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  • 51.
    Jekabsons, Normunds
    et al.
    Ventspils University College.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Locans, Uldis
    Ventspils University College.
    Mechanical properties of a rapidly prototyped composite: numerical modeling and parametric analysis2012In: Proceedings of Mechanics of Composite Materials XVII International Conference, Riga, 2012Conference paper (Refereed)
  • 52.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Annual International SICOMP Conference on Manufacturing and Design of Composites2010Conference paper (Other (popular science, discussion, etc.))
  • 53.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Annual International SICOMP Conference on Manufacturing and Design of Composites2010Conference paper (Other (popular science, discussion, etc.))
  • 54.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Mechanics of Composite Materials, XVI International Conference2010Conference paper (Other (popular science, discussion, etc.))
  • 55.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Mechanics of Composite Materials, XVI International Conference2010Conference paper (Other (popular science, discussion, etc.))
  • 56.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Mechanics of Composite Materials, XVI International Conference2010Conference paper (Other (popular science, discussion, etc.))
  • 57.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Behavior of non-crimp fabric composites in shear2009In: Experimental techniques and design in composite materials (ETDCM8) with regular papers: [15 papers selected from the eighth edition of the seminar ... which took place on October 3 - 6, 2007 at the Sant'Elmo Beach Hotel (Castiadas), Sardinia, Italy] / [ed] Pierluigi Priolo, Amsterdam: Elsevier, 2009, p. 9-10Conference paper (Other academic)
  • 58.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage accumulation and stiffness degradation in composite laminates1999Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the presented thesis analytical modeling of elastic properties degradation and modeling of damage evolution in composite laminates is performed. Transverse cracking initiation on a fiber/matrix level is briefly studied by testing model composites and cross-ply laminates. Analysis of obtained results show limitation in application of model composites. Models originally obtained for cross-ply laminates (shear lag, variational) are adapted for more general lay-up of laminates [S,90n]s, where S can be any balanced sub-laminate. Closed form expressions to calculate elastic properties changes as a function of crack density are derived. These models are verified by comparing prediction with experimental results. In this work damage evolution modeling is performed by Monte-Carlo simulation. Two approaches that use different failure criteria are executed. One method employs strength failure criteria and second uses fracture mechanics approach. Experimental data for [+Q/-Q/90/90/90/90]s laminates are used to verify correctness of the applied models. Influence of the thickness of 90-layer on damage evolution is also analyzed. The investigation of laminates containing off-axis layers is performed. The results show that degradation of the shear modulus occurs in those layers during the loading. Degradation of the shear modulus influences longitudinal modulus of the whole laminate as well as stress distribution in 90-layer.

  • 59.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage development in noncrimp fiber composites under various loading conditions2006In: Book of abstracts: Fourteenth International Conference Mechanics of Composite Materials, May 29 - June 2, 2006, Riga, Latvia / [ed] V. Tamuzs, Institute of Polymer Mechanics, University of Latvia , 2006Conference paper (Other academic)
  • 60.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Durability of natural fiber composites2007In: Conference programme & List of abstracts / International conference on innovative natural fibre composites for industrial applications: October 10-13, 2007, University of Rome "La Sapienza", Faculty of engineering, Rome, Italy, 2007, p. 34-Conference paper (Other academic)
  • 61.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of test methods on measured compressive strength of polymer composites2000In: Proceedings of the 3rd Nordic Meeting on Materials and Mechanics: 8-11 May 2000 / [ed] Ryszard Pyrz; Nikolaj Vejen; Jan Schödt-Thomsen, Aalborg: Inst of mechanical engineering, Aalborg university , 2000Conference paper (Refereed)
  • 62.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Matrix cracking and interfacial debonding in polymer composites1996Licentiate thesis, comprehensive summary (Other academic)
  • 63.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanical properties and failure of non-crimp fabric composites subjected to shear stress2008In: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Conference paper (Other academic)
    Abstract [en]

    2-5 June 2008, Stockholm, Sweden

  • 64.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Performance of non-crimp fabric composites in shear2010In: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 425, p. 45-59Article in journal (Refereed)
    Abstract [en]

