| 1. |
- Correa, E., et al.
(författare)
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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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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 38:11, s. 2260-2269
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Tidskriftsartikel (refereegranskat)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.
(författare)
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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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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 20:2, s. 761-770
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Tidskriftsartikel (refereegranskat)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.
(författare)
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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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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 60:16, s. 2883-2894
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Tidskriftsartikel (refereegranskat)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.
(författare)
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The potential of wood fibers as reinforcement in cellular biopolymers
- 2012
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Ingår i: Journal of cellular plastics (Print). - 0021-955X .- 1530-7999. ; 48:1, s. 71-103
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Glass fiber reinforced high glass transition temperature thiol-ene networks
- 2011
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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 42:11, s. 1800-1808
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Tidskriftsartikel (refereegranskat)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.
(författare)
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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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Ingår i: Materials & design. - : Elsevier BV. - 0264-1275 .- 1873-4197. ; 205
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Tidskriftsartikel (refereegranskat)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.
(författare)
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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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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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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| 11. |
- Afshar, R, et al.
(författare)
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Comparison of experimental testing and finite element modelling of a replica of a section of the Vasa warship to identify the behaviour of structural joints
- 2017
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Ingår i: Engineering structures. - : ELSEVIER SCI LTD. - 0141-0296 .- 1873-7323. ; 147, s. 62-76
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Tidskriftsartikel (refereegranskat)abstract
- Modelling in design of new support systems necessitates the joint stiffness of the existing wooden structures. In valuable structures, e.g. in cultural heritage, or structures with inaccessible joints, these stiffness values must be estimated, e.g. by testing joints in tailored replicas of the original parts. Although a simplified structure, the replica, can call for finite element (FE) modelling to capture the stiffness parameters. The first step in such a process is to compare FE predictions with experimental tests, for validation purposes. The reasons for unavoidable differences in load-displacement behaviour between model predictions and experimental test should be identified, and then possibly remedied by an improved model. Underlying causes like the complex shape of joints, geometrical uncertainties, contact mechanisms and material nonlinearity are generally too computationally expensive to be included in a full-scale model. It is therefore convenient to collect such effects in the contact penalty stiffness in the joint contact areas where stresses are high, which influences the resulting joint stiffness. A procedure for this is here illustrated for the case of the 17th century Vasa shipwreck A replica of a section of the ship has been constructed, and its joints were tested in bending-compression, in-plane shear and rotation. The FE simulations showed stiffer behaviour than the experimental results. Therefore, a normal penalty stiffness in contact surfaces of the joint were introduced, and used as a calibration parameter to account for the simplifying assumptions or indeliberate imprecision in the model, e.g. concerning boundary conditions, material properties and geometrical detail. The difference between numerical predictions and experimental results could then be significantly reduced, with a suitable normal penalty stiffness value. Once an acceptable finite element model has been obtained, it is shown how this can be used to identify stiffness values for joints in the physical structure with compensation for degradation of material properties due to aging and conservation treatment.
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| 12. |
- Afshar, Reza, et al.
(författare)
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Creep in oak material from the Vasa ship: : verification of linear viscoelasticity and identification of stress thresholds
- 2020
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Ingår i: European Journal of Wood and Wood Products. - : Springer Nature. - 0018-3768 .- 1436-736X. ; 78:6, s. 1095-1103
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Tidskriftsartikel (refereegranskat)abstract
- Creep deformation is a general problem for large wooden structures, and in particular for shipwrecks in museums. In this study, experimental creep data on the wooden cubic samples from the Vasa ship have been analysed to confrm the linearity of the viscoelastic response in the directions where creep was detectable (T and R directions). Isochronous stress–strain curves were derived for relevant uniaxial compressive stresses within reasonable time spans. These curves and the associated creep compliance values justify that it is reasonable to assume a linear viscoelastic behaviour within the tested ranges, given the high degree of general variability. Furthermore, the creep curves were ftted with a one-dimensional standard linear solid model, and although the rheological parameters show a fair amount of scatter, they are candidates as input parameters in a numerical model to predict creep deformations. The isochronous stress–strain relationships were used to defne a creep threshold stress below which only negligible creep is expected. These thresholds ranges were 0.3–0.5 MPa in the R direction and 0.05–0.2 MPa in the T direction.
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| 13. |
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| 14. |
- Almgren, Karin M., et al.
(författare)
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Characterization of interfacial stress transfer ability by dynamic mechanical analysis of cellulose fiber based composite materials
- 2010
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Ingår i: Composite interfaces (Print). - 0927-6440 .- 1568-5543. ; 17:9, s. 845-861
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Tidskriftsartikel (refereegranskat)abstract
- The stress transfer ability at the fiber-matrix interface of wood fiber composites is known to affect the mechanical properties of the composite. The evaluation of interface properties at the level of individual fibers is however difficult due to the small dimensions and variability of the fibers. The dynamical mechanical properties of composite and constituents, in this case wood fibers and polylactide matrix, was here used together with micromechanical modeling to quantify the stress transfer efficiency at the fiber-matrix interface. To illustrate the methodology, a parameter quantifying the degree of imperfection at the interface was identified by inverse modeling using a micromechanical viscoelastic general self-consistent model with an imperfect interface together with laminate analogy on the composite level. The effect of moisture was assessed by comparison with experimental data from dynamic mechanical analysis in dry and moist state. For the wood fiber reinforced polylactide, the model shows that moisture absorption led to softening and mechanical dissipation in the hydrophilic wood fibers and biothermoplastic matrix, rather than loss of interfacial stress transfer ability.
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| 15. |
- Almgren, Karin M., et al.
