| 1. |
- Correa, E., et al.
(author)
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Effects of the presence of compression in transverse cyclic loading on fibre-matrix debonding in unidirectional composite plies
- 2007
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In: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 38:11, s. 2260-2269
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Journal article (peer-reviewed)abstract
- Fatigue of composite materials is of great concern in load-carrying structures. The first type of damage to appear is generally transverse cracks in off-axis plies. These cracks form when fibre-matrix debonds coalesce. The underlying mechanism is hence fatigue growth of debonds at the fibre-matrix interfaces. In the present study, debond growth has been characterized under tensile and compressive cyclic loading of single glass fibres embedded in polymer matrix. The debond length was deter-mined by in situ microscopy with transmitted polarized light showing the more damaging effect of tension-compression cyclic loading than tension-tension cyclic loading. A boundary element model has been developed and interfacial fracture mechanics concepts applied over the numerical results aiming to give an explanation of this experimental fact. These results may be used to formulate a fatigue growth law at a local microscopic level, at a stage prior to the formation of any visible damage, i.e. transverse cracks. Ideas of how to develop this methodology further are also discussed.
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| 2. |
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| 3. |
- Josefsson, Gabriella, et al.
(author)
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Prediction of elastic properties of nanofibrillated cellulose from micromechanical modeling and nano-structure characterization by transmission electron microscopy
- 2013
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In: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 20:2, s. 761-770
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Journal article (peer-reviewed)abstract
- Cellulose-based materials have a great potential in terms of mechanical performance, since crystalline cellulose is known to have excellent stiffness along the main axis. This potential is not completely fulfilled in structural wood materials and in composite materials, due to structural inhomogeneities, misalignment, voids etc. on several length scales. This study investigates the difference in stiffness of nanofibrillated cellulose (NFC) compared to that of cellulose crystallites, based on nanostructural characterization, image analysis and micromechanical modeling. Nanofibrillated cellulose is believed to be composed of a distribution of crystallites in an amorphous matrix, and it is assumed to represent the distribution of the crystalline allomorph I-beta. To predict the elastic properties of NFC, a micromechanical model based on a Mori-Tanaka approach and self-consistent scheme was used. The input data, i.e. orientation distribution, aspect ratio and volume fraction of these crystalline regions, were estimated from image analysis of transmission electron micrographs. The model predicts a ca. 56 % loss of stiffness of NFC compared to that of cellulose crystals along the main axis.
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| 4. |
- Lindhagen, J. E., et al.
(author)
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Application of bridging-law concepts to short-fibre composites Part 3 : Bridging law derivation from experimental crack profiles
- 2000
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In: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 60:16, s. 2883-2894
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Journal article (peer-reviewed)abstract
- This is the third paper in a series of four where notch sensitivity, fracture energy and bridging laws are studied in short-fibre polymer composites. Here, bridging laws are derived from experimental crack-opening profiles in centre-hole notched tensile specimens. The materials studied are three types of commercial glass-mat composites with different reinforcement structures and matrices. The materials have softening bridging laws and the calculated fracture energies from bridging laws are in good agreement with values determined directly by experiment. The calculated maximum local bridging stress is found to be higher than the uniaxial tensile strength. An outline of a failure criterion for notched specimens based on the crack-bridging approach is presented.
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| 5. |
- Neagu, R. Cristian, et al.
(author)
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The potential of wood fibers as reinforcement in cellular biopolymers
- 2012
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In: Journal of cellular plastics (Print). - 0021-955X .- 1530-7999. ; 48:1, s. 71-103
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Journal article (peer-reviewed)abstract
- Wood fiber-reinforced polylactic acid composite foams have been successfully produced using supercritical carbon dioxide. The addition of fibers had a strong effect on microstructure of the foams. An increase in wood fiber content implied smaller average cell size and higher average cell wall thickness as estimated from image analysis of scanning electron microscopy micrographs. Addition of 10 wt% wood fibers seemed to be a limit to obtain foams, with the used processing conditions. The stiffness properties of the foams in compression improved upon addition of wood fibers. A significant increase of specific stiffness was achieved by adding 5-10 wt% wood fibers. It was shown that the stiffness was about 50% higher in the transverse direction for reinforced foams. The strength in the transverse direction increased for foams with unmodified wood fibers but decreased for foams with two types of treated wood fibers as compared with the strength of the pure polylactic acid foam of similar density. A butyl tetracarboxylic acid treatment followed by an additional surfactant treatment results in reduced wood fiber network-forming ability and reduced fiber-matrix adhesion. This contributes to the inferior observed strength properties in this study. The experimental stiffness was comparable with a superposed micromechanical model for a three-phase fiber-reinforced foam. The model shows that increasing the relative density, that is, the ratio of the density of the foam to the density of the composite material, by adding wood fibers results in a noteworthy increase in the transverse compression stiffness of the foams but only at relative density values above 0.2 for the used processing conditions in this study. The key factor for reinforcement is the relation between foam relative density and fiber volume fraction in the preform. The foaming conditions have to be adapted for each wood fiber content to obtain foams with the desired relative density.