    Experimental results of performance of non-crimp fabric composites are presented in this paper. In order to characterize in-plane shear properties of these composites, tensile and compression tests of [±452]S specimens were carried out. Two directions of loading are considered: warp (Shear warP: SP) and weft (Shear wefT: ST). This corresponds to laminates with the following lay-up: SP = [+45/-45/+45/-45]S; ST = [-45/+45/-45/+45]S. Although both types of laminates are cut from the same plate, mechanical performance in tension is rather different: laminate cut in weft direction exhibit higher shear modulus, shear strength and shear strain at failure than samples cut in warp direction. Fractography showed that different behavior of the materials is caused by much earlier damage initiation and accumulation in SP laminates. Analysis of images obtained from optical microscopy indicated that premature failure is most likely caused by stitches that prevent free rotation of bundles (shear deformation).

  • 65.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Testing, treatment and processing of fibres2007In: Forum proceedings: Biocomposites, JEC publications , 2007, p. 12-29Conference paper (Other academic)
  • 66.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, J.
    Sparnins, Edgars
    Wallström, Lennart
    Cellulose-based fibers and their polymer composites characterization and prediction of properties2005In: 8th International Conference on Woodfiber-Plastic Composites (and Other Natural Fibers): May 23 - 25, 2005, Monona Terrace Community & Convention Center, Madison, Wisconsin, USA, Madison, Wis., 2005, p. 25-36Conference paper (Refereed)
  • 67.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, J.
    Institute of Polymer Mechanics, University of Latvia.
    Wallström, Lennart
    Interfacial shear strength of flax fiber/thermoset polymers estimated by fiber fragmentation tests2005In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 40, no 9-10, p. 2721-2722Article in journal (Refereed)
    Abstract [en]

    We study flax/thermoset polymer ISS [interfacial shear strength] for most common types of thermosets: vinylester, polyester, and epoxy. The effect of fiber surface treatment on ISS in such systems reinforced with flax fibers is investigated. Apparent ISS is evaluated from SFF tests by the Kelly-Tyson approach utilizing fiber strength at the critical length. Fiber strength at such small lengths is hardly accessible to direct testing, therefore it is usually extrapolated from test results at larger gauge lengths. We present ISS of flax fibers subjected to different treatments and thermoset polymers, derived from SFF tests using comprehensive fiber strength data.

  • 68.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Mechanical Characterization and Properties of Cellulose Fibers2014In: Handbook of Green Materials: Processing Technologies, Properties and Applications, Singapore: World Scientific Publishing Co Pte Ltd , 2014, p. 7-23Chapter in book (Refereed)
    Abstract [en]

    The objective of this chapter is to present experimental methods that are suitable for characterization of cellulose fibers. An overview of different techniques to obtain and analyze fiber strength distribution is presented. The differences of internal structure and mechanical behavior between natural and synthetic fibers, applicability and validity of experimental methods, as well as test conditions are discussed. Some of the techniques developed for synthetic fibers can be directly applied to the characterization of natural fibers, while others should be modified in order to adapt for the specific limitations of reinforcement with natural origin.

  • 69.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Mechanical performance of thermoplastic matrix natural fibre composites2008In: Properties and performance of natural fibre composites, Cambridge: Woodhead Publishing Materials , 2008, p. 402-459Chapter in book (Other academic)
  • 70.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Modeling the tensile strength of hemp fibers and short hemp fiber reinforced composites2015In: Natural Filler and Fibre Composites: Development and Characterisation, Southampton: WIT Press, 2015, p. 13-26Chapter in book (Refereed)
  • 71.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Nordström, Ylva
    Innventia AB, SE-114 86 Stockholm, Sweden.
    Sjöholm, Elisabeth
    Innventia AB, SE-114 86 Stockholm, Sweden.
    Analysis of the Tensile Strength of Softwood Lignin Based Carbon Fibers by Use of the Weibull Statistics2013Conference paper (Refereed)
    Abstract [en]

    This paper presents analysis of mechanical properties of carbon fibers produced from softwood kraft lignin. The average tensile strength of these fibers is approximately 300 MPa. The analysis is based on the Weibull statistical distribution, the experimental results and predictions based on Weibull statistics show a fairly good fit.

