Ändra sökning
Avgränsa sökresultatet
1 - 45 av 45
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Träffar per sida
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
Markera
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1.
    Agde Tjernlund, Jessica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Gamstedt, Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Gudmundson, Peter
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Length-scale effects on damage development in tensile loading of glass-sphere filled epoxy2006Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 43, nr 24, s. 7337-7357Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Particle-reinforced polymers are widely used in load-carrying applications. The effect of particle size on damage development in the polymer is still relatively unexplored. In this study, the effect of glass-sphere size on the damage development in tensile loaded epoxy has been investigated. The diameter of the glass spheres ranged from approximately 0.5-50 mu m. The first type of damage observed was debonding at the sphere poles, which subsequently grew along the interface between the glass spheres and epoxy matrix. These cracks were observed to kink out into the matrix in the radial direction perpendicular to the applied load. The debonding stresses increased with decreasing sphere diameter, whereas the length to diameter ratio of the resulting matrix cracks increased with increasing sphere diameter. These effects could not be explained by elastic stress analysis and linear-elastic fracture mechanics. Possible explanations are that a thin interphase shell may form in the epoxy close to the glass spheres, and that there is a length-scale effect in the yield process which depends on the strain gradients. Cohesive fracture processes can contribute to the influence of sphere size on matrix-crack length. Better knowledge on these underlying size-dependent mechanisms that control damage development in polymers and polymer composites is useful in development of stronger materials. From a methodology point of view, the glass-sphere composite test can be used as an alternative technique (although still in a qualitative way) to hardness vs. indentation depth to quantify length-scale effects in inelastic deformation of polymers.

  • 2.
    Agde Tjernlund, Jessica
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Gamstedt, Kristofer
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Xu, Zhi-Hui
    KTH, Tidigare Institutioner, Materialvetenskap.
    Influence of molecular weight on strain-gradient yielding in polystyrene2004Ingår i: Polymer Engineering and Science, ISSN 0032-3888, E-ISSN 1548-2634, Vol. 44, nr 10, s. 1987-1997Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Experimental observations have indicated that the presence of strain gradients has an influence on the inelastic behavior of polymers as well as in other materials such as ceramics and metals. The present study has experimentally quantified length-scale effects in inelastic deformations of the polymer material polystyrene (PS) with respect to the molecular length. The experimental technique that has been used is nano-indentation to various depths with a Berkovich indenter. The hardness has been calculated with the method by Oliver and Pharr, and also by direct measurements of the area from atomic force microscopy. The experiments showed that the length-scale effects in inelastic deformations exist in polystyrene at ambient conditions. The direct method gave a smaller hardness than the Oliver-Pharr method. It was also shown that the length-scale parameter according to Nix and Gao increases with increasing molecular weight. For high molecular weights above a critical value of entanglement, there was no pertinent increase in the length-scale parameter. The length-scale parameter for strain-gradient plasticity has a size of around 0.1 μm for polystyrene.

  • 3. Almgren, Karin
    et al.
    Gamstedt, Kristofer E.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Nygård, P.
    Malmberg, F.
    Lindblad, J.
    Lindström, M.
    Role of fibre-fibre and fibre-matrix adhesion in stress transfer in composites made from resin-impregnated paper sheets.2009Ingår i: International Journal of Adhesion and Adhesives, ISSN 0143-7496, E-ISSN 1879-0127, Vol. 29, nr 5, s. 551-557Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 4.
    Almgren, Karin M.
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Träkemi och massateknologi.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Träkemi och massateknologi.
    Characterization of interfacial stress transfer ability by dynamic mechanical analysis of cellulose fiber based composite materials2010Ingår i: Composite interfaces (Print), ISSN 0927-6440, E-ISSN 1568-5543, Vol. 17, nr 9, s. 845-861Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 5.
    Almgren, Karin M.
    et al.
    Innventia.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Träkemi och massateknologi.
    Berthold, Fredrik
    Innventia.
    Lindström, Mikael
    Innventia.
    Moisture uptake and hygroexpansion of wood fiber composite materials with polylactide and polypropylene matrix materials2009Ingår i: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 30, nr 12, s. 1809-1816Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 6. Almgren, Karin M.
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Träkemi och massateknologi.
    Varna, Janis
    Luleå tekniska universitet, LTU.
    Contribution of wood fiber hygroexpansion to moisture induced thickness swelling of composite plates2010Ingår i: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 31, nr 5, s. 762-771Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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

