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  • 1.
    Almgren, Karin M
    et al.
    STFI-Packforsk AB, Box 5604, SE-114 86 Stockholm.
    Gamstedt, Kristofer
    Department of Fiber and Polymer Technology, Royal Institute of Technology - KTH.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Contribution of wood fiber hygroexpansion to moisture induced thickness swelling of composite plates2010In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 31, no 5, p. 762-771Article in journal (Refereed)
    Abstract [en]

    One of the main drawbacks of wood fiber-based composite materials is their propensity to swell due to moisture uptake. Because the wood fibers are usually the main contributor to hygroexpansion, it is of interest to quantify the hygroexpansion coefficient of wood fibers, to compare and rank different types of fibers. This investigation outlines an inverse method to estimate the transverse hygroexpansion coefficient of wood fibers based on measurements of moisture induced thickness swelling of composite plates. The model is based on composite micromechanics and laminate theory. Thickness swelling has been measured on polylactide matrix composites with either bleached reference fibers or crosslinked fibers. The crosslinking modification reduced the transverse hygroexpansion of the composites and the transverse coefficient of hygroexpansion of the fibers was reduced from 0.28 strain per relative humidity for reference fibers to 0.12 for cross-linked fibers.

  • 2.
    Andersons, J.
    et al.
    University of Latvia, Riga.
    Sparnins, Edgars
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stiffness and strength of flax fiber/polymer matrix composites2006In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 27, no 2, p. 221-229Article in journal (Refereed)
    Abstract [en]

    Flax fiber composites with thermoset and thermoplastic polymer matrices have been manufactured and tested for stiffness and strength under uniaxial tension. Flax/polypropylene and flax/maleic anhydride grafted polypropylene composites are produced from compound obtained by coextrusion of granulated polypropylene and flax fibers, while flax fiber mat/vinylester and modified acrylic resin composites are manufactured by resin transfer molding. The applicability of rule-of-mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers is considered.

  • 3.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sparnins, Edgars
    Rubenis, Oskars
    Institute of Polymer Mechanics, University of Latvia.
    Progressive cracking mastercurves of the transverse ply in a laminate2009In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 30, no 8, p. 1175-1182Article in journal (Refereed)
    Abstract [en]

    In this study, progressive cracking of a transverse layer in a cross-ply composite laminate subjected to tensile loading is considered. Using the results of a probabilistic cracking model, approximate relations for crack density as a function of stress are derived for initiation-controlled and propagation-controlled cracking. It is shown that the crack density evolution in the transverse ply can be represented by a mastercurve in suitably normalized coordinates. The mastercurve approach is applied to progressive cracking in glass/epoxy laminates

  • 4.
    Andersson, H.M.
    et al.
    Luleå tekniska universitet.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gebart, Rikard
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Synnergren, Per
    Luleå tekniska universitet.
    Application of digital speckle photography to measure thickness variations in the vacuum infusion process2003In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 24, no 3, p. 448-455Article in journal (Refereed)
    Abstract [en]

    A new method to measure the movement of the flexible bag used in vacuum infusion is presented. The method is based on an in-house developed stereoscopic digital speckle photography system (DSP). The advantage with this optical method, which is based on cross-correlation, is that the deflection of a large area can be continuously measured with a great accuracy (down to 10 μm. The method is at this stage most suited for research but can in the long run also be adopted in production control and optimization. By use of the method it was confirmed that a ditch is formed at the resin flow front and that there can be a considerable and seemingly perpetual compaction after complete filling. The existence of the ditch demonstrates that the stiffness of the reinforcement can be considerably reduced when it is wetted. Hence, the maximum fiber volume fraction can be larger than predicted from dry measurements of preform elasticity. It is likely that the overall thickness reduction after complete filling emanates from lubrication of the fibers combined with an outflow of the resin. Besides, the cross-linking starts and the polymer shrinks. Hence, the alteration in height will continue until complete cross-linking is reached.