(författare)
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Contribution of wood fiber hygroexpansion to moisture induced thickness swelling of composite plates
- 2010
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 31:5, s. 762-771
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Tidskriftsartikel (refereegranskat)abstract
- One of the main drawbacks of wood fiber-based composite materials is their propensity to swell due to moisture uptake. Because the wood fibers are usually the main contributor to hygroexpansion, it is of interest to quantify the hygroexpansion coefficient of wood fibers, to compare and rank different types of fibers. This investigation outlines an inverse method to estimate the transverse hygroexpansion coefficient of wood fibers based on measurements of moisture induced thickness swelling of composite plates. The model is based on composite micromechanics and laminate theory. Thickness swelling has been measured on polylactide matrix composites with either bleached reference fibers or crosslinked fibers. The crosslinking modification reduced the transverse hygroexpansion of the composites and the transverse coefficient of hygroexpansion of the fibers was reduced from 0.28 strain per relative humidity for reference fibers to 0.12 for cross-linked fibers
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| 16. |
- Almgren, Karin M., et al.
(författare)
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Moisture uptake and hygroexpansion of wood fiber composite materials with polylactide and polypropylene matrix materials
- 2009
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 30:12, s. 1809-1816
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Tidskriftsartikel (refereegranskat)abstract
- Effects of butantetracarboxylic acid (BTCA) modification, choice of matrix, and fiber volume fraction on hygroexpansion of wood fiber composites have been investigated. Untreated reference wood fibers and BTCA-modified fibers were used as reinforcement in composites with matrices composed of polylactic acid (PLA), polypropylene (PP), or a mixture thereof. The crosslinking BTCA modification reduced the out-of-plane hygroexpansion of PLA and PLA/PP composites, under water-immersed and humid conditions, whereas the swelling increased when PP was used as matrix material. This is explained by difficulties for the BTCA-modified fibers to adhere to the PP matrix. Fiber volume fraction was the most important parameter as regards out-of-plane hygroexpansion, with a high-fiber fraction leading to large hygroexpansion. Fiber-matrix wettability during processing and consolidation also showed to have a large impact on the dimensional stability and moisture uptake. POLYM. COMPOS., 30:1809-1816, 2009.
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| 17. |
- Almgren, Karin, et al.
(författare)
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Role of fibre-fibre and fibre-matrix adhesion in stress transfer in composites made from resin-impregnated paper sheets.
- 2009
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Ingår i: International Journal of Adhesion and Adhesives. - : Elsevier BV. - 0143-7496 .- 1879-0127. ; 29:5, s. 551-557
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Tidskriftsartikel (refereegranskat)abstract
- Paper-reinforced plastics are gaining increased interest as packaging materials, where mechanical properties are of great importance. Strength and stress transfer in paper sheets are controlled by fibre-fibre bonds. In paper-reinforced plastics, where the sheet is impregnated with a polymer resin, other stress-transfer mechanisms may be more important. The influence of fibre-fibre bonds on the strength of paper-reinforced plastics was therefore investigated. Paper sheets with different degrees of fibre-fibre bonding were manufactured and used as reinforcement in a polymeric matrix. Image analysis tools were used to verify that the difference in the degree of fibre-fibre bonding had been preserved in the composite materials. Strength and stiffness of the composites were experimentally determined and showed no correlation to the degree of fibre-fibre bonding, in contrast to the behaviour of unimpregnated paper sheets. The degree of fibre-fibre bonding is therefore believed to have little importance in this type of material, where stress is mainly transferred through the fibre-matrix interface.
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| 18. |
- Bengtsson, Rhodel, et al.
(författare)
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A basic orthotropic viscoelastic model for composite and wood materials considering available experimental data and time-dependent Poisson's ratios
- 2020
-
Ingår i: IOP Conference Series. - : IOP Publishing. - 1757-8981 .- 1757-899X. ; 942
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Tidskriftsartikel (refereegranskat)abstract
- Long-term deformation in creep is of significant engineering importance. For anisotropic materials, such as wood, composites and reinforced concrete, creep testing in several axial directions including shear is necessary to obtain a creep model which is able to predict deformation in the basic orthotropic case. Such a full set of experimental data is generally not available, and simplifying assumptions are typically made to conceive a useful 3D model. These assumptions should preferably be made based on the material behaviour and sound engineering arguments. This problem appears to be addressed in many different ways and sometimes the assumptions are not well justified. In the present study, we examine 3D creep of wood and composite materials. Particular emphasis is made on explaining the choices made in developing the model, considering practicality, incomplete material data and the specific behaviour of wood and composites. An orthotropic linear viscoelastic model is implemented as a material model in a commercial FE software. The constitutive equations are derived in the 1D case using a hereditary approach, then later generalized to the 3D formulation. Guidelines are shown how to implement it into the FE software to predict creep of components and structures. Although the model itself is conventional, the effect of considering time-dependent Poisson's ratios is investigated here, as well an optimization approach when inserting inevitably asymmetric experimental creep data into the model. As far as the authors know, creep of wooden materials have not been defined using this approach before. The model of interest is calibrated against experimental data. Examples using experimental results from solid wood data and a unidirectional fiber composite are demonstrated. The results show that the model is able to capture the orthotropic behaviour adequately. Orthotropy requires symmetry of the creep compliance matrix, which typically is not the case experimentally. It is shown that in rendering the matrix symmetric, one needs to decide which direction is more important. It is also shown that the frequently employed assumption of constant Poisson's ratios should be made with caution.
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| 19. |
- Bengtsson, Rhodel, et al.
