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  • 1.
    Basso, Margherita
    et al.
    Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Milan, Italy. Research Hub by Electrolux Professional, Pordenone, Italy.
    Piselli, Agnese
    Research Hub by Electrolux Professional, Pordenone, Italy. Politecnico di Milano, Department of Design, Milan, Italy.
    Simonato, Michele
    Research Hub by Electrolux Professional, Pordenone, Italy.
    Furlanetto, Riccardo
    Research Hub by Electrolux Professional, Pordenone, Italy.
    Pupure, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    De Nardo, Luigi
    Politecnico di Milano, Department of Design, Milan. INSTM—National Interuniversity Consortium of Materials Science and Technology, Firenze, Italy.
    Effect of food chemicals and temperature on mechanical reliability of bio-based glass fibers reinforced polyamide2019In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 157, p. 140-149Article in journal (Refereed)
    Abstract [en]

    This paper presents an experimental study to assess the effects of food chemicals and temperature on the mechanical performance of glass fiber reinforced bio-based polyamide. The diffusion of food chemicals was mainly driven by thermal energy, following Arrhenius law in all tested environments. Degradation of mechanical properties and decrease in reliability were assessed, due to the plasticization of polymer matrix. Secondary but not negligible effect on flexural strength degradation is given by the different chemical interaction between polymeric chains and molecules of food chemicals. Colour change was measured and resulted to be positively correlated to diffusion.

  • 2.
    Basso, Margherita
    et al.
    Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering, Milan, Italy;The Research Hub by Electrolux Professional, Pordenone, Italy.
    Pupure, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Simonato, Michele
    The Research Hub by Electrolux Professional, Pordenone, Italy.
    Furlanetto, Riccardo
    The Research Hub by Electrolux Professional, Pordenone, Italy.
    De Nardo, Luigi
    Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering, Milan, Italy;INSTM—National Interuniversity Consortium of Materials Science and Technology, Firenze, Italy.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nonlinear creep behaviour of glass fiber reinforced polypropylene: Impact of aging on stiffness degradation2019In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 163, p. 702-709Article in journal (Refereed)
    Abstract [en]

    Nonlinear creep behavior of one commercial short glass fiber reinforced polypropylene was investigated using tensile creep tests and stiffness degradation measurements. The impact of thermal aging and following quenching was evaluated on the latter mechanical property. Experimental results were modeled applying nonlinear viscoelastic model used by Pupure et al. (2013) and developed by Lou and Schapery [1,2]. Results showed that this model can describe nonlinear behavior of short glass fiber reinforced polymer composites, where microdamage is given by debonding of fiber-matrix interfaces already at low strains, where cracks propagate and lead to tensile creep fracture.

  • 3.
    Doroudgarian, Newsha
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanical behavior of bio-based composites and their constituents at different humidity levels2013Conference paper (Refereed)
    Abstract [en]

    During the few past years the development of natural fiber composites for structural applications has gained the momentum. Mainly the efforts in development of these materials were focused on the composites with synthetic matrices. But most recently several bio-based resins (Tribest, EpoBioX etc.) have been introduced, allowing production of whole bio-based composites. The latest results demonstrated that these composites are comparable with glass fiber reinforced polymers in terms of stiffness. However due to variability of fiber properties and limited filament length it is complicated to arrange and control fiber alignment in composites as well as ensure stable, predictable composite properties. Therefore, another type of reinforcement with natural origin has caught attention of researchers – Regenerated Cellulose Fibers (RCF). These fibers are continuous with constant, reproducible cross-section and properties but with one significant disadvantage - they exhibit highly non-linear behavior. Thus, this reinforcement should be treated as material with time-dependent properties. Schepary developed model for time-dependent materials. This model has been successfully applied to short fiber (natural and synthetic) and long synthetic fiber composites. However, in order to apply this model, large number of time-consuming tests on studied composites must be performed. Therefore, our objective is to improve this model in such a way that only input of properties of constituents is required to predict behavior of material with any composition. Visco-elasticity and viscoplasticity has been analyzed by performing creep tests at different time steps and stress levels, and extent of damage is evaluated by performing stiffness degradation tests of fibers, matrix and composite.