  • 7. Almgren, Karin M.
    et al.
    Åkerholm, Margaretha
    Gamstedt, Kristofer
    Salmén, Lennart
    Lindström, Mikael
    Effects of Moisture on Dynamic Mechanical Properties of Wood Fiber Composites Studied by Dynamic FT-IR Spectroscopy2008Ingår i: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 27, nr 16-17, s. 1709-1721Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wood fiber reinforced polylactide is a biodegradable composite where both fibers and matrix are from renewable resources. In the development of such new materials, information on mechanical behavior on the macroscopic and the molecular level is useful. In this study, dynamic Fourier transform infrared (FT-IR) spectroscopy is used to measure losses at the molecular level during cyclic tensile loading for bonds that are characteristic of the cellulosic fibers and the polylactid matrix. This molecular behavior is compared with measured macroscopic hysteresis losses for different moisture levels. The results show that moisture ingress will transfer the load from the fibers to the matrix, and that a more efficient fiber-matrix interface would diminish mechanical losses. Although the dynamic FT-IR spectroscopy method is still qualitative, this investigation shows that it can provide information on the stress transfer of the constituents in wood fiber reinforced plastics.

  • 8. Asp, Leif
    et al.
    Gamstedt, Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Gibson, Geoff
    13th European Conference on Composite Materials2009Ingår i: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 38, nr 2-4, s. 47-48Artikel i tidskrift (Övrigt vetenskapligt)
  • 9.
    Bjurhager, Ingela
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Halonen, Helena
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Lindfors, E. -L
    Iversen, Tommy
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Almkvist, G.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Berglund, Lars A.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    State of degradation in archeological oak from the 17th century vasa ship: Substantial strength loss correlates with reduction in (holo)cellulose molecular weight2012Ingår i: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 13, nr 8, s. 2521-2527Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 10.
    Bjurhager, Ingela
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Ljungdahl, Jonas
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Wallstrom, Lennart
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Berglund, Lars A.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Biokompositer.
    Towards improved understanding of PEG-impregnated waterlogged archaeological wood: A model study on recent oak2010Ingår i: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 64, nr 2, s. 243-250Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 11.
    Bjurhager, Ingela
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Ljungdahl, Jonas
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Wallström, Lennart
    Division of Polymer Engineering, Luleå University of Technology (LTU).
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Effects of polyethylene glycol treatment on the mechanical properties of oakManuskript (Övrigt vetenskapligt)
  • 12. Blanco, N.
    et al.
    Gamstedt, E. Kristofer
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Asp, L. E.
    Costa, J.
    Mixed-mode delamination growth in carbon-fibre composite laminates under cyclic loading2004Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, nr 15, s. 4219-4235Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 13. Blanco, N.
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Costa, J.
    Mechanical hinge system for delamination tests in beam-type composite specimens2008Ingår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, nr 7-8, s. 1837-1842Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 14. Blanco, N.
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Costa, J.
    Trias, D.
    Analysis of the mixed-mode end load split delamination test2006Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 76, nr 1-2, s. 14-20Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 15.
    Bogren, Karin M.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Gamstedt, E. Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Neagu, R. Cristian
    Åkerholm, Margaretha
    Lindström, Mikael
    STFI–Packforsk AB.
    Dynamic-mechanical properties of wood-fibre reinforced polyactide: experimental characterization and micro-mechanical modelling2006Ingår i: Journal of Thermoplastic Composite Materials, ISSN 0892-7057, E-ISSN 1530-7980, Vol. 19, nr 6, s. 613-638Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 16. Correa, E.
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Paris, F.
    Mantic, V.
    Effects of the presence of compression in transverse cyclic loading on fibre-matrix debonding in unidirectional composite plies2007Ingår i: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 38, nr 11, s. 2260-2269Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 17. Duanmu, Jie
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Biokompositer.
    Pranovich, Andrey
    Rosling, Ari
    Studies on mechanical properties of wood fiber reinforced cross-linked starch composites made from enzymatically degraded allylglycidyl ether-modified starch2010Ingår i: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 41, nr 10, s. 1409-1418Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 18. Duanmu, Jie
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Rosling, Ari
    Hygromechanical properties of composites of crosslinked allylglycidyl-ether modified starch reinforced by wood fibres2007Ingår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 67, nr 15-16, s. 3090-3097Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 19. Duanmu, Jie
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Rosling, Ari
    Synthesis and preparation of crosslinked allylglycidyl ether-modified starch-wood fibre composites2007Ingår i: Starke (Weinheim), ISSN 0038-9056, E-ISSN 1521-379X, Vol. 59, nr 11, s. 523-532Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 20. Gamstedt, E. Kristofer
    Effects of debonding and fiber strength distribution on fatigue-damage propagation in carbon fiber-reinforced epoxy2000Ingår i: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 76, nr 4, s. 457-474Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 21. Gamstedt, E. Kristofer
    et al.
    Redon, O.
    Brondsted, P.
    Fatigue dissipation and failure in unidirectional and angle-ply glass fibre/carbon fibre hybrid laminates2002Ingår i: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 221-2, s. 35-48Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 22.
    Gamstedt, E. Kristofer
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Sjogren, B. A.
    An experimental investigation of the sequence effect in block amplitude loading of cross-ply composite laminates2002Ingår i: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 24, nr 04-feb, s. 437-446Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 23.
    Gamstedt, E. Kristofer
    et al.
    KTH, Tidigare Institutioner                               , Hållfasthetslära.
    Skrifvars, M.
    Jacobsen, T. K.
    Pyrz, R.
    Synthesis of unsaturated polyesters for improved interfacial strength in carbon fibre composites2002Ingår i: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 33, nr 9, s. 1239-1252Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Carbon fibres are gaining use as reinforcement in glass fibre/polyester composites for increased stiffness as a hybrid composite. The mechanics and chemistry of the carbon fibre-polyester interface should be addressed to achieve an improvement also in fatigue performance and off-axis strength. To make better use of the versatility of unsaturated polyesters in a carbon fibre composite, a set of unsaturated polyester resins have been synthesized with different ratios of maleic anhydride, o-phthalic anhydride and 1,2-propylene glycol as precursors. The effective interfacial strength was determined by micro-Raman spectroscopy of a single-fibre composite tested in tension. The interfacial shear strength with untreated carbon fibres increased with increasing degree of unsaturation of the polyester, which is controlled by the relative amount of maleic anhydride. This can be explained by a contribution of chemical bonding of the double bonds in the polymer to the functional groups of the carbon fibre surface.