(författare)
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An applicable orthotropic creep model for wood materials and composites
- 2022
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Ingår i: Wood Science and Technology. - : Springer Nature. - 0043-7719 .- 1432-5225. ; 56:6, s. 1585-1604
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Tidskriftsartikel (refereegranskat)abstract
- Despite the engineering importance of creep of composite materials and other fibrous anisotropic load-carrying materials like wood, there is an apparent lack in useful experimental data in 3D. Proposed creep models are generally not commensurate with realistic data from experimental characterization. In the present study, an orthotropic linear viscoelastic model is presented and examined on its performance of predicting the time-dependent nature of wood and composite materials. The constitutive equations are presented using the hereditary approach. A clear description of the finite element implementation of the material model is given. Since constant Poisson's ratios are a common assumption for viscoelastic composites due to lack of data, this study presents the effects of time-dependent Poisson's ratio in the study. The model is calibrated against inevitably asymmetric experimental creep data using an optimization approach. With time-dependent Poisson's ratios, the results show that the model is able to simultaneously capture the time-dependent behaviour in three material axis of orthotropic materials such as European beech wood and a fibre-reinforced composite. However, a relatively poor match was found when the Poisson's ratios were set to be constant. Thus, the frequently employed assumption of constant Poisson's ratios should be made with caution.
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| 20. |
- Bengtsson, Rhodel
(författare)
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Creep aspects of softwood from the cell-wall level to structures
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis addresses the intricate mechanical behaviour of natural materials, with a particular focus on wood. Despite millennia of use, understanding the mechanical behaviour of wood materials remains challenging due to their complex microstructures. For instance, they exhibit variations in properties among samples, nonlinear behaviour under elevated loads, and are sensitive to alterations in moisture content.Wood and related natural biobased materials hold immense potential due to their renewability, cost-effectiveness, eco-friendliness, and ease of use in sustainable construction. Wood boasts remarkable stiffness and strength along its primary axis, surpassing many man-made materials in strength-to-weight ratios. However, its anisotropic and heterogeneous nature gives rise to challenges, necessitating the consideration of multiple parameters for accurate characterization to be used in design.Wood is intrinsically heterogeneous, leading to considerable variations in local stresses and deformations during loading. To address these microstructural effects on macroscopically measurable phenomena, mathematical homogenization methods, established since the 1970s, have found applications in material mechanics, including both fibre composites and wood.In recent years, there has been a growing focus on the viscoelastic behaviour of composites and timber structures, given their increased long-term use in load-carrying applications. While numerous investigations have explored the relationship between the microstructure of wood and its elastic properties, few studies have explored the connection between microstructure and viscoelastic properties.The thesis focuses on the static and, more notably, on the time-dependent mechanical properties of wood, bridging the gap from cell-wall creep to structures. It includes experiments and numerical work, culminating in the development of a material model suitable for orthotropic materials like wood. The multiscale model establishes a link between microstructural parameters and macroscopic properties, potentially applicable to various softwood species. Given the lack of shear creep data in the literature, the thesis introduces straightforward methods to characterize shear creep properties, addressing a significant knowledge gap.Furthermore, the thesis progresses from material-level experiments to higher length scales, demonstrating how the results can be applied to larger wooden structures, such as the tower for a counter-rotating axis tilted turbine. While these results require further validation in the absence of experimental data for wooden wind turbine structures, they offer useful insights into simulating creep behaviour in such applications.In conclusion, this thesis highlights the multifaceted nature of a natural material like wood, its mechanical challenges, and the promising research avenues for comprehensive understanding and practical use. The outcome provides contributions to the efficient utilization of wood in load-carrying structures and underlines the importance of ongoing research in this field.
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| 21. |
- Bengtsson, Rhodel, et al.
(författare)
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Evaluating the viscoelastic shear properties of clear wood via off-axis compression testing and digital-image correlation
- 2023
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Ingår i: Mechanics of time-dependant materials. - 1385-2000 .- 1573-2738.
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Tidskriftsartikel (refereegranskat)abstract
- Highly anisotropic materials like wood and unidirectional polymer composite structures are sensitive to shear deformations, in particular close to fixed joints. Large wooden structures in buildings and, e.g. wind-turbine blades, are designed to last for decades, and hence are susceptible to unwanted creep deformations. For improved structural design, the shear-creep properties of the material are needed. These are rarely available in the literature, possibly because of technical difficulties to achieve a well-defined shear-stress state in test specimens. For cost-efficient testing, this goal of a pure stress state necessarily needs to be compromised. In the present study, we propose a simple test method based on uniaxial compression on wooden cubes, but is equally applicable for fibre composites. The viscoelastic shear properties of Norway spruce (Picea abies) under off-axis creep compression tests have been characterised in all three directions. The tests are performed in a controlled climate chamber and the creep strains are captured using digital-image correlation.
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| 22. |
- Bengtsson, Rhodel, et al.
(författare)
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Viscoelastic behavior of softwood based on a multiscale computational homogenization
- 2023
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 179
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, a numerical multiscale model is made to show how the hierarchical structure of softwood affect its macroscopic viscoelastic properties. The performance of the model is demonstrated for two softwood species — Norway spruce and Japanese cypress, whose creep behavior has been characterized experimentally. The results show that by using the same transversely isotropic viscous properties of the cell wall for both species, it is possible to predict creep deformation relatively close to experimental creep measurements for both species. Assuming that the variability is larger on the microstructural level (density, cell-wall geometry, microfibril angle, composition of wood tissues) than on the cell-wall level, it is possible to predict the macroscopic creep behavior based on the microstructural parameters alone. Such predictions can potentially save cost and time, since creep characterization in all material directions is demanding.
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| 23. |
- Bermejo, Daniel, 1985-, et al.
(författare)
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First Aldol-Crosslinked Hyaluronic Acid Hydrogel: Fast and Hydrolytically Stable Gel with Tissue Adhesive Properties
-
Ingår i: Chemical Sciences Journal. - 2150-3494.
