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  • 1.
    Al-Ramahi, Nawres
    et al.
    Luleå University of Technology, Sweden ; Middle Technical University, Iraq.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Swerea, Swerea SICOMP. Luleå University of Technology, Sweden .
    Varna, Janis
    Luleå University of Technology, Sweden .
    Investigation of end and edge effects on results of numerical simulation of single lap adhesive joint with non-linear materials2018In: International Journal of Adhesion and Adhesives, ISSN 0143-7496, E-ISSN 1879-0127, Vol. 87, p. 191-204Article in journal (Refereed)
    Abstract [en]

    This paper presents systematic numerical study of stresses in the adhesive of a single-lap joint with the objective to improve understanding of the main material and geometrical parameters determining performance of adhesive joints. For this purpose a 3D model as well as 2D model, optimized with respect to the computational efficiency by use of novel displacement coupling conditions able to correctly represent monoclinic materials (off-axis layers of composite laminates), are employed. The model accounts for non-linearity of materials (adherend and adhesive) as well as geometrical non-linearity. The parameters of geometry of the joint are normalized with respect to the dimensions of adhesive (e.g. thickness) thus making analysis of results more general and applicable to wide range of different joints. Optimal geometry of the single-lap joint allowing to separate edge effect from end effects is selected based on results of the parametric analysis by using peel and shear stress distributions in the adhesive layer as a criterion. Three different types of single lap joint with similar and dissimilar (hybrid) materials are considered in this study: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). The influence of the abovementioned parameters on peel and shear stress distributions in the adhesive layer is examined carefully and mechanical parameters governing the stress concentrations in the joint have been identified, this dependence can be described by simple but accurate fitting function. The effect of the used material model (linear vs non-linear) on results is also demonstrated.

  • 2.
    Al-Ramahi, Nawres
    et al.
    Luleå University of Technology, Sweden; Middle Technical University, Iraq.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP. Luleå University of Technology, Sweden.
    Varna, Janis
    Luleå University of Technology, Sweden.
    Numerical stress analysis in adhesive joints under thermo-mechanical load using model with special boundary conditions2019In: IOP Conference Series: Materials Science and Engineering, Volume 518, Mechanical and Materials Engineering: Materials Science and Engineering, Institute of Physics Publishing , 2019, no 3Conference paper (Refereed)
    Abstract [en]

    A numerical study of the adhesivejoint made of similar and dissimilar adherends subjected to thermo-mechanical loading is presented. A comprehensive numerical model was used for this purpose with the novel displacement coupling conditions which are able to correctly represent monoclinic materials (off-axis layers of composite laminates). The geometrical nonlinearity as well as nonlinear material model are also taken into account. Three different types of single-lap and double-lap adhesive joints are considered in this study: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). This paper focuses on the parameters which have the majoreffect on the peel and shear stress distribution within adhesive layer at the overlap ends. The comparison of behaviour of single-and double-lap joints in relation to these parameters is made. The master curves for maximum stress (peel and shear) at the ends of the overlap with respect to the bending stiffness and axial modulus of the adherends are constructed by analysing stress distributions in the middle of the adhesive.The main conclusions of this paper are: the maximum peel stress value for SLJ is reduced with increase of the adherend bending stiffness and for DLJ,similar behaviour was observed at the end next to the inner plate corner, while, at the end next to the outer plate corner peel stress is reduced with increase of adherend axial modulus.

  • 3.
    Andersons, J.
    et al.
    University of Latvia.
    Sparniņš, E.
    University of Latvia.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Scale effect of the tensile strength of flax-fabric-reinforced polymer composites2011In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 30, no 23, p. 1969-1974Article in journal (Refereed)
    Abstract [en]

    The development of UD natural fiber composites, considered for application as structural materials, necessitates evaluation of the scale effect of their strength. Alignment of the fibers in flax bast fiber composites can be achieved by employing textile reinforcement, such as yarns and fabrics. Cutting specimens for mechanical tests out of such textile-reinforced composite plates results in a complex non-uniform reinforcement structure at their edges, which may affect the strength of specimens. Scale effect of the tensile strength in the fiber direction of flax-fabric-reinforced composites is studied in the current work. A model accounting for both volume and edge effect of the specimens on their tensile strength is proposed. © The Author(s) 2011.