  • 4.
    Doroudgarian, Newsha
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Moisture uptake and resulting mechanical response of biobased composites: II. Composites2015In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 36, no 8, p. 1510-1519Article in journal (Refereed)
    Abstract [en]

    The durability of entirely bio-based composites with respect to the exposure to elevated humidity was evaluated. Different combinations of bio-based resins (Tribest, EpoBioX, Envirez) and cellulosic fibers (flax and regenerated cellulose fiber rovings and fabrics) were used to manufacture unidirectional and cross-ply composite laminates. Water absorption experiments were performed at various humidity levels (41%, 70%, and 98%) to measure apparent diffusion coefficient and moisture content at saturation. Effect of chemical treatment (alkali and silane) of fibers as protection against moisture was also studied. However, fiber treatment did not show any significant improvement and in some cases the performance of the composites with treated fibers was lower than those with untreated reinforcement. The comparison of results for neat resins and composites showed that moisture uptake in the studied composites is primarily due to cellulosic reinforcement. Tensile properties of composites as received (RH = 24%) and conditioned (RH = 41%, 70%, and 98%) were measured in order to estimate the influence of humidity on behavior of these materials. Results were compared with data for glass fiber reinforced composite, as a reference material. Previous results from study of unreinforced polymers showed that resins were resistant to moisture uptake. Knowing that moisture sorption is primarily dominated by natural fibers, the results showed that some of the composites with bio-based resins performed very well and have comparable properties with composites of synthetic epoxy, even at elevated humidity.

  • 5.
    Doroudgarian, Newsha
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sensivity to moisture and its effect on mechanical behavior of bio-based resins reinforced with cellulosic fibers2013In: 21st anniversary of the bio-environmental polymer society, 2013Conference paper (Refereed)
  • 6.
    Gong, Guan
    et al.
    Swerea SICOMP AB, Piteå, Sweden.
    Nyström, Birgitha
    Swerea SICOMP AB, Piteå, Sweden.
    Sandlund, Erik
    Swerea SICOMP AB, Piteå, Sweden.
    Eklund, Daniel
    Swerea SICOMP AB, Piteå, Sweden.
    Noël, Maxime
    Swerea SICOMP AB, Piteå, Sweden.
    Westerlund, Robert
    Swerea SICOMP AB, Piteå, Sweden.
    Stenberg, Sofia
    Swerea SICOMP AB, Piteå, Sweden.
    Pupure, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP AB, Piteå, Sweden.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP AB, Piteå, Sweden.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    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.
    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)
  • 8.
    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.

  • 9.
    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.

  • 10.
    Mannberg, Peter
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Service life assessment and moisture influence on composites from renewable feedstock2012In: Proceedings of Mechanics of Composite Materials XVII International Conference, 2012Conference paper (Refereed)
    Abstract [en]

    Composite materials are an important and growing material group in a large number of industries such as aeronautics, marine, automotive, energy production and infrastructure. The most common composite materials is manufactured from fibres such as glass fibres and carbon fibres together with oil based resins i.e. polyester, vinyl ester and epoxy. An increasing use calls for alternative environmental friendly, biobased, constituents. The new biobased materials have not only to compete in mechanical properties but it also has to restrain environmental loads like moisture and temperature over time. In the present work are predictions of the long term properties of biobased resins made. The work also presents the influence of moisture, comparison with creep test data and comparison with oil based resin. The long term property prediction is made by using dynamic mechanical thermal analysis, DMTA, measurements and time temperature superposition, TTSP, [1,2]. The procedure is to make DMTA measurements in 3p-bening mode of the storage modulus at different frequencies at increasing temperature. The method used in this case was to measure the modulus at 0.1, 0.3, 1, 3 and 10 Hz at temperatures 25 – 175°C with 5°C intervals. The frequencies where then transformed to time by Eq. (1) below t=2/πω (1)where ω is the frequency. 25°C were chosen as reference temperature where as all other curves where shifted horizontally creating a continuous master curve. The modulus time master curve was then inverted creating creep compliance time master curve, this curve is compared to creep test. The moisture behaviour is characterised in form of water uptake, change in glass transition temperature, Tg, and change in dynamic response. The water uptake is determined by submersion into water and tracking the weight change over time. The glass transition temperature is determined in DMTA by conducting a temperature sweep at constant frequency. The change in Tg is used as a knock down factor and a vertical shift of the master curve. An attempt to determine the dynamic response of wet samples has also been made with the insight that higher temperatures will dry the sample during testing leading to a non homogenous moisture distribution within the sample. The results from the study show on differences between TTSP and creep data, it also shows the differences between different biobased resins and a comparison to oil based epoxy. REFERENCES1. Christensen R.M., “Theory of viscoelasticity, an introduction” 2nd edition, Academic Press, 1982.2. Ward I.M. and Hadley D.W., “An Introduction to the mechanical properties of solid polymers” 1st edition, John Wiley & Sons, 1993.