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Tidskriftsartikel (refereegranskat)abstract
- Currently, there are limited approaches to tailor 3D scaffolds crosslinked with a stable covalent C-C bond that does not require any catalysts or initiators. We present here the first hydrogels employing aldol condensation chemistry that exhibit exceptional physicochemical properties. We investigated the aldol-crosslinking chemistry using two types of aldehyde-modified hyaluronic acid (HA) derivatives, namely; an enolizable HA-aldehyde (HA-Eal) and a non-enolizable HA-aldehyde (HA-Nal). Hydrogels formed using HA-Eal demonstrate inferior crosslinking efficiency (due to intramolecular loop formation), when compared with hydrogels formed by mixing HA-Eal and HA-NaI leading to a cross-aldol product. The change in mechanical properties as a result of crosslinking at different pH is determined using rheological measurements and is interpreted in terms of molecular weight between cross-links (Mc). The novel HA cross-aldol hydrogels demonstrate excellent hydrolytic stability and favorable mechanical properties but allow hyaluronidase mediated enzymatic degradation. Interestingly, residual aldehyde functionality within the aldol product leads to adhesion to tissue as demonstrated by bonding two bone tissues. The aldehyde functionality also permits facile post-synthetic modifications with nucleophilic reagents such as Alexa FluorTM 488. Finally, we demonstrate that the novel hydrogel is biocompatible with encapsulated stem cells that show a linear rate of expansion in our 3–6 days of study.
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| 24. |
- Bermejo-Velasco, Daniel, 1985-, et al.
(författare)
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First Aldol Cross-Linked Hyaluronic Acid Hydrogel : Fast and Hydrolytically Stable Hydrogel with Tissue Adhesive Properties
- 2019
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 11:41, s. 38232-38239
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Tidskriftsartikel (refereegranskat)abstract
- Currently, there are limited approaches to tailor 3D scaffolds cross-linked with a stable covalent C-C bond that does not require any catalysts or initiators. We present here the first hydrogels employing aldol condensation chemistry that exhibit exceptional physicochemical properties. We investigated the aldol-cross-linking chemistry using two types of aldehyde-modified hyaluronic acid (HA) derivatives, namely, an enolizable HA-aldehyde (HA-EaI) and a non-enolizable HA-aldehyde (HA-NaI). Hydrogels formed using HA-EaI demonstrate inferior cross linking efficiency (due to intramolecular loop formation), when compared with hydrogels formed by mixing HA-EaI and HA-NaI leading to a cross-aldol product. The change in mechanical properties as a result of cross-linking at different pH values is determined using rheological measurements and is interpreted in terms of molecular weight between cross-links (Me). The novel HA cross-aldol hydrogel demonstrate excellent hydrolytic stability and favorable mechanical properties but allow hyaluronidase-mediated enzymatic degradation. Interestingly, residual aldehyde functionality within the aldol product rendered the tissue adhesive properties by bonding two bone tissues. The aldehyde functionality also facilitated facile post-synthetic modifications with nucleophilic reagents. Finally, we demonstrate that the novel hydrogel is biocompatible with encapsulated stem cells that show a linear rate of expansion in our 3-6 days of study.
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| 25. |
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| 26. |
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| 27. |
- Bjurhager, Ingela, et al.
(författare)
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State of degradation in archeological oak from the 17th century vasa ship : Substantial strength loss correlates with reduction in (holo)cellulose molecular weight
- 2012
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Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; :8, s. 2521-2527
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Tidskriftsartikel (refereegranskat)abstract
- In 1628, the Swedish warship Vasa capsized on her maiden voyage and sank in the Stockholm harbor. The ship was recovered in 1961 and, after polyethylene glycol (PEG) impregnation, it was displayed in the Vasa museum. Chemical investigations of the Vasa were undertaken in 2000, and extensive holocellulose degradation was reported at numerous locations in the hull. We have now studied the longitudinal tensile strength of Vasa oak as a function of distance from the surface. The PEG-content, wood density, and cellulose microfibril angle were determined. The molar mass distribution of holocellulose was determined as well as the acid and iron content. A good correlation was found between the tensile strength of the Vasa oak and the average molecular weight of the holocellulose, where the load-bearing cellulose microfibril is the critical constituent. The mean tensile strength is reduced by approximately 40%, and the most affected areas show a reduction of up to 80%. A methodology is developed where variations in density, cellulose microfibril angle, and PEG content are taken into account, so that cell wall effects can be evaluated in wood samples with different rate of impregnation and morphologies.
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| 28. |
- Bjurhager, Ingela, et al.
(författare)
-
Towards improved understanding of PEG-impregnated waterlogged archaeological wood : A model study on recent oak
- 2010
-
Ingår i: Holzforschung. - 0018-3830 .- 1437-434X. ; 64:2, s. 243-250
-
Tidskriftsartikel (refereegranskat)abstract
- To prevent deformation and cracking of waterlogged archaeological wood, polyethylene glycol (PEG) as a bulk impregnation agent is commonly applied. PEG maintains the wood in a swollen state during drying. However, swelling of wood can reduce its mechanical properties. In this study, the cellular structure of oak and cell wall swelling was characterized by scanning electron microscopy (SEM) of transverse cross-sections, and the microfibril angle of oak fibers was determined by wide angle X-ray scattering (WAXS). Samples of recent European oak (Quercus robur L) impregnated with PEG (molecular weight of 600) were tested in axial tension and radial compression. Mechanical tests showed that axial tensile modulus and strength were only slightly affected by PEG, whereas radial compressive modulus and yield strength were reduced by up to 50%. This behavior can be explained by the microstructure and deformation mechanisms of the material. Microfibril angles in tensile test samples were close to zero. This implies tensile loading of cellulose microfibrils within the fiber cell walls without almost any shear in the adjacent amorphous matrix. These results are important because they can help separate the impact of PEG on mechanical properties from that of chemical degradation in archaeological artifacts, which display only small to moderate biological degradation.