  • 72.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    IPM.
    Sparnins, Edgars
    Applicability of Weibull strength distribution for cellulose fibers with highly non-linear behaviour2009In: ICCM 17, Edinburgh: 17th International Conference on Composite Materials ; 27 Jul 2009 - 31 Jul 2009, Edinburgh International Convention Centre, Edinburgh, UK, London: IOM Communications , 2009Conference paper (Refereed)
    Abstract [en]

    It is shown that tensile strength of cellulose fibers with the same length follow Weibull distribution. However, size effect predicted by the Weibull distribution for fibers with various lengths is not observed. This is most likely due to the highly non-linear behaviour of cellulose fibers. Applicability of the Weibull distribution is studied on fibers with different length via single fiber as well as bundle tensile tests.

  • 73.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Wallström, Lennart
    Flax fibres for structural composites2003Conference paper (Other academic)
  • 74.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Wallström, Lennart
    Single fiber fragmentation experiment performed on flax fibers2006In: Experimental techniques and design in composite materials (ETDCM6): ["Sixth International Seminar on Experimental Techniques and Design in Composite Materials" wich was held at Padova in June 2003] / [ed] Marino Quaresimin, Amsterdam: Elsevier, 2006Conference paper (Other academic)
  • 75.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Wallström, Lennart
    Strength and adhesion characteristics of elementary flax fibres with different surface treatments2003In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 34, no 7, p. 603-612Article in journal (Refereed)
    Abstract [en]

    It is known that the best flax fibres can compete in terms of mechanical properties with glass fibres. However, during the manufacturing process flax fibres are often damaged, and hence, the properties can be lowered. Furthermore, these properties change from batch to batch (depending on the time and place of harvest), which means that they are somewhat unpredictable. The most affected fibre property is strength, which can vary in very wide interval due to defects introduced by the manufacturing process. Therefore, there is a need for a simple but reliable testing procedure that allows the estimation of the strength of flax fibres, so called quality control. Regarding the final goal, that is the development of natural fibre composites, another crucial property is the fibre/matrix adhesion. The objective of this study is to investigate the possibility to use the single fibre fragmentation test to characterize strength distribution of flax fibres and to evaluate the adhesion. Untreated flax fibres and fibres coated by a special surface treatment are used. Fragmentation tests are performed on flax fibres embedded in thermoset, vinylester and polyester, resins. Results show that there is a definite improvement in interfacial strength when a fibre surface treatment is applied. Fibre strength distribution is obtained from SFFT and compared with limited results available from single flax fibre tests.

  • 76.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Wallström, Lennart
    Strength distribution of flax fibers obtained from single fiber fragmentation test2002In: Book of abstracts / Twelfth International Conference Mechanics of Composite Materials, June 9 - 13, 2002, Riga, Latvia / [ed] V. Tamuzs, 2002Conference paper (Other academic)
  • 77.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP.
    Forsberg, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lycksam, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjögren, Anders
    Lund University.
    Characterization of defects in polymer composites used in medical devices by means of x-ray microtomography2017In: 3rd International Conference on Tomography of Materials and Structures (ICTMS2017), 2017Conference paper (Refereed)
    Abstract [en]

    This paper presents a study on micro-structural characterization of carbon fibre-reinforced plastics used inmedical devices. The focus of the investigation is on determination of void content in the materials, since voids act asdefects and will affect the service life of the composites/devices. The results show that x-ray microtomography is anaccurate and powerful technique to identify defects in composites, and it is of great value in quality control.

