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| 3. |
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| 4. |
- Girlanda, Orlando, et al.
(författare)
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Analysis of the Micromechanical Deformation in Pressboard performed by X-ray Microtomography
- 2015
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Ingår i: IEEE Electrical Insulation Conference (EIC). - Piscataway, NJ : IEEE Communications Society. - 9781479973545 ; , s. 89-92
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Konferensbidrag (refereegranskat)abstract
- A large number of electrical insulation components are produced in paper-based materials. Paper combines good insulating properties with the necessary mechanical and chemical stability. Paper consists of a system of fibers binding to each other creating a strong network. The presence of large open pores allows for impregnability of the material but also causes mechanical weakness in particular in the out-of-plane direction of the material. This aspect is important for pressboard components, where the resistance to compression stress is relevant for e.g. transformer windings. It is therefore relevant to understand the mechanisms that underlay the out-of-plane deformation of pressboard. In order to get a clear picture of the deformation patterns within the material, X-ray micro-computed tomography was used. Pressboard test pieces were subjected to in-situ out-of-plane compressive loading. 3D images of the sample could be captured before, during and after the loading sequence. Image analysis allowed for the definition of strain fields. The results revealed a strong correlation between the density variation within the sample and the strain calculated from the 3D images.
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| 5. |
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| 6. |
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| 7. |
- Joffre, Thomas, 1987-, et al.
(författare)
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A 3D in-situ investigation of the deformation in compressive loading in the thickness direction of cellulose fiber mats
- 2015
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 22:5, s. 2993-3001
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Tidskriftsartikel (refereegranskat)abstract
- Fiber mat materials based on cellulose natural fibers combines a useful set of properties, including renewability, stiffness, strength and dielectric insulation, etc. The dominant in-plane fiber orientation ensures the in-plane performance, at the expense of reduced out-of-plane behavior, which has not been studied as extensively as the in-plane behavior. Quantitative use of X-ray micro-computed tomography and strain analyses under in-situ loading open up possibilities to identify key mechanisms responsible for deformation. In the present investigation, focus is placed on the out-of-plane deformation under compressive loading of thick, high density paper, known as pressboard. The samples were compressed in the chamber of a microtomographic scanner. 3D images were captured before and after the loading the sample. From sequential 3D images, the strain field inside the material was calculated using digital volume correlation. Two different test pieces were tested, namely unpolished and surface polished ones. The first principal strain component of the strain tensor showed a significant correlation with the density variation in the material, in particular on the top and bottom surfaces of unpolished samples. The manufacturing-induced grooves generate inhomogeneities in the microstructure of the surface, thus creating high strain concentration zones which give a sensible contribution to the overall compliance of the unpolished material. More generally, the results reveal that, on the micrometer scale, high density fiber pressboard behaves as a porous material rather than a low density fiber network.
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| 8. |
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| 9. |
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| 10. |
- Joffre, Thomas, 1987-, et al.
(författare)
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Characterization of interfacial stress transfer ability in acetylation-treated wood fibre composites using X-ray microtomography
- 2017
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Ingår i: Industrial crops and products (Print). - : Elsevier BV. - 0926-6690 .- 1872-633X. ; 95, s. 43-49
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Tidskriftsartikel (refereegranskat)abstract
- The properties of the fibre/matrix interface contribute to stiffness, strength and fracture behaviour of fibre-reinforced composites. In cellulosic composites, the limited affinity between the hydrophilic fibres and the hydrophobic thermoplastic matrix remains a challenge, and the reinforcing capability of the fibres is hence not fully utilized. A direct characterisation of the stress transfer ability through pull-out tests on single fibres is extremely cumbersome due to the small dimension of the wood fibres. Here a novel approach is proposed: the length distribution of the fibres sticking out of the matrix at the fracture surface is approximated using X-ray microtomography and is used as an estimate of the adhesion between the fibres and the matrix. When a crack grows in the material, the fibres will either break or be pulled-out of the matrix depending on their adhesion to the matrix: good adhesion between the fibres and the matrix should result in more fibre breakage and less pull-out of the fibres than poor adhesion. The effect of acetylation on the adhesion between the wood fibres and the PLA matrix was evaluated at different moisture contents using the proposed method. By using an acetylation treatment of the fibres it was possible to improve the strength of the composite samples soaked in the water by more than 30%.
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| 11. |
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| 12. |
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| 13. |
- Joffre, Thomas, et al.