  • 11.
    Miettinen, Arttu
    et al.
    University of Jyväskylä.
    Joffe, Roberts
    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.
    Madsen, Bo
    Technical University of Denmark, Risø Campus, Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark.
    Identification of true microstructure of composites based on various flax fibre assemblies by means of three-dimensional tomography2015Conference paper (Refereed)
    Abstract [en]

    Lately it has been demonstrated that natural fibres may be an environmentally superior alternative for, e.g., glass fibres. In order to estimate properties of composite materials made of natural fibres, models designed for synthetic fibres are often used. The models usually do not account for irregularities in the material, e.g., suboptimal fibre orientation due to the twisting angle of fibres in yarns. Use of models without taking those features into account might lead to unreliable results. Methods to quantify the microstructural properties of natural fibre composites with X-ray microtomography and three-dimensional image analysis are demonstrated in this work. The methods are applied to flax fibre composites made from three different kinds of pre-forms. Microstructural parameters estimated with the methods are used in micromechanical models for the stiffness of the composite. Comparison between rule-of-mixtures and classical laminate theory is made, highlighting the requirement for accurate parameter estimation and use of a model that accounts for significant structural features of the material.

  • 12.
    Miettinen, Arttu
    et al.
    University of Jyväskylä.
    Joffe, Roberts
    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.
    Madsen, Bo
    Technical University of Denmark, Risø Campus, Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark.
    X-Ray Microtomography Of Natural Fibre Composites2015In: Proceedings of 7th International Conference on Composites Testing and Model Identification / [ed] C. González; C. López; J. LLorca, Madrid, Spain: IMDEA, Madrid (SPAIN) , 2015Conference paper (Refereed)
  • 13.
    Nyström, Birgitha
    et al.
    Swerea SICOMP AB.
    Joffe, Roberts
    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.
    Development of bio-based advanced composites with transport structures in mind2011Conference paper (Other academic)
    Abstract [en]

    A world wide eco race is taking place in the transportation sectors. This race is driven by factors such as climate change awareness and oil prices. The main goal is reduction of fossil raw material dependency and minimizing the overall carbon foot-print. As well as the entering on the market of new types of hybrid engines with low fuel consumption and electric cars there is an increasing push from many car producers to incorporate renewable, recyclable lightweight materials in their vehicles. Swerea SICOMP and Luleå University of Technology have been involved in R&D on bio-based materials for transportation applications for more than 10 years. The results from ongoing research projects with focus on bio-based structural applications for transportation applications are promising. Fibres that are being studied include manmade regenerated cellulose, cellulose nano fibrils and whiskers as well as high quality flax fibre fabrics. Several new bio-based or partly bio-based resins that are entering the market are being evaluated. Results regarding processability, mechanical performance, durability and design & manufacturing show that the latest development in the field of bio-based constituents (reinforcing fibres, core materials and resins) for composites are taking large steps forward and new green options for the transportation sector will soon be available.

  • 14.
    Pupure, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Non-linear model applied on composites exhibiting inelastic behavior: development and validation2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The polymeric composite materials are in high demand by industries where light and strong materials are required. Although manmade fiber (e.g. glass, carbon, aramid fibers) are most often used to reinforce polymers, natural fibers due to their environmental friendliness and sustainability have been also considered. Natural fiber composites have shown to have great potential as a substitute for conventional glass fiber materials. However, bio-based composites exhibit highly non-linear behavior, besides they are very sensitive to elevated moisture and temperature. Therefore, careful design and optimization of composite properties defined by constituents, composition and internal structure is needed to meet requirements of real-life applications. This can be done by using accurate models that can take into account factors responsible for inelastic behavior of these materials. The initial part of this thesis is dealing with development of phenomenological approach to predict inelastic behavior of composites in tension. Viscoelasticity and viscoplasticity was analyzed in short term creep tests and modulus degradation in stiffness degradation tests. Schapery’s model for viscoelasticity and Zapa’s model for viscoplasticity was used to characterize nonlinearity. This method was then validated on short, randomly oriented fiber composites with different cellulosic fibers (flax, viscose) and bio-polymers (PLA, Lignin). The elastic modulus, tensile stress-strain curves and failure were analyzed at different humidity and temperature levels. Results showed high sensitivity to moisture and temperature and highly non-linear behavior of these materials. Modeling showed good agreement between experimental data and simulations.Since there is need for simulations of strain controlled tests, this model was rewritten in inverted incremental form. Simulations of stress-strain curves showed, that predictions are more accurate, when characterization of viscoelastic and viscoplastic parameters was done at stresses close to failure. However, due to creep rapture it was not always possible to characterize material at high stresses and in this case viscoelastic functions have to be extrapolated. The stress-strain curves can be then used to further adjust extrapolation of model parameters.The model developed in the first part of the thesis proved to be capable of predicting behavior of short fiber composites with good accuracy. However, in order to carry out simulations input parameters have to be experimentally obtained and it has to be done for every composite that is studied. The second part of this thesis is dedicated to development of constitutive model which uses parameters of constituents to predict behavior of material with any composition. This model then is applied on semi-structural natural fiber composites consisting of bio-based resins reinforced with continuous cellulosic fibers. Mechanical properties of different bio-based thermoset resins and regenerated cellulose fibers have been analyzed. Results showed comparable properties of bio-based and synthetic epoxy resins, even at elevated humidity levels, but high scattering of properties from sample to sample. They also showed that bio-based resin exhibit limited non-linearity whereas regenerated cellulose fiber is highly non-linear.In order to avoid large scatter typical for bio-based materials and improve accuracy of the model, methodology for parameter identification for viscoplastic model with use of only one sample has been suggested.The objective here is to simulate strain controlled tests and the most convenient way to do it is with Schapery’s strain formulation model. The parameters for such model can be obtained from relaxation tests, where viscoelastic strain is kept constant but due to presence of viscoplastic strain component such experiments are difficult to perform. Instead, constituents exhibiting viscoplastic behavior have been characterized in creep and viscoelastic parameters for Schapery’s strain formulation are obtained from simulations of relaxation tests with inverted incremental model. Then these parameters are used to simulate behavior of composite subjected to iso-strain conditions.