  • 4.
    Gong, Guan
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Development of polyethylene/nanoclay masterbatch for use in wood-plastic composites2013Conference paper (Refereed)
    Abstract [en]

    In this work, organoclay reinforced high density polyethylene (HDPE) nanocomposites were prepared at laboratory scale using a batch mixer. Processing conditions, maleic anhydride modified polyethylene (MAPE) type and MAPE/clay weight ratio were optimised. The microstructure of the resultant nanocomposites was analysed by X-ray diffraction and melt rheology tests, and flexural properties and thermal stability were evaluated. Three types of MAPEs with different melt flow indices (MFI) and maleic anhydride contents all improved the interaction between HDPE and clay and promoted clay dispersion. Nanocomposites where the MAPE with MFI most similar to HDPE was used showed the best exfoliation of clay and the strongest HDPE/clay interface. Mechanical properties were slightly improved, while thermal stability was distinctly enhanced in these HDPE nanocomposites compared with neat HDPE and HDPE nanocomposite without MAPE. The prepared HDPE nanocomposites show the potential to improve the thermal stability of wood-plastic composites for structural applications. © 2013 Institute of Materials, Minerals and Mining.

  • 5.
    Gong, Guan
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Enhanced thermal stability and flame retarding properties of recycled polyethylene based wood composites via addition of polyethylene/nanoclay masterbatch2013In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, no 6, p. 244-255Article in journal (Refereed)
    Abstract [en]

    Barrier and mechanical properties of wood powder composites based on recycled polyethylene (RPE) were modified using a commercial nanoclay masterbatch. X-ray diffraction, dynamic rheology and thermogravimetric analysis measurements showed that nanoclay from the selected masterbatch was well dispersed and formed a percolation network in both virgin and RPEs. The resulting nanocomposites promoted the thermal stability of matrix significantly. Modification efficiency of nanoclay, however, was evidently influenced by the type of matrix, where the strongest effect was achieved in a low viscosity virgin high density PE. The masterbatch was incorporated into an industrial formula designed extrusion quality RPE/wood flour composite. Processing procedures, mainly compounding cycles, and material composition, mainly clay content and type of coupling agent, were optimised. Two extrusion cycles led to higher uniformity of resulting composites than one cycle. Addition of a coupling agent, which has medium viscosity and plenty functional groups, led to enhanced tensile strength. The twice compounded composites were well stiffened and strengthened via combination of 6 wt-% clay and medium viscosity coupling agent. All composites without the addition of nanoclay burned faster after ignition and dripped much earlier and more compared to the composites containing nanoclay even with as small amount as 3 wt-% and being compounded once. The material with 6 wt-% clay showed the best sample integrity and burned slowest of all the tested composites. Furthermore, no dripping during combustion was seen for this material. This study shows that the incorporation of nanoclay using the selected masterbatch can effectively improve the flame retarding properties of RPE based wood composites. © Institute of Materials, Minerals and Mining 2013.

  • 6.
    Gong, Guan
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Sandlund, Erik
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Eklund, Daniel
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Noel, Maxime
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Westerlund, Robert
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Stenberg, Sofia
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Pupure, Liva
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP. Luleå University of Technology, Sweden.
    Pupurs, Andrejs
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP. Luleå University of Technology, Sweden.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP. Luleå University of Technology, Sweden.
    Development of electrophoretic deposition prototype for continuous production of carbon nanotube-modified carbon fiber fabrics used in high-performance multifunctional composites2018In: Fibers, ISSN 2079-6439, Vol. 6, no 4, article id 71Article in journal (Refereed)
    Abstract [en]

    An electrophoretic deposition (EPD) prototype was developed aiming at the continuous production of carbon nanotube (CNT) deposited carbon fiber fabric. Such multi-scale reinforcement was used to manufacture carbon fiber-reinforced polymer (CFRP) composites. The overall objective was to improve the mechanical performance and functionalities of CFRP composites. In the current study, the design concept and practical limit of the continuous EPD prototype, as well as the flexural strength and interlaminar shear strength, were the focus. Initial mechanical tests showed that the flexural stiffness and strength of composites with the developed reinforcement were significantly reduced with respect to the composites with pristine reinforcement. However, optical microscopy study revealed that geometrical imperfections, such as waviness and misalignment, had been introduced into the reinforcement fibers and/or bundles when being pulled through the EPD bath, collected on a roll, and dried. These defects are likely to partly or completely shadow any enhancement of the mechanical properties due to the CNT deposit. In order to eliminate the effect of the discovered defects, the pristine reinforcement was subjected to the same EPD treatment, but without the addition of CNT in the EPD bath. When compared with such water-treated reinforcement, the CNT-deposited reinforcement clearly showed a positive effect on the flexural properties and interlaminar shear strength of the composites. It was also discovered that CNTs agglomerate with time under the electric field due to the change of ionic density, which is possibly due to the electrolysis of water (for carboxylated CNT aqueous suspension without surfactant) or the deposition of ionic surfactant along with CNT deposition (for non-functionalized CNT aqueous suspension with surfactant). Currently, this sets time limits for the continuous deposition.