(författare)
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Effects of defects on the tensile strength of short-fibre composite materials
- 2014
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Ingår i: Mechanics of materials. - : Elsevier BV. - 0167-6636 .- 1872-7743. ; 75, s. 125-134
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Tidskriftsartikel (refereegranskat)abstract
- Heterogeneous materials tend to fail at the weakest cross-section, where the presence of microstructural heterogeneities or defects controls the tensile strength. Short-fibre composites are an example of heterogeneous materials, where unwanted fibre agglomerates are likely to initiate tensile failure. In this study, the dimensions and orientation of fibre agglomerates have been analysed from three-dimensional images obtained by X-ray microtomography. The geometry of the specific agglomerate responsible for failure initiation has been identified and correlated with the strength. At the plane of fracture, a defect in the form of a large fibre agglomerate was almost inevitably found. These new experimental findings highlight a problem of some existing strength criteria, which are principally based on a rule of mixture of the strengths of constituent phases, and not on the weakest link. Only a weak correlation was found between stress concentration induced by the critical agglomerate and the strength. A strong correlation was however found between the stress intensity and the strength, which underlines the importance of the size of largest defects in formulation of improved failure criteria for short-fibre composites. The increased use of three-dimensional imaging will facilitate the quantification of dimensions of the critical flaws.
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| 14. |
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| 15. |
- Joffre, Thomas, et al.
(författare)
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Microscopic strain fields at crack tips in porous materials analyzed by a gradient-enhanced elasticity theory
- 2016
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Ingår i: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 168, s. 160-173
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Tidskriftsartikel (refereegranskat)abstract
- The microstructural influence on the strain field at opening mode crack tips in porous materials, and especially its practical implication for understanding macroscopic failure, i.e. on a scale above, is investigated. Theoretical subscale microstrain fields are approximated using a gradient-enhanced elasticity theory and compared to microstrain fields computed in discrete high-resolution finite element microstructural models having varying pore densities but similar macroscopic geometry and boundary conditions as the theoretical gradient-enhanced model. The numerical elastic microstrain and microstress fields are non-singular in strong contrast to the singular macroscopic fields in classical linear elastic fracture theories. Experimentally approximated microstrain fields, estimated with a digital image correlation algorithm on images obtained in X-ray computational tomography fracture tests on a small wood specimen, are used to contrast the.numerical analyses. A key observation is that an internal length parameter, used in the gradient-enhanced model, seems to be linked to the average pore diameter, allowing for direct bridging between scales.
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| 16. |
- Joffre, Thomas, et al.
(författare)
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Modelling of the hygroelastic behaviour of normal and compression wood tracheids
- 2014
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Ingår i: Journal of Structural Biology. - : Elsevier BV. - 1047-8477 .- 1095-8657. ; 185:1, s. 89-98
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Tidskriftsartikel (refereegranskat)abstract
- Compression wood conifer tracheids show different swelling and stiffness properties than those of usual normal wood, which has a practical function in the living plant: when a conifer shoot is moved from its vertical position, compression wood is formed in the under part of the shoot. The growth rate of the compression wood is faster than in the upper part resulting in a renewed horizontal growth. The actuating and load-carrying function of the compression wood is addressed, on the basis of its special ultrastructure and shape of the tracheids. As a first step, a quantitative model is developed to predict the difference of moisture-induced expansion and axial stiffness between normal wood and compression wood. The model is based on a state space approach using concentric cylinders with anisotropic helical structure for each cell-wall layer, whose hygroelastic properties are in turn determined by a self-consistent concentric cylinder assemblage of the constituent wood polymers. The predicted properties compare well with experimental results found in the literature. Significant differences in both stiffness and hygroexpansion are found for normal and compression wood, primarily due to the large difference in microfibril angle and lignin content. On the basis of these numerical results, some functional arguments for the reason of high microfibril angle, high lignin content and cylindrical structure of compression wood tracheids are supported.