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| 29. |
- Blanco, N., et al.
(författare)
-
Analysis of the mixed-mode end load split delamination test
- 2006
-
Ingår i: Composite structures. - : Elsevier BV. - 0263-8223 .- 1879-1085. ; 76:1-2, s. 14-20
-
Tidskriftsartikel (refereegranskat)abstract
- Composite delaminations are commonly characterized using the double cantilever beam test for mode 1, the end-notched flexure test or the end load split test for mode 11 and the mixed-mode bending test for mixed-mode. For all these tests, the mode mix remains constant and does not vary with the crack length. However, in the mixed-mode end load split test (MMELS), the delamination propagates under a varying mode mix that depends on the crack extension, which is a more realistic scenario. The MMELS test has been previously analysed by different researchers but the resulting expressions are not equivalent. A more accurate alternative analysis of the test, based on the finite element method and the virtual crack closure technique, is used in the present work for comparison. The results are compared to the predictions of approaches presented in the literature and significant findings are found for materials characterization using the MMELS test.
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| 30. |
- Blanco, N., et al.
(författare)
-
Mechanical hinge system for delamination tests in beam-type composite specimens
- 2008
-
Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 68:7-8, s. 1837-1842
-
Tidskriftsartikel (refereegranskat)abstract
- During the experimental study of composite delaminations external loads are usually applied by means of steel or aluminium parts bonded to the surface of beam-type specimens. The bonded joints between the metallic parts and the composite specimen might fail, especially when the tests are carried out under extreme temperatures or fatigue conditions. In addition, the point of application of the external load does not coincide with the neutral axis of the specimen beam, inducing non-linear effects that can lead, for example, to incorrect estimations of fracture toughness. In this paper, the relative importance of the non-linear effects in delamination tests is evaluated and the corresponding correction factors discussed. Next, the design of an improved mechanical hinge that avoids non-linear effects, eliminates bonded joints and can be adapted to different specimen thicknesses is introduced.
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| 31. |
- Blanco, N., et al.
(författare)
-
Mixed-mode delamination growth in carbon-fibre composite laminates under cyclic loading
- 2004
-
Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 41:15, s. 4219-4235
-
Tidskriftsartikel (refereegranskat)abstract
- Delamination growth under fatigue loads in real composite components generally develops in a non-constant propagation mode. The aim of the investigation described in this article was to develop a model capable of predicting the fatigue delamination growth in a general case, under varying mode mix conditions. The crack growth development in essentially unidirectional laminates of carbon-fibre reinforced epoxy was analysed in terms of the Paris law for different constant propagation modes: mode I (double-cantilever beam test), mode II (end-notched flexure test) and different mixed-modes I/II (mixed-mode bending test). The dependence of the Paris law parameters oil mode mix is compared with the existing models in the literature. It is shown that these models do not reproduce the non-monotonic dependence on mode mix which has been observed in experimental data. Therefore, an improved phenomenological model is introduced and compared with the experimental data obtained by other researchers. To check the ability of the model to predict variable mixed-mode fatigue delamination, the mixed-mode end-loaded split test was employed and the experimental results were compared to the predictions of the model. The underlying mechanisms responsible for the dependency of the crack propagation rates on the degree of mode mix are also discussed on the basis of fractographic analysis.
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| 32. |
- Bogren, Karin M., et al.
(författare)
-
Dynamic-mechanical properties of wood-fibre reinforced polyactide : experimental characterization and micro-mechanical modelling
- 2006
-
Ingår i: Journal of Thermoplastic Composite Materials. - : SAGE Publications. - 0892-7057 .- 1530-7980. ; 19:6, s. 613-638
-
Tidskriftsartikel (refereegranskat)abstract
- Wood-fiber reinforced polylactide is a biodegradable compositewhere both fibers and matrix are from renewableresources. When designing new materials of this kind, itis useful to measure the influence of fiber–matrixinterface properties on macroscopic mechanicalproperties. In particular, a quantitative measure of thedynamic stress transfer between the fibers andthe matrix when the material is subjected tocyclic loading would simplify the development of wood-fibercomposites. This is obtained by comparing themechanical dissipation of the composite with avalue predicted by a viscoelastic micromechanical model basedon perfect interfacial stress transfer. Theloss factors predicted by the model are 0.12 and 0.16 at dryand humid conditions, respectively, which amountto 63 and 66% of the experimentally determinedvalues. For Young's moduli the predicted values are 1.01 and0.88 GPa, which correspond to 92% of the experimentallydetermined values. The mismatch between thepredicted and experimental values may be attributed toimperfect interfaces with restrained stress transfer.Loss factors are also determined for specificmolecular bonds using dynamic Fourier transform infrared(FT-IR) spectroscopy. These values show the sametrends with regard to moisture content as themacroscopically determined loss factors.
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| 33. |
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| 34. |
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| 35. |
- da Costa, Marcus Vinicius Tavares, et al.
(författare)
-
Prediction of loss of barrier properties in cracked thin coatings on polymer substrates subjected to tensile strain
- 2021
-
Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 426
-
Tidskriftsartikel (refereegranskat)abstract
- Thin brittle coatings on polymer films are a potentially useful material combination for food packaging applications. The brittle coatings inevitably risk cracking when the package is converted. This strain-induced cracking leads to a loss of the key barrier properties. In design of packaging materials, it would be useful to predict the loss of the oxygen transmission rate (OTR) as a function of the applied tensile strain, which are linked by the crack opening and crack spacing in the coating. Previous works have presented a model that predicts the effect of strain on the OTR in the presence of cracks in the coating. This work uses an improved numerical model based on finite element method (FEM) to predict the oxygen permeability more accurately, especially for thin coatings with high crack densities. The numerical predictions show reasonable correspondence with experimental results for SiOx coatings. These results as well as predictions for previously tested metal-oxide coated polymer films show a significant increase in OTR at crack onset, which suggests that efforts should be made to make the coatings more ductile with higher crack onset strains if the barrier performance should be maintained in converted packages. The quantitative link from deformation over the damage state to barrier properties indicate that mechanics could provide a tool to aid the design of improved food packages with retained barrier capacity.