  • 15.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Al-Maqdasi, Zainab
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gong, Guan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of Nano-reinforcement on the Time-dependent Properties of Graphene Modified High Density PolyethyleneManuscript (preprint) (Other academic)
  • 16.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Doroudgarian, Newsha
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Moisture uptake and resulting mechanical response of biobased composites: I. Constituents2014In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 35, no 6, p. 1150-1159Article in journal (Refereed)
    Abstract [en]

    The mechanical properties of the biobased fiber and resins have been characterized and moisture influence on the behavior of these materials has been studied. Commercially available biobased thermoset resins (Tribest, EpoBioX, Palapreg, Envirez SA, and Envirez SB) and regenerated cellulose fibers (Cordenka) have been conditioned at different levels of relative humidity (as received, dried, 41, 70, and 90%) to obtain materials with different moisture content. The following properties of polymers were measured: tensile, flexural (3P-bending), impact strength (unnotched Charpy), and fracture toughness (compact tension). The results of characterization of biobased thermosets were compared against data for epoxy Araldite LY556, which is used as reference resin. RCF bundles (with and without twist, extracted from fabric) as well as single fibers separated from these bundles were tested in tension. In general biobased resins performed well, moreover EpoBioX showed better properties than synthetic epoxy. POLYM. COMPOS

  • 17.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    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.
    Identification Of Parameters For Direct And Inverted Model To Predict Performance Of Materials Exhibiting Non-Linear Viscoelastic Behavior2015In: Proceedings of 7th International Conference on Composites Testing and Model Identification / [ed] C. González; C. López; J. LLorca, Madrid, Spain: IMDEA, Madrid (SPAIN) , 2015Conference paper (Refereed)
  • 18.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    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.
    Nyström, Birgitha
    Development of constitutive model for composites exhibiting time dependent properties2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, IOP Publishing Ltd , 2013, article id 12007Conference paper (Refereed)
    Abstract [en]

    Regenerated cellulose fibres and their composites exhibit highly nonlinearbehaviour. The mechanical response of these materials can be successfully described by themodel developed by Schapery for time-dependent materials. However, this model requiresinput parameters that are experimentally determined via large number of time-consuming testson the studied composite material. If, for example, the volume fraction of fibres is changed wehave 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 thecomposite behaviour based on behaviour of constituents of the composite. This paper gives anoverview of problems and difficulties, associated with development, implementation andverification of such model.

  • 19.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Saseendran, Sibin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP, Piteå, Sweden.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Basso, Margherita
    Department of Chemistry, Materials and Chemical Engineering, Politecnico di Milano, Italy. The Research Hub by Electrolux Professional, Pordenone, Italy.
    Effect of degree of cure on viscoplastic shear strain development in layers of [45/−45]s glass fibre/ epoxy resin composites2018In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 24, p. 3277-3288Article in journal (Refereed)
    Abstract [en]

    Effect of degree of cure on irreversible (viscoplastic) shear strain development in layers of glass fibre/ epoxy resin (LY5052 epoxy resin) [+45 °/−45 °]s laminate is studied performing a sequence of constant stress creep and viscoelastic strain recovery tests. For fixed values of degree of cure in range from 79.7% to 100%, the viscoplastic strains were measured as dependent on time and stress and Zapa's integral representation was used to characterize the observed behaviour. It is shown that at all degrees of cure the viscoplastic behaviour can be described by Zapa's model with parameters dependent on degree of cure. It is shown that for degree of cure lower than 80% the viscoplastic strains grow much faster and are much more sensitive to the increase of the applied shear stress. These irreversible strains developing in the final phase of the curing can significantly alter the residual stress state in the composite structure.