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| 17. |
- Joffre, Thomas, 1987-
(författare)
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Structure and Mechanical Behaviour of Wood-Fibre Composites
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wood fibres have several advantages compared to man-made synthetic fibres: they have high specific stiffness, are renewable, relatively inexpensive, available in industrial quantities and biodegradable. However, to increase and diversify their utilisation, it is necessary to increase the understanding on what controls their mechanical properties.In this work, the hygroelastic behaviour of isolated wood fibres has been investigated using an analytical model and a finite element model based on three dimensional images obtained using synchrotron-based X-ray micro-computed tomography. It was thus possible to show how the cell wall responds to a mechanical load or a change in ambient relative humidity.The wood fibres were then mixed with a biopolymer aiming to produce a cost-efficient, 100% renewable composite material. The microstructure of the produced composites has been characterised using X-ray microtomography and digital image processing. It was for instance possible to measure the moisture-induced swelling of fibres embedded in a polymeric matrix. The experimental results have then been successfully compared with prediction obtained with a finite element model. The length of the fibres inside the composite has also been measured from three dimensional images, aiming to understand how each step of the processing chain is affecting the degradation of the aspect ratio of the reinforcing fibres.The presence of defects inside the composite has also been quantified using X-ray microtomography. The effects of the defects on the tensile strength have been predicted using an analytical model. The results have been compared with the measured tensile strength on each sample, showing that the size and orientation of the critical defect controls the tensile strength of the material.Finally, wood-fibre mats without any matrix material were compressed in the chamber of a microtomographic scanner. Sequential images were taken during the test. Using digital volume correlation, it was possible to calculate the local strain field inside the material. The effects of heterogeneities on the strain field have then been investigated. The applied compressive load resulted in transport of material from high to low density regions.
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| 18. |
- Joffre, Thomas, et al.
(författare)
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Trabecular deformations during screw pull-out : a micro-CT study of lapine bone
- 2017
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Ingår i: Biomechanics and Modeling in Mechanobiology. - : SPRINGER HEIDELBERG. - 1617-7959 .- 1617-7940. ; 16:4, s. 1349-1359
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical fixation of endosseous implants, such as screws, in trabecular bone is challenging because of the complex porous microstructure. Development of new screw designs to improve fracture fixation, especially in high-porosity osteoporotic bone, requires a profound understanding of how the structural system implant/trabeculae interacts when it is subjected to mechanical load. In this study, pull-out tests of screw implants were performed. Screws were first inserted into the trabecular bone of rabbit femurs and then pulled out from the bone inside a computational tomography scanner. The tests were interrupted at certain load steps to acquire 3D images. The images were then analysed with a digital volume correlation technique to estimate deformation and strain fields inside the bone during the tests. The results indicate that the highest shear strains are concentrated between the inner and outer thread diameter, whereas compressive strains are found at larger distances from the screw. Tensile strains were somewhat smaller. Strain concentrations and the location of trabecular failures provide experimental information that could be used in the development of new screw designs and/or to validate numerical simulations.
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| 19. |
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| 20. |
- Marais, Andrew, et al.
(författare)
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New insights into the mechanisms behind the strengthening of lignocellulosic fibrous networks with polyamines
- 2014
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 21:6, s. 3941-3950
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Tidskriftsartikel (refereegranskat)abstract
- Polyelectrolytes have been used extensively in the papermaking industry for various purposes. Although recent studies have shown that polyamines can be efficient dry-strength additives, the mechanism governing the strength enhancement of paper materials following the adsorption of polyamines onto pulp fibres is still not well understood. In this study, the effect of the adsorption of polyallylamine hydrochloride (PAH) onto the surface of unbleached kraft pulp fibres was investigated on both the fibre and the network scale. Isolated fibre crosses were mechanically tested to evaluate the impact of the chemical additive on the interfibre joint strength on the microscopic scale and the effect was compared with that previously observed on the paper sheet scale. X-ray microtomography was used to understand structural changes in the fibrous network following the adsorption of a polyamine such as PAH. Using image analysis methods, it was possible to determine the number of interfibre contacts (or joints) per unit length of fibre as well as the average interfibre joint contact area. The results showed that the median interfibre joint strength increased by 18 % upon adsorption of PAH. This can be achieved both by a larger molecular contact area in the contact zones and by a stronger molecular adhesion. The addition of the polymer also increased the number of efficient interfibre contacts per sheet volume. This combination of effects is the reason why polyamines such as PAH can increase the dry tensile strength of paper materials.
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| 21. |
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| 22. |
- Peaucelle, Marc, et al.
