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  • 1.
    Andersons, J.
    et al.
    University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Estimation of the tensile strength of an oriented flax fiber-reinforced polymer composite2011In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 42, no 9, p. 1229-1235Article in journal (Refereed)
    Abstract [en]

    Unidirectional orientation of natural fibers in a polymer composite ensures the highest efficiency of reinforcement. Flax fiber reinforcement is discontinuous due to limited fiber length and heterogeneous due to the presence of elementary fibers and their bundles. In order to assess the upper limit of tensile strength of such slightly misoriented, nominally UD natural fiber composite, a statistical strength model of continuous UD fiber reinforced composites is applied. It is found that the experimental strength of UD flax composites, produced from rovings or manually aligned fibers, approaches the theoretical limit only at relatively low fiber volume fraction ca. 0.2, being markedly below it at higher fiber content.

  • 2.
    Ansari, Farhan
    et al.
    Department of Fiber and Polymer Technology, KTH Royal Institute of Technology.
    Granda, Luis A.
    Laboratory of Paper Engineering and Polymer Materials (LEPAMAP) Group, Department of Chemical Engineering, University of Girona.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Department of Fiber and Polymer Technology, KTH Royal Institute of Technology.
    Vilaseca, Fabiola
    Department of Fiber and Polymer Technology, KTH Royal Institute of Technology.
    Experimental evaluation of anisotropy in injection molded polypropylene/wood fiber biocomposites2017In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 96, p. 147-154Article in journal (Refereed)
    Abstract [en]

    Although the anisotropy of wood fibers is reasonably well established, the anisotropy of injection molded wood fiber composites is not well understood. This work focuses on chemo-thermomechanical pulp (CTMP) reinforced polypropylene (PP) composites. A kinetic mixer (Gelimat) is used for compounding CTMP/PP composites, followed by injection molding. Effects from processing induced orientation on mechanical properties are investigated. For this purpose, a film gate mold was designed to inject composites in the shape of plates so that specimens in different directions to the flow could be evaluated in tensile tests. Observations from tensile tests were complemented by performing flexural tests (in different directions) on discs cut from the injected plates. SEM was used to qualitatively observe the fiber orientation in the composites. At high fiber content, both modulus and tensile strength could differ by as much as 40% along the flow and transverse to the flow. The fiber orientation was strongly increased at the highest fiber content, as concluded from theoretical analysis.

  • 3. Asp, Leif
    et al.
    Nilsson, Sören
    Aeronautical Research Institute of Sweden.
    Singh, Sunil
    Mechanical Sciences Sector, DERA Farnborough.
    An experimental investigation of the influence of delamination growth on the residual strength of impacted laminates2001In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 32, no 9, p. 1229-1235Article in journal (Refereed)
    Abstract [en]

    This study concerns an experimental investigation to establish data for validation of residual strength models for impacted composite panels. The work focuses on compression tests of panels with embedded artificial delaminations at various depths. Accompanying tests on undamaged and impact-damaged panels are reported and the relevance of the tests on artificially delaminated panels is assessed. In the experiments both the artificially delaminated and the impacted plates failed by delamination growth. Consequently, the same mechanism governed failure in the two cases. Hence, the artificially delaminated plate test is reliable for validation of methods developed for analysis of the residual strength of impact-damaged panels. However, for impacted plates, the load at global plate buckling was consistently 10% lower than that of the artificially delaminated plates and 20% lower than that of the undamaged plates. Hence, conservative prediction of the global buckling load of an impacted composite panel requires methods that consider influence of stiffness reduction of the damaged zone

  • 4.
    Bengtsson, Magnus
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    LeBaillif, Marie
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Extrusion and mechanical properties of highly filled cellulose fibre-polypropylene composites2007In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 38, no 8, p. 1922-1931Article in journal (Refereed)
    Abstract [en]

    This study focused on manufacturing of highly filled cellulose fibre-polypropylene composites and evaluation of the mechanical properties of the composites. Cellulose fibre reinforced polypropylene composites with up to 60 wt-% of fibres with and without coupling agent were manufactured by extrusion. In order to achieve consistent feeding of the fibres into the extruder a pelletization technique was used where the fibres were pressed into pellets. Two commercial grades of cellulose fibres were used in the study, bleached sulfite and bleached kraft fibres. Fibre dimension measurements showed that the pelletization process and extrusion at high fibre loading caused the most severe fibre breakage. Flexural testing showed that increased fibre loading made the composites stiffer but reduced the toughness. Addition of maleic anhydride grafted coupling agent increased the stiffness and strength of the composites significantly. In general, there was no significant difference in the mechanical properties between the composites with kraft and sulfite fibres. Scanning electron microscopy showed that addition of coupling agent improved the interfacial adhesion between the fibres and polypropylene matrix.