  • 5. Andersson, Magnus
    et al.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gebart, Rikard
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Långström, R.
    Swedish Institute of Composites, Piteå.
    Flow-enhancing layers in the vacuum infusion process2002In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 23, no 5, p. 895-901Article in journal (Refereed)
    Abstract [en]

    The current trend towards increased use of vacuum infusion molding for large surface-area parts has increased the interest in an advanced modeling of the process. Because the driving pressure is limited to 1 atmosphere, it is essential to evaluate possible ways to accelerate the impregnation. One way of doing this is to use layers of higher permeability within the reinforcing stack, i.e. flow-enhancing layers. We present an experimental investigation of the flow front shape when using such layers. The through-thickness flow front was observed by making a number of color marks on the glass-mats forming the reinforcing stack, which became visible when the resin reached their position. The in-plane flow front was derived from observations of the uppermost layer. It turned out that existing analytical models agree very well with the experiments if effective permeability data is used, that is, permeability obtained from vacuum infusions. However, the fill-time was nearly twice as long as predicted from permeability data obtained in a stiff tool. This rather large discrepancy may be due to certain features of a flexible mold half and is therefore a topic for further research. The lead-lag to final thickness ratio is dependent on the position of the flow front and ranges form 5 to 10 for the cases tested. Interestingly the lead-lag has a miximum close to the inlet.

  • 6. Andersson, Magnus
    et al.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Langhans, N.
    EADS Military Aircraft, Munich.
    Computational fluid dynamics applied to the vacuum infusion process2005In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 26, no 2, p. 231-239Article in journal (Refereed)
    Abstract [en]

    An all-purpose computational fluid dynamics software is used for simulations of the vacuum infusion process. The study comprises simulations of a full three-dimensional two-phase flow through a porous medium. The medium that has an anisotropic, spatial- and time-dependent permeability is located in a complex mold with moving boundaries. With this generalization, different material combinations, processing conditions, and even other manufacturing techniques can be evaluated. The strength of the presented approach is exemplified by simulations of mold filling of a real part, using a typical vacuum infusion set-up. In addition to the overall development of the model, a number of specific aspects and phenomena are investigated and evaluated. Local lead of the flow front and a minor influence in overall flow front lead-lag, with no influence on the fill time, is the result of simulations of edge effects due to poor preform fitting.

  • 7.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sjögren, B.A.
    Berglund, Lars A.
    Prediction of failure initiation in polypropylene with glass beads1997In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 18, no 1, p. 9-15Article in journal (Refereed)
    Abstract [en]

    The effect of glass bead content and residual stresses on failure initiation in isotactic polypropylene composites has been investigated by finite element analysis for the cases of interfacial debonding, plastic yielding, and cavitation. Residual thermal stresses are demonstrated to have a large effect on global failure initiation stress. Yielding and cavitation occur at higher global stresses than debonding. Modeling results, as well as previous experimental data, support debonding as the initial failure mechanism

  • 8.
    Bengtsson, Magnus
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Stark, Nicole
    Forest Products Laboratory.
    Oksman, Kristiina
    Profile Extrusion and Mechanical Properties of Crosslinked Wood-Thermoplastic Composites2006In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 27, no 2, p. 184-194Article in journal (Refereed)
    Abstract [en]

    Challenges for wood-thermoplastic composites to be utilized in structural applications are to lower product weight and to improve the long-term load performance. Silane crosslinking of the composites is one way to reduce the creep during long-term loading and to improve the mechanical properties. In this study, silane crosslinked wood-polyethylene composites were produced by reactive extrusion and subsequently manufactured into rectangular profiles. The silane crosslinked composites were stored in a sauna at 90 °C to increase the degree of crosslinking. The toughness of the silane crosslinked composites was significantly higher than for the non-crosslinked composites. Improved adhesion between the wood and polyethylene phases is most likely the reason for the improved toughness of the crosslinked composites. There was no significant difference in flexural modulus between the crosslinked and non-crosslinked composites. In addition, impact testing showed that the impact strength of the crosslinked composites was considerable higher (at least double) than the non-crosslinked. The effect of temperature on the impact strength of the composites indicated slightly higher impact strength at _30 °C than at 0° and at 25 °C, and then an incrase in impact strength at 60 °C. Crosslinking also reduced the creep response during short-term loading. Moreover, scanning electron microscopy on the fracture surface of the crosslinked composites revealed good adhesion between the polyethylene and wood phases.

  • 9.
    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.