(författare)
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Covariations between plant functional traits emerge from constraining parameterization of a terrestrial biosphere model
- 2019
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Ingår i: Global Ecology and Biogeography. - : Wiley. - 1466-822X .- 1466-8238. ; 28:9, s. 1351-1365
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Tidskriftsartikel (refereegranskat)abstract
- Aim: The mechanisms of plant trait adaptation and acclimation are still poorly understood and, consequently, lack a consistent representation in terrestrial biosphere models (TBMs). Despite the increasing availability of geo-referenced trait observations, current databases are still insufficient to cover all vegetation types and environmental conditions. In parallel, the growing number of continuous eddy-covariance observations of energy and CO2 fluxes has enabled modellers to optimize TBMs with these data. Past attempts to optimize TBM parameters mostly focused on model performance, overlooking the ecological properties of ecosystems. The aim of this study was to assess the ecological consistency of optimized trait-related parameters while improving the model performances for gross primary productivity (GPP) at sites. Location: Worldwide. Time period: 1992–2012. Major taxa studied: Trees and C3 grasses. Methods: We optimized parameters of the ORCHIDEE model against 371 site-years of GPP estimates from the FLUXNET network, and we looked at global covariation among parameters and with climate. Results: The optimized parameter values were shown to be consistent with leaf-scale traits, in particular, with well-known trade-offs observed at the leaf level, echoing the leaf economic spectrum theory. Results showed a marked sensitivity of trait-related parameters to local bioclimatic variables and reproduced the observed relationships between traits and climate. Main conclusions: Our approach validates some biological processes implemented in the model and enables us to study ecological properties of vegetation at the canopy level, in addition to some traits that are difficult to observe experimentally. This study stresses the need for: (a) implementing explicit trade-offs and acclimation processes in TBMs; (b) improving the representation of processes to avoid model-specific parameterization; and (c) performing systematic measurements of traits at FLUXNET sites in order to gather information on plant ecophysiology and plant diversity, together with micro-meteorological conditions.
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| 23. |
- Recio-Blanco, A., et al.
(författare)
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The Gaia-ESO Survey: the Galactic thick to thin disc transition
- 2014
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 567
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The nature of the thick disc and its relation to the thin disc is presently an important subject of debate. In fact, the structural and chemo-dynamical transition between disc populations can be used as a test of the proposed models of Galactic disc formation and evolution. Methods. We used the atmospheric parameters, [alpha/Fe] abundances, and radial velocities, which were determined from the Gaia-ESO Survey GIRAFFE spectra of FGK-type stars (first nine months of observations) to provide a chemo-kinematical characterisation of the disc stellar populations. We focussed on a subsample of 1016 stars with high-quality parameters, covering the volume vertical bar Z vertical bar < 4.5 kpc and R in the range 2-13 kpc. Results. We have identified a thin to thick disc separation in the [alpha/Fe] vs. [M/H] plane, thanks to the presence of a low-density region in the number density distribution. The thick disc stars seem to lie in progressively thinner layers above the Galactic plane, as metallicity increases and [alpha/Fe] decreases. In contrast, the thin disc population presents a constant value of the mean distance to the Galactic plane at all metallicities. In addition, our data confirm the already known correlations between V-phi and [M/H] for the two discs. For the thick disc sequence, a study of the possible contamination by thin disc stars suggests a gradient up to 64 +/- 9 km s(-1) dex(-1). The distributions of azimuthal velocity, vertical velocity, and orbital parameters are also analysed for the chemically separated samples. Concerning the gradients with galactocentric radius, we find, for the thin disc, a flat behaviour of the azimuthal velocity, a metallicity gradient equal to -0.058 +/- 0.008 dex kpc(-1) and a very small positive [alpha/Fe] gradient. For the thick disc, flat gradients in [M/H] and [alpha/Fe] are derived. Conclusions. Our chemo-kinematical analysis suggests a picture where the thick disc seems to have experienced a settling process, during which its rotation increased progressively and, possibly, the azimuthal velocity dispersion decreased. At [M/H] approximate to -0.25 dex and [alpha/Fe] approximate to 0.1 dex, the mean characteristics of the thick disc in vertical distance to the Galactic plane, rotation, rotational dispersion, and stellar orbits' eccentricity agree with that of the thin disc stars of the same metallicity, suggesting a possible connection between these two populations at a certain epoch of the disc evolution. Finally, the results presented here, based only on the first months of the Gaia-ESO Survey observations, confirm how crucial large high-resolution spectroscopic surveys outside the solar neighbourhood are today for our understanding of the Milky Way history.
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| 24. |
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| 25. |
- Tummala, Gopi Krishna, 1986-, et al.