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| 6. |
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| 7. |
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| 8. |
- Trey, Stacy M., et al.
(author)
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Glass fiber reinforced high glass transition temperature thiol-ene networks
- 2011
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In: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 42:11, s. 1800-1808
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Journal article (peer-reviewed)abstract
- Although thiol-ene polymers have highly desirable processing properties the networks usually are limited to having characteristically low glass transition temperatures with low strength. This study is one of the first studies to examine a thiol-ene polymer thermoset matrix, having many industrial advantages compared to conventional polymer matrices, reinforced with continuous E-glass fibers. In order to control the interphase, a mercapto functional sizing of 1 wt% is applied to the glass fibers. The resulting composites of 12 vol% fibers are comparable to glass fiber reinforced polyesters in terms of strength with Young's modulus. This work contributes to the furthering of thiol-ene ultra-violet cure systems, with their range of advantageous properties, for use in a broader scope of applications by way of creating a stronger material based on a novel class of thermoset matrix.
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| 9. |
- Zendejas Medina, León, et al.
(author)
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Enhancing corrosion resistance, hardness, and crack resistance in magnetron sputtered high entropy CoCrFeMnNi coatings by adding carbon
- 2021
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In: Materials & design. - : Elsevier BV. - 0264-1275 .- 1873-4197. ; 205
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Journal article (peer-reviewed)abstract
- This study explores carbon addition as a materials design approach for simultaneously improving the hardness, crack resistance, and corrosion resistance of high entropy thin films. CoCrFeMnNi was selected as a starting point, due to its high concentration of weak carbide formers. The suppression of carbides is crucial to the approach, as carbide formation can decrease both ductility and corrosion resistance. Films with 0, 6, and 11 at.% C were deposited by magnetron co-sputtering, using a graphite target and a sintered compound target. The samples with 0 at.% C crystallized with a mixture of a cubic closed packed (ccp) phase and the intermetallic chi-phase. With 6 and 11 at.% C, the films were amorphous and homogenous down to the nm-scale. The hardness of the films increased from 8 GPa in the carbon-free film to 16 GPa in the film with 11 at.% C. Furthermore, the carbon significantly improved the crack resistance as shown in fragmentation tests, where the crack density was strongly reduced. The changes in mechanical properties were primarily attributed to the shift from crystalline to amorphous. Lastly, the carbon improved the corrosion resistance by a progressive lowering of the corrosion current and the passive current with increasing carbon concentration.
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| 10. |
- Afshar, Reza, et al.
(author)
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Characterization of mechanical properties of Vasa oak and their application in a full-scale numerical model for support assessment
- 2019
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Conference paper (other academic/artistic)abstract
- The 17th century Vasa shipwreck is a well-known object of cultural heritage. According to geodetic measurements inside and outside of the ship as well as on the support structure, the ship is sinking onto its cradle. The analysis of measurements showed the ship undergoes continued deformation with increasing strain. Previous research projects on the Vasa ship have largely been focused on the chemical degradation of the Vasa oak, which concerns the waterlogged polyethylene glycol (PEG) impregnated oak wood. The main goal was to provide understanding of the degradation mechanisms and possible remedies to mitigate the chemical decay. In this paper, a review is presented of previous research in term of characterization of mechanical properties, and effects of PEG and moisture on the mechanical behaviour of the Vasa oak. In addition, a full-scale finite-element model of the Vasa ship has been developed to assess its current and future structural behaviour, as well as a tool to design an improved support structure. The mechanical properties, defined in the model in terms of orthotropic elastic engineering constants, have been determined in previous work. Moreover, creep properties of the archaeological wood material have been and are being characterized, so that the model can be extended by extrapolation to predict future deformation. Geodetic measurements have been used for validation of the static model. The approach undertaken in this project could hopefully be useful in design strategies of improved support for other aging and deforming wood structures in cultural heritage.
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