  • 20.
    Pupure, Liva
    et al.
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Applications and limitations of non-linear viscoelastic model for simulation of behaviour of polymer composites2015Conference paper (Refereed)
    Abstract [en]

    There are two alternative formulation of non-linear viscoelastic model to describe strain and stress controlled tests. Both models for non-linear viscoelastic materials are not compatible, and cannot be directly inverted if so required in certain cases. In order to do it numerical procedures has to be employed. Methodology for simulating nonlinear stress-strain response in iso-strain situations of fiber composites based on properties on constituents is presented.

  • 21.
    Pupure, Liva
    et al.
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Methodology for macro-modeling of bio-based composites with inelastic constituents2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 163, p. 41-48Article in journal (Refereed)
    Abstract [en]

    Methodology for development of a macro-scale model (with strain as an input) for Regenerated Cellulose fiber (RCF) composites with highly non-linear (viscoelastic (VE) and viscoplastic (VP)) constituents is presented and demonstrated. The VE is described by Schapery's models and Zapas' model is used for VP. For a purely VE constituent the model can be identified from stress relaxation in constant strain tests. In the presence of VP the constant strain test does not render VE stress relaxation functions, because part of the applied strain is VP and the VE strain is changing. As an alternative creep and strain recovery tests are suggested to find the plasticity law and also the nonlinear creep compliances to identify the VE model where stress is an input. The incremental form of this model is then inverted and used to simulate the VE relaxation tests and the simulated relaxation functions are used to identify the VE model with VE strain as an input.

    Models for constituents are used in micromechanics simulations of the composite behavior in arbitrary ramps including the composite VE relaxation test. Using the latter, a macro-model is developed and its validity and accuracy are demonstrated.

  • 22.
    Pupure, Liva
    et al.
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modeling of Natural Fiber Composites2015In: Natural Fiber Composites, CRC Press, Taylor & Francis Group, , 2015, p. 221-253Chapter in book (Refereed)
  • 23.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Division of Materials Science, Composite Centre Sweden, Luleå University of Technology.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Division of Materials Science, Composite Centre Sweden, Luleå University of Technology.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Division of Materials Science, Composite Centre Sweden, Luleå University of Technology.
    Modelling of mechanical behaviour of polymeric composites with nonlinear constituents2014In: 16th European Conference on Composite Materials, ECCM 2014: Seville, Spain, 22 - 26 June 2014, European Conference on Composite Materials, ECCM , 2014Conference paper (Refereed)
    Abstract [en]

    There is need to model behaviour of time-dependent non-linear material in stress or/and strain controlled experiments. This study explores possibility to apply model in both forms developed by Schapery for viscoelastic materials. Viscoelasticity has been analysed using experimental data from creep and relaxation tests. Incremental simulation procedure, which inverts model, where strains are expressed through stresses, is used to simulate relaxation curves for bio-based polymer. Comparison of viscoelastic parameters obtained from simulated and experimental relaxation curves has been performed.

  • 24.
    Pupure, Liva
    et al.
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Natural fiber composite: Challenges simulating inelastic response in strain-controlled tensile tests2016In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 50, no 5, p. 575-587Article in journal (Refereed)
    Abstract [en]

    Problems occurring, when nonlinear time-dependent material model with parameters identified in creep tests is applied to simulate high-strain response in strain-controlled tests, are described and analyzed. Reasons for discrepancies with experimental loading curves are revealed. Presented numerical/experimental examples deal with three bio-based composites showing highly nonlinear behavior due to damage, nonlinear viscoelasticity and viscoplasticity. Schapery's approach for viscoelasticity and Zapas' model for viscoplasticity are used. The model is generalized to include microdamage effect. It is shown that the main problem in simulations at high stresses is the reliability of data from creep test for model identification in this region because creep rupture limits the available data region and extrapolation to higher stresses is rather uncertain. Alternative solution is to employ relaxation tests at high strains to obtain the missing information. However, it would work only in absence of viscoplastic strains: viscoelastic relaxation functions cannot be determined by maintaining constant total strain if viscoplastic-strain is developing. Based on sensitivity analysis of composite response to variations of the elastic modulus, damage, viscoelastic and viscoplastic parameters, suggestions are made for improving (further “tuning”) the model in high stress region by using tensile stress–strain curves in quasi-static loading.