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| 36. |
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| 37. |
- Duanmu, Jie, et al.
(författare)
-
Allyloxy-modified starch with low degree of substitution for fiber reinforced thermoset starch composites
- 2011
-
Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 71:4, s. 520-527
-
Tidskriftsartikel (refereegranskat)abstract
- In the present work dough moulding compound premixes of allyl glycidyl ether modified (AGE)-potato starch, (DS) = 0.2, has been prepared and tested for its fiber reinforced composite properties. The AGE-starch was hydrolyzed with a-amylase under neutral condition for 6 h at 45 degrees C for improved process ability. The grafting and hydrolytic scission was confirmed by nuclear magnetic resonance (NMR) and size exclusion chromatography (SEC), respectively. Homogeneous composite premixes of AGE-starch, wood fibers, various amount of glycerol and ethylene glycol dimethacrylate were successfully mixed with a Brabender-kneader at 55 degrees C and cured by compression molding at 150 degrees C using 2 wt% of dibenzoyl peroxide. Adding 5 wt% of glycerol did not reduce the ultimate strength of the composites: 10% glycerol reduced the strength from 60 MPa to 40 MPa, and 16% glycerol to 14 MPa. The results with 5 wt% glycerol are comparable with earlier achieved results. The water absorption rate increased with increased glycerol content and the mechanical strength of the composites was lost completely when the moisture uptake reached 15 wt%.
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| 38. |
- Duanmu, Jie, et al.
(författare)
-
Bulk composites from microfibrillated cellulose-reinforced thermoset starch made from enzymatically degraded allyl glycidyl ether-modified starch
- 2012
-
Ingår i: Journal of composite materials. - : SAGE Publications. - 0021-9983 .- 1530-793X. ; 46:25, s. 3201-3209
-
Tidskriftsartikel (refereegranskat)abstract
- Microfibrillated cellulose consists of nanoscale bundles of elementary microfibrils prepared, e.g. by the defibrillation of delignified wood pulp fibres in high-pressure homogenizers. In this study, microfibrillated cellulose was used to reinforce a thermoset starch plastic. The starch was modified with allyl glycidyl ether with a degree of substitution of 1.3, which was further hydrolyzed with alpha-amylase for 18 h yielding significantly improved processing properties. Dry premixes of all constituents were prepared by a stepwise drying process before sample manufacturing. The composite was cured by ethylene glycol dimethacrylate initiated with benzoyl peroxide during compression moulding at 150 degrees C. Scanning electron microscopy revealed some degree of porosity in the samples, where the dispersed microfibrillated cellulose network was detectable. Microfibrillated cellulose, even in relatively small additions (2 wt%, 5 wt% and 10 wt%), resulted in composites with rather good hygromechanical properties. The ultimate strength increased with microfibrillated cellulose content and reached values of comparable composites with 40 wt% softwood fibre. Importantly, the dimensional stability in water was much improved compared to similar composites reinforced with substantially larger weight fractions of softwood fibres.
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| 39. |
- Duanmu, Jie, et al.
(författare)
-
Hygromechanical properties of composites of crosslinked allylglycidyl-ether modified starch reinforced by wood fibres
- 2007
-
Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 67:15-16, s. 3090-3097
-
Tidskriftsartikel (refereegranskat)abstract
- In a previous work a new family of thermoset composites of allylglycidyl ether modified starch as matrix, an ethylene glycol dimethacrylate as cross-linker and a wood fibre as reinforcement were prepared. The aim of the present work was to study the hygromechanical properties of the new composites including density, dimensional stability in water, water uptake, stiffness, and ultimate strength in three-point bending. It was shown that the samples with a starch matrix of a high degree of substitution (DS = 2.3), HDS, absorbed less water, were more stable in water and had also higher stiffness and strength than corresponding composite samples with a starch matrix of low degree of substitution (DS = 1.3), LDS. Overall, the fibre addition improved water stability. An increased fibre content from 40 to 70% by weight had a negligible impact on the water uptake. An increase in fibre content did, however, improve the mechanical properties. The HDS-sample with highest fibre content, 70% by weight showed the highest Young's modulus (3700 MPa) and strength (130 MPa), which are markedly higher compared with the samples based on the pure HDS matrix (Young's modulus of 360 MPa and strength of 15 MPa). The measured Young's modulus and tensile strength values were roughly one order of magnitude higher than earlier reported cellulosic fibre reinforced natural polymer composites.
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| 40. |
- Duanmu, Jie, et al.
(författare)
-
Studies on mechanical properties of wood fiber reinforced cross-linked starch composites made from enzymatically degraded allylglycidyl ether-modified starch
- 2010
-
Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 41:10, s. 1409-1418
-
Tidskriftsartikel (refereegranskat)abstract
- In a previous work we introduced a new family of thermoset composites of softwood fiber and allylglycidyl ether modified potato starch (AGE-starch with a degree of substitution of 1.3 and 2.3) prepared by hot pressing. To improve the processability of AGE-starch with a DS = 1.3 (LDS-3) and to increase hygromechanical properties, the LDS-3 matrix has now been partially degraded by a-amylase at 45 degrees C (pH 6) for 0.5, 6 and 18 h. The study shows that already a 30 min enzymatic hydrolysis has a marked effect on the modified starch molecular weight and its thermal properties. The new composites with enzyme hydrolyzed AGE-starch, generically named D-LDS-3, showed good fiber dispersion and excellent interface between the fiber and matrix as studied by SEM. Premixes of D-LDS-3 matrix and fiber showed improved processability. The water vapor absorption was evaluated at 43.2% and 82.2% RH and the stiffness and strength properties were measured. The water uptake was shown to be reduced. The strength of neat matrix D-LDS-3-6 at ambient 68% RH reached 63 MPa and Young's modulus 3200 MPa and with 40 wt.% wood fiber reinforcement impressive 128 MPa and 4500 MPa, respectively.