  • 78.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hajlane, Abdelghani
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Kaddami, Hamid
    Cadi Ayyad University, Faculty of Sciences and Techniques.
    Effect of surface modification of regenerated cellulose fibers on moisture absorption and fiber/matrix adhesion2016In: ECCM 2016: Proceeding of the 17th European Conference on Composite Materials, European Conference on Composite Materials , 2016Conference paper (Refereed)
    Abstract [en]

    This paper describes the effect of surface modification of regenerated cellulose fibers by chemical treatment and deposition of cellulose nano-crystals onto fibers. The effect of this modification on themoisture absorption by fibers and interface properties with epoxy matrix has been studies. The preliminary results show positive trends in reducing moisture uptake by fibers and improving interfacial shear strength.The deposition of cellulose nano-crystals at different concentrations onto regenerated cellulose fibers creates a network covering surface of fibers and interconnecting them. This resulted in rather significant reduction of the moisture absorption compare to untreated fibers (8% vs 12% respectively)and improving interfacial shear strength of regenerated cellulose fiber/epoxy system. The increase of the interfacial shear strength measured from bundle pull-out test on fibers conditioned at 64% relative humidity has been observed. Thus, the hierarchical structure created by grafting nano-crystals onmicro-sized cellulose fibers resulted in improvement of fiber/matrix adhesion by reducing water absorption.

  • 79.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Varna, Janis
    COD-based simulation of transverse cracking and stiffness reduction in [S/90n]s laminates2001In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, no 5, p. 637-656Article in journal (Refereed)
    Abstract [en]

    Closed-form expressions for the thermo-elastic properties of [S/90n]s laminates with transverse cracks in the 90° layers are derived. Provided the normalised average crack-opening displacement (COD) is known, expressions contain only crack density, geometrical parameters and elastic constants of layers. The average COD dependence on the crack spacing and constraint effect of adjacent sub-laminates is analysed by using finite-element method in plane stress formulation. It is found that the out-of-plane elastic constants have an insignificant effect on COD. A simple power law relating average COD to elastic and geometrical parameters of constituents is derived. The obtained power law and the developed methodology are successfully used to predict the reduction of thermo-elastic properties and damage evolution of [±θ/904]s laminates. The crack-closure technique and Monte-Carlo simulations are used to model the damage development. The 90° layer is divided in to a large number of elements and Gc values are assigned to each element according to a Weibull distribution. Parameters in the Weibull distribution are determined by using experimental crack density versus strain curve for glass-fibre/epoxy [02/904]s cross-ply laminates. Damage development in [S/904]s laminates of the same material, containing sub-laminates with ±θ layers only, is modelled by using these Weibull parameters and the results are in good agreement with test data. The effect of the thickness of the 90° layer on damage development is discussed in strength and fracture mechanics formulation.

  • 80.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Madsen, Bo
    Risø, DTU.
    Nättinen, Kalle
    VTT.
    Tensile properties of cellulosic fiber/starch avetate composites with variable fiber and plasticizer content2011In: Composite materials for structural performance: towards higher limits: proceedings of the 32nd Risø International Symposium on Materials Science ; 5 - 9 September 2011, Roskilde, Denmark / [ed] S. Faester; D. Juul Jensen; B. Ralph; B.F. Soerensen, Roskilde: Risø National Laboratory for Sustainable Energy, Danmarks Tekniske Universitet , 2011, p. 329-337Conference paper (Refereed)
    Abstract [en]

    In this experimental study, the performance of injection-molded short cellulosic fiber/plasticized starch acetate composites is analyzed in terms of stiffness and strength. Parameters involved in the analysis are a variable fiber and plasticizer content. The measured strength of the composites varies in the range 12–51 MPa, whereas the stiffness varies in the range 1.1-8.3 GPa, which is significantly higher than the properties of unreinforced matrix.

  • 81.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Madsen, Bo
    Technical University of Denmark, Risø Campus.
    Nättinen, Kalle
    VTT Technical Research Centre of Finland, Espoo.
    Miettinen, Arttu
    University of Jyväskylä.
    Strength of cellulosic fiber/starch acetate composites with variable fiber and plasticizer content2015In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 49, no 8, p. 1007-1017Article in journal (Refereed)
    Abstract [en]

    In this experimental study, the performance of injection-molded short flaxand hemp fibers in plasticized starch acetate were analyzed in terms ofstrength. Parameters involved in the analysis are a variable fiber andplasticizer content. The measured strength of the composites varies in therange of 12–51 MPa for flax fibers and 11-42 MPa for hemp fibers, which issignificantly higher than the properties of the unreinforced starch acetatematrix. The micro-structural parameters used in modeling of compositestrength were obtained from optical observations and indirectmeasurements. Some of these parameters were qualitatively verified by Xraymicrotomography.