  • 25.
    Pupure, Liva
    et al.
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    On viscoplasticity characterization of natural fibres with high variability2015In: Advanced Composites Letters, ISSN 0963-6935, Vol. 24, no 6, p. 125-129Article in journal (Refereed)
    Abstract [en]

    Zapas model has shown good results in characterizing viscoplasticity in polymers and composites. Typically all viscoplastic parameters for this model are obtained in creep and strain recovery tests at different stress levels and at different test length. Obtaining parameters for highly variable bio-based materials is more challenging: trends are hidden in the scatter and data fitting is complicated. This paper suggests "single specimen methodology" for complete viscoplastic characterization and identification

  • 26.
    Pupure, Liva
    et al.
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP, Piteå.
    Berthold, Fredrik
    Rise Bioeconomy/Innventia AB, Stockholm.
    Miettinen, Arttu
    Department of Physics, University of Jyväskylä.
    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.

  • 27.
    Pupure, Liva
    et al.
    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.
    Joffe, Roberts
    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.
    An analysis of the nonlinear behavior of lignin-based flax composites2013In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 49, no 2, p. 139-154Article in journal (Refereed)
    Abstract [en]

    A lignin composite reinforced with 30% flax fibers at two levels of relative humidity, 34 and 66%, was used in this study. The nonlinearity of the composite was analyzed by studying the degradation of its modulus and the development of viscoelastic and viscoplastic strains. The reduction in the modulus of lignin-based composites in tension starts before the maximum in the stress-strain curve is reached and can be as large as 50%. With increasing relative humidity, these effects are slightly magnified. The time-dependent phenomena in tension were examined in short-term creep and strain recovery tests, demonstrating a rather high viscoplastic strain in lignin composites. Both viscoelastic and viscoplastic strains are larger at a higher relative humidity

  • 28.
    Rozite, Liva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Non-linear behavior of bio-based composite: characterization and modeling2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The development and application of bio-based composite materials have been frequently studied. Most of the work is done on quasi-static performance of these materials. However, these composites are highly non-linear therefore there is need for investigation of their viscelastic and viscoplastic behavior. This thesis is dealing with characterization and modeling of behavior of bio-based composite. The effect of temperature and relative humidity on mechanical behavior of natural fiber reinforced bio-based matrix composites subjected to tensile loading was investigated. Composites with different natural fibers (flax, viscose) and bio-based matrices (PLA, Lignin) were studied. Elastic modulus, the nonlinear tensile stress-strain curves and failure were analyzed showing that all materials are temperature sensitive. The nonlinearity was evaluated by studying modulus degradation as well as development of viscoelastic and viscoplastic strains as a function of applied load. The time-dependent phenomena were investigated in short term creep and strain recovery tests at several high stress levels. These tests demonstrated significantly higher viscoplastic strain in lignin than PLA based composites. Both, viscoelastic and viscoplastic strains are larger at higher relative humidity. The observed nonlinearity was attributed to microdamage, viscoelastic and viscoplastic response suggesting Schapery’s type of model for viscoelasticilty and Zapas’ model for viscoplasticity. PLA and lignin based flax fiber composites have been analyzed in order to obtain parameters needed for model. It was found that in PLA based composites after loading at stress levels below the maximum possible the elastic modulus is not affected and, therefore, damage does not need to be included in the material model. The modulus reduction in lignin based composites in tension starts before the maximum in stress-strain curve is reached and it can be as large as 50%. With increasing relative humidity these effects are slightly magnified. It appears that there is no region of linear viscoelasticity for PLA based composites. Nonlinear elasticity, viscoelasticity and viscoplasticity are equally responsible for observed nonlinearity in tensile tests.

  • 29.
    Rozite, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Analysis of the nonlinear behavior of bio-based polymers reinforced with flax fibers2012Conference paper (Refereed)
    Abstract [en]

    In last years competitive bio-based composite materials have been developed to reduce the dependence on non-renewable oil-based polymers and composites. This work is performed on composites with two bio-based matrix materials reinforced with flax fibers, Polylactic acid (PLA) and lignin. PLA is one of most commonly used bio-based thermoplastics, whereas lignin is relatively new material for composite applications.The stiffness of bast fibers (e.g. flax, hemp) in longitudinal direction is comparable to that of glass fibers (for example flax fibers have stiffness 50-100 GPa vs 72 GPa for E-glass fibers). The direct benefits of use of natural fibers in composites are light weight, reduced wear on the processing equipment and lower impact on the environment. However, their mechanical properties are subject to large variation and composites made of these fibers are sensitive to moisture and humidity. Study [1] has shown that in natural fiber composites the mechanical properties of fibers and the matrix are inherently nonlinear and the composite exhibits complex time dependent stress-strain behavior with loading rate effects and hysteresis loops. The phenomena dominating mechanical behavior may also include evolving microdamage (cracks, debonds etc) resulting in elastic properties degradation and development of irreversible strains. Therefore, material models of these materials should account for viscoelasticy and viscoplasticity accompanied with microdamage.In the present study systematic investigation of mechanical behaviour of flax fiber reinforced Lignin and PLA composites in tension is done. The objective is to indentify material model for these composites and to analyze the significance of viscoelastic, viscoplastic and stiffness reduction effects on mechanical behaviour.REFERENCES1. Sparnins E., Varna J., Joffe R., Nättinen K. and Lampinen J., “Time dependent behaviour of flax/starch composites,” Mechanics of Time-Dependent Materials, in press, 2006.

  • 30.
    Rozite, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nystöm, Birgitha
    Swerea SICOMP AB.
    Damage accumulation and degradation of mechanical properties in polymer composites reinforced with highly non-linear cellulosic fibers2011In: 11th International Conference on Wood and Biofiber Plastic CompositesMadison: Madison, Wisconsin, USA, 16-17 May 2011, Madison, WI: Forest Products Society, 2011Conference paper (Other academic)
  • 31.
    Rozite, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    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.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Analysis of the time-dependent behavior of bio-based composites made from highly non-linear constituents2012In: Proceedings of Mechanics of Composite Materials XVII International Conference, Riga, 2012Conference paper (Refereed)
    Abstract [en]

    Large number of studies has been made about renewable bio-based (e.g. flax, hemp reinforced composites) composites during the last years. It has been showed that mechanical properties of bio-based composites compares well with glass fiber composites, especially if the properties are normalized with density. However, there are some drawbacks with respect to utilization of natural fiber properties to the extent that composites based on these reinforcements also can compete with synthetic materials, such as limited fiber length, sensitivity to the moisture, temperature etc. But probably one of the major drawbacks of natural fibers is variability of properties due to growth, processing conditions etc. Therefore, another type of reinforcement with high cellulose content is of interest – regenerated cellulose fibers (RCF). These fibers are manmade, but unlike materials with fossil origin, they are made out of the natural polymer directly. RCF is continuous and therefore it is easy to arrange them into fabrics with stable orientation and geometry. But RCF fiber and also bio-based resin performance is highly non-linear with presence of very significant viscoelastic and viscoplastic component.As basis material model developed by Schapery [1,2] was used in order to analyse time dependent behaviour of these materials. Model has been further modified to account for microdamage [3] In order to obtain all parameters needed for this model large number of experiments needs to be carried out. The numerous combinations of matrix and fibers are possible. Therefore it would be more convenient if material model is based on performance of constituents and test on composites should be performed only for verification of the model.The main objective of this investigation is to predict mechanical behaviour of these composites and their constituents by generalizing existing models to capture their time-dependent behaviour. In order to identify and quantify parameters needed for the modelling, extensive damage tolerance tests as well as creep experiments are carried out. REFERENCES1. Lou Y.C. and Schapery R.A., “Viscoelastic Characterization of a Nonlinear Fibre-reinforced Plastic,” Journal of Composite Materials, Vol. 5, pp. 208-234, 1971. 2. Schapery R.A., “Viscoelastic and Viscoplastic Constitutive Equations Based on Thermodynamics,” Mechanics of Time-Dependent Materials, Vol. 1, pp. 209-240, 1997. 3. Marklund E., Eitzenberg J. and Varna J. “Nonlinear viscoelastic viscoplastic material model including stiffness degradation for hemp/lignin composites,” Composite Science and Technology, Vol. 68, pp. 2156-2162, 2008.

  • 32.
    Rozite, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    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.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Characterization and analysis of time dependent behavior of bio-based composites made out of highly non-linear constituents2013In: 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] B. Antoun; H.J. Qi; R. Hall; G.P. Tandon; H. Lu; C. Lu, New York: Encyclopedia of Global Archaeology/Springer Verlag, 2013, Vol. 2, p. 109-115Conference 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.

  • 33.
    Rozite, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    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.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Characterization and modeling of performance of polymer composites reinforced with highly non-linear cellulosic fibers2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: IOP Publishing Ltd , 2012, p. 12005-12014Conference 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.