  • 10.
    Enqvist, Evelina
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ramanenka, Dmitrij
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Grácio, José
    TEMA – NRD, Mechanical Engineering Department, University of Aveiro, 3810-193 Aveiro, Portugal. Aveiro Institute of Nanotechnology (AIN).
    Marques, Paula
    TEMA – NRD, Mechanical Engineering Department, University of Aveiro, 3810-193 Aveiro, Portugal. Aveiro Institute of Nanotechnology (AIN).
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The effect of ball milling time and rotational speed on ultra high molecular weight polyethylene reinforced with multiwalled carbon nanotubes2016In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 37, no 4, p. 1128-1136Article in journal (Refereed)
    Abstract [en]

    Ultra high molecular weight polyethylene (UHMWPE) composites reinforced with multiwalled carbon nanotubes (MWCNT) were produced using planetary ball milling. The aim was to develop a more wear resistant composite with increased mechanical properties to be used in stress bearing joints. The manufacturing technique, using ball-milling to incorporate MWCNT into UHMWPE matrix was investigated. The effect of manufacturing parameters such as effect of ball milling time and rotational speed on final composite was analyzed by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), particle size distribution and contact angle measurements. Ball milling as mixing technique for UHMWPE based composites is not a new approach but yet, the effect of time, rotational speed, loading of milling jar and type of ball mill has not been reported properly for UHMWPE. 0.5 and 1 wt% UHMWPE/MWCNTs were manufactured at different rotational speed and mixing time. The results indicate that rotational speed rather than mixing time is important for dispersing MWCNTs

  • 11.
    Ericson, Mats L.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    The effect of microstructure on the elastic modulus and strength of performed and commercial GMTs2003In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 14, no 1, p. 35-41Article in journal (Refereed)
    Abstract [en]

    A new technique was used to fabricate performed glass fiber/polypropylene GMTs. The method utilizes thermoplastic powder and fiber roving in a spray-up procedure in which a porous perform in fabricated, heated and molded. The objective was to compare the properties of various preformed GMT composition with two commercial GMTs and to relate the mechanical properties to the microstructure of the materials. Preformed GMTs were fabricated with various fiber lengths and with or without a fiber/matrix adhesion promotor. Processing observation, microstructure, tensile creep modulus, and tensile strength of these preformed GMTs are reported. Fiber length and the addition of a fiber/matrix adhesion promotor had generally little effect on the modulus and strength of the preformed GMT. Comparisons with two structurally different commercial GMTs also showed negligible effects on modulus and strength. The major reason for this is suggested to be the inhomogeneities of the materials. The mechanical properties are controlled by local inhomogeneities rather than by the general microstructure of the material. These inhomogeneities arise from the fiber arrangement in the semi-finished sheet or perform. Since the microstructure of preformed GMT can be controlled, this material is well suited for future studies on the effect of better fiber dispersion.

  • 12.
    Eslah, Farnaz
    et al.
    Department of Wood and Paper Science and Technology, Faculty of Natural Resources, University of Tehran.
    Jonoobi, Mehdi
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Department of Wood and Paper Science and Technology, Faculty of Natural Resources, University of Tehran, Karaj, Iran.
    Faezipour, Mehdi
    Department of Wood and Paper Science and Technology, Faculty of Natural Resources, University of Tehran.
    Ashori, Alireza
    Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), Tehran.
    Chemical modification of soybean flour-based adhesives using acetylated cellulose nanocrystals2018In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 39, no 10, p. 3618-3625Article in journal (Refereed)
    Abstract [en]

    In this study, two types of new bioadhesives formulated from abundant and renewable soybean flour (SF), acetylated soybean flour based adhesive (ASF), and soybean flour-based adhesive, were modified with acetylated cellulose nanocrystal (ACNC). The apparent viscosity and morphology of the adhesive formulations were characterized. The chemical composition of the formulations was evaluated by FT-IR spectroscopy and the effect of polyethilenimine (PEI) on the formulations was investigated using the proton nuclear magnetic resonance (1H NMR) spectra. Moreover, water resistance of produced plywood composites bonded with the bioadhesives was measured. The results of FT-IR and 1H NMR confirmed that chemical modifications of the SF occurred. The scanning electron microscopy (SEM) images showed less holes and cracks on the cross section of the ASF/PEI/NaOH and SF/PEI/NaOH/ACNC formulations. The results showed that the plywood specimens bonded with formulations of the ASF/PEI/NaOH (with a dry weight ratio of ASF/PEI: 5/1 and 6/1), and SF/PEI/NaOH/ACNC had good resistance to water. POLYM. COMPOS., 2017. © 2017 Society of Plastics Engineers