  • 5.
    Bengtsson, Magnus
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Oksman, Kristiina
    The use of silane technology in crosslinking polyethylene/wood flour composites2006In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 37, no 5, p. 752-765Article in journal (Refereed)
    Abstract [en]

    In this study, the use of silane technology in crosslinking polyethylene-wood flour composites have been investigated. Composites were produced in a one-step process using a co-rotating twin-screw extruder. The composites were stored in a sauna and at room temperature to study the effect of humidity on the degree of crosslinking. Crosslinked composites showed improved toughness and creep properties compared to non-crosslinked composites. The flexural modulus, on the other hand, was lower in the crosslinked samples than in the non-crosslinked ones. FTIR was used to study the crosslinking reaction in the samples. X-ray mapping of the silicon signal was also performed to locate the silane in the composites. This study provides a basis for proposing, that part of the silane is grafted onto polyethylene and wood thereby creating a crosslinked network in the matrix with chemical bonds (covalent and hydrogen bonding) to wood. The other part of the silane remains un-reacted and blends into the system.

  • 6.
    Bondeson, Daniel
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Polylactic acid/cellulose whisker nanocomposites modified by polyvinyl alcohol2007In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 38, no 12, p. 2486-2492Article in journal (Refereed)
    Abstract [en]

    The aim of this study was to produce biodegradable polylactic acid/cellulose whisker nanocomposites by compounding extrusion and investigate the possibility to use polyvinyl alcohol to improve the dispersion of whiskers in the matrix. Two feeding methods of polyvinyl alcohol and cellulose nanowhiskers were used and evaluated, dry-mixing with polylactic acid prior extrusion or pumping as suspension directly into the extruder. Various microscopic techniques, tensile testing, and dynamic mechanical thermal analysis were used to study the structure and properties of the nanocomposites. Due to immiscibility of the polymers, phase separation occurred with a continuous polylactic acid phase and a discontinuous polyvinyl alcohol phase. The whiskers were primarily located in the polyvinyl alcohol phase and only a negligible amount was located in the polylactic acid phase. This inadequate dispersion of whiskers in the polylactic acid phase was probably the reason why no improvements in thermal properties were seen for the nanocomposites. The relative small improvements in tensile modulus, tensile strength, and elongation to break for the nanocomposites also indicated that it was principally the polyvinyl alcohol phase that was reinforced with whiskers but not the polylactic acid phase.

  • 7.
    Butylina, Svetlana
    et al.
    Department of Mechanical Engineering, Lappeenranta University of Technology.
    Hyvärinen, Marko
    Department of Mechanical Engineering, Lappeenranta University of Technology.
    Kärki, Timo
    Department of Mechanical Engineering, Lappeenranta University of Technology.
    Accelerated weathering of wood-polypropylene composites containing minerals2012In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 43, no 11, p. 2087-2094Article in journal (Refereed)
    Abstract [en]

    Accelerated weathering tests were carried out on wood-polypropylene composites containing minerals. Three different mineral fillers were studied: calcium carbonate, wollastonite and talc. Colour changes were evaluated after distinct periods; the total time of exposure of the composites to UV irradiation was 2000 h. The weathering resulted in significant colour fading of the composites. The composites containing mineral fillers had higher changes of colour (lightness) than the reference composite. Scanning electron microscopy analysis revealed deterioration of the polymer surface layer in all weathered composites. Exposure of the reference composite to UV irradiation resulted in the disappearance of the polypropylene surface layer and disclosure of wood fibres, which led to a higher drop in the lignin content of this composite compared to mineral-containing composites. A substitution of part of the wood with mineral fillers resulted in decreased water absorption and thickness swelling of mineral-containing composites, compared to the reference composite. Exposure to water immersion-freeze-thaw cyclic treatment and UV irradiation led to a decrease in the Charpy impact strength of the composites, except for the composite containing talc