  • 34.
    Rozite, Liva
    et al.
    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.
    Joffe, Roberts
    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 PLA and lignin based flax composites subjected to tensile loading2013In: Journal of Thermoplastic Composite Materials, ISSN 0892-7057, E-ISSN 1530-7980, Vol. 26, no 4, p. 476-496Article in journal (Refereed)
    Abstract [en]

    The effect of temperature (T = 22°C, 30°C and 35°C) and relative humidity (RH = 34% and 66%) on mechanical behavior of natural fiber reinforced bio-based matrix composites subjected to tensile loading was investigated. Three composites were studied (a) polylactic acid (PLA) composite with 10% weight fraction of flax fibers; (b) PLA composite containing 5% viscose fibers (filaments of regenerated cellulose); (c) lignin-based composite with 30% of flax fibers. Elastic modulus, the nonlinear tensile stress–strain curves and failure were analyzed showing that all materials are temperature sensitive. The nonlinearity was analyzed studying modulus degradation as well as development of viscoelastic and viscoplastic strains. The modulus reduction in PLA-based composites starts after reaching the stress maximum and is not significant, whereas the modulus reduction in lignin-based composites starts before the maximum and it can reach 50%. With increasing RH these effects are slightly larger. The time-dependent phenomena were analyzed in short-term creep and strain recovery tests demonstrating significantly higher viscoplastic strain in lignin composites. Both viscoelastic and viscoplastic strains are larger at higher RH.

  • 35.
    Varna, Janis
    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.
    Characterization of viscoelasticity, viscoplasticity, and damage in composites2019In: Creep and Fatigue in Polymer Matrix Composites / [ed] Rui Miranda Guedes, Elsevier, 2019, 2, p. 497-530Chapter in book (Refereed)
    Abstract [en]

    Empirical inelastic constitutive material models for composites and testing methodology for parameter determination in these models are analyzed. Short fiber as well as long unidirectional fiber reinforced composites are analyzed in situations when the main sources of inelastic behavior are a combination of (a) nonlinear viscoelasticity; (b) nonlinear viscoplasticity; and (c) microdamage-induced reduction of thermoelastic properties, all three evolving with time and stress. These phenomena are included in a common material model. The necessary tests for model identification are tensile quasistatic loading-unloading tests and creep tests at different stress levels with recorded strain recovery after load removal. The methodology is demonstrated presenting models for (a) shear in layers of [45/−45]s laminates; (b) response of short fiber composites (SMC with glass fiber bundles; composites with natural or man-made cellulosic fibers in bio-based resins).

  • 36.
    Varna, Janis
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Incremental forms of Schapery’s model: convergence and inversion to simulate strain controlled ramps2016In: Mechanics of time-dependant materials, ISSN 1385-2000, E-ISSN 1573-2738, Vol. 20, no 4, p. 535-552Article in journal (Refereed)
    Abstract [en]

    Schapery’s nonlinear viscoelastic model is written in incremental form, and three different approximations of nonlinearity functions in the time increment are systematically analysed with respect to the convergence rate. It is shown that secant slope is the best approximation of the time shift factor, leading to significantly higher convergence rate. This incremental form of the viscoelastic model, Zapas’ model for viscoplasticity, supplemented with terms accounting for damage effect is used to predict inelastic behaviour of material in stress controlled tests. Then the incremental formulation is inverted to simulate stress development in ramps where strain is the input parameter. A comparison with tests shows good ability of the model in inverted form to predict stress–strain response as long as the applied strain is increasing. However, in strain controlled ramps with unloading, the inverted model shows unrealistic hysteresis loops. This is believed to be a proof of the theoretically known incompatibility of the stress and strain controlled formulations for nonlinear materials. It also shows limitations of material models identified in stress controlled tests for use in strain controlled tests.

  • 37.
    Varna, Janis
    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.
    Joffe, Roberts
    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.
    Non-linear behaviour of PLA based flax composites2012In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 41, no 2, p. 49-60Article in journal (Refereed)
    Abstract [en]

    The mechanical behaviour of polylactic acid/flax fibre composite in tension was investigated by analysing elastic properties and loading curves. The observed non-linearity was attributed to microdamage, viscoelastic and viscoplastic response, suggesting Schapery's type of model for viscoelasticilty and Zapas' model for viscoplasticity. It was found that after loading at stress levels below the maximum possible, the elastic modulus is not affected, and therefore, damage does not need to be included in the material model. Viscoplastic and viscoelastic strain development was analysed in creep and strain recovery tests at several high stress levels. The identified and validated material model is non-linear viscoelastic and viscoplastic with slight non-linearity even in the elastic strain term. It appears that there is no region of linear viscoelasticity for this material. Non-linear elasticity, viscoelasticity and viscoplasticity are equally responsible for the observed non-linearity in the tensile tests

1 - 37 of 37
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