  • 82.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mattsson, David
    Compressive failure of non-crimp fabric composites: influence of bundle out-of-plane misalignment2004In: Mechanics of composite materials :: XIII international conference, May 16 - 20, 2004, Riga ; MCM 2004, 2004Conference paper (Other academic)
  • 83.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mattsson, David
    Methodology for characterization of internal structure of NCF composite and its influence on mechanical properties2006In: Proceedings of the 3rd International Conference on Composites Testing and Model Identification: CompTest 2006, Faculty of Engineering, University of Porto, Portugal , 2006, p. 64-65Conference paper (Other academic)
  • 84.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mattsson, David
    NCF cross-ply laminates: damage accumulation and degradation of elastic properties2006In: Fracture of Nano and Engineering Materials and Structures: Proceedings of the 16th European Conference of Fracture, Alexandroupolis, Greece, July 3-7, 2006 / [ed] E. E. Gdoutos, Encyclopedia of Global Archaeology/Springer Verlag, 2006, Vol. Paper 450, p. 1281-1288Conference paper (Refereed)
  • 85. Joffe, Roberts
    et al.
    Mattsson, David
    Modniks, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Compressive failure analysis of non-crimp fabric composites with large out-of-plane misalignment of fiber bundles2005In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 36, no 8, p. 1030-1046Article in journal (Refereed)
    Abstract [en]

    Failure initiation under compressive loading in non-crimpfabric composites containing bundles with out-of-plane orientation imperfections was analyzed using FEM in plane stress and linear elastic formulation. The bundle orientation imperfection in a composite unit was described by a sine function. Failure initiation strain was determined comparing failure functions corresponding to two alternative failure mechanisms: (a) plastic microbuckling in bundle due to mixed compressive and shear load; (b) plastic matrix yielding according to von Mises criterion. Parameters for compressive failure initiation analysis were bundle misalignment angle, fiber volume fraction inside the bundle and bundle volume fraction inside the composite unit. The support effect of the neighbouring material was analyzed varying boundary conditions and solving cases with particular configuration of surrounding material. Model prepreg tape GF/EP composite with different introduced levels of out-of-plane waviness of layers was used to validate the conclusions from parametric analysis.

  • 86.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mattsson, David
    Varna, Janis
    Damage tolerance of cross-ply non-crimp fabric composites2005In: The fifth Canadian international composites conference, CACSMA , 2005Conference paper (Other academic)
  • 87.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Suitability of man made cellulose fibers as reinforcement for bio-based polymer composites2010Conference paper (Other academic)
    Abstract [en]

    It has been demonstrated that natural fibers, such as flax and hemp for example, have mechanical properties competitive to those of glass fibers. This makes such fibers very goodpotential reinforcement for polymer composites. However, there are certain difficulties to utilize those properties on the extent that composites based on these fibers become alsocompetitive with synthetic materials in terms of mechanical performance. One of the problems is poor adhesion between matrix and fibers, which can be improved by additionalfiber treatment or by use of more compatible polymers (for example bio-based resins).Another issue is related to the length or the fibers and their orientation. Normally natural fibers are fairly short and thus it is difficult to maintain their pre-define orientation in thecomposite. Finally, maybe one of the biggest drawbacks of natural fibers is variability of their properties: quality of fibers will differ depending on location of growth and time of harvest.Moreover, large variability of properties can be observed even within the same batch of fibers. Sometimes these disadvantages can overshadow the main advantage of these materials– they are environmentally friendlier than synthetic materials and they are sustainable.Due to these concerns, the other type of reinforcement with plant origin (and high cellulose content, similar to flax and hemp fibers) has recently caught an attention as alternativereinforcement for bio-based composites - regenerated cellulose fibers. Although these are manmade fibers, they are made out of the natural polymer directly in contrary to the fibersmade out materials with fossil origin. These fibers are continuous and it is easy to arrange them into fabrics with stable orientation and geometry. These fibers have been discovered for more than a century ago but until now they have not been seriously considered for use in high performance polymer composites. At present the main application of these fibers is within textile industry and in tire processing.The current study focuses on characterization of regenerated cellulose fibers (namely Cordenka fibers) in order to evaluate them as perspective reinforcement for bio-based polymer composites. General mechanical performance, statistical strength distribution as well as viscoelastic behaviour of the Cordenka fibers are presented. Influence of moisture andtemperature on mechanical properties is also examined. Suitability of regenerated cellulose fibers as reinforcement for bio-based polymer composites is discussed.