  • 13. Fällström, Karl-Evert
    Determining material properties in anisotropic plates using Rayleigh's method1991In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 12, no 5, p. 306-314Article in journal (Refereed)
    Abstract [en]

    Material parameters of anisotropic plates are determined. Rectangular anisotropic plates are tuned by changing the quotient between the length of the sides, so that the second and third modes of vibration, for free-free boundary conditions, degenerate into the well-known cross-shaped and ring-shaped modes, respectively. The first three modes of vibrations for these plates are determined by optical methods. The propagation of bending waves, generated by the impact of a ballistic pendulum on the plates, has also been optically studied. It is found that bending waves generated in the middle of the tuned plates will reach the boundaries of the plates simultaneously. This gives a relationship between main material parameters. Using this relation and Rayleigh's method for the first three modes of vibrations, the main material parameters for the plates, that is, the effective two Young's moduli, the shear modulus, and the Poisson's ratio, are determined

  • 14. Fällström, Karl-Evert
    et al.
    Jonsson, Mikael
    A nondestructive method to determine material properties in anisotropic plates1991In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 12, no 5, p. 293-305Article in journal (Refereed)
    Abstract [en]

    Material parameters in anisotropic rectangular plates are determined in a nondestructive way. Real-time, TV-holography is used to determine frequencies and shapes of the first five modes of vibration of plates with free-free boundary conditions. According to rules given in the paper, finite element analysis is then used to determine two effective Young's moduli, the shear modulus, and the Poisson's ratio

  • 15. Fällström, Karl-Evert
    et al.
    Molin, Nils-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nondestructive method to determine material properties in orthotropic plates1987In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 8, no 2, p. 103-108Article in journal (Refereed)
    Abstract [en]

    An electronic speckle pattern interferometer (ESPI) is used to determine modes of vibration in rectangular, orthotropic, free-free-plates; that is using a noncontact, nondestructive, optical method. It is shown, using the finite element method (FEM), that each of the first three modes of vibration in rectangular orthotropic plates has a strong dependence upon only one of the main material parameters, namely the in-plane shear modulus and the two Young's moduli, respectively. With this one-variable dependence it is a simple task to determine the effective material parameters. This method has several obvious advantages compared to the use of test bars and it can be extended to give a measure of the damping parameters and probably also be used for production control. Preliminary results are presented and discussed

  • 16. Gebart, Rikard
    et al.
    Lidström, Peder
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Measurement of in-plane permeability of anisotropic fiber reinforcements1996In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 17, no 1, p. 43-51Article in journal (Refereed)
    Abstract [en]

    Three of the most common methods (two with parallel flow and one with radial flow) for determination of the in-plane permeability tensor are studied both theoretically and experimentally. An error analysis shows that the difference between the methods is negligible if the error levels are equal. However, the radial flow method is found to be susceptible to large errors from mold deflection in an experimental comparison between the methods. Additional experiments with the radial flow method in a stiffer mold show that the method gives the same values for the permeability tensor as the other two methods. A new method with multiple cavities in parallel is proposed that combines the simplicity of the radial flow method with the stiff mold of the parallel flow method. Only mass and time need to be measured in one experiment and it eliminates the need to measure fluid viscosity, temperature, and injection pressure. The method depends on the availability of a reference material with known permeability.

  • 17.
    Gebart, Rikard
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sandlund, Erik
    In-plane permeability measurements on fiber reinforcements by the multi-cavity parallel flow technique1999In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 20, no 1, p. 146-154Article in journal (Refereed)
    Abstract [en]

    This report discusses the advantages and drawbacks of the multi-cavity parallel flow technique for permeability measurements. An experimental series with repeated measurements on material from the same roll shows that the repeatability of the technique is very good considering the manufacturing variability of the fabric. The measured standard deviation in the repeatability study is about 10%. It is, however, shown that the permeability can vary considerably- between reinforcements of similar geometry. Furthermore, computer simulations were used to estimate the errors when highly anisotropic materials are oriented at an angle to the material principal direction in the parallel flow technique. The conclusion based on the simulations is that the length to width ratio of the cavity should be larger than the anisotropy of the reinforcement for an acceptable error.