  • 24. Gamstedt, E. Kristofer
    et al.
    Östlund, Sören
    KTH, Tidigare Institutioner                               , Hållfasthetslära.
    Fatigue propagation of fibre-bridged cracks in unidirectional polymer-matrix composites2001Ingår i: Applied Composite Materials, ISSN 0929-189X, E-ISSN 1573-4897, Vol. 8, nr 6, s. 385-410Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 25. Hofstetter, Karin
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Hierarchical modelling of microstructural effects on mechanical properties of wood. A review COST Action E35 2004-2008: Wood machining - micromechanics and fracture2009Ingår i: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 63, nr 2, s. 130-138Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    Wood exhibits a hierarchical architecture. Its macroscopic properties are determined by microstructural features at different scales of observation. Recent developments of experimental micro-characterisation techniques have delivered further insight into the appearance and the behaviour of wood at smaller length scales. The improved knowledge and the availability of increasingly powerful micromechanical modelling techniques and homogenisation methods have stimulated rather comprehensive research on multiscale modelling of wood. Linking microstructural properties to macroscopic characteristics is expected to improve the knowledge of the mechanical behaviour of wood and to serve as the basis for the development of innovative wood-based products and for biomimetic material design. Moreover, understanding fundamental aspects of wood machining requires multiscale approaches which can take into account the heterogeneity, anisotropy and hierarchies of wood and wood composites. In this review, recent developments in the field of hierarchical modelling of the hygroelastic behaviour of wood are discussed, and a short outline of the theoretical background is given. Much focus is placed on composite micromechanical models for the wood cell wall and on multiscale models for wood resting upon hierarchical finite element models and on the application of continuum micromechanics, respectively. These models generally lead to the specification of equivalent homogeneous continua with effective material properties. Finally, current deficiencies and limitations of hierarchical models are sketched and possible future research directions are specified.