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| 41. |
- Duanmu, Jie, et al.
(författare)
-
Synthesis and preparation of crosslinked allylglycidyl ether-modified starch-wood fibre composites
- 2007
-
Ingår i: Starke (Weinheim). - : Wiley. - 0038-9056 .- 1521-379X. ; 59:11, s. 523-532
-
Tidskriftsartikel (refereegranskat)abstract
- Native potato starch has been modified with allylglycidyl ether (AGE) under various reaction conditions including different sodium hydroxide and AGE concentrations, reaction temperatures and times. H-1-NMR and FT-IR were used to analyze the products. AGE-modified starch, with two degrees of substitution (DS), namely DS = 1.3 and DS = 2.3, was synthesized and used for preparation of a new family of crosslinked composites reinforced with various amounts of bleached softwood fibres. Composite premixes of modified starch, wood fibres and ethylene glycol dimethacrylate (EGDA) were cured in a hot press using 2% (w/w) of benzoyl peroxide at 150 degrees C under high pressure for 10 min. The matrix with high degree of substitution exhibited good processability and was easily processed even for the highest fibre contents, up to 70% (w/w). In addition, scanning electron micrographs showed good dispersion and adhesion between the starch matrix with high degree of substitution and fibre. The original poor mechanical properties of the cured modified starch were markedly improved by the addition of wood fibres. In the extractions tests cured high-DS and low-DS composite samples showed weight losses in the range of 1 and 15% (w/w), respectively. No unreacted crosslinker ethylene glycol dimethacrylate was detected in the solutes as determined by NMR.
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| 42. |
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| 43. |
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| 44. |
- Gamstedt, E. Kristofer, et al.
(författare)
-
An experimental investigation of the sequence effect in block amplitude loading of cross-ply composite laminates
- 2002
-
Ingår i: International Journal of Fatigue. - 0142-1123 .- 1879-3452. ; 24:04-feb, s. 437-446
-
Tidskriftsartikel (refereegranskat)abstract
- The Palmgren-Miner rule has been shown to be inexact in many cases for various composite materials. Several empirical models have been conceived to account for this discrepancy, as well as the effect of block sequence. The approach taken here is based on the underlying mechanisms. A cross-ply laminate was used as a model material. In general, composites show both initiatory and progressive mechanisms under fatigue loading. The former is active at high static stresses, whereas the latter predominates at lower stress amplitudes where they are given sufficient time to propagate, Initiatory mechanisms give rise to damage from which the progressive mechanisms can start, and conversely the progressive mechanisms continually alter the local stress state which results in further damage accumulation caused by the initiation controlled mechanisms. In a cross-ply laminate, the initiatory mechanism is the formation of transverse cracks, and the progressive mechanism is mainly delamination growth initiated from the transverse cracks. In an experimental investigation of carbon fiber/epoxy cross-ply laminates, the interaction of these mechanisms has shown why a sequence of high-low amplitude levels results in shorter lifetimes than a low-high order. Such a sequence effect seems to be a common behavior for many other composite materials, and can be mechanistically explained by a similar kind of interaction. Advantages and drawbacks of the mechanistic approach compared with empirical rules are also discussed.
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| 45. |
- Gamstedt, E. Kristofer, et al.
(författare)
-
Characterization of interfacial stress transfer ability of particulate cellulose composite materials
- 2011
-
Ingår i: Mechanics of materials. - : Elsevier BV. - 0167-6636 .- 1872-7743. ; 43:11, s. 693-704
-
Tidskriftsartikel (refereegranskat)abstract
- Composites with cellulose reinforcements are steadily gaining increased use. The stress transfer ability between reinforcement and polymer matrix has a strong influence on mechanical properties like strength and fracture toughness. This work presents a method to assess the stress transfer ability between cellulose and polymer matrix from a model material with cellulose spheres embedded in a polymer matrix. Such a material show smaller variability compared with composites based on natural cellulose fibres, and is less cumbersome than single fibre tests with regard to interfacial characterization. Measured elastic moduli of particulate composites is compared with predicted values from a micromechanical model based on a composite sphere assembly in a self-consistent scheme with only a spring constant of an imperfect interface as fitting parameter expressed in Pa/m. This interface parameter is identified through inverse modelling and used to quantify stress-transfer ability of cellulose/polylactide and cellulose/polystyrene composite interfaces. A higher degree of interfacial interaction was found for the former. This ranking was corroborated by adhesive force measurements using a micrometre sized cellulose sphere attached to the end of a cantilever in an atomic force microscope. With the model microstructure of a cellulose-sphere composite, an interfacial efficiency parameter can be backed out from stiffness measurements to be used in e.g. ranking of different fibre surface treatments and choice of matrix in the development of stronger natural-fibre composites.