  • 7.
    Gong, Guan
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Sandlund, Erik
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Eklund, Danierl
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Noel, Maxime
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Westerlund, Robert
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP. Luleå University of Technology, Sweden.
    Pupure, Liva
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP. Luleå University of Technology, Sweden.
    Pupurs, Andrejs
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP. Luleå University of Technology, Sweden.
    SCALING-UP PRODUCTION OF CNT-COATEDFIBRE REINFORCEMENT USING CONTINUOUS EPDFOR HIGH-PERFORMANCE ANDMULTIFUNCTIONAL COMPOSITES2018Conference paper (Refereed)
    Abstract [en]

    Itis important within the composite community to improve out-of-plane performance ofcomposites dominated by polymer matrix and properties of matrix-rich regions formed in the gapsbetween the interlaced fibre bundles. These properties are difficult to modify with traditional fibrereinforcement. Various nanoscale materials have been explored for such purpose, among which carbonnanotube (CNT) has been suggested as an ideal candidate because of its outstanding mechanical,electrical and thermal properties (1). Electrophoretic deposition (EPD) is considered as a cost-effectivemethod to deposit CNTs onto substrates with mild working conditions, requiring relatively simpleequipment and being amenable to scaling up (2,3). Due to the shortcoming of existing laboratory setupwhich corresponds to a non-continuous process, EPD has not been used at even a pilot plant scale fornano-coated fibre reinforcement. The current work presents the development of a prototype andmethod for continuous EPD process. Geometric defect of fibre reinforcement introduced during thedeposition, which can shadow the reinforcing effect of CNT deposit, was discovered. Enhancement ofcomposite properties by the CNT deposit was hence shown.

  • 8.
    Mannberg, Peter
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Service life assessment and moisture influence on bio-based thermosetting resins2014In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 49, no 10, p. 3687-3693Article in journal (Refereed)
    Abstract [en]

    In this study, three different types of bio-based resins are compared to a conventional oil-based epoxy in terms of moisture uptake, long-term properties and its influence of moisture and glass transition temperature, T g. Moisture uptake is determined by means of gravimetric method, time temperature superposition (TTSP), and T g data obtained in dynamic mechanical thermal analysis (DMTA). Moisture uptake show Fickian diffuison behavour for all resins, saturation level and diffusion coefficient however differ. The long-term properties is characterised by creep compliance master curves created by means of TTSP. The examined bio-based resins are compatible to the reference epoxy in term of stability up to 3-10 years. Comparison between master curves for virgin, wet, and dried material show that moisture present in the specimen increases creep rate, and that some of this increase remains after drying of samples. T g measurements show that moisture inside the specimen decreases T g; this is anticipated because of the plasticizing effect of water. The overall conclusions are that the bio-based resins of polyester, and epoxy type are comparable in performance with oil-based epoxy, LY556 and they can be used to develop high-performance composites. © 2014 Springer Science+Business Media New York.

  • 9.
    Mannberg, Peter
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Wallström, L.
    LTU.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Service life assessment and moisture influence on bio-based composites2014In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 49, no 15, p. 5265-5270Article in journal (Refereed)
    Abstract [en]

    A race towards a more sustainable society is going on worldwide, and decreasing dependence on fossil resources in energy and transport sectors are main goals. One path to decreased oil usage is development of lightweight materials from renewable resources like bio-based composites. However, these new bio-based materials not only have to compete in mechanical performance but also have to restrain environmental loads like moisture and temperature over time. In this study, two bio-based composites have been compared to an oil-based composite in terms of long-term properties and water absorption behaviour. The long-term behaviour is determined by dynamic mechanical thermal analysis, DMTA and time temperature superposition, TTSP. The water uptake is determined by submersion of specimens into water and tracking their weight change over time. The moisture influence is characterised in form of water uptake and change in the master curves created by TTSP procedure. The results show that there is a significant difference in long-term performance between the bio-based and oil-based composites. It is realised that the bio-based composites can be a good alternative for some applications especially when taking their eco-friendly nature into account. © 2014 Springer Science+Business Media New York.

