molecular Self-Assembly with Additive Manufacturing
- 2017
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Ingår i: Small. - : Wiley. - 1613-6810 .- 1613-6829. ; 13:28
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Tidskriftsartikel (refereegranskat)abstract
- Biological composites display exceptional mechanical properties owing to a highly organized, heterogeneous architecture spanning several length scales. It is challenging to translate this ordered and multiscale structural organization in synthetic, bulk composites. Herein, a combination of top-down and bottom-up approach is demonstrated, to form a polymer-ceramic composite by macroscopically aligning the self-assembled nanostructure of polymerizable lyotropic liquid crystals via 3D printing. The polymer matrix is then uniformly reinforced with bone-like apatite via in situ biomimetic mineralization. The combinatorial method enables the formation of macrosized, heterogeneous composites where the nanostructure and chemical composition is locally tuned over microscopic distances. This enables precise control over the mechanics in specific directions and regions, with a unique intrinsic-extrinsic toughening mechanism. As a proof-of-concept, the method is used to form large-scale composites mimicking the local nanostructure, compositional gradients and directional mechanical properties of heterogeneous tissues like the bone-cartilage interface, for mechanically stable osteochondral plugs. This work demonstrates the possibility to create hierarchical and complex structured composites using weak starting components, thus opening new routes for efficient synthesis of high-performance materials ranging from biomaterials to structural nanocomposites.
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| 2. |
- Sandström, Carl, 1978, et al.
(författare)
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A two-scale finite element formulation of Stokes flow in porous media
- 2013
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : Elsevier BV. - 0045-7825. ; 261, s. 96-104
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Tidskriftsartikel (refereegranskat)abstract
- Seepage through saturated porous material with an open pore system is modeled as a non-linear Stokes flow through a rigid matrix. Based on variationally consistent homogenization, the resulting macroscale problem becomes a Darcy-type flow. The prolongation of the Darcy flow fulfills a macrohomogeneity condition, which in a Galerkin context implies a symmetric macroscale problem. The homogenization is of 1st order and periodic boundary conditions are adopted on a Representative Volume Element. A nonlinear nested multiscale technique, in which the subscale problem is used as a constitutive model, is devised. In the presented numerical investigation, the effects of varying physical parameters as well as of the discretization are considered. In particular, it is shown that the two-scale results agree well with those of the fully resolved fine-scale problem.
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| 3. |
- Sandström, Carl, 1978, et al.
(författare)
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Homogenization of coupled flow and deformation in a porous material
- 2016
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : Elsevier BV. - 0045-7825. ; 308, s. 535-551
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a framework for computational homogenization of the fluid-solid interaction that pertains to the coupled deformation and flow of pore fluid in a fluid-saturated porous material. Large deformations are considered and the resulting problem is established in the material setting. In order to ensure a proper FE-mesh in the fluid domain of the RVE, we introduce a fictitious elastic solid in the pores; however, the adopted variational setting ensures that the fictitious material does not obscure the motion of the (physical) solid skeleton. For the subsequent numerical evaluation of the RVE-response, hyperelastic properties are assigned to the solid material, whereas the fluid motion is modeled as incompressible Stokes' flow. Variationally consistent homogenization of the standard first order is adopted. The homogenization is selective in the sense that the resulting macroscale (upscaled) porous media model reminds about the classical one for a quasi-static problem with displacements and pore pressure as the unknown macroscale fields. Hence, the (relative) fluid velocity, i.e. seepage, "lives" only on the subscale and is part of the set of unknown fields in the RVE-problem. As to boundary conditions on the RVE, a mixture of Dirichlet and weakly periodic conditions is adopted. In the numerical examples, special attention is given to an evaluation of the Biot coefficient that occurs in classical phenomenological models for porous media.
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| 4. |
- Sandström, Carl, 1978, et al.
(författare)
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Homogenization of Stokes Flow in Porous Media
- 2012
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Ingår i: ECCOMAS 2012, European Congress on Computational Methods in Science and Engineering, (1 p. abstract).
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 5. |
- Sandström, Carl, 1978, et al.
(författare)
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Multiscale modeling of porous media
- 2011
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Ingår i: Proceedings of NSCM-24, the 24th Nordic Seminar on Computational Mechanics. ; , s. 111-114
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Konferensbidrag (refereegranskat)
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| 6. |
- Sandström, Carl, 1978, et al.
(författare)
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Multiscale Modeling of Porous Media
- 2012
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Ingår i: Proceeding of the 25th Nordic Seminar on Computational Mechanics. ; , s. 137-140, s. 137-140
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 7. |
- Sandström, Carl, 1978, et al.