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| 46. |
- Gamstedt, E. Kristofer
(författare)
-
Effects of debonding and fiber strength distribution on fatigue-damage propagation in carbon fiber-reinforced epoxy
- 2000
-
Ingår i: Journal of Applied Polymer Science. - 0021-8995 .- 1097-4628. ; 76:4, s. 457-474
-
Tidskriftsartikel (refereegranskat)abstract
- In order to design new fatigue-resistant composites, the underlying fatigue damage mechanisms must be characterized and the controlling microstructural properties should be identified. The fatigue-damage mechanisms of a unidirectional carbon fiber-reinforced epoxy has been studied under tension-tension loading. A ubiquitous form of damage was one or a few planar fiber breaks from which debonds or shear yield zones grew in the longitudinal direction during fatigue cycling. This leads to a change in stress profile of the neighboring fibers, and an increase in failure probability of these fibers. The breakage of fibers in the composite is controlled by the fiber strength distribution. The interaction between the fiber strength distribution and debond propagation leading to further fiber breakage was investigated by a numerical simulation. It was found that a wider distribution of fiber strength and a higher debond rate lead to more distributed damage and a higher fracture toughness. Implications to fatigue life behavior are discussed, with reference to constituent microstructure.
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| 47. |
- Gamstedt, E. Kristofer, et al.
(författare)
-
Fatigue dissipation and failure in unidirectional and angle-ply glass fibre/carbon fibre hybrid laminates
- 2002
-
Ingår i: Key Engineering Materials. - 1013-9826 .- 1662-9795. ; 221-2, s. 35-48
-
Tidskriftsartikel (refereegranskat)abstract
- The tensile fatigue behaviour of unidirectional 0degrees, [+/-10](4S) and [+/-45](4S) carbon fibre/glass fibre hybrid composite has been investigated. The dissipation was measured by the stiffness, hysteresis loss and temperature field of the specimen surface in an insulated testing chamber. The hysteresis loss correlates well with the increase temperature. Microscopic studies show frictional sliding of longitudinal crack faces between carbon and glass fibre bundles to be the main source of dissipation for on-axis specimens. With increasing off-axis angle the primary loss mechanism became cyclic shear deformation of the polymer matrix. With a finer dispersion of the constituents of the hybrid, the growth of these longitudinal cracks or of zones of inelastic matrix shear deformation. would be suppressed, which would result in a more fatigue resistant material. A localisation of heat generation sets in just prior to final failure. Damage and heat localisation lead to impending failure. If the parameters that control localisation were better understood, it would be possible to improve the fatigue resistance of the material by sensible microstructural design.
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| 48. |
- Gamstedt, E.Kristofer, et al.
(författare)
-
Fatigue mechanisms in unidirectional glass-fibre-reinforced polypropylene
- 1999
-
Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 59:5, s. 759-768
-
Tidskriftsartikel (refereegranskat)abstract
- Polypropylene (PP) and polypropylene modified with maleic anhydride (MA-PP) reinforced by continuous longitudinal glass fibres have been investigated. The most prominent effect of the modification with maleic anhydride in the composite is a stronger fibre/matrix interface. The effects of interfacial strength on fatigue performance and on the underlying micromechanisms have been studied for these composite systems. Tension-tension fatigue tests (R=0.1) were carried out on 0 glass-fibre/PP and glass-fibre/MA-PP coupons. The macroscopic fatigue behaviour was characterized in terms of stiffness reduction and fatigue-life curves. The results showed that the longitudinal Young's modulus degraded more rapidly for glass-fibre/PP, which was caused by a higher degree of damage growth and accumulation. The improvement in monotonic strength was negligible, but the fatigue life was prolonged by about one decade for the composite with the stronger interface by use of the maleic-anhydride grafted polypropylene matrix. During the fatigue testing, the microscopic mechanisms were monitored intermittently by a surface replication technique. From microscopic observations, it could be concluded that the better fatigue resistance of glass-fibre/MA-PP can be attributed to the greater interfacial strength and the resistance to debond propagation
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| 49. |
- Gamstedt, E. Kristofer, et al.
(författare)
-
Fatigue propagation of fibre-bridged cracks in unidirectional polymer-matrix composites
- 2001
-
Ingår i: Applied Composite Materials. - 0929-189X .- 1573-4897. ; 8:6, s. 385-410
-
Tidskriftsartikel (refereegranskat)abstract
- In order to improve the fatigue resistance of polymer-matrix composites by materials design, or to conceive micromechanics based models for life predictions, the underlying micromechanisms must be understood. Experimental investigations have revealed fibre-bridged cracking as a toughening micromechanism that retards further fatigue crack growth in a unidirectional 0 degrees carbon-fibre-reinforced epoxy. The bridging fibres exert a closing traction on the crack surfaces, thereby reducing the driving force for crack growth. In this study, the growth of bridged cracks has been quantified by a surface replication technique. The da/dN-DeltaK curve defined in terms of nominal stress-intensity factors shows a crack retarding behaviour. The crack growth curve can be replotted in terms of the effective stress-intensity factor where the contribution of the cohesive crack surface forces from the bridging fibres are taken into account. This curve falls somewhat closer to that of the neat matrix material, but the difference is still considerable, and it shows a decelerating propagation. Therefore, there must be other active toughening mechanisms besides fibre bridging, that slow the crack propagation down, and account for the fatigue resistant behaviour of the tested material. Ways by microstructural design to promote the fatigue resistant mechanisms are discussed.
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| 50. |
- Gamstedt, E. Kristofer
(författare)
-
Moisture induced softening and swelling of natural cellulose fibres in composite applications
- 2016
-
Ingår i: 37th Riso International Symposium on Materials Science.
-
Konferensbidrag (refereegranskat)abstract
- Composites based on natural cellulose fibres are susceptible to moisture. The fibres as well as the composite will inevitably soften and swell as moisture is absorbed. The intention of the present paper is to shed some light on the mechanisms behind softening and swelling. Also references to modelling work are made, to predict the moisture-induced dimensional stability. Characterisation techniques and models of such kind can be useful in choosing suitable fibres for improved moisture resistance, and identifying the main controlling parameters which affect the engineering consequences of moisture absorption. Understanding of the mechanisms and the main contributions to swelling can rationalise materials development. The examples shown in this review attempt to show the benefits by experimental mechanics and modelling in development of moisture resistant cellulose composites.
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