  • 26. Lindhagen, J. E.
    et al.
    Gamstedt, E. Kristofer
    Berglund, Lars A.
    Application of bridging-law concepts to short-fibre composites Part 3: Bridging law derivation from experimental crack profiles2000Ingår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, nr 16, s. 2883-2894Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 27. Lindström, M.
    et al.
    Berthold, F.
    Gamstedt, Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Nobell, N.
    Razavi, F.
    Wickholm, K.
    Sustainable materials design2008Ingår i: 2008 Nordic Wood Biorefinery Conference, NWBC 2008 - Proceedings, 2008, s. 144-149Konferensbidrag (Refereegranskat)
    Abstract [en]

    Traditionally new materials have been developed in order to solve technological problems within the aerospace, military or medicine fields. Renewable materials are developed from another perspective; they are developed in order to meet the demands from a sustainable society. If they are developed to become replacement materials, they have to be better or cheaper in order to take market shares or there has to be a political willingness to promote them. Economical subsidies are never sound and a better strategy to introduce these new materials is to identify their unique properties and possibilities and to market them. One such strategy, materials design, will be discussed in this paper. Renewable materials open for new strategies in design. Traditionally, in the LCA perspective materials should be recycled as materials and preferably as the original material quality. This implies that there has to be a limited number of materials and they need to be identifiable in order to be sorted. Renewable, natural fibre reinforced materials will degrade during recycling materials and recycling. The consequence of this is that these materials should be recycled as energy. An integrated materials and product design will therefore be a new sustainable strategy. Researchers involved in R&D of new materials have been involved in discussions and collaborations with the R&D department of companies producing and marketing materials. In parallel, industrial designers and engineers involved in product development and design have had on-going discussion with the same companies, but through their marketing department. No communication between R&D and end-product designers. This paper suggest an R&D&D concept, Research and Development and Design, a hierarchical, integrated materials and product design strategy.

  • 28. Marklund, Erik
    et al.
    Varna, Janis
    Neagu, Cristian R.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Gamstedt, Kristofer E.
    Stiffness of aligned wood fiber composites: Effects of microstructure and phase properties2008Ingår i: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 42, nr 22, s. 2377-2405Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness is analyzed. An analytical model for an N-phase composite with orthotropic properties of constituents is developed and used. This model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. It was found that most macro-properties are governed by only one property of the cell wall which is very important in attempts to back-calculate the fiber properties. The role of lumen (whether it filled by resin or not) has a very large effect on the composite shear properties. It is shown that several of the unknown anisotropic constants characterizing wood fiber are not affecting the stiffness significantly and rough assumptions regarding their value would suffice. The errors introduced by application of the Hashin's model and neglecting the orthotropic nature of the material behavior in cylindrical axes are evaluated. The effect of geometrical deviations from circular cross-section, representing, for example, collapsed fibers, is analyzed using the finite element method (FEM) and the observed trends are discussed.

  • 29.
    Neagu, Cristian
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Gamstedt, Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Berthold, Fredrik
    Stiffness contribution of various wood fibers to composite materials2006Ingår i: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Journal of Composites Materials, Vol. 40, nr 8, s. 663-699Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wood pulp fibers can serve as useful reinforcement of plastics for increased stiffness. To assess the potential of various wood fibers as reinforcement, a method has been developed to determine the contribution of the fibers to the elastic properties of the composite. A micromechanical composite model and classical laminate mechanics are used to relate the elastic properties of the fibers to the elastic properties of the composite. A large variety of composites made of various wood pulp fibers in an epoxy vinyl ester matrix was manufactured. From the tensile test results of the composites, the contributing Young's moduli of the fibers in the longitudinal direction are back-calculated and summarized. One finding is that there is an optimum in fiber stiffness as a function of lignin content. It is also found that industrially pulped hardwood fibers have higher stiffness than the corresponding softwood fibers. One example is kraft-cooked Norway spruce fiber, for which a Young's modulus of 40 GPa is found. The effects of hornification, prehydrolysis, and sulfite processing are also investigated. The results indicate that mild defibration process should be used, that does not damage the cell wall structure so that the inherent high stiffness of the native fibers can be retained. It can be concluded that the proposed method works well to rank the wood fiber candidates in terms of their contribution to the composite stiffness.