  • 8. Duchemin, Benoit
    et al.
    Mathew, Aji P.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    All-cellulose composites by partial dissolution in the ionic liquid 1-butyl-3-methylimidazolium chloride2009In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 40, no 12, p. 2031-2037Article in journal (Refereed)
    Abstract [en]

    Fully bio-based and biodegradable all-cellulose composites were prepared in the form of films by partial dissolution of two cellulose sources: a commercially available microfibrillated cellulose (MFC) and filter paper (FP). The solvent selected for this work was the ionic liquid 1-butyl-3-methylimidazolium chloride ([C4mim]Cl). Both cellulose sources were partially dissolved at 80 °C and consolidated by partial dissolution, resulting in excellent mechanical properties. X-ray diffraction and electron microscopy demonstrated that partial dissolution was a viable path to transform FP into a continuous paracrystalline matrix reinforced with cellulose I crystallites. In contrast, partially dissolved MFC was not as thoroughly dissolved and large amounts of undissolved material were still visible along the center line of the films after the longest dissolution times. Consequently, partially dissolved MFC retained its initially high crystallinity. The degree of polymerization of the materials after dissolution was significantly reduced.

  • 9.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Asp, Leif
    Approximate analytical constitutive model for non-crimp fabric composites2005In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 36, no 2, p. 173-181Article in journal (Refereed)
    Abstract [en]

    In this paper, a study set on development and validation of constitutive models to account for out-of-plane fibre waviness in Non-crimp fabric (NCF) composites is presented. For this purpose, a mathematical model based on Timoshenko beam theory applied on curved beams, representing wavy tows in a NCF composite layer is employed. Stiffness knock-down factors operating at the ply level are established and introduced in laminate theory. The developed models are validated on laminates by comparison between predictions and experimental data as well as by comparison with numerical results for a cross-ply laminate. Application of the models on NCF composite laminates (cross-ply and quasi-isotropic) reveals that the models successfully predict laminate elastic properties.

  • 10.
    Farge, L.
    et al.
    Laboratoire de Science et Génie des Surfaces, EEIGM.
    Ayadi, Z.
    Laboratoire de Science et Génie des Surfaces, EEIGM.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Optically measured full-field displacements on the edge of a cracked composite laminate2008In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 39, no 8, p. 1245-1252Article in journal (Refereed)
    Abstract [en]

    The main objectives of this study are to visualize the displacement field on the edge of a [0, 554, -554]s GF/EP laminate specimen with multiple transverse cracks and to analyze the crack opening displacement dependence on the applied mechanical load. The specimen full-field displacement measurement was carried out using ESPI (Electronic Speckle Pattern Interferometry) and phase-shifting. ESPI is an optical technique that provides the displacement for every point on a surface. The measurement resolution is roughly 20 nm. The displacement measurement is along the tensile axis and takes place on the specimen edge. Using the displacement map, it is possible to obtain the displacement profiles along the tensile-axis. The different profiles were drawn along the specimen edge at several distances from the mid-plane corresponding to the different plies. Studies of the displacement discontinuities make it possible to carry out a measurement of the crack opening displacements corresponding to the cracks in the measurement field. The experimental results are in a good agreement with idealized straight crack model in low stress region and much larger in the high stress region, which is attributed to development of local inter-ply delaminations.

  • 11.
    Fernberg, Patrik
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Effects of glass fiber size composition (film-former type) on transverse cracking in cross-ply laminates2000In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 31, no 10, p. 1083-1090Article in journal (Refereed)
    Abstract [en]

    Transverse cracking is an important phenomenon in the context of fluid leakage in pipes and pressure vessels. Multiple transverse cracking in [0/90]S glass-fiber-reinforced vinylester and epoxy laminates with six different fiber surface treatments (size) is examined. Film-former composition is the variable since this component can be easily changed also in commercial size formulations. The influence of the film-former polymer on transverse cracking is significant in epoxy laminates and very strong in vinylester laminates. Both onset of transverse cracking and slope of crack density vs. strain are influenced. Remarkably low crack densities were observed in some vinylester laminates. Micromechanisms of cracking are interpreted. Correlation is established between transverse cracking behavior and interfacial shear strength measured by single fragmentation tests. The strong film-former effect is proposed to be due to a combination of improved interfacial adhesion and the plasticizing effect from the film-former on the interphase region.

  • 12. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Jakovics, A.
    University of Latvia.
    Bubble motion through non-crimp fabrics during composites manufacturing2008In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 39, no 2, p. 243-251Article in journal (Refereed)
    Abstract [en]

    Bubbles motion through inter bundle channels in bi-axial non crimp fabrics is modelled. The scenario is that formed bubbles move with the resin through these channels and are trapped if the channels become too narrow. By usage of a permeability network model, existing criteria on bubble deformation and a variety of analytical and probabilistic methods it is found that the paths of the bubbles depend significantly on the position of the threads keeping the fabric together and the number of fibres crossing the interbundle channels. Another result is that the pressure difference over a trapped bubble increases with 50 % in a 3D geometry possible helping the bubble to escape. A third result is that, on average, the bubbles move biased to the direction of the tows. Finally it is found that the predicted void distribution of bubbles after a major part of bubbles have moved through the system are in qualitative agreement with experimental data.

  • 13.
    Giannadakis, Konstantinos
    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.
    Analysis of non-linear shear stress-strain response of unidirectional GF/EP composite2014In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 62, p. 67-76Article in journal (Refereed)
    Abstract [en]

    Reasons for nonlinear stress-strain curves in shear of unidirectional glass fibre composite are analysed. Laminate with stacking sequence [45/-45]s is used in tensile quasi-static as well as in tensile creep and strain recovery tests to study the development of viscoelastic and viscoplastic shear strains in local coordinates of the ply. It is shown that Zapa’s integral representation of viscoplasticity is applicable for this material and methodology for parameter determination is given. Schapery’s nonlinear viscoelastic material model was used for shear response characterization and the nonlinearity parameters’ dependence on the shear stress was determined and described by fitting functions. Microdamage development is quantified by measuring axial modulus and Poisson’s ratio of the laminate. The obtained nonlinear viscoelastic, viscoplastic model with included effect of microdamage was successfully used to predict the nonlinear shear stress-strain curve in strain controlled tensile loading.

  • 14.
    Gong, Guan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pyo, Jinkyung
    Luleå tekniska universitet.
    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.
    Tensile behavior, morphology and viscoelastic analysis of cellulose nanofiber-reinforced (CNF) polyvinyl acetate (PVAc)2011In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 42, no 9, p. 1275-1282Article in journal (Refereed)
    Abstract [en]

    Cellulose nanofiber-reinforced (CNF) polyvinyl acetate (PVAc) composites were prepared using the twin-screw extrusion technique. The influence of CNF content on nanocomposites morphology, tensile, and viscoelastic properties was studied. The tensile modulus and strength increased with increasing CNF content, being 59% and 21% higher in 10 wt% CNF composite compared to neat PVAc. The activation volume at yielding of PVAc was decreased by CNFs, indicating restricted chain mobility. The fracture surfaces of nanocomposites showed bridging of CNFs inside the micro-cracks. The storage modulus increased for all nanocomposites compared to the matrix, being more significant in the rubbery state. Also, the activation energy for the transition increased with increased CNF content. A slight shift and broadening was observed in the tan delta peak for 10 wt% CNFs composite. The creep strain of PVAc was reduced, whereas the creep elasticity and viscosity calculated from Burger’s model were increased by the addition of CNFs.Keywords: A. Polymer-matrix composites (PMCs); B. Creep, Mechanical properties; E. Extrusion

  • 15.
    Grubbström, Göran
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Holmgren, Allan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Silane-crosslinking of recycled low-density polyethylene/wood composites2010In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 41, no 5, p. 678-683Article in journal (Refereed)
    Abstract [en]

    The aim of this work was to study silane-crosslinking of recycled low-density polyethylene wood composites and its effect on composites properties. The composites were produced in a one-step twin-screw extrusion process and the silane-peroxide solution was pumped into the extruder. Degree of crosslinking, mechanical properties, short-term creep, fractured surfaces and nature of crosslinking were studied to understand the relationship between composite structure and properties. The results showed that crosslinked composite strength, toughness and creep resistance were improved compared to uncrosslinked composites. The flexural strength was doubled compared to uncrosslinked samples and the creep strain was reduced. The crosslinked composites stored under room conditions showed the highest strength, whereas storage in a sauna resulted in a higher degree of crosslinking. The Fourier transform infrared spectroscopy indicated formation of silane-bridges between wood and polyethylene, accordingly improving the interfacial adhesion between the wood and LDPE. The low concentration of peroxide in the silane-solution was shown to be a preferred composition to limit unintentional crosslinking during the process.