(författare)
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MULTISCALE MODELING OF POROUS MEDIA
- 2010
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Ingår i: Proceedings of NSCM-23, Anders Eriksson och Gunnar Tibert, Nordic Seminar on Computational Mechanics, Sockholm, 21-22 October 2010. - 0348-467X. ; 2010, s. 177-180
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Konferensbidrag (refereegranskat)abstract
- Homogenization of flow in porous media is studied. The continuity equation in conjunction with Darcy’s law as a constitutive relation on the macroscale, where the permeability is a function of the pressure gradient, is applied to a macroscopic domain. On the heterogenousmesoscale, a Stokes flow problem is formulated on a Representative Volume Element with a prescribed pressure gradient and suitable boundary condition. The numerical procedure for finite element simulations of the two-scale problem is outlined and illustrated by a few exampleproblems.
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| 8. |
- Sandström, Carl, 1978, et al.
(författare)
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On bounded approximations of periodicity for computational homogenization of Stokes flow in porous media
- 2017
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Ingår i: International Journal for Numerical Methods in Engineering. - : Wiley. - 0029-5981 .- 1097-0207. ; 109:3, s. 307-325
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Tidskriftsartikel (refereegranskat)abstract
- By separation of scales and the homogenization of a flow through porous media, a two-scale problem arises where a Darcy-type flow is present on the macroscale and a Stokes flow on the subscale. In this paper, the problem is given as the minimization of a potential. Additional constraints imposing periodicity in a weak sense are added using Lagrange multipliers. In particular, the upper and lower energy bounds for the corresponding strongly periodic problem are produced, quantifying the accuracy of the weakly periodic boundary conditions. A numerical example demonstrates the evaluation of energy bounds and the performance of weakly periodic boundary conditions on a representative volume element.
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| 9. |
- Sandström, Carl, 1978
(författare)
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On Computational Homogenization of Flow in Porous Media
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Porous materials are present in many natural as well as engineered structures. Engineering examples of porous materials include sandstone in which oil can be stored, earth dams, composite materials, air filters and sanitary products with fiber networks (e.g. diapers). The members of this class of materials possess a strongly heterogeneous substructure consisting of a fluid contained in a solid matrix. As the scale of the substructural features is considerably smaller than the scale of the engineered component, taking the complete substructure into account when performing computations is virtually impossible due to limitations in computer power and memory.The traditional approach to the modeling of seepage in a porous medium is to adopt a phenomenological model, the simplest one being the (linear) Darcy’s law, which is calibrated using experimental data. However, the absence of fundamental physical interpretation of the model implies that a change in the fluid phase calls for new experiments. This thesis concerns the modeling of porous media using a two-scale model, where a Stokes flow is present on the heterogeneous subscale. Homogenization of the subscale problem is carried out on a Representative Volume Element (RVE) comprising the subscale phases. As a result, the single relevant balance equation on the macroscale is that of mass balance and Darcy’s permeability model is recovered. Both a priori homogenization (upscaling) and concurrent multiscale computations, in which the RVE problem is solved in each gausspoint on the macro level (FE2 ), are carried out. Hence, in the numerical simulations, both linear and nonlinear subscale flows are considered.
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| 10. |
- Sandström, Carl, 1978
(författare)
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On Computational Homogenization of Fluid-filled Porous Materials
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Porous materials are present in many natural as well as engineered structures. Engineering examples include filters, sanitary products and foams while examples of natural occurrences are oil reservoirs and biological tissue. These materials possess a strongly heterogeneous microstructure consisting of a contiguous solid skeleton, more or less saturated with fluid. The scale of the substructural features is normally much smaller than that of the engineering structure. For instance, groundwater flow takes place at a length scale of kilometers while the pores and channels where the fluid is transported have a length scale of millimeters. Thus, taking the complete microstructure into consideration when performing analysis on such structures is simply too computationally demanding.Traditionally, computations on porous materials are performed using phenomenological models, the simplest one being the linear Darcy's law which relates the seepage and pressure gradient. However, this thesis concerns the modeling of porous materials using homogenization, where the macroscale properties are derived from the subscale. This technique can either be used to calibrate existing phenomenological models or in a fully concurrent setting where a subscale model replaces the macroscale material model in each Gauss point in an FE-setting. The latter constitutes the so-called FE2-approach. The obvious drawback of FE2 is that the computational cost, while smaller than the fully resolved case, is still high. Due to its mathematical and physical consistency, the method used is the Variationally Consistent Homogenization method.The ultimate goal of this work is to predict the mechanical behaviour of a two-phase material consisting of fluid that flows through a deformable open-pore solid. An important feature is that interaction between the solid and the fluid phases is taken into account. It is required that the modeling is performed in 3D, since the solid phase in a 2D model of a porous material is not connected and can, therefore, not sustain mechanical loading. The issue of imposing periodic boundary conditions on a unstructured, non-periodic mesh is addressed. Numerical results include the assesment of how the pore characteristics affect the macroscopic permeability, comparison of solutions pertaining to the fully resolved problem versus the homogenized problem, performance of weakly periodic boundary conditions and the interaction of fluid and deforming solid.
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