  • 30.
    Neagu, Cristian
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Gamstedt, Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Lindström, Mikael
    Characterization methods for elastic properties of wood fibers from mats for composite materials2006Ingår i: Wood and Fiber Science, ISSN 0735-6161, Vol. 38, nr 1, s. 95-111Artikel i tidskrift (Refereegranskat)
    Abstract [en]

     Wood fibers offer excellent specific properties at low cost and are of interest as reinforcement in composites. This work compares two alternative test methods to determine the stiffness of wood fibers from simple macroscopic tests oil fiber mats. One method is compression of the fiber trial in the thickness direction, which uses a statistical micromechanical model based oil first-order beam theory to describe the deformation. The other method is tensile testing of fiber trials and back calculation of the fiber stiffness with I laminate model. Experiments include compression tests and tensile stiffness index tests as well as determination of fiber content, orientation, and dimensional distribution. For trials with unbleached softwood kraft fibers, all effective value of the Young's modulus of 20.1 GPa determined by the compression method call be compared with values of 17.4-19.0 GPa obtained from tensile tests. These are ill agreement with values for similar cellulosic fibers found in literature. The compression method is more appropriate for low-density fiber mats, while the tensile test works better for well-consolidated high-density fiber mats. The two methods have different ranges of applicability and are complementary to one another. Limitations of the methods are also discussed. The main advantage of the methods is that they are quantitative. The potential as stiffening reinforcement of various types of Fibers can be systematically investigated, even if the fiber mat microstructures are different.

  • 31.
    Neagu, Cristian
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Gamstedt, Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Lindström, Mikael
    Influence of wood-fibre hygroexpansion on the dimensional instability of fibre mats and composites2005Ingår i: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 36, nr 6, s. 772-788Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wood fibres are gaining increased use as reinforcement in plastics. One of the disadvantages of cellulose-based fibres is their propensity to absorb water and swell. This inevitably leads to undesired dimensional instability of the composite and its fibre-mat preform. A measure of the hygroexpansion behaviour of the fibres could serve to rank the suitability of different kinds of cellulosic fibres with regard to dimensionally stable composites. A method has been developed to determine the hygroexpansion coefficient of wood fibres. Since fibre mats manufactured with conventional techniques generally have a thickness gradient of fibre orientation, fibre mats and composites will curl if the moisture content varies. Models based on laminate mechanics and micromechanics of hygroexpansion have been used to estimate the transverse hygroexpansion coefficient of the fibres from experimental results of curvature and thickness changes, and tensile stiffness of fibre mats based on unbleached sulphate cooked softwood. The fibre orientation distribution through the fibre-mat thickness was characterized with a tape-splitting technique and image analysis. The transverse hygroexpansion coefficient of the wood fibres was found to depend on the assumed ratios of the elastic parameters of the transversely isotropic fibres. Values in the range of 0.10-0.15 strain per relative change in moisture content were found for plausible degrees of elastic anisotropy of the fibres. Means of how to suppress dimensional instability of cellulose based composites and fibre mats are addressed.

  • 32. Neagu, Cristian R.
    et al.
    Gamstedt, Kristofer E.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Modelling of effects of ultrastructural morphology on the hygroelastic properties of wood fibres2007Ingår i: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 42, nr 24, s. 10254-10274Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wood fibres constitute the structural framework of e.g. wood, paper, board and composites, where stiffness and dimensional stability are of importance. An analytical modelling approach has been used for prediction of hygroelastic response, and assessment of the stresses in thick-walled cylinder models of wood fibres. A wood fibre was idealised as a multilayered hollow cylinder made of orthotropic material with helical orientation. The hygroelastic response of the layered assembly due to axisymmetric loading and moisture content changes was obtained by solving the corresponding boundary value problem of elasticity. A simple solution scheme based on the state space approach and the transfer matrix method was employed. This was combined with an analytical ultrastructural homogenisation method, used to link hygroelastic properties of constituent wood polymers to properties of each layer. Predicted hygroelastic response captured experimentally measured behaviour. Fibres that were constrained not to twist showed a stiffer response than fibres allowed twisting under uniaxial loading. It was also shown that the ultrastructure, i.e. the microfibril angle, will control the hygroexpansion in the same way as it controls the compliance of the cell wall. Qualitative failure trends comparable with experimental observations could be established with stress analysis and a simple plane-stress failure criterion.