  • 88.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Marec, Pierre Erwan Le
    Exchange Student LTU.
    Investigation of damage accumulation in flax fibre composite laminates2009Conference paper (Other academic)
  • 89.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nystöm, Birgitha
    Swerea SICOMP AB.
    Rozite, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanical performance of polymer composites reinforced with nonlinear cellulosic fibers2011Conference paper (Refereed)
  • 90.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pupure, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berthold, Fredrik
    Innventia AB.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Micro-structure and Mechanical Properties in PLA Reinforced with Cellulosic Fiber Sheets Made by Wet Forming Method2017In: 8th International Conference on Composites Testing and Model Identification: CompTest2017 / [ed] S.V . Lomov, L. Gorbatikh, Y. Swolfs, 2017Conference paper (Refereed)
    Abstract [en]

    The current paper presents results of study of composites produced by compression moulding of sheets of comingled PLA and reinforcing fibers. The dynamic sheet former is employed to produce fiber mats (with the PLA in the form of fibers) which are consolidated into composite plates by using hot press. This processing route ensures that initial length of the fibers is preserved during the manufacturing and preferential fiber orientation is achieved. However, the internal structure of the composites in question is very complex and somewhat unpredictable, which complicates design of these materials. The main objectives of this paper are application and validation of micro-mechanical models on composites produced within this study as well as direct (experimental) and indirect (back-calculation) identification of input parameters to be used in the modelling. The main input parameters considered in this study are fiber orientation and porosity. The estimation of these parameters is done through micro–computed tomography but also by using micro-mechanical expressions in combination with experimental results (e.g. composite density, stiffness). The input parameters identified by different approaches are compared, then these parameters are used in the micro-mechanical models to predict stiffness of composites with different types of fibers and various fiber contents.

  • 91.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Rozite, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nonlinear behavior of natural fiber/bio-based matrix composites2013In: Challenges in Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials: Proceedings of the 2012 Annual Conference on Experimental and Applied Mechanics / [ed] Bonnie Antoun; H. Jerry Qi; Richard Hall; G.P. Tandon; Hongbing Lu; Charles Lu, New York: Encyclopedia of Global Archaeology/Springer Verlag, 2013, Vol. 2, p. 131-137Conference paper (Refereed)
    Abstract [en]

    The rising concern about the dependence on synthetic polymers and oil has motivated research on competitive bio-based replacement materials. Some of the often considered bio-based thermoplastics are starch, polylactic acid (PLA) and rather recently, lignin. These bio-plastics in combination with natural fibers (flax, hemp, wood) are used to manufacture whole bio-based composites. Although there are certain direct benefits to use natural fibres in composites, their performance is often very nonlinear. Moreover, properties of these materials are very sensitive to moisture and temperature. The behavior of these composites has to be studied and mechanisms occurring during the loading must be identified. The effect of temperature and relative humidity on mechanical behavior of natural fiber reinforced bio-based matrix composites subjected to the tensile loading was investigated. Time dependent behavior of these materials is analyzed. Testing methodology is suggested to identify sources of nonlinearities observed in stress-strain curves. It was found that microdamage accumulation and stiffness reduction is significant for some of the composites but the major nonlinear phenomena are related to nonlinear viscoelasticity and viscoplasticity. Material models accounting for these effects are proposed and their predictive capability is demonstrated.

  • 92.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sjögren, Anders
    Luleå tekniska universitet.
    Effect of fiber/matrix interface on transverse cracking in glassfiber/vinylester composites1997In: Polymeric composites - expanding the limits: proceedings of the 18th Risø International Symposium on Materials Science, 1-5 September 1997 / [ed] S.I. Andersen, Roskilde: Risø National Laboratory , 1997, p. 381-386Conference paper (Refereed)
  • 93.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sjögren, Anders
    Effects of internal structure on the mechanical properties of NCF composites2002In: Book of abstracts / Twelfth International Conference Mechanics of Composite Materials, June 9 - 13, 2002, Riga, Latvia / [ed] V. Tamuzs, 2002Conference paper (Other academic)
  • 94.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Analytical modeling of elastic properties reduction in symmetric and balanced lamintes due to cracks in 90 layers1998In: Book of abstracts: Tenth International Conference Mechanics of Composite Materials : April 20 - 23, 1998, Riga, Latvia / [ed] V. Tamuzs, 1998Conference paper (Refereed)
  • 95.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Analytical modeling of stiffness reduction in symmetric and balanced laminates due to cracks in 90° layers1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 11, p. 1641-1652Article in journal (Refereed)
    Abstract [en]

    Stiffness reduction in [S,90n]s symmetric laminates, containing orthotropic sub-laminates (S) and cracked 90° layer, is analyzed. Closed-form expressions relating stiffness changes to the transverse crack density are derived. They contain only material properties, laminate geometry and a stress-perturbation function that is proportional to the normalized average crack-opening displacement. Stress-distribution models [shear lag, based on variational approach, and finite-element analysis (FEA)] are adopted for the [S,90n]s configurations and used to calculate the stress-perturbation function. Predictions are compared with experimental data for [θ, 904]s θ=0, 15, 30, 40 glass-fiber/epoxy-resin laminates. Generally the FEA model slightly underestimates stiffness reduction whereas both of the variational models used lead to similar results, slightly lower than experimental. Even the shear-lag model may be successfully used if the shear-lag parameter is first obtained from fitting test results for cross-ply laminate of the same material.

  • 96.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Compressive strength of unidirectional polymer composites obtained from test of multidirectional laminates2001In: 2001 Mechanics and Materials Summer Conference: June 27-29, 2001 Sheraton San Diego Hotel and Marina ; technical program and book of abstracts ; MMC2001, 2001Conference paper (Refereed)
  • 97.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Damage evolution modeling in multidiractional laminates and the resulting nonlinear response1999In: ICCM-12 Europe: 12th International Conference on Composite Materials ; Palais des Congrès, Paris [July 5th - 9th] 1999 / [ed] Thierry Massard, Tours: ICCM , 1999Conference paper (Refereed)
  • 98.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Effect of bundle waviness on compressive strength and notch sensitivity of non-crimp fabric composites2002Conference paper (Refereed)
    Abstract [en]

    Non-Crimp Fabrics (NCF) and vinylester resins seem to give lower compression strength data than similar prepreg-based composites. However, the lower data turned out to be not only material issue, but a problem associated with testing methodology, namely design of test fixtures and specimen configuration. Specimen geometry optimization study led to research concerning notch sensitivity of NCF. Investigations were performed testing specimens with circular center holes and symmetric sharp edge notches. In all cases reduction of compressive strength, corresponding to the net cross-section, due to introduced notches was not observed. Although NCF composites showed rather low compressive strength if compared with similar prepreg materials, they also exhibit almost complete notch insensitivity. This can be explained by irregular and uneven internal structure of NCF composite. Due to complex internal geometry NCF composite fails close to an internal defect, prior to the failure in vicinity of the notch, where stress concentration takes place.

  • 99.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Failure analysis of fiber bundles with waviness in composites under compression2004In: MACM 2004 conference proceedings: Proceedings of the Tenth International Conference on Marine Applications of Composite Materials, Composites education association , 2004Conference paper (Refereed)
  • 100.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Fiber/matrix interface fracture toughness identification in single fiber fragmentation test1997In: Material identification using mixed numerical experimental methods: proceedings of the EUROMECH Colloquium held in Kerkrade, The Netherlands, 7-9 April 1997 / [ed] Hugo Sol, Encyclopedia of Global Archaeology/Springer Verlag, 1997Conference paper (Refereed)
1234 51 - 100 of 194
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