  • 10.
    Nyström, Birgitha
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Långström, Runar
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Microstructure and strength of injection molded natural fiber composites2007In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 26, no 6, p. 579-599Article in journal (Refereed)
    Abstract [en]

    Injection molded short natural fiber composites (NFC) are studied in this article. Micromechanical models are employed to investigate effects of fiber length distribution, fiber orientation, and fiber/matrix adhesion on mechanical performance of NFCs. A parametric study showed that length/diameter ratio strongly affects the composite strength, whereas fiber strength has very little influence on strength of the composites when fiber/matrix adhesion is poor. In order to verify conclusions based on theoretical predictions, modeling results are compared with experimental results of wood powder-, flax-, pulp-, and glass fiber composites. Recommendations on improvement of strength of NFC through change of the internal structure of the materials are formulated based on results of this investigation. © 2007 SAGE Publications.

  • 11.
    Pupure, L.
    et al.
    Luleå University of Technology.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Varna, J.
    Luleå University of Technology.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Development of constitutive model for composites exhibiting time dependent properties2013Conference paper (Refereed)
    Abstract [en]

    Regenerated cellulose fibres and their composites exhibit highly nonlinear behaviour. The mechanical response of these materials can be successfully described by the model developed by Schapery for time-dependent materials. However, this model requires input parameters that are experimentally determined via large number of time-consuming tests on the studied composite material. If, for example, the volume fraction of fibres is changed we have a different material and new series of experiments on this new material are required. Therefore the ultimate objective of our studies is to develop model which determines the composite behaviour based on behaviour of constituents of the composite. This paper gives an overview of problems and difficulties, associated with development, implementation and verification of such model.

  • 12.
    Pupure, LIva
    et al.
    Luleå University of Technology, Sweden.
    Varna, Janis
    Luleå University of Technology, Sweden.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Swerea, Swerea SICOMP. Luleå University of Technology, Sweden.
    Berthold, Fredrik
    RISE - Research Institutes of Sweden, Bioeconomy, Biobased Materials.
    Miettinen, Arttu
    University of Jyväskylä, Finland; Paul Scherrer Institute, Switzerland.
    Mechanical properties of natural fiber composites produced using dynamic sheet former2018In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280Article in journal (Refereed)
    Abstract [en]

    Composites formed from wood fibers and man-made cellulosic fibers in PLA (polylactic acid) matrix, manufactured using sheet forming technique and hot pressing, are studied. The composites have very low density (due to high porosity) and rather good elastic modulus and tensile strength. As expected, these properties for the four types of wood fiber composites studied here improve with increasing weight fraction of fibers, even if porosity is also increasing. On the contrary, for man-made cellulosic fiber composites with circular fiber cross-section, the increasing fiber weight fraction (accompanied by increasing void content) has detrimental effect on stiffness and strength. The differences in behavior are discussed attributing them to fiber/ fiber interaction in wood fiber composites which does not happen in man-made fiber composites, and by rather weak fiber/matrix interface for man-made fibers leading to macro-crack formation in large porosity regions.

  • 13.
    Rozite, L.
    et al.
    Luleå University of Technology.
    Joffe, Roberts
    RISE, Swerea, SICOMP. Luleå University of Technology.
    Varna, J.
    Luleå University of Technology.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Characterization and analysis of time dependent behavior of bio-based composites made out of highly non-linear constituents2013Conference paper (Refereed)
    Abstract [en]

    The objective of this investigation is to predict mechanical behavior of bio-based composites and their constituents by generalizing existing models to capture their time-dependent behavior. In order to identify and quantify parameters needed for the modeling, extensive damage tolerance tests as well as creep experiments are carried out. © The Society for Experimental Mechanics Inc. 2013.

  • 14.
    Rozite, L.
    et al.
    Luleå University of Technology.
    Joffe, Roberts
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Varna, J.
    Luleå University of Technology.
    Nyström, Birgitha
    RISE - Research Institutes of Sweden, Materials and Production, SICOMP.
    Characterization and modeling of performance of polymer composites reinforced with highly non-linear cellulosic fibers2012Conference paper (Refereed)
    Abstract [en]

    The behaviour of highly non-linear cellulosic fibers and their composite is characterized. Micro-mechanisms occurring in these materials are identified. Mechanical properties of regenerated cellulose fibers and composites are obtained using simple tensile test. Material visco-plastic and visco-elastic properties are analyzed using creep tests. Two bio-based resins are used in this study - Tribest and EpoBioX. The glass and flax fiber composites are used as reference materials to compare with Cordenka fiber laminates.

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