  • 33.
    Neagu, Cristian R.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Gamstedt, Kristofer E.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Bardage, Stig L.
    Lindström, Mikael
    Ultrastructural features affecting mechanical properties of wood fibres2006Ingår i: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 1, nr 3-4, s. 146-170Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure-property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure-property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.

  • 34. Neagu, R. C.
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Micromechanical modelling of wood-polymer composites2008Ingår i: Wood-Polymer Composites, Elsevier, 2008, s. 118-141Kapitel i bok, del av antologi (Refereegranskat)
  • 35.
    Nilsson, Helena
    et al.
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Galland, Sylvain
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi, Biokompositer. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Larsson, Per Tomas
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Gamstedt, E. Kristofer
    Uppsala University.
    Iversen, Tommy
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Compression molded wood pulp biocomposites: A study of hemicellulose influence on cellulose supramolecular structure and material properties2012Ingår i: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, nr 3, s. 751-760Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this study, the importance of hemicellulose content and structure in chemical pulps on the property relationships in compression molded wood pulp biocomposites is examined. Three different softwood pulps are compared; an acid sulfite dissolving grade pulp with high cellulose purity, an acid sulfite paper grade pulp and a paper grade kraft pulp, the latter two both containing higher amounts of hemicelluloses. Biocomposites based the acid sulfite pulps exhibit twice as high Young's modulus as the composite based on paper grade kraft pulp, 11-12 and 6 GPa, respectively, and the explanation is most likely the difference in beating response of the pulps. Also the water retention value (WRV) is similarly low for the two molded sulfite pulps (0.5 g/g) as compared to the molded kraft pulp (0.9 g/g). The carbohydrate composition is determined by neutral sugar analysis and average molar masses by SEC. The cellulose supramolecular structure (cellulose fibril aggregation) is studied by solid state CP/MAS 13C-NMR and two forms of hemicellulose are assigned. During compression molding, cellulose fibril aggregation occurs to higher extent in the acid sulfite pulps as compared to the kraft pulp. In conclusion, the most important observation from this study is that the difference in hemicellulose content and structure seems to affect the aggregation behaviour and WRV of the investigated biocomposites.

  • 36.
    Nilsson, Helena
    et al.
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Galland, Sylvain
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center. KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Biokompositer.
    Larsson, Per Tomas
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center. KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Gamstedt, E. Kristofer
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center. KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Biokompositer.
    Nishino, Takashi
    Dept. of Chem. Sci. and Engng., Kobe Univ. Rokko, Nada, Kobe, Japan.
    Berglund, Lars A.
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center. KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Biokompositer.
    Iversen, Tommy
    KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    A non-solvent approach for high-stiffness all-cellulose biocomposites based on pure wood cellulose2010Ingår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 70, nr 12, s. 1704-1712Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    All-cellulose composites are commonly prepared using cellulose solvents. In this study, moldable all-cellulose I wood fiber materials of high cellulose purity (97%) were successfully compression molded. Water is the only processing aid. The material is interesting as a "green" biocomposite for industrial applications. Dissolving wood fiber pulps (Eucalyptus hardwood and conifer softwood) are used and the influence of pulp origin, beating and pressing temperature (20-180 degrees C) on supramolecular cellulose nanostructure is studied by solid state CP/MAS C-13 NMR. Average molar mass is determined by SEC to assess process degradation effects. Mechanical properties are determined in tensile tests. High-density composites were obtained with a Young's modulus of up to 13 GPa. In addition, nanoscale cellulose fibril aggregation was confirmed due to processing, and resulted in a less moisture sensitive material.