  • 18.
    Gebart, Rikard
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Vahlund, C. Fredrik
    Luleå tekniska universitet.
    Analysis of an image processing method for fiber orientation in polymer composites2001In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 22, no 3, p. 327-336Article in journal (Refereed)
    Abstract [en]

    The errors in fiber orientation distribution measurements of compression molding materials have been investigated, for generated as well as for real distributions. Because of the size of the sample, only a finite number of fibers are visible in a picture leading to a statistical error in the measurement. A method is proposed to express this error as function of the number of visible fibers and the number of subintervals the distribution is divided into. Studies of the error In a Fraunhofer/FT based fiber orientation distribution analysis have been performed, including effects of increasing number of fibers (fiber-fiber intersections) in generated pictures and investigation of the errors in real fiber images. All steps in a fiber orientation distribution analysis of SMC/GMT kind of material are described, including suitable equipment, image enhancement methods and investigation of the errors present in analysis of a real image as well as comparisons to hand-measured data.

  • 19.
    Gong, Guan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mathew, Aji P.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Toughening effect of cellulose nanowhiskers on polyvinyl acetate: Fracture toughness and viscoelastic analysis2011In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 32, no 10, p. 1492-1498Article in journal (Refereed)
    Abstract [en]

    The toughening effect of cellulose nanowhiskers (CNWs) on modified polyvinyl acetate (PVAc) was analyzed with the help of morphology, relaxation, and creep behavior. The CNWs together with bound moisture at the matrix/whisker interfaces resulted in significant improvement in resistance to crack initiation and propagation. The magnitude of plastic deformation of the nanocomposites was higher than that of the neat PVAc. The relaxation temperature decreased, while the width of the damping peak increased with increasing CNW and moisture contents. The results from creep modeling showed that the instantaneous elastic modulus first increased and then decreased with the addition of CNWs, while the time-dependent elasticity and viscosity decreased. The results suggested that the reinforcing effect of the CNWs was overwhelmed by the plasticizing effect of the bound moisture. Furthermore, low concentrations of CNWs significantly improved the fracture toughness of PVAc at the minor cost of strength, stiffness, and creep resistance. In this article, we present a novel approach to studying the toughening effect of CNWs on polymers using fracture tests and viscoelastic modeling

  • 20.
    Hagstrand, P.-O.
    et al.
    Chalmers University of Technology.
    Oksman, Kristiina
    Mechanical properties and morphology of flax fiber reinforced melamine-formaldehyde composites2001In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 22, no 4, p. 568-578Article in journal (Refereed)
    Abstract [en]

    The mechanical performance of natural fiber reinforced polymers is often limited owing to a weak fiber-matrix interface. In contrast, melamine-formaldehyde (MF) resins are well known to have a strong adhesion to most cellulose containing materials. In this Paper, nonwoven flax fiber mat reinforced and particulate filled MF composites processed by compression molding are studied and compared to a similar MF composite reinforced with glass fibers. Using flax instead of glass fibers has a somewhat negative effect on tensile performance. However, the difference is relatively small, and if density and material cost are taken into account, flax fibers become competitive. Tensile damage is quantified from the stiffness reduction during cyclic straining. Compared to glass fibers, flax fibers generate a material with a considerably lower damage rate. From scanning electron microscopy (SEM), it is found that microcracking takes place mainly in the fiber cell walls and not at the fiber-matrix interface. This suggests that the fiber-matrix adhesion is high. The materials are also compared using dynamic mechanical thermal analysis (DMTA) and water absorption measurements

  • 21.
    Hammami, A.
    et al.
    U. A. E. University, Al-Ain.
    Gebart, Rikard
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Analysis of the vacuum infusion molding process2000In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 21, no 1, p. 28-40Article in journal (Refereed)
    Abstract [en]

    The vacuum infusion molding process is becoming increasingly popular for the production of large composite parts. A comprehensive model of the process has not been proposed yet, making its optimization difficult. The flexible nature of the vacuum bag coupled to the varying pressure inside the mold cavity results in a variation of the cavity thickness during the impregnation. A complete simulation model must incorporate this phenomenon. In this paper, a complete analysis of the vacuum infusion molding process is presented. The analysis is not restricted to the theoretical aspects but also reviews the effect of the main processing parameters. The parameters investigated in this paper are thought to be those of most interest for the process, i.e. the compaction of the reinforcement, the permeability, the infusion strategy and the presence of flow enhancement layers. Following the characterization experiments, a 1-D model for the vacuum infusion molding process is presented. This model is derived assuming that an elastic equlibrium holds in the mold cavity during mold filling. Even though good agreement was found between simulation results and experiments, it is concluded that additional work is needed on the numerical model to integrate interesting findings from the experimental part.

  • 22.
    Lindhagen, Johan
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Notch sensitivity and damage mechanisms of glass mat reinforced polypropylene1997In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 18, no 1, p. 40-47Article in journal (Refereed)
    Abstract [en]

    Notch Sensitivities of two structurally different glass-mat reinforced polypropylene materials (GMT) were investigated. Center hole notches in tensile specimens were used in the tests. The results show that the short fiber material has higher unnotched strength, but also higher notch sensitivity compared to a swirled mat material. Reasons for this are discussed, focusing on microstructural differences. Predictions from four different failure criteria were compared with the experimental notched strength data. The point stress and damage zone criteria were found to give the best descriptions.

  • 23. Lundström, Staffan
    Bubble transport through constricted capillary tubes with application to resin transfer molding1996In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 17, no 6, p. 770-779Article in journal (Refereed)
    Abstract [en]

    This paper describes and expounds a theoretical and experimental study of bubble motion through constricted capillary tubes. In the experiment, two liquidfilled capillaries are used. They have unequal radii and are glued together. Gas bubbles are injected into the larger capillary. Then the pressure required to force the bubbles through the constriction is measured for various liquids, bubble lengths, capillary radii and constriction geometry. It appears that the pressure directly follows Young's-Laplace law for capillary pressure. The results of the study are used to understand the bubble transport through fiber reinforcements, which generally takes place during the manufacturing of composites. The bubbles are carried if: (i) the pressure gradient is high enough, (ii) the surface tension of the liquid is low enough, (iii) the cross-sectional area of the channels in the reinforcement is sufficiently uniform. The theory reveals that the bubbles are more likely to be trapped on a small scale, i.e. within fiber bundles rather than on a large scale, i.e. between the bundles. It is also concluded that, if the bubbles are trapped at the resin flow front, a converging flow is better for the transport of the voids than a diverging flow.

  • 24. Lundström, Staffan
    et al.
    Gebart, Rikard
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Influence from process parameters on void formation in resin transfer molding1994In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 15, no 1, p. 25-33Article in journal (Refereed)
    Abstract [en]

    The influence of different process variables on the void content in resin transfer modling (RTM) has been investigated experimentally. The moldings were made in a flat mold filled by a parallel flow from one edge of the laminate to the other. The viods were found concentrated in a narrow region close to the ventilation side of the laminate. The void volume fraction in this region was almost constant and dropped over a short distance to basically no voids in the rest of the laminate. Micrographs from cross sections in different directions revealed that the voids were of two different types, long cylinderical bubbles inside the fiber bundles. An efficient way of reducing the void content was to use vacuum assistance during mold filling. This technique was benefical both for the magnitude of the void content and for the extent of the void region. The void content with the highest level of vacuum assistance (≈ 1 kPa absolute pressure), was practically negligible. Strong indications for void generation by mechanical entrapment at the flow front was found. The lowering of the void content with vacuum assistance can be interpreted as aresult of compression of voids when the vacuum is released and a higher mobility of voids created at a lower pressure.

  • 25.
    Nilsson, Greger
    et al.
    Luleå tekniska universitet.
    Ericson, Mats L.
    Volvo Car Components Corporation Body Component Division SE-293 80 Olofström.
    Holmberg, Anders
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Flow induced fiber orientation in compression molded glass mat thermoplastics2000In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 21, no 6, p. 1007-1013Article in journal (Refereed)
    Abstract [en]

    This study examines how the mechanical properties in GMT are affected by axisymmetric flow during compression molding. Two types of GMT with different architecture are used, swirled mat and short fiber GMT. Tree different grades are tested for each fiber architecture 20, 30, and 40% fiber content by weight. These are in principle the grades of GMT commercially available today. It is found that the flow reduced the tensile strength by 30 to 50% and the tensile modulus up to 30% in the flow direction. The reduction in mechanical properties, which is mainly caused by flow-induced fiber orientation, is larger at high fiber contents. The study also showed that there is no major difference in behavior between swirled mat and short fiber GMT regarding flow induced fiber orientation.

  • 26. Nordlund, Markus
    et al.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    An investigation of particle deposition mechanisms during impregnation of dual-scale fabrics with micro particle image velocimetry2010In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 31, no 7, p. 1232-1240Article in journal (Refereed)
    Abstract [en]

    Injection moulding processing of composite materials most often includes infiltration of a thermoset resin into a multi-scale porous fabric. Controlling the fluid flow within the multi-scale fabric is essential for the quality of the final composite material, since the transport of fluid between regions with different scales is of importance for phenomena such as void formation and filtration of particle doped resins. Hence, the transient flow behaviour in dual scale porous media is investigated in detail with Micro Particle Image Velocimetry. These experiments show that the fluid transport between the two scales can be controlled by the injection velocity. Validation of the measured velocity fields furthermore shows excellent agreement with theory and that transport between the two scales can be substantial at the flow front but negligible up-stream it. POLYM. COMPOS

  • 27.
    Nättinen, Kalle
    et al.
    VTT Technical Research Centre of Finland, Espoo.
    Hyvärinen, Sari
    VTT Technical Research Centre of Finland, Espoo.
    Joffe, Roberts
    Wallström, Lennart
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Madsen, Bo
    Department of Materials Research, Riso National Laboratory for Sustainable Energy, Technical University of Denmark.
    Naturally compatible: starch acetate/cellulosic fibre composites. Part I: Processing and properties2010In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 31, no 3, p. 524-535Article in journal (Refereed)
    Abstract [en]

    Composite compounds based on hemp and flax fibers in triethyl citrate plasticized starch acetate were prepared by melt processing. For better properties and processability, compounds with plasticizer contents in the range 20-35 wt% were screened. Composites were prepared with fiber contents up to 50 wt%. The composite mechanical properties were measured from injection molded test specimens. A Young's modulus of 8.3 GPa and stress at maximum load of 51 MPa were obtained with 40 wt% flax fiber in a plasticized starch acetate with 20 wt% triethyl citrate. Decreasing the plasticizer and increasing the fiber content, the tensile properties were consistently improved. An almost linear relation between fiber content and the tensile properties was found. The increase of the fiber content first improved the impact strength, but at higher fiber contents resulted in a reduction of impact strength. The quality of the produced materials was found to be good; the variation in properties between duplicated compounds was acceptable low, the variation in density and fiber content along a single tensile specimen was low, and finally, the porosity content was low even at high fiber content. The latter result was verified with scanning electron microscope images of fracture surfaces of the composites.

  • 28.
    Oldenbo, Magnus
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A constitutive model for non-Linear behavior of SMC accounting for linear viscoelasticity and micro-damage2005In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 26, no 1, p. 84-97Article in journal (Refereed)
    Abstract [en]

    An approach for modeling sheet molding compound (SMC) composites as viscoelastic damageable material is presented. Continuum damage mechanics theory by Chow and Wang (Int. J. Fract., 33, 3 (1987)) was used in combination with linear viscoelasticity. The model was applied to a modern SMC composite material containing both hollow glass spheres for low density and toughening additive for improved impact resistance. Tensile tests and uniaxial creep test were employed to build the constitutive model. Validation was done by comparing test data with simulations of uniaxial creep on material with different degrees of damage. The model has good accuracy at moderate damage levels under controlled time-dependent crack propagation. Tensile testing at two different fixed strain rates was simulated using quasi-elastic method to calculate relaxation modulus. The model predicts the stress-strain curve with good accuracy until the region is close to failure, where new mechanisms not accounted for are taking place. Finally, a simulation of a cyclic tensile test with increasing maximum strain per cycle was performed, and since both damage and viscoelasticity are included in the model, the slope change, accumulation of residual strain, and hysteresis in the stress-strain, loading-unloading curve are predicted

  • 29.
    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

  • 30.
    Sjögren, B.A.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Failure mechanisms in polypropylene with glass beads1997In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 18, no 1, p. 1-8Article in journal (Refereed)
    Abstract [en]

    The effect of glass beads on the stress-strain behavior of isotactic polypropylene has been examined. Poisson's ratio and secant compliance as a function of strain have been measured. Both sets of data are consistent with interfacial debonding as the initial damage mechanism. Interfacial debonding is then followed by extensive plastic yielding of the matrix at the debond sites. The maximum stress and strain to failure decrease with glass bead content and glass bead diameter. Impact properties correlate with the ability of the composites to reach high strain to failure. The proposed failure mechanisms are supported by fractography and in-situ deformation studies by scanning electron microscopy.

  • 31.
    Sjögren, B.A.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Toughening mechanisms in rubber-modified glass fiber/unsaturated polyester composites1999In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 20, no 5, p. 705-712Article in journal (Refereed)
    Abstract [en]

    An important case of composite failure is the leakage of pipes and pressure vessels subjected to internal pressure. The primary damage mechanism leading to leakage is transverse cracking parallel to the fibers. Effects of matrix toughening on transverse cracking strains were therefore investigated for GF/UP cross-ply laminates with matrices of different liquid rubber content. The strain to the first transverse crack was increased from 0.2 to 0.6% with 10 wt% rubber. Debonding occurred at similar strains in GF/UP and GF/UP-rubber. However, whereas debonding was almost simultaneous with transverse cracking in GF/UP, gradual growth of debonds to short cracks took place initially in GF/UP-rubber. This was followed by slow extension of short cracks to a critical flaw size corresponding to unstable growth

  • 32.
    Sparnins, Edgars
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Pupurs, Andrejs
    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.
    Nättinen, Kalle
    VTT Technical Research Centre of Finland, Espoo.
    Lampinen, Johanna
    VTT Technical Research Centre of Finland, Espoo.
    The moisture and temperature effect on mechanical performance of flax/starch composites in quasi-static tension2011In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 32, no 12, p. 2051-2061Article in journal (Refereed)
    Abstract [en]

    The effect of temperature and moisture on mechanical behavior of flax fiber/starch based composites was investigated experimentally. Elastic modulus, the nonlinear tensile loading curves, and failure strain were analyzed. Neat matrix and composites with 20 and 40% weight content of fibers were tested. It was found, performing tests with different amplitudes, that microdamage development with stress is rather limited and the related elastic modulus reduction in this type of compositesis not significant. It was shown that the composite elastic modulus and failure stress are linearly related to the maximum tensile stress in resin. The sensitivity of the maximum stress of the resin with respect to temperature and moisture is the source of composites sensitivity to these parameters. Constant interface stress shear lag model for stress transfer assuming matrix yielding at the fiber/matrix interface has been successfully used to explain the tensile test data. It indicates that the sensitivity of the used composite with respect to the matrix properties change could be significantly reduced by increasing the average fiber length from 0.9 mm to 1.5 mm.

  • 33.
    Tanpichai, Supachok
    et al.
    Learning Institute King Mongkut's University of Technology Thonburi Bangkok .
    Witayakran, Suteera
    Kasetsart Agricultural and Agro-Industrial Product Improvement Institute Kasetsart University Bangkok.
    All-cellulose composites from pineapple leaf microfibers: Structural, thermal, and mechanical properties2018In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 39, no 3, p. 895-903Article in journal (Refereed)
    Abstract [en]

    Pineapple leaf microfibers (PALM) were used to prepare all‐cellulose composites using the surface selective dissolution method. Effect of dissolution times on mechanical and physical properties of the all‐cellulose composites was investigated. The structural transformation from cellulose I to cellulose II was observed when the fibers were dissolved in a mixed solution of lithium chloride (LiCl) and N,N‐dimethylacetamide (DMAc). Values of 42.8 MPa and 1.2 GPa for tensile strength and Young's modulus, respectively, were obtained from the composites with 120 min dissolution time in the LiCl/DMAc solution, whereas the tensile strength and Young's modulus of the undissolved PALM mats were found to be only 1.5 MPa and 0.1 GPa, respectively. The failure mechanism of the composites was changed from the fiber pull‐out to the fiber breakage when the dissolution time was longer. However, lower thermal stability and degree of crystallinity of the composites were caused by the change of the cellulose structure. The composites prepared in this work can be called as biodegradable materials, and could be the potential candidates to replace nonbiodegradable materials.

1 - 33 of 33
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