  • 16.
    Hietala, Maiju
    et al.
    Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu.
    Pelletized cellulose fibres used in twin-screw extrusion for biocomposite manufacturing: Fibre breakage and dispersion2018In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 109, p. 538-545Article in journal (Refereed)
    Abstract [en]

    Pelletizing is effective in compacting cellulose fibres, but it also causes fibre breakage and poor dispersion due to increased hydrogen bonding. This study investigated whether fibre dispersion and length could be improved by the addition of a lubricant, a commonly used composite processing aid, into cellulose pellets, or by using pelletized fibres that have not been completely dried to reduce hydrogen bonding. Cellulose pellets with different lubricant and moisture contents were prepared and compounded using twin-screw extrusion with polypropylene with 5 wt% fibre and 50 wt% fibre contents. The fibre dispersion, morphology and mechanical properties of the prepared composites were analysed. Dispersion and composite strength were improved with the addition of 4–6 wt% of lubricant while moisture had a negative effect on both properties. This study demonstrated that pelletization in the presence of a lubricant is a promising way to compact cellulose fibres and enable their continuous processing into biocomposites with improved mechanical properties.

  • 17.
    Hietala, Maiju
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Samuelsson, Erik
    Luleå University of Technology.
    Niinimäki, Jouko
    University of Oulu.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    The effect of pre-softened wood chips on wood fibre aspect ratio and mechanical properties of wood-polymer composites2011In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 42, no 12, p. 2110-2116Article in journal (Refereed)
    Abstract [en]

    The objective of this work was to study the effect of chemical pre-treatment and moisture content of wood chips on the wood particle aspect ratio after compounding in a twin-screw extruder and on the mechanical properties of wood-polymer composites (WPC). Composites with 50 wt% wood content were manufactured using pre-treated and untreated wood chips. The effect of wood moisture content on composite properties was studied by using dried and undried wood chips. The mechanical properties and fracture surfaces of the composites as well as the microstructure and aspect ratio of wood particles after compounding were studied. The highest wood particle aspect ratio after extrusion was achieved by using pre-treated, undried wood chips as raw material. The chemical pre-treatment was found to enhance the defibration of wood chips as well as the mechanical properties of the composites.

  • 18.
    Holmberg, J.A.
    et al.
    Swedish Institute of Composites.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Manufacturing and performance of RTM U-beams1997In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 28, no 6, p. 513-521Article in journal (Refereed)
    Abstract [en]

    Geometrical limitations and robustness of resin transfer moulding were investigated for single curved laminates. The design of experiments approach was used to determine how the out of plane tensile strength is affected by variations in performing method, radius, fibre content and vacuum assistance. U-beams with an inner radius of 0.8 mm were manufactured and demonstrated good mechanical performance. The strength of these beams was in fact less sensitive to defects than the strength of beams with a 5 mm radius. Different preforming methods can have a strong influence on beam strength due to effects on the void content in the finished laminates. The particular preforming problems encountered are described and possible solutions are suggested and tested. For example utilisation of vacuum assistance during mould filling reduces void content, which in turn tends to reduce the sensitivity of the beam strength to other defects such as wrinkling.

  • 19.
    Holmberg, J.A.
    et al.
    Swedish Institute of Composites.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Micromechanisms of delamination failure in RTM U-beams1997In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 28, no 8, p. 709-717Article in journal (Refereed)
    Abstract [en]

    In a previous study, resin transfer moulded U-beams were subjected to opening tensile loads applied to the flanges. The failure behaviour was sensitive to the void content of the material. In the present study, we use optical microscopy of polished edges to observe mechanisms of failure in-situ as delaminations develop in the beam radius. Descriptions of these failure mechanisms are presented. Voids were found to facilitate the growth phase of the delamination process in the beam radius. The mechanisms described will be used in a forthcoming stress analysis to explain the experimentally observed failure behaviour of beams with different radii and void contents.

  • 20.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Wallström, Lennart
    Strength and adhesion characteristics of elementary flax fibres with different surface treatments2003In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 34, no 7, p. 603-612Article in journal (Refereed)
    Abstract [en]

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

  • 21. Joffe, Roberts
    et al.
    Mattsson, David
    Modniks, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Compressive failure analysis of non-crimp fabric composites with large out-of-plane misalignment of fiber bundles2005In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 36, no 8, p. 1030-1046Article in journal (Refereed)
    Abstract [en]

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

  • 22.
    Jonoobi, Mehdi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Aitomäki, Yvonne
    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.
    Thermoplastic polymer impregnation of cellulose nanofibre networks: Morphology, mechanical and optical properties2014In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 58, p. 30-35Article in journal (Refereed)
    Abstract [en]

    Biobased nanocomposite sheets of cellulose nanofibres (CNF) and cellulose acetate butyrate (CAB) were prepared using a resin impregnation technique. Porous nanofibre networks together with a low viscosity thermoplastic resin were the key elements in the processing. SEM images of the network before the impregnation showed high porosity and after the impregnation indicated impregnated fibre network. A significant improvement in the visible light transmittance was observed for the nanocomposite compared to the nanofibre network, which is explained on the filling of the pores with a transparent matrix. The tensile tests showed an increase of 364% and 145% for stiffness and strength respectively for nanocomposites with 60 wt.% CNF when compared to CAB. Dynamic mechanical properties showed a good interaction between the CAB and cellulose nanofibres. These results show that CAB impregnated cellulose nanofibre networks are promising biocomposite that could be used in applications where transparency and good mechanical properties are of interest.

  • 23.
    Koivuranta, Elisa
    et al.
    Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu.
    Hietala, Maiju
    Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu.
    Ämmälä, Ari
    Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Illikainen, Mirja
    Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu .
    Improved durability of lignocellulose-polypropylene composites manufactured using twin-screw extrusion2017In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 101, p. 265-272Article in journal (Refereed)
    Abstract [en]

    The objective of this study was to investigate the use of peat as a potential lignocellulose source in composites manufactured by twin-screw extrusion. The effects of peat decomposition rate and particle size on the mechanical properties and moisture resistance of peat-polypropylene (PP) composites under cyclic conditions were evaluated. The properties of the peat-PP composites were compared to commercial lignocellulosic fibre products, namely wood-plastic composite (WPC), medium density fibreboard (MDF) and hardboard (HB). The results show that prior cyclic freeze-thaw testing peat-PP composites had properties equal to commercial WPC, but their mechanical permanence was better after freeze-thaw conditioning. When moderately decomposed, smaller particle–size peat was used, peat-PP composites had better dimensional stability, though particle size did not affect as much as the decomposition degree. Thus, the chemical structure of peat has a greater influence on composite durability, as better water and weather resistance are achieved with peat that is more decomposed.

  • 24.
    Loukil, Mohamed Sahbi
    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.
    Ayadi, Zoubir
    Institut Jean Lamour, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Engineering expressions for thermo-elastic constants of laminates with high density of transverse cracks2013In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 48, no 1, p. 37-46Article in journal (Refereed)
    Abstract [en]

    Thermo-elastic constants of symmetric and balanced laminates with intralaminar cracks in 90-layers depend on the opening displacement (COD) of the crack. The COD dependence on the interaction between cracks in the same layer is studied using FEM. The COD dependence on crack density is described by interaction function in form of tanh(). This interaction function multiplied with COD of non-interactive crack is the input parameter in analytical model for thermo-elastic properties of damaged symmetric and balanced laminates. Predictions performed for cross-ply laminates with cracks in inside and in surface layers and for quasi-isotropic laminates with different position of the 90-layer are in a very good agreement with direct FEM calculations.

  • 25. Lundström, Staffan
    Measurement of void collapse during resin transfer moulding1997In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 28, no 3, p. 201-214Article in journal (Refereed)
    Abstract [en]

    This paper considers the dissolution of cylindrical voids trapped between fibres during the resin transfer moulding process. To study the behaviour of the tiny voids, a system is used which consists of a transparent mould loaded with a glass fibre reinforcement. The mould is placed under a microscope and a small area of the preform is monitored during the injection of a vinylester resin. The length of the voids decreases at a relatively fast rate and they disappear in the order of minutes due to diffusion. The experimental results are rationalized through a theory showing how certain parameters influence the rate of dissolution. The investigation shows the significance of degassing the resin before injection. This is especially important when low pressures are utilized during injection.