  • 37.
    Pettersson, Kaj
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Neumeister, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Gamstedt, Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Öberg, Hans
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Stiffness reduction, creep, and irreversible strains in fiber composites tested in repeated interlaminar shear2006Ingår i: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 76, nr 1-2, s. 151-161Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Interlaminar shear of a unidirectional carbon fiber/epoxy composite was studied experimentally by use of a tensile inclined double notch shear (IDNS-) test-setup. The non-linear constitutive behavior, the degradation and stiffness reduction, under monotonic and repeated loading with successively increasing peak strains were investigated. Shear strains (average values) up to 7-8% prior to failure were observed in the test region together with irreversible strains approaching 2.5% when unloaded. Hysteresis loops of notable width, increasing with higher peak strains, were observed. Several shear moduli measures were investigated. A notable decay in unloading secant moduli (similar to 30-40%) was present. Tangent (re-)loading and unloading moduli could be reliably measured only with a 2 h hold time prior to each load reversal. A quite early drop of > 10% was observed in the elastic reloading tangent modulus, but not for the unloading tangent modulus. To the knowledge of the authors, no previous experiments have achieved global interlaminar shear strain levels of such magnitudes, and moreover, stress levels were higher than obtained with previously established methods. Several mechanisms were deemed to contribute to the overall strains; such were higher elastic-but also higher irreversible shear strains in the epoxy matrix and the fibers; damage within the epoxy; conceivably also in the fibers; appearance of microcracks in epoxy-rich regions; possibly separation of the interfaces, and also notable creep deformation at higher loads. None of these phenomena alone could account for the dominating part of the obtained strains. Matrix cracks (shear hackles) were found post mortem, but could not be confirmed during the test: a different damage mechanism is responsible for the early loss of elastic stiffness. Friction-like processes (sliding, viscosity) contributed substantially to the total strains.

  • 38. Sorensen, B. F.
    et al.
    Gamstedt, E. Kristofer
    Jacobsen, T. K.
    Equivalence of J integral and stress intensity factor approaches for large scale bridging problems2000Ingår i: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 104, nr 4, s. L31-L36Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The application of the J integral to problems involving large scale crack bridging is discussed. Using some simple examples for which close form analytical solutions exist, it is shown that there is a complete agreement between analysis based on stress intensity factors and the J integral. A simple method for deriving the bridging law from J integral measurements is outlined.

  • 39. Sorensen, Bent F.
    et al.
    Gamstedt, E. Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Ostergaard, Rasmus C.
    Goutianos, Stergios
    Micromechanical model of cross-over fibre bridging - Prediction of mixed mode bridging laws2008Ingår i: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 40, nr 4-5, s. 220-234Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The fracture resistance of fibre composites can be greatly enhanced by crack bridging. In situ observations of mixed mode crack growth in a unidirectional carbon-fibre/epoxy composite reveal crack bridging by single fibres and by beam-like ligaments consisting of several fibres. Based on the observed bridging mechanism, a micromechanical model is developed for the prediction of macroscopic mixed mode bridging laws (stress-opening laws). The model predicts a high normal stress for very small openings, decreasing rapidly with increasing normal and tangential crack opening displacements. In contrast, the shear stress increases rapidly, approaching a constant value with increasing normal and tangential openings. The solutions for the bridging laws and the resulting toughening due to the bridging stresses are obtained in closed analytical form.

  • 40.
    Ståhlberg, Daniel
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Junestam, Per
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Gamstedt, Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Johansson, Mats
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Relaxation properties of particle filled coatings: Experimental study and modelling of a screw joint2006Ingår i: Progress in organic coatings, ISSN 0300-9440, E-ISSN 1873-331X, Vol. 55, nr 2, s. 112-118Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The present study describes the mechanical behaviour of powder coatings used under very high compressive loads in clamping force joints. Carboxyl functional polyester powder coatings cured with hydroxyl functional P-hydroxyalkylamides with variations in amount of filler have been studied. The coatings were subjected to relaxation tests in tension and in compression. The tests in compression were performed in specially designed tests developed to study the behaviour of powder coatings under compressive loads in clamping force joints. The relaxation results for the matrix were used in a unit cell in micromechanical finite element (FE) model to predict the homogenised viscoelastic properties of the particle composite. These constitutive properties were subsequently used to evaluate the behaviour on a macromechanical scale in a screw joint. The model corresponds well with experimental data at ambient temperature. When increasing the temperature above the glass transition of the coating, however, the model predictions and experimental data differ. Experiments in compression show a much lower relaxation its compared to the FE model. The relaxation simulations of the coating under compressive loads from screw joints showed a significant sensitivity to the Poisson's ratio of the polymer matrix. As the Poisson's ratio approaches 0.5, the matrix becomes hydrostatically incompressible, which resulted in a negligible relaxation of the coating at the screw joint.