(författare)
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Hyperelastic Nanocellulose-Reinforced Hydrogel of High Water Content for Ophthalmic Applications
- 2016
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Ingår i: ACS Biomaterials Science & Engineering. - : American Chemical Society (ACS). - 2373-9878. ; 2:11, s. 2072-2079
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Tidskriftsartikel (refereegranskat)abstract
- A nanocellulose-reinforced poly(vinyl alcohol) hydrogel material of exceptionally high water content for ophthalmic applications is presented (>90 wt %), which also features a hitherto unprecedented combination of optical, mechanical, viscoelastic, oxygen permeability, and biocompatibility properties. The hydrogel combines the desired softness with remarkable strain-dependent mechanical strength and thereby demonstrates hyperelastic, rubber-like mechanical properties. The observed unusual mechanical behavior is due to both high water content and the combination of relatively stiff cellulose nanowhiskers entangled in a soft polymer matrix of poly(vinyl alcohol) (PVA), thus mimicking the structural characteristics of the cornea's main constituents, i.e., water and collagen.
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| 26. |
- Tummala, Gopi Krishna, 1986-, et al.
(författare)
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Strain-induced stiffening of nanocellulose-reinforced poly(vinyl alcohol) hydrogels mimicking collagenous soft tissues
- 2017
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-683X .- 1744-6848. ; 13:21, s. 3936-3945
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Tidskriftsartikel (refereegranskat)abstract
- Soft tissues possess remarkable mechanical strength for their high water content, which is hard to mimic in synthetic materials. Here, we demonstrate how strain-induced stiffening in hydrogels plays a major role in mimicking the mechanical properties of collagenous soft tissues. In particular, nanocellulose reinforced polyvinyl alcohol (PVA) hydrogels of exceptionally high water content (90-93 wt%) are shown to exhibit collagen-like mechanical behavior typical for soft tissues. High water content and co-existence of both soft and rigid domains in the gel network are the main factors responsible for strain-induced stiffening. This observed effect due to the alignment of rigid components of the hydrogel is simulated through modeling and visualized through strain-induced birefringence experiments. Design parameters such as nanocellulose aspect ratio and solvent composition are also shown to be important to control the mechanical properties. In addition, owing to their transparency (90-95% at 550 nm) and hyperelastic properties (250-350% strain), the described hydrogels are promising materials for biomedical applications, especially in ophthalmology.
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| 27. |
- Wernersson, Erik, et al.
(författare)
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Swelling of cellulose fibres in composite materials: Constraint effects of the surrounding matrix
- 2013
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Ingår i: Composites Science and Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 74, s. 52-59
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Tidskriftsartikel (refereegranskat)abstract
- Wood fibres have several highly desirable properties as reinforcement in composite materials for structural applications, e.g. high specific stiffness and strength, renewability and low cost. However, one of the main drawbacks is the swelling of these hydrophilic fibres due to moisture uptake. Since the fibres in the composite are generally embedded in a relatively hydrophobic matrix, the surrounding matrix should restrain the swelling of the fibres. The present study investigates this constraint effect and establishes a micromechanical model to predict the swelling of embedded fibres based on experimentally characterised microstructural parameters and hygroelastic properties of the constituents. The predicted swelling is in concert with direct measurement of various wood-pulp fibre composites by means of three-dimensional X-ray microtomographic images. (c) 2012 Elsevier Ltd. All rights reserved.
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| 28. |
- Wu, Dan, 1990-, et al.
(författare)
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A combined experimental and numerical method to estimate the elastic modulus of single trabeculae
- 2022
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Ingår i: Journal of The Mechanical Behavior of Biomedical Materials. - : Elsevier. - 1751-6161 .- 1878-0180. ; 125
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Tidskriftsartikel (refereegranskat)abstract
- The elastic modulus at the single trabecular level is an important parameter for the understanding of the mechanical behavior of trabecular bone. Current methods are commonly limited by the irregular trabecular shape and the accuracy of displacement measurement. The aim of this study was to propose a method to estimate the trabecular modulus overcoming some of these limitations. For high-precision displacement measurements, insitu compression within a synchrotron radiation based X-ray tomograph was used. Trabecular displacements were subsequently estimated by a global digital volume correlation algorithm, followed by high-resolution finite element analyses to account for the irregular geometry. The trabecular elastic moduli were then estimated by comparing the loads from the finite element analyses with those of the experiments. With this strategy, the average elastic modulus was estimated to 3.83 +/- 0.54 GPa for three human trabeculae samples. Though limited by the sample size, the demonstrated method shows a potential to estimate the mechanical properties at the single trabecular level.
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