  • 26. Lundström, Staffan
    Permeability of non-crimp stitched fabrics2000In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 31, no 12, p. 1345-1353Article in journal (Refereed)
    Abstract [en]

    Non-crimp stitched fabrics are becoming increasingly popular as reinforcing materials in high-performance composites due to their high mechanical properties. Hence, it is of interest to investigate the processability of this type of fabrics. Focus is here set on the permeability of the fabrics. The methods of this investigation are: (i) a theoretical analysis; (ii) microscopic studies of the geometry of the fabrics and (iii) measurements of the permeability. From the experiments, it is found that only small variations of the geometry change the permeability up to three times. Furthermore, the simple model proposed works surprisingly well in certain cases, while in others it fails to predict the permeability. From the experimental results and a comparison between the model and the experiments, some guidelines are presented how to design a high-permeable fabric.

  • 27.
    Lundström, Staffan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Frishfelds, Vilnis
    Jakovics, Andris
    Faculty of Physics and Mathematics, University of Latvia.
    Bubble formation and motion in non-crimp fabrics with perturbed bundle geometry2010In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 41, no 1, p. 83-92Article in journal (Refereed)
    Abstract [en]

    The motion of the liquid front during impregnation of non-crimp fabrics has been considered by using Sethian's level set method. Particular attention is put on the creation of bubbles at the liquid front and a virtual 3D model mimicking biaxial fabrics has been built for this purpose. The saturated fluid flow is governed by the Navier-Stokes Equations and Darcy law, while capillary pressure has been accounted for at the liquid flow front and continuity maintained. The influence of perturbation in the bundle geometry has been investigated. Local correlations of the dimensions of neighbouring gaps formed between the bundles are of paramount importance. Focus is on inter-bundle bubbles. An existing model for bubble dynamics is used based on a probabilistic approach for bubbles moving, splitting, merging, and dissolving. The same approach was used for intra-bundle bubbles, the difference being that their motion appears to be much slower. The obtained void fractions of inter-bundle bubbles at different vacuum levels applied at the liquid flow front are compared to those from real mouldings with a high degree of conformity.

  • 28. Lundström, Staffan
    et al.
    Stenberg, R.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Bergström, R.
    Ahlström Glassfibre OY.
    Partanen, H.
    Ahlström Glassfibre OY.
    Birkeland, P. A.
    Devold AMT, Langevåg.
    In-plane permeability measurements: a Nordic round-robin study2000In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 31, no 1, p. 29-43Article in journal (Refereed)
    Abstract [en]

    Three methods to measure the in-plane permeability are compared. The methods are based on the wetting and saturated parallel flow techniques and the wetting radial flow technique. It is found that the stability of all three methods is very good. The best repeatability in the results is obtained with the wetting parallel flow technique. A main drawback of this method is, however, that the samples are not saturated hence, the saturated parallel flow method is suggested for continued evaluation. The standard deviation from repeatable measurements of this method is 14 and 20% for the two fabrics tested. By a round-robin comparison at three laboratories it is found that the reproducibility of the saturated parallel flow technique is in the same range as the repeatability of the method. Furthermore, it is shown that the liquid tested and the pressure applied have very small influence on the results. It is clear that most of the scatter is due to the sample preparation and variations in the material. Accordingly, a continued work should be concentrated on improvements of the sample preparation, and development of methods to reduce the influence, from small errors, in sample preparation on the results.

  • 29.
    Melin, L. Gunnar
    et al.
    Luleå tekniska universitet.
    Levin, K.
    Aeronautical Research Institute of Sweden.
    Nilsson, S.
    Aeronautical Research Institute of Sweden.
    Palmer, S.J.P.
    Cavendish Laboratory.
    Rae, P.
    Cavendish Laboratory.
    A study of the displacement field around embedded fibre optic sensors1999In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 30, no 11, p. 1267-1275Article in journal (Refereed)
    Abstract [en]

    Deformation fields around optical fibres embedded in carbon fibre/epoxy composite specimens have been measured using moiré interferometry. The inclusion of the optical fibre resulted in large strain gradients. Calculated displacements from finite element analysis were compared to the experimental results. The numerical analysis showed that the displacement field on the specimen surface is smoothed out through the moiré grating thickness, an effect which is most pronounced at the material interfaces. With this influence taken into consideration a reasonable good quantitative agreement between the experiments and the finite element analysis was obtained. The finite element analysis also showed that the grating stiffness did not affect the measured displacements as long as the grating had a lower stiffness than the specimen