  • 41.
    Tanem, Bjorn S.
    et al.
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Li, Yanjun
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Gamstedt, Kristofer
    KTH.
    Vullum, Per Erik
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Walmsley, John C.
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Elastic properties of MFC: Advanced electron microscopy combined with micromechanical modeling2009Ingår i: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 237Artikel i tidskrift (Övrigt vetenskapligt)
  • 42.
    Tanem, Bjorn S.
    et al.
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Li, Yanjun
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Gamstedt, Kristofer
    KTH. KTH, SE-10044 Stockholm, Sweden..
    Vullum, Per Erik
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Walmsley, John C.
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Stenstad, Per M.
    SINTEF Mat & Chem, NO-7465 Trondheim, Norway..
    Structure and properties of MFC reinforced nanocomposites: Advanced electron microscopy combined with micromechanical modeling2009Ingår i: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 237Artikel i tidskrift (Övrigt vetenskapligt)
  • 43. Thuvander, F.
    et al.
    Gamstedt, E. Kristofer
    Ahlgren, P.
    Distribution of strain to failure of single wood pulp fibres2001Ingår i: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 16, nr 1, s. 46-56Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The single-fibre composite test has been applied to characterize the distribution of strain to failure of individual fibres from unbleached softwood kraft pulp. The fibre was moulded into a thermoset matrix which is subjected to a tensile test, whereupon the fibre fragmented with increasing applied strain. The measurements of mean fragment lengths and corresponding strain levels were used to determine the Weibull parameters of the distribution of strain to failure of the fibres by a least squares method. Three groups of fibres of the same kind were tested, which have been exposed to increased beating. The beating process in a PFI mill made the variability of strain to failure decrease, whereas the average value remained constant until it dropped rapidly for over-beaten fibres. This can be interpreted as a result of interfibrillar debonding induced by beating.

  • 44.
    Tjernlund, J. Agde
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Gamstedt, E. Kristofer
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Gudmundson, Peter
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    An experimental investigation of the relationship between molecular structure and length scales in inelastic deformation of an amorphous thermoplastic2006Ingår i: IUTAM Symposium on Size Effects on Material and Structural Behavior at Micron- and Nano-Scales / [ed] Sun, QP; Tong, P, DORDRECHT: SPRINGER , 2006, Vol. 142, s. 143-150Konferensbidrag (Refereegranskat)
  • 45.
    Yang, Ting
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Long, Hui
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Malkoch, Michael
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Gamstedt, E. Kristofer
    Berglund, Lars
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Hult, Anders
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Characterization of Well-Defined Poly(ethylene glycol) Hydrogels Prepared by Thiol-ene Chemistry2011Ingår i: Journal of Polymer Science Part A: Polymer Chemistry, ISSN 0887-624X, E-ISSN 1099-0518, Vol. 49, nr 18, s. 4044-4054Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Considering the large number of applications for hydrogels, a better understanding of the relation between molecular structure and mechanical properties for well-defined hydrogel is essential. A new library has been compiled of poly(ethylene glycol) polymers (PEG) of different length end functionalized with diallyl, dithiol, and dimethacrylate, and crosslinked with complementary trifunctional crosslinkers. In this study, the hydrogels were initially analyzed by FT-Raman and NMR to study the conversion ratio of the functional groups. The effects of solvent type, solid content concentration, curing time and length of the PEG chains on the final leaching, swelling and tensile properties of the hydrogels were studied.

1 - 45 av 45
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf