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  • 1. Andersson, B.
    et al.
    Sjogren, A.
    Berglund, Lars A.
    Micro- and meso-level residual stresses in glass-fiber/vinyl-ester composites2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 10, p. 2011-2028Article in journal (Refereed)
    Abstract [en]

    Residual stresses in glass-fiber composites were studied on the micro and meso scales by computational and experimental methods. Transmitted polarized light images of thin sections were compared with 3D finite-element solutions of a sample containing 1410 fibers. Calculated point-wise stresses were derived from a linear thermoelastic model with negligibly small numerical errors. Regions with calculated maximum compressive stresses showed good agreement with experimentally observed optical bands. A material with poor interfacial adhesion showed weaker optical effects indicating fiber/matrix debonding. On the basis of these results it stems likely that irreversible matrix deformation and debonding can take place in the curing phase.

  • 2.
    Ansari, Farhan
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.
    Lindh, Erik L.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. Innventia AB, Sweden.
    Furo, Istvan
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Johansson, Mats K.G.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Interface tailoring through covalent hydroxyl-epoxy bonds improves hygromechanical stability in nanocellulose materials2016In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 134, p. 175-183Article in journal (Refereed)
    Abstract [en]

    Wide-spread use of cellulose nanofibril (CNF) biocomposites and nanomaterials is limited by CNF moisture sensitivity due to surface hydration. We report on a versatile and scalable interface tailoring route for CNF to address this, based on technically important epoxide chemistry. Bulk impregnation of epoxide-amine containing liquids is used to show that CNF hydroxyls can react with epoxides at high rates and high degree of conversion to form covalent bonds. Reactions take place inside nanostructured CNF networks under benign conditions, and are verified by solid state NMR. Epoxide modified CNF nanopaper shows significantly improved mechanical properties under moist and wet conditions. High resolution microscopy is used in fractography studies to relate the property differences to structural change. The cellulose-epoxide interface tailoring concept is versatile in that the functionality of molecules with epoxide end-groups can be varied over a wide range. Furthermore, epoxide reactions with nanocellulose can be readily implemented for processing of moisture-stable, tailored interface biocomposites in the form of coatings, adhesives and molded composites.

  • 3.
    Ansari, Farhan
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.
    Skrifvars, M.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Nanostructured biocomposites based on unsaturated polyester resin and a cellulose nanofiber network2015In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 117, p. 298-306Article in journal (Refereed)
    Abstract [en]

    Biocomposites reinforced by natural plant fibers tend to be brittle, moisture sensitive and have limited strength. Wood cellulose nanofibers (CNF) were therefore used to reinforce an unsaturated polyester matrix (UP) without the need of coupling agents or CNF surface modification. The nanostructured CNF network reinforcement strongly improves modulus and strength of UP but also ductility and toughness. A template-based prepreg processing approach of industrial potential is adopted, which combines high CNF content (up to 45 vol%) with nanoscale CNF dispersion. The CNF/UP composites are subjected to moisture sorption, dynamic thermal analysis, tensile tests at different humidities, fracture toughness tests and fractography. The glass transition temperature (T-g) increases substantially with CNF content. Modulus and strength of UP increase about 3 times at 45 vol% CNF whereas ductility and apparent fracture toughness are doubled. Tensile properties at high humidity are compared with other bio-composites and interpreted based on differences in molecular interactions at the interface.

  • 4.
    Asp, Leif E.
    et al.
    Polymer Engineering, Luleå University of Technology.
    Berglund, Lars A.
    Polymer Engineering, Luleå University of Technology.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Effects of a composite-like stress state on the fracture of epoxies1995In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 53, no 1, p. 27-37Article in journal (Refereed)
    Abstract [en]

    The strain to failure of a transversely loaded composite is much lower than for the pure matrix in uniaxial tension. Several studies of composites suggest the triaxial matrix stress state as one of the explanations. In order to investigate this experimentally, a triaxial tensile test previously used for rubbers (the poker-chip test) was successfully applied to four epoxies in the glassy state. The chosen specimen geometry mimicked the most severe stress state in the matrix as determined by finite element analysis of a transversely loaded glass-fiber/epoxy composite. The poker-chip strains to failure in the primary loading direction were 0.5-0.8%, whereas uniaxial strains to failure were 1.8-7%. The triaxial stress state in composite matrices may therefore by itself be a sufficient explanation for low values of transverse composite strains to failure.

  • 5. Badia, J. D.
    et al.
    Reig-Rodrigo, P.
    Teruel-Juanes, R.
    Kittikorn, Thorsak
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. Prince of Songkla University, Thailand.
    Strömberg, Emma
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Ek, Monica
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Karlsson, Sigbritt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Ribes-Greus, A.
    Effect of sisal and hydrothermal ageing on the dielectric behaviour of polylactide/sisal biocomposites2017In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 149, p. 1-10Article in journal (Refereed)
    Abstract [en]

    The dielectric properties of virgin polylactide (PLA) and its reinforced composites with different weight amounts of sisal fibres were assessed at broad temperature (from −130 °C to 130 °C) and frequency ranges (from 10−2–107 Hz), before and after being subjected to accelerated hydrothermal ageing. The synergetic effects of both the loading of sisal and hydrothermal ageing were analysed by means of dielectric relaxation spectra. The relaxation time functions were evaluated by the Havriliak-Negami model, substracting the ohmic contribution of conductivity. The intramolecular and intermolecular relaxations were respectively analysed by means of Arrhenius and Vogel-Fulcher-Tammann-Hesse thermal activation models. The addition of fibre increased the number of hydrogen bonds, which incremented the dielectric permittivity and mainly hindered the non-cooperative relaxations of the biocomposites by increasing the activation energy. Hydrothermal ageing enhanced the formation of the crystalline phase at the so-called transcrystalline region along sisal. This fact hindered the movement of the amorphous PLA fraction, and consequently decreased the dielectric permittivity and increased the dynamic fragility.

  • 6. Blanco, N.
    et al.
    Gamstedt, E. Kristofer
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Costa, J.
    Mechanical hinge system for delamination tests in beam-type composite specimens2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 7-8, p. 1837-1842Article in journal (Refereed)
    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.

  • 7. Carlsson, L A
    et al.
    Lindstrom, T
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    A shear-lag approach to the tensile strength of paper (vol 65, pg 183, 2005)2005In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 65, no 7-8, p. 1321-1321Article in journal (Refereed)
  • 8. Carlsson, L. A.
    et al.
    Lindström, Tom
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    A shear-lag approach to the tensile strength of paper2005In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 65, no 2, p. 183-189Article in journal (Refereed)
    Abstract [en]

    A shear-lag approach to the prediction of the tensile strength of paper is outlined and examined. It is demonstrated that transition of fiber strength to paper strength requires long fibers for sheets with weak fiber-fiber bonds, or low relative bonded area. Fiber pull-out is encountered even for highly bonded sheets. Predictions of tensile strength for papers of the same fiber length, but with different beating degrees, and for papers with different fiber lengths and beating degrees, are in quantitative agreement with previously published data.

  • 9.
    Castro, Daniele Oliveira
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. MoRe Research Örnsköldsvik AB, Örnsköldsvik, Sweden.
    Karim, Zoheb
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. MoRe Research Örnsköldsvik AB, Örnsköldsvik, Sweden.
    Medina, Lilian
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Häggström, J. -O
    Carosio, F.
    Svedberg, A.
    Wågberg, Lars
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Söderberg, Daniel
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    The use of a pilot-scale continuous paper process for fire retardant cellulose-kaolinite nanocomposites2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 162, p. 215-224Article in journal (Refereed)
    Abstract [en]

    Nanostructured materials are difficult to prepare rapidly and at large scale. Melt-processed polymer-clay nanocomposites are an exception, but the clay content is typically below 5 wt%. An approach for manufacturing of microfibrillated cellulose (MFC)/kaolinite nanocomposites is here demonstrated in pilot-scale by continuous production of hybrid nanopaper structures with thickness of around 100 μm. The colloidal nature of MFC suspensions disintegrated from chemical wood fiber pulp offers the possibility to add kaolinite clay platelet particles of nanoscale thickness. For initial lab scale optimization purposes, nanocomposite processing (dewatering, small particle retention etc) and characterization (mechanical properties, density etc) were investigated using a sheet former (Rapid Köthen). This was followed by a continuous fabrication of composite paper structures using a pilot-scale web former. Nanocomposite morphology was assessed by scanning electron microscopy (SEM). Mechanical properties were measured in uniaxial tension. The fire retardancy was evaluated by cone calorimetry. Inorganic hybrid composites with high content of in-plane oriented nanocellulose, nanoclay and wood fibers were successfully produced at pilot scale. Potential applications include fire retardant paperboard for semi structural applications.

  • 10.
    Doddapaneni, Venkatesh
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Saleemi, Mohsin
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Ye, Fei
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Gati, R.
    Toprak, Muhammet Sadaka
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Engineered PMMA-ZnO nanocomposites for improving the electric arc interruption capability in electrical switching applications: Unprecedented experimental insights2017In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 141, p. 113-119Article in journal (Refereed)
    Abstract [en]

    Polymer inorganic nanocomposites (PINCs) have been engineered for controlling the electrical arc and to improve the arc interruption capability of the electrical switching applications, like circuit breakers. Several PINCs are fabricated by formation of ZnO quantum dots (QDs) in a poly (methyl methacrylate) (PMMA) matrix via in-situ polymerization method to avoid agglomeration of QDs, leading to a good spatial distribution of QDs in the polymer matrix. These PINCs have been characterized in detail for the morphology of QDs, interaction between QDs and polymer matrix, and ultraviolet (UV) radiation absorption. ZnO QDs have been assessed to have particle diameter of 3.5 nm, and their presence in the PMMA is revealed by the unique luminescence characteristics of the QDs under UV light. The presence of ZnO QDs broadened the range of UV radiation absorption of PMMA and the absorption edge is gradually shifted from 270 nm to 338 nm with step-wise loading of ZnO QDs. The PINCs are tested to determine their reproducibility and impact on the electrical arcs of current 1.6 kA generated using a specially designed test-setup. Interaction of PINCs with the electrical arcs generates ablation of chemical species towards core of the electrical arc, resulting in increase of voltage leading to cool-down the arc temperature. This experimental study demonstrates for the first time that these PINCs are reproducible, reliable and provides superior arc interruption capability.

  • 11.
    Doddapaneni, Venkatesh
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Ye, Fei
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Saleemi, Mohsin
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Gati, Rudolf
    ABB Switzerland.
    Toprak, Muhammet Sadaka
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    New experimental insights for controlling the electrical arcs in electrical switching applications: a comparative study on PMMA nanocomposites of Au and ZnO2017In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050Article in journal (Other academic)
    Abstract [en]

    Polymer inorganic nanocomposites (PINCs) are developed, not only due to scientific interest but also improving theelectric arc interruption process in the electrical switching applications like circuit breakers. The novelty of this work isin integrating the current developments in PINCs into electrical switching application in order to extend the limits of thepower switching devices. Several PINCs are fabricated by using pre-synthesized Au nanoparticles (NPs) of size 2.75 ±0.4 nm and poly (methyl methacrylate) (PMMA) matrix via in-situ polymerization method. Six homogeneous PINCsamples with ultra-low wt% of Au NPs varying from 0.0003 to 0.005 wt% have been fabricated. We find that thepresence of Au NPs improved the convective heat transfer and visible optical radiation absorption of PMMA. Thefabricated PINCs are tested for their arc interruption performance and the results are compared with ZnO PINCs in ourearlier work. The results of the experiments insights demonstrate the impact of PINCs on the electrical arcs and theirpotential advantages of having PINCs for the electric arc interruption process in high power switching devices.

  • 12. Duanmu, Jie
    et al.
    Gamstedt, E. Kristofer
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Rosling, Ari
    Hygromechanical properties of composites of crosslinked allylglycidyl-ether modified starch reinforced by wood fibres2007In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 67, no 15-16, p. 3090-3097Article in journal (Refereed)
    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.

  • 13.
    Edgren, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Asp, Leif E.
    Joffe, Roberts
    Failure of NCF Composites Subjected to Combined Compression and Shear Loading2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, no 15, p. 2865-2877Article in journal (Refereed)
    Abstract [en]

    Earlier studies have shown that formation of kink bands is the mechanism that is likely to govern failure of compression loaded non-crimp fabric (NCF) composite laminates. Because of this, a failure criterion for prediction of failure caused by kinking under multiaxial (axial compression and shear) loading has been adapted to a NCF composite system. The criterion has been validated for compression tests of quasi-isotropic laminates tested in uniaxial compression. By performing compression tests of the laminate at different off-axis angles, it was possible to vary the ratio of compressive axial stress/shear stress in the specimens. The test results proved that the criterion works well for predictions of kinking governed failure for the present material system. Detailed fractographic studies confirmed that formation of kink bands was the mechanism responsible for specimen failure. Kink bands were also found to develop at loads significantly lower than load at specimen failure.

  • 14. Eitelberger, J.
    et al.
    Hofstetter, K.
    Dvinskikh, Sergey V.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Physical Chemistry.
    A multi-scale approach for simulation of transient moisture transport processes in wood below the fiber saturation point2011In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 71, no 15, p. 1727-1738Article in journal (Refereed)
    Abstract [en]

    This paper presents a new, physically-based model for transport processes in wood for conditions below the fiber saturation point. The macroscopic mathematical description of these processes involves three coupled differential equations: two mass balance laws for the bound water and the water vapor phase, and an energy balance equation. These governing equations and the corresponding boundary conditions are expressed in terms of the state variables bound water concentration, water vapor concentration, and temperature. Macroscopic material properties are estimated based on a multiscale approach in the framework of continuum micromechanics. The phase change between the two water phases and the thus resulting coupling between the differential equations cannot be suitably captured in a purely macroscopic description, therefore a microscale sub-model is presented. Numerical solutions of the model equations are derived by means of the Finite Element Method. Finally, the model is applied to prediction of moisture profiles in a wood sample under transient environmental conditions. Comparing these results with corresponding profiles obtained non-destructively by proton magnetic resonance imaging (MRI) validates the model and confirms suitability of the underlying physical assumptions.

  • 15.
    Ekh, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Schön, Joakim
    Effect of Secondary Bending on Strength Prediction of Composite, Single Shear Lap Joints2005In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 65, p. 953-965Article in journal (Refereed)
    Abstract [en]

    The influence of secondary bending on strength prediction of composite joints has been studied by means of detailed finite element analyses. Secondary bending was changed in magnitude by altering the length of the overlap region, in a two-fastener specimen, while leaving all other joint parameters unchanged. It was found that secondary bending influences the various macroscopic failure modes, and thus has the potential to change the mode of failure and affect the ultimate failure load. Under tensile loading, secondary bending increased the contact area between the fasteners and the hole edges which reduced the bearing stresses and increased the bearing strength. Secondary bending also increased the bending of the plates and thus generated more severe net-tensile stresses which resulted in reduced net-tensile strength. Under compressive loading, secondary bending reduced the net-compressive strength due to the increased plate bending. The effect of secondary bending on bearing strength is difficult to predict and must be evaluated on an individual basis. It was concluded that secondary bending needs to be accounted for in accurate strength predictions.

  • 16.
    Ekh, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Schön, Joakim
    Load Transfer in Multirow, Single Shear, Composite-to-Aluminium Lap Joints2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, p. 875-885Article in journal (Refereed)
    Abstract [en]

    A three-dimensional finite element model has been developed in order to determine the load transfer in multifastener single shear joints. The model is based on continuum elements and accounts for all important mechanisms involved in load transfer, such as bolt-hole clearances, bolt clamp-up and friction. In particular, member plates can be of different thickness and stiffness and with different coefficient of thermal expansion. An experimental programme was conducted in order to validate the finite element model through measurements of fastener loads, by means of instrumented fasteners. Good agreement between simulations and experiments was achieved and it was found that bolt-hole clearance is the most important factor in terms of load distribution between the fasteners. Any variation in clearance between the different holes implies that load is shifted to the fastener where the smallest clearance occurs. Sensitivity to this variation in clearance was found to be large, so that temperature changes could significantly affect the load distribution if member plates with different thermal expansion properties are used. It was found that good accuracy in load transfer predictions requires that all aforementioned factors are taken into consideration and that nonlinear kinematics is accounted for in the solution process.

  • 17. Fernberg, S. P.
    et al.
    Berglund, Lars A.
    Bridging law and toughness characterisation of CSM and SMC composites2001In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, no 16, p. 2445-2454Article in journal (Refereed)
    Abstract [en]

    This work presents an experimental investigation of the fracture properties of three different short-fiber-reinforced composites [one chopped strand mat (CSM) and two sheet molding compound (SMC) materials]. Fracture tests are performed on double-cantilever beam (DCB) specimens loaded with pure bending moments. In this experimental configuration, the bridging law for the material can be derived directly from measurements. No significant dependency on specimen height was observed in our results. The bridging laws determined can, therefore, be considered as material properties. The coupling between microstructure and fracture behaviour is discussed through the measured bridging laws. The beneficial effect (in terms of fracture energy) of increasing tendency for pull-out is confirmed for one SMC, referred to as Flex-SMC, which shows remarkably high fracture energy, J(c) = 56.0 kJ/ml, compared to a standard SMC, termed Std-SMC, J(c)= 25.9 kJ/m(2). This increasing tendency for pull-out is observed to shift the bridging law towards larger crack openings. On the basis of our observations we find the concept of characterising the failure behaviour in terms of bridging laws attractive since it can be used as a tool for the tailoring of the microstructure towards desired fracture behaviour.

  • 18.
    Fu, Qiliang
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Yan, Min
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Jungstedt, Erik
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Yang, Xuan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Li, Yuanyuan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Transparent plywood as a load-bearing and luminescent biocomposite2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 164, p. 296-303Article in journal (Refereed)
    Abstract [en]

    Transparent wood (TW) structures in research studies were either thin and highly anisotropic or thick and isotropic but weak. Here, transparent plywood (TPW) laminates are investigated as load-bearing biocomposites with tunable mechanical and optical performances. Structure-property relationships are analyzed. The plies of TPW were laminated with controlled fiber directions and predetermined stacking sequence in order to control the directional dependence of modulus and strength, which would give improved properties in the weakest direction. Also, the angular dependent light scattering intensities were investigated and showed more uniform distribution. Luminescent TPW was prepared by incorporation of quantum dots (QDs) for potential lighting applications. TPW can be designed for large-scale use where multiaxial load-bearing performance is combined with new optical functionalities.

  • 19.
    Gazzotti, Stefano
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. Univ Milan, Dept Chem, Via Golgi 19, I-20133 Milan, Italy.;K;Univ Milan, Dept Chem, CRC Mat Polimer LaMPO, Via Golgi 19, I-20133 Milan, Italy..
    Rampazzo, Riccardo
    Univ Milan, Dept Chem, Via Golgi 19, I-20133 Milan, Italy.;Univ Milan, Dept Chem, CRC Mat Polimer LaMPO, Via Golgi 19, I-20133 Milan, Italy..
    Hakkarainen, Minna
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology.
    Bussini, Daniele
    INNOVHUB SSI Paper Dept, Via Giuseppe Colombo 83, I-20133 Milan, Italy..
    Ortenzi, Marco Aldo
    Univ Milan, Dept Chem, Via Golgi 19, I-20133 Milan, Italy.;Univ Milan, Dept Chem, CRC Mat Polimer LaMPO, Via Golgi 19, I-20133 Milan, Italy..
    Farina, Hermes
    Univ Milan, Dept Chem, Via Golgi 19, I-20133 Milan, Italy.;Univ Milan, Dept Chem, CRC Mat Polimer LaMPO, Via Golgi 19, I-20133 Milan, Italy..
    Lesma, Giordano
    Univ Milan, Dept Chem, Via Golgi 19, I-20133 Milan, Italy.;Univ Milan, Dept Chem, CRC Mat Polimer LaMPO, Via Golgi 19, I-20133 Milan, Italy..
    Silvani, Alessandra
    Univ Milan, Dept Chem, Via Golgi 19, I-20133 Milan, Italy.;Univ Milan, Dept Chem, CRC Mat Polimer LaMPO, Via Golgi 19, I-20133 Milan, Italy..
    Cellulose nanofibrils as reinforcing agents for PLA-based nanocomposites: An in situ approach2019In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 171, p. 94-102Article in journal (Refereed)
    Abstract [en]

    One-pot in situ polymerization approach was explored for the preparation of polylactide (PLA)-cellulose nano-fibril (CNF) bio-nanocomposites. CNF were first prepared through enzymatic and mechanical treatment of bleached hardwood kraft pulp. The bio-nanocomposites- were then fabricated through ring opening polymerization (ROP) of L-lactide, in the presence of various amounts of fibrils. Molecular weight, thermal properties, surface morphology, mechanical and wettability properties of the PLA-CNF nanocomposites were evaluated. DSC analysis demonstrated the effect of CNF on crystallization and crystalline morphology of PLA. Improved modulus for the nanocomposites with respect to standard PLA was demonstrated, however, the differences in tensile stress were small probably due to the counteracting effects of reinforcement from CNF and the decreasing molecular weight as a function of CNF concentration. The absence of pulled-out fibers was assessed, highlighting the strong interface and covalent attachment of PLA chains on CNF surface. Finally, the covalent bonding of PLA chains on CNF surface was demonstrated by isolating the non-soluble part, consisting of PLA-grafted CNF, and characterization of this residue.

  • 20.
    Gudmundson, Peter
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Alpman, Johan
    Structures Department, The Aero. Res. Institute.
    Initiation and growth criteria for transverse matrix cracks in composite laminates2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 2, p. 185-195Article in journal (Refereed)
    Abstract [en]

    Cecchin and Monti [Cecchin E, Monti A. Experimental techniques for analysis of matrix crack evolution in composite laminates. Masters thesis, Department of Solid Mechanics, Royal Institute of Technology (KTH), Stockholm, Sweden, 1998.] have presented experimental results using off-axis loaded tensile specimens that made it possible to introduce transverse matrix cracks which started from one edge of a specimen and arrested before reaching the other edge. Matrix crack positions and lengths were registered as functions of the applied load through optical images which were captured at regular external load intervals. In the present paper, the experimental data from Cecchin and Monti are analysed. Since several transverse matrix cracks appeared in each specimen and crack growth can be observed at different load levels, a single macroscopic specimen generates lots of data for crack initiation and propagation. Glass/epoxy cross-ply laminates with two different transverse ply thicknesses are investigated. The results strongly indicate that the energy release rate is a relevant measure for prediction of crack growth whereas the local strain controls matrix crack initiation.

  • 21.
    Gudmundson, Peter
    et al.
    SWEDISH INST. COMPOSITE.
    Östlund, Sören
    SWEDISH INST. COMPOSITE.
    Prediction Of Thermoelastic Properties Of Composite Laminates With Matrix Cracks.1992In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 44, no 2, p. 95-105Article in journal (Refereed)
    Abstract [en]

    A theory is presented for the determination of changes in the elastic compliance tensor and the thermal expansion coefficients due to matrix cracking. Effects of residual stresses are taken into account. The theory is valid for two- or three-dimensional laminates of arbitrary lay-ups and is asymptotically exact for dilute micro-crack densities. For large micro-crack densities an alternative model is described which is asymptotically exact for large crack densities. Combined, the two models cover to a good approximation the whole range of micro-crack densities. The theory is applied to a micro-cracking cross-ply GFRP laminate for which the stress/strain curve is calculated. Data are also presented for changes in thermomechanical properties of angle ply laminates as functions of the lay-up angle.

  • 22.
    Hagberg, Johan
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Applied Electrochemistry.
    Maples, Henry A.
    Polymer and Composite Engineering (PaCE) Group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna, Währinger Straße 42, A-1090 Vienna, Austria .
    Alvim, Kayne S. P.
    Polymer and Composite Engineering (PaCE) Group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna, Währinger Straße 42, A-1090 Vienna, Austria .
    Xu, Johanna
    Polymeric Composite Materials, Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-97187 Luleå, Sweden .
    Johannisson, Wilhelm
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Bismarck, Alexander
    Polymer and Composite Engineering (PaCE) Group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna, Währinger Straße 42, A-1090 Vienna, Austria ; Polymer and Composite Engineering (PaCE) Group, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK .
    Zenkert, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Lindbergh, Göran
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Applied Electrochemistry.
    Lithium iron phosphate coated carbon fiber electrodes for structural lithium ion batteries2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 162, p. 235-243Article in journal (Refereed)
    Abstract [en]

    A structural lithium ion battery is a material that can carry load and simultaneously be used to store electrical energy. We describe a path to manufacture structural positive electrodes via electrophoretic deposition (EPD) of LiFePO4 (LFP), carbon black and polyvinylidene fluoride (PVDF) onto carbon fibers. The carbon fibers act as load-bearers as well as current collectors. The quality of the coating was studied using scanning electron microscopy and energy dispersive X-ray spectroscopy. The active electrode material (LFP particles), conductive additive (carbon black) and binder (PVDF) were found to be well dispersed on the surface of the carbon fibers. Electrochemical characterization revealed a specific capacity of around 60–110 mAh g−1 with good rate performance and high coulombic efficiency. The cell was stable during cycling, with a capacity retention of around 0.5 after 1000 cycles, which indicates that the coating remained well adhered to the fibers. To investigate the adhesion of the coating, the carbon fibers were made into composite laminae in epoxy resin, and then tested using 3-point bending and double cantilever beam (DCB) tests. The former showed a small difference between coated and uncoated carbon fibers, suggesting good adhesion. The latter showed a critical strain energy release rate of ∼200–600 J m−2 for coated carbon fibers and ∼500 J m−2 for uncoated fibers, which also indicates good adhesion. This study shows that EPD can be used to produce viable structural positive electrodes.

  • 23.
    Hajian, Alireza
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Fu, Qiliang
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Berglund, Lars
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Recyclable and superelastic aerogels based on carbon nanotubes and carboxymethyl cellulose2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 159, p. 1-10Article in journal (Refereed)
    Abstract [en]

    Deformation mechanisms are largely unknown for superelastic carbon nanotube (CNT) aerogels, and this hampers materials design efforts. The CNT network in the cell walls is typically crosslinked or connected by a thermoset polymer phase. In order to create a recyclable superelastic aerogel, unmodified single or multi-walled CNTs were dispersed in water by adding to aqueous carboxymethyl cellulose (CMC) solution. Directional freeze-drying was used to form honeycombs with cell walls of random-in-the-plane CNTs in CMC matrix. Cell wall morphology and porosity were studied and related to CNT type and content, as well as elastic or plastic buckling of the cell walls under deformation. CMC acts as a physical crosslinker for the CNTs in a porous cell wall. Aerogel structure and properties were characterized before and after recycling. The conductivity of the composite aerogel with a density of 10 kg/m3, 99% porosity and 50 wt % single-walled CNT exceeds 0.5 S/cm. The potential of these superelastic and conductive aerogels for applications such as mechanoresponsive materials was examined in cyclic conductivity tests at different strains. This opens a new route for recyclable superelastic CNT composite aerogels, avoiding material loss, chemical treatment or addition of other components.

  • 24.
    Hassel, Ivon
    et al.
    Laboratory of Structural Function, Research Institute for Sustainable Humanosphere, Kyoto University.
    Berard, Pierre
    Laboratory of Sustainable Materials, Research Institute for Sustainable Humanosphere, Kyoto University.
    Modén, Carl S.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.
    The single cube apparatus for shear testing: Full-field strain data and finite element analysis of wood in transverse shear2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 7-8, p. 877-882Article in journal (Refereed)
    Abstract [en]

    The design and analysis of wood structures require accurate data for shear properties, where transverse shear in particular has been neglected in the past. The single cube apparatus (SCA) was applied to transverse shear of Norway spruce (Picea Abies), due to the importance of this species in wood structures, such as glulam, and also its allegedly low value of GRT . Full-field strain data and FEA were used to analyze the potential of the method. The presence of a large central region of homogeneous and close to pure shear strain was confirmed. The SCA method is therefore a strong candidate for improved shear test procedures in wood and other materials, where porosity (gripping problems), heterogeneity on mm-scale and polar orthotropy (annual ring curvature) may cause particular difficulties. In contrast to many other shear test studies, the accuracy of the present GRT data is supported by documented large and homogeneous specimen stress- and strain-fields in almost pure shear, direct measurements of strain field, and careful stress analysis based on FEA.

  • 25.
    Hassel, Ivon
    et al.
    Laboratory of Structural Function, Research Institute for Sustainable Humanosphere, Kyoto University.
    Modén, Carl S.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.
    Functional gradient effects explain the low transverse shear modulus in spruce: Full-field strain data and a micromechanics model2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 14, p. 2491-2496Article in journal (Refereed)
    Abstract [en]

    An important failure mechanism in glulam beams is cracking caused by out-of-plane transverse loads. It has been demonstrated that the low transverse shear modulus G(RT) in spruce contributes to large transverse strain inhomogeneities due to the annual ring structure in combination with shear coupling effects. In the present study, improved understanding of annual ring effects is achieved by the development of a micromechanical model. It relates the functional density gradient in spruce annual rings to shear modulus GRT. The geometrical basis is a hexagonal cell model, and in shear it is demonstrated to deform primarily by cell wall bending. Full-field strain measurements by digital speckle photography (DSP) show very strong correlation with predicted shear strains at the annual ring scale. Predictions are obtained by implementation of the micromechanics model in a finite element (FE) model developed for the single cube apparatus shear specimen. The low GRT of spruce is due to the strong dependence of GRT on relative density rho/rho(s)(G(RT) proportional to (rho/rho(s))(3)). This is particularly important in spruce. Even though average density is typically quite high, the functional gradient structure includes local densities as low as 200 kg/m(3).

  • 26.
    Hedenqvist, Mikael S.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Backman, A.
    Gallstedt, M.
    Boyd, R. H.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Morphology and diffusion properties of whey/montmorillonite nanocomposites2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, no 13, p. 2350-2359Article in journal (Refereed)
    Abstract [en]

    The diffusion of methanol in solution-cast whey-protein-isolate/poly(vinylidene pyrrolidone)-coated montmorillonite composite films was studied. The filler content was 0-4.8 vol%. Desorption experiments on samples saturated in methanol were conducted in a nitrogen-purged chamber at 294 K or in a thermogravimeter at 333, 348 and 363 K. The desorption curves were fitted using a two-phase diffusion model; where diffusion occurred first in the rubbery and later in the glassy state. A free volume model was used to analyze desorption at 363 K which occurred only within the rubbery phase and it enabled the geometrical impedance factor and free volume constraints from the nanoparticles to be assessed. Transmission electron microscopy showed a partial exfoliation of the nanoparticles. The geometrical impedance factor increased markedly with increasing filler content in accordance with the high aspect ratio of the nanoparticle aggregates. Transmission electron microscopy, in combination with model calculations, revealed that the clay sheets were oriented preferentially in the plane of the film which was an important factor underlying the high geometrical impedance factor obtained for the nanocomposites. A small but significant reduction in the fractional free volume of the polymer matrix was observed for the rubbery polymer in the presence of montmorillonite.

  • 27.
    Henriksson, Marielle
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Fogelström, Linda
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Johansson, Mats K. G.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Hult, Anders
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Novel nanocomposite concept based on cross-linking of hyperbranched polymers in reactive cellulose nanopaper templates2011In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 71, no 1, p. 13-17Article in journal (Refereed)
    Abstract [en]

    Cellulosic fibers offer interesting possibilities for good interfacial adhesion due to the high density of hydroxyl groups at the surface. in the present study, the potential of a new nanocomposite concept is investigated, where a porous cellulose nanofiber network is impregnated with a solution of reactive hyperbranched polyester. The polymer is chemically cross-linked to form a solid matrix. The resulting nanocomposite structure is unique. The matrix surrounds a tough nanopaper structure consisting of approximately 20 nm diameter nanofibers with an average interfiber distance of only about 6 nm. The cross-linked polymer matrix shows strongly altered characteristics when it is cross-linked in the confined space within the nanofiber network, including dramatically increased T-g, and this must be due to covalent matrix-nanofiber linkages.

  • 28.
    Huang, Hui
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Dobryden, Illia
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Thorén, Per Anders
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Ejenstam, Lina
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. SP Technical Research Institute of Sweden.
    Pan, Jinshan
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Fielden, Matthew
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Claesson, Per Martin
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. SP Technical Research Institute of Sweden.
    Local surface mechanical properties of PDMS-silica nanocomposite probed with Intermodulation AFM2017In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 150, p. 111-119Article in journal (Refereed)
    Abstract [en]

    The mechanical properties of polymeric nanocomposites are strongly affected by the nature of the interphase between filler and matrix, which can be controlled by means of surface chemistry. In this report, we utilize intermodulation atomic force microscopy (ImAFM) to probe local mechanical response with nanometer-scale resolution of poly(dimethylsiloxane) (PDMS) coatings with and without 20 wt% of hydrophobic silica nanoparticles. The data evaluation is carried out without inferring any contact mechanics model, and is thus model-independent. ImAFM imaging reveals a small but readily measurable inhomogeneous mechanical response of the pure PDMS surface layer. The analysis of energy dissipation measured with ImAFM showed a lowering of the viscous response due to the presence of the hydrophobic silica nanoparticles in the polymer matrix. An enhanced elastic response was also evident from the in-phase stiffness of the matrix, which was found to increase by a factor of 1.5 in presence of the nanoparticles. Analysis of dissipation energy and stiffness in the immediate vicinity of the nanoparticles provides an estimate of the interphase thickness. Because the local stiffness varies significantly near the nanoparticle, AFM height images contain artifacts that must be corrected in order to reveal the true surface topography. Without such a correction the AFM height images erroneously show that the stiff particles protrude from the surface, whereas corrected images show that they are actually embedded in the matrix and likely covered with a thin layer of polymer.

  • 29.
    Jacques, Eric
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kjell, Maria H.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
    Zenkert, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Lindbergh, Göran
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
    Behm, Mårten
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
    Willgert, Markus
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Impact of electrochemical cycling on the tensile properties of carbon fibres for structural lithium-ion composite batteries2012In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 72, no 7, p. 792-798Article in journal (Refereed)
    Abstract [en]

    Carbon fibres are particularly well suited for use in a multifunctional lightweight design of a structural composite material able to store energy as a lithium-ion battery. The fibres will in this case act as both a high performance structural reinforcement and one of the battery electrodes. However, the electrochemical cycling consists of insertions and extractions of lithium ions in the microstructure of carbon fibres and its impact on the mechanical performance is unknown. This study investigates the changes in the tensile properties of carbon fibres after they have been subjected to a number of electrochemical cycles. Consistent carbon fibre specimens were manufactured with polyacrylonitrile-based carbon fibres. Sized T800H and desized IMS65 were selected for their mechanical properties and electrochemical capacities. At the first lithiation the ultimate tensile strength of the fibres was reduced of about 20% but after the first delithiation some strength was recovered. The losses and recoveries of strength remained unchanged with the number of cycles as long as the cell capacity remained reversible. Losses in the cell capacity after 1000 cycles were measured together with smaller losses in the tensile strength of the lithiated fibres. These results show that electrochemical cycling does not degrade the tensile properties which seem to depend on the amount of lithium ions inserted and extracted. Both fibre grades exhibited the same trends of results. The tensile stiffness was not affected by the cycling. Field emission scanning electron microscope images taken after electrochemical cycling did not show any obvious damage of the outer surface of the fibres.

  • 30.
    Johannisson, Wilhelm
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Ihrner, Niklas
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology.
    Zenkert, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Johansson, Mats
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology.
    Carlstedt, D.
    Asp, L. E.
    Sieland, F.
    Multifunctional performance of a carbon fiber UD lamina electrode for structural batteries2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 168, p. 81-87Article in journal (Refereed)
    Abstract [en]

    In electric transportation there is an inherent need to store electrical energy while maintaining a low vehicle weight. One way to decrease the weight of the structure is to use composite materials. However, the electrical energy storage in today's systems contributes to a large portion of the total weight of a vehicle. Structural batteries have been suggested as a possible route to reduce this weight. A structural battery is a material that carries mechanical loads and simultaneously stores electrical energy and can be realized using carbon fibers both as a primary load carrying material and as an active battery electrode. However, as yet, no proof of a system-wide improvement by using such structural batteries has been demonstrated. In this study we make a structural battery composite lamina from carbon fibers with a structural battery electrolyte matrix, and we show that this material provides system weight benefits. The results show that it is possible to make weight reductions in electric vehicles by using structural batteries. 

  • 31.
    Johnson, Mikael
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Broad-band transient recording and characterization of acoustic emission events in composite laminates2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 15, p. 2803-2818Article in journal (Refereed)
    Abstract [en]

    Acoustic emission (AE) transients due to different kinds of microdamage, such as matrix cracking, fiber breakage and local delaminations, have been recorded in glass/epoxy composite laminates. Different stacking sequences, [0 degrees, 90(2)degrees](S), [90(2)degrees, 0 degrees](S), [+45 degrees, -45 degrees](S) and [0(4)degrees], were used in order to trigger different crack mechanisms during tensile testing. The AE transients were recorded as functions of time by the use of broad-band AE transducers. In the experiments, it was observed that different types of cracks generated acoustic emission signals varying in amplitude, duration time and frequency content. It was also seen that the same type of crack produced signals with different characteristics depending on the layer in which the crack appeared and its orientation compared to the location of the transducer. The majority of signals from the tests could be divided into a few main groups. Typical signals and characteristics representing these groups are presented in the paper. The observations show that there is a potential in future development of quantitative methods for identification of damage development in composite laminates.

  • 32.
    Johnson, Mikael
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Experimental and theoretical characterization of acoustic emission transients in composite laminates2001In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, no 10, p. 1367-1378Article in journal (Refereed)
    Abstract [en]

    Transient wave propagation resulting from transverse matrix cracking in cross-ply composite tensile test specimens are investigated both theoretically and experimentally. In the experiments, broad-band transducers and a fast data-acquisition system enable measurements of transients in a frequency interval up to 1 MHz. The theoretical predictions are based on a model which utilizes a finite-element discretization of the cross-section of the specimen and a Fourier representation of the axial and time dependence. The comparisons between experimental and theoretical results are encouraging. Weak points in the experimental technique and in the theoretical model are identified.

  • 33.
    Kaufmann, Markus
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Zenkert, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Mattei, Christophe
    Cost optimization of composite aircraft structures including variable laminate qualities2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 13, p. 2748-2754Article in journal (Refereed)
    Abstract [en]

    Composite structures can lower the weight of an airliner significantly. The increased production cost, however, requires the application of cost-effective design strategies in which cost, weight and the desired laminate quality are taken into account. This paper proposes an optimization framework for composite aircraft structures that minimizes the direct operating cost on a part level. In addition to previous models, a non-destructive testing model is implemented that calculates design allowables of a laminate based on ultrasonic scan parameters. In a case study, the effect of the laminate quality on the direct operating cost is discussed. It is investigated how the permissible flaw size and therefore the scan pitch of a composite laminate can influence the optimal solution in terms of cost and weight; thus, the manufacturing cost, the non-destructive testing cost and the weight of a component can be balanced by optimizing the laminate quality in an early design phase.

  • 34. Kazemahvazi, S.
    et al.
    Khokar, N.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Wadley, H. N. G.
    Deshpande, V. S.
    Confluent 3D-assembly of fibrous structures2016In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 127, p. 95-105Article in journal (Refereed)
    Abstract [en]

    The ability to independently control fiber alignments and structural geometry is critical for design of optimal three-dimensional (3D) fibrous structures. We present a novel method to 3D-assemble carbon fiber structures, containing no seams or adhesive joints, using a confluence of several textile methodologies. A variety of complex structural shapes with tailored fiber topologies are demonstrated to be achievable. These optimized structures are shown to have unprecedented static and dynamic strength as well as damage tolerance and ductility. For example, the energy absorption capacity of a 3D-assembled carbon fiber prismatic sandwich structure is shown to be 300% greater than a high performance metallic counterpart.

  • 35.
    Kazemahvazi, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kiele, Jörn
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Zenkert, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Tensile strength of UD-composite laminates with multiple holes2010In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 70, no 8, p. 1280-1287Article in journal (Refereed)
    Abstract [en]

    The residual strength of glass fibre reinforced vinyl-ester laminates with multiple holes was investigated through an experimental programme. Different types of structured hole patterns and hole densities were investigated and analysed using digital image correlation strain measuring technique. Three different failure modes could be observed when the hole patterns and the hole densities were a altered. These three failure modes were used as the foundation for a simple yet effective analytical model in order to predict the residual strength of damaged composite specimens. Finally, a number of laminates with randomly distributed holes were tested experimentally. The analytical model can predict the failure mode and failure strength of the experiments with sufficiently good fidelity.

  • 36.
    Kazemahvazi, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Tanner, Daniel
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Zenkert, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Corrugated all-composite sandwich structures. Part 2: Failure mechanisms and experimental programme2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 7-8, p. 920-925Article in journal (Refereed)
    Abstract [en]

    A novel corrugated composite core, referred to as a hierarchical corrugation, has been developed and tested experimentally. Hierarchical corrugations exhibit a range of different failure modes depending on the geometrical properties and the material properties of the structures. In order to understand the different failure modes the analytical strength model, developed in part I of this paper, was used to make collapse mechanism maps for the different corrugation configurations. If designed correctly, the hierarchical structures can have more than 7 times higher weight specific strength compared to its monolithic counter part. The difference in strength arises mainly from the increase in buckling resistance of the sandwich core members compared to the monolithic version. The highest difference in strength is seen for core configurations with low overall density. As the density of the core increases, the monolithic core members get stockier and more resistant to buckling and thus the benefits of the hierarchical structure reduces.

  • 37.
    Kazemahvazi, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Zenkert, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Corrugated all-composite sandwich structures. Part 1: Modeling2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 7-8, p. 913-919Article in journal (Refereed)
    Abstract [en]

    An analytical model for the compressive and shear response of monolithic and hierarchical corrugated composite cores has been developed. The stiffness model considers the contribution in stiffness from the bending- and the shear deformations of the core members in addition to the stretching deformation. The strength model is based on the normal stress and shear stress distribution over each core member when subjected to a shear or compressive load condition. The strength model also accounts for initial imperfections. In part 1 of this series, the analytical model is described and the results are compared to finite element predictions. In part 2, the analytical model is compared to experimental results and the behaviour of the corrugated structures is investigated more thoroughly using failure mechanism maps.

  • 38.
    Kazemahvazi, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Zenkert, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Burman, Magnus
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Notch and Strain Rate Sensitivity of Non-Crimp Fabric Composites2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 6, p. 793-800Article in journal (Refereed)
    Abstract [en]

    The notch and strain rate sensitivity of noncrimp glass fibre/vinyl-ester laminates subjected to uniaxial tensile loads has been investigated experimentally. Two set of notch configurations were tested; one where circular holes were drilled and another where fragment simulating projectiles were fired through the plate creating a notch. Experiments were conducted for strain rates rangingfrom 10-4/s to 102/s using servo hydraulic machines. A significant increase in strength with increasing strain rate was observed for both notched and unnotched specimens. High speed photography revealed changes in failure mode, for certain laminate configurations, as the strain rate increased. The tested laminate configurations showed fairly small notch sensitivity for the whole range of strain rates.

  • 39. Koissin, Vitaly
    et al.
    Shipsha, Andrey
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Residual dent in locally loaded foam core sandwich structures: Analysis and use for NDI2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 1, p. 57-74Article in journal (Refereed)
    Abstract [en]

    This paper addresses the residual denting in the face sheet and corresponding core damage in a locally loaded flat sandwich structure with foam core. The problem is analytically considered in the context of elastic bending of the face sheet accompanied by non-linear deformation of the crushed foam core. The plane and axisymmetric formulations are studied. The obtained solutions allow for the estimation of the magnitude of the residual face dent through the size of the crushed core zone (or vice versa). Thus, the analytical background is proposed for a non-destructive inspection of locally indented/impacted sandwich structures. The solutions are verified experimentally.

  • 40. Koissin, Vitaly
    et al.
    Shipsha, Andrey
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Skvortsov, Vitaly
    Compression strength of sandwich panels with sub-interface damage in the foam core2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 13, p. 2231-2240Article in journal (Refereed)
    Abstract [en]

    This paper addresses the effect of a local quasi-static indentation or a low-velocity impact on the residual strength of foam core sandwich panels subjected to edgewise compression. The damage is characterized by a local zone of crushed core accompanied by a residual dent in the face sheet. Experimental studies show that such damage can significantly alter the compressive strength. Theoretical analysis of the face sheet local bending is performed for two typical damage modes (with or without a face-core debonding). The solutions allow estimation of the onset of (a) an unstable dent growth (local buckling) or (b) a compressive failure in the face sheet. The theoretical results are in agreement with the test data for two considered sandwich configurations.

  • 41. Lee, Koon-Yang
    et al.
    Aitomäki, Yvonne
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Oksman, Kristiina
    Bismarck, Alexander
    On the use of nanocellulose as reinforcement in polymer matrix composites2014In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 105, p. 15-27Article in journal (Refereed)
    Abstract [en]

    Nanocellulose is often being regarded as the next generation renewable reinforcement for the production of high performance biocomposites. This feature article reviews the various nanocellulose reinforced polymer composites reported in literature and discusses the potential of nanocellulose as reinforcement for the production of renewable high performance polymer nanocomposites. The theoretical and experimentally determined tensile properties of nanocellulose are also reviewed. In addition to this, the reinforcing ability of BC and NFC is juxtaposed. In order to analyse the various cellulose-reinforced polymer nanocomposites reported in literature, Cox-Krenchel and rule-of-mixture models have been used to elucidate the potential of nanocellulose in composite applications. There may be potential for improvement since the tensile modulus and strength of most cellulose nanocomposites reported in literature scale linearly with the tensile modulus and strength of the cellulose nanopaper structures. Better dispersion of individual cellulose nanofibres in the polymer matrix may improve composite properties.

  • 42.
    Leijonmarck, Simon
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
    Carlson, T.
    Lindbergh, Göran
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
    Asp, L. E.
    Maples, H.
    Bismarck, A.
    Solid polymer electrolyte-coated carbon fibres for structural and novel micro batteries2013In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 89, p. 149-157Article in journal (Refereed)
    Abstract [en]

    We report a method to deposit a thin solid polymer electrolyte (SPE) coating around individual carbon fibres for the realization of novel battery designs. In this study an electrocoating method is used to coat methacrylate-based solid polymer electrolytes onto carbon fibres. By this approach a dense uniform, apparently pinhole-free, poly(methoxy polyethylene glycol (350) monomethacrylate) coating with an average coating thickness of 470. nm was deposited around carbon fibres. Li-triflate, used as supporting electrolyte remained in the coating after the electrocoating operation. The Li-ion content in the solid polymer coating was found to be sufficiently high for battery applications. A battery device was built employing the SPE coated carbon fibres as negative electrode demonstrating reversible specific capacity of 260. mA. h/g at low currents (C/10), suggesting that these coated carbon fibres can be employed in future structural composite batteries.

  • 43. Lindhagen, J. E.
    et al.
    Berglund, Lars A.
    Application of bridging-law concepts to short-fibre composites - Part 1: DCB test procedures for bridging law and fracture energy2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 6, p. 871-883Article in journal (Refereed)
    Abstract [en]

    This is the first paper in a series of four where notch sensitivities, fracture energies and bridging laws of short-fibre polymer composites are investigated. In the context of crack-bridging, the bridging law is an important material parameter. The bridging law can be used in combination with a stress analysis to address failure problems, for instance large-scale bridging, where linear elastic fracture mechanics is not valid. The bridging law of the material is sensitive to material composition and fibre architecture. Owing to the lack of established procedures, it is of interest to develop experimental and analytical methods for determination of the bridging law and fracture energy of short-fibre polymer composites. A method based on a large DCB specimen loaded by pure bending moments is used. Commercial GMT and SMC materials are investigated in addition to chopped-strand-mat laminates based on glass fibres of two different lengths and two thermoset matrices of different ductility. Fracture energies and bridging law data are successfully determined. All materials demonstrate softening bridging laws and this is discussed on the basis of observed mechanisms of failure and existing micromechanical models.

  • 44. Lindhagen, J. E.
    et al.
    Berglund, Lars A.
    Application of bridging-law concepts to short-fibre composites - Part 2. Notch sensitivity2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 6, p. 885-893Article in journal (Refereed)
    Abstract [en]

    This is the second paper in a series of four where notch sensitivities, fracture energies and bridging laws are studied in short-fibre polymer composites. Estimates based on an order-of-magnitude criterion indicate that previous notch sensitivity studies on short-fibre composites are limited to small notch sizes in the ductile region. For this reason, centre-hole notch sensitivity is studied experimentally as a function of relatively large notch diameters in the range 15-60 mm. The materials have different matrices, glass-fibre content and fibre lengths. The onset of notch sensitivity is observed for all materials (glass-mat thermoplastics, sheet-moulding compounds and chopped-strand-mat laminates), although large notch sizes are required. The reasons for this are discussed as well as the influence of different material parameters. On the basis of the material bridging law and laminate Young's modulus, it was possible to rank different short-fibre composites with respect to notch sensitivity.

  • 45. 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 profiles2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 16, p. 2883-2894Article in journal (Refereed)
    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.

  • 46. Lindhagen, J. E.
    et al.
    Jekabsons, N.
    Berglund, Lars A.
    Application of bridging-law concepts to short-fibre composites 4. FEM analysis of notched tensile specimens2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 16, p. 2895-2901Article in journal (Refereed)
    Abstract [en]

    This is the fourth paper in a series of four where notch sensitivities, fracture energies and bridging laws in short-fibre polymer composites are investigated. In this paper finite-element modelling (FEM) of centre-hole-notched tensile specimens is performed, with different bridging laws governing crack growth. Crack lengths, crack profiles and stress distributions are predicted. The results are compared with experimentally determined crack shapes from an earlier investigation. Only with softening bridging laws can the experimental results be matched. The predicted crack lengths are sensitive to bridging-law parameters. When bridging laws determined by the double cantilever beam (DCB) method are applied, the predicted crack lengths and profiles show good correlation with the experimental results. The results support the validity of the DCB method to determine bridging laws in short-fibre composites.

  • 47.
    Lundgren, Jan-Erik
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Moisture absorption in glass-fibre/epoxy laminates with transverse matrix cracks1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 13, p. 1983-1991Article in journal (Refereed)
    Abstract [en]

    Moisture uptake in cross-ply glass-fibre/epoxy laminates containing matrix cracks in transverse plies has been investigated. Experimental results are compared to finite-element calculations and an approximate analytical model. The experimental results show that crack closure may occur early in the absorption process. This is confirmed by finite-element calculations which indicate that even small amounts of absorbed moisture (average moisture concentration < 10% of saturation level) may result in crack closure. As a result of crack closure, moisture penetration through the crack channels is prevented. The experimentally observed small differences in moisture uptake between cracked and uncracked laminates thus support the crack-closure hypothesis. Direct microscopic observations of crack closure versus time furthermore confirm the mechanism. A previously developed analytical model has been extended to include the case of crack-closure effects. The model compares favourably with experimental observations.

  • 48.
    Lönnberg, Hanna
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Fogelström, Linda
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Zhou, Qi
    KTH, School of Biotechnology (BIO), Glycoscience.
    Hult, Anders
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Malmström, Eva
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Investigation of the graft length impact on the interfacial toughness in a cellulose/poly(ε-caprolactone) bilayer laminate2011In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 71, no 1, p. 9-12Article in journal (Refereed)
    Abstract [en]

    Interfacial adhesion between immiscible cellulose–polymer interfaces is a crucial property for fibrous biocomposites. To tailor the interfacial adhesion, the grafting of polymers from cellulose films was studied using ring-opening polymerization of ε-caprolactone. The poly(ε-caprolactone) (PCL) grafted cellulose was analyzed with FTIR, AFM and via water CA measurements. The graft length was varied by the addition of a free initiator, enabling tailoring of the interfacial toughness. Films of microfibrillated cellulose were grafted with PCL and hot-pressed together with a PCL-film to form a bilayer laminate. Interfacial peeling toughness correlates very strongly with PCL degree of polymerization (DP). PCL grafts form physical entanglements in the PCL matrix and promote significant plastic deformation in the PCL bulk, thus increasing interfacial peeling energy.

  • 49.
    Modén, Carl S.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.
    A two-phase annual ring model of transverse anisotropy in softwoods2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 14, p. 3020-3028Article in journal (Refereed)
    Abstract [en]

    Transverse anisotropy in softwoods is an important phenomenon of both scientific and industrial interest. Simple one-phase hexagonal honeycomb cell models for transverse moduli of softwoods are based on cell wall bending as the only deformation mechanism. In the present study, a two-phase annual ring model is developed and includes both cell wall bending and stretching as deformation mechanisms. The proportion of cell wall bending and stretching for different cases is analysed and the importance of stretching is confirmed. A two-phase annual ring model is presented based on fixed densities for earlywood and latewood. Such a model is motivated by the large difference in density between earlywood and latewood layers. Two-phase model predictions show much better agreement with experimental data than predictions from a one-phase model. Radial modulus is dominated by bending at low density and by stretching at high density. For tangential modulus, bending is more important at all densities.

  • 50.
    Nilsson, Fritjof
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Modelling the Relative Permittivity of Anisotropic Insulating Composites2011In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 71, no 2, p. 216-221Article in journal (Refereed)
    Abstract [en]

    Three models have been developed for predicting the dielectric permittivity of insulating composites with inclusions of different lengths (from nm and larger) and different shapes. Firstly, for approximately periodic materials, a finite element model based on a smallest repeating box method was used in order to mimic frameworks with fibres, crystals, clay platelets, foams and lamellar layers. The introduction of parameters for relative aspect ratio, overlap, rotation and packing density made the model very flexible while maintaining its simplicity. Secondly, a finite element composite model with oriented, randomly positioned particles of different shapes was constructed. Thirdly, an analytical relationship to approximate the effective permittivity of two- or three-phase insulators with brick-shaped inclusions was derived. For a wide range of volume fractions, permittivity ratios and packing conditions, this model gave solutions very close to corresponding finite element simulation data for lamellae, much closer than all the other analytical relationships found in the literature. Results obtained by simulation were in agreement with experimental data from the literature for composites of micrometre-sized hollow glass spheres in epoxy and nanocomposites of mica platelets in polyimide, provided that a third (interfacial) component was introduced.

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