  • 30.
    Melin, L.Gunnar
    et al.
    Luleå tekniska universitet.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effects of strain rate on transverse tension properties of a carbon/epoxy composite: Studied by moire photography1999In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 30, no 3, p. 305-316Article in journal (Refereed)
    Abstract [en]

    The dependence on strain rate of the mechanical properties of a high performance carbon fibre/epoxy composite loaded in transverse tension has been investigated. Dog-bone shaped specimens have been tested in quasi-static and dynamic loading conditions. The dynamic tests were performed in a split Hopkinson bar at strain rates between 100 and 800 s-1. A moiré technique combined with high-speed photography, at framing rates of 0.25-1 MHz, was used for extraction of the local strain fields. The transverse mechanical properties were found to have weak or no dependence on strain rate. The average transverse modulus did not depend on strain rate, whereas the strain to and stress at failure were found to increase slightly with increased strain rate. For these dog-bone shaped specimens the strain evaluated by conventional Hopkinson bar technique was found to underestimate the true strain field measured by moiré technique. Finally, the moiré technique facilitated crack-propagation monitoring in real time. Crack speeds up to 2300 m s-1 were measured at transverse crack propagation.

  • 31.
    Miettinen, Arttu
    et al.
    University of Jyväskylä.
    Ojala, Antti
    VTT Technical Research Centre of Finland, Espoo.
    Wikström, Lisa
    VTT Technical Research Centre of Finland, Espoo.
    Joffe, Roberts
    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.
    Nättinen, Kalle
    VTT Technical Research Centre of Finland, Espoo.
    Kataja, Markku
    Department of Physics, University of Jyväskylä.
    Non-destructive automatic determination of aspect ratio and cross-sectional properties of fibres2015In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 77, p. 188-194Article in journal (Refereed)
    Abstract [en]

    We propose a novel method for computerized estimation of the length/diameter distribution of fibres in short-fibre reinforced composite. Having its basis in X-ray micro-computed tomography, the method is non-destructive and does not require user intervention. In addition to the aspect ratio, the method is also capable of estimating other geometrical properties of fibre cross-sections. Based on results on specially fabricated model material, the accuracy and precision of the method seems to be reasonable. An application to the manufacturing process of wood fibre reinforced thermoplastic composite is also shown, indicating a significant decrease in the aspect ratio of fibres during the processing steps.

  • 32.
    Nilsson, Greger
    et al.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Fernberg, S. Patrik
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Strain field inhomogeneities and stiffness changes in GMT containing voids2002In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 33, no 1, p. 75-85Article in journal (Refereed)
    Abstract [en]

    During compression moulding of glass mat thermoplastics (GMT), voids may form. However, it is not clear whether voids are as critical to mechanical performance in GMT as in thermoset composites. The present investigation also considers the general problem of damage mechanisms in GMT. Conventional tensile tests, acoustic emission, a stiffness degradation test and a speckle technique for strain field measurements are used as well as optical microscopy of polished cross-sections. The void content (up to 5%) does not significantly influence the strength or stiffness degradation process. The reason is the large inhomogeneity of the strain fields in GMT. Failure occurs in locally soft regions and void effects are of secondary importance. Details of the failure process are discussed, emphasising the large local strains in matrix-rich regions.

  • 33.
    Nissilä, Tuukka
    et al.
    Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu, Oulu, Finland.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu, Oulu, Finland.Mechanical & Industrial Engineering (MIE), University of Toronto, 5 King’s College Road, Toronto, ON, Canada.
    Hietala, Maiju
    Fibre and Particle Engineering Research Unit, Faculty of Technology, University of Oulu, Oulu, Finland.
    A method for preparing epoxy-cellulose nanofiber composites with an oriented structure2019In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 125, article id 105515Article in journal (Refereed)
    Abstract [en]

    A method was developed for processing cellulose nanocomposites using conventional vacuum infusion. Porouscellulose nanofiber networks were prepared via ice-templating and used as preforms for impregnation with a bioepoxyresin. Microscopy studies showed a unidirectionally oriented micrometer-scale pore structure that facilitatedthe infusion process by providing flow channels for the resin. The permeability of the preforms wascomparable to that of natural fiber mats, and the infusion time significantly decreased after optimizing theprocessing temperature. The flexural modulus of the bio-epoxy increased from 2.5 to 4.4 GPa, the strengthincreased from 89 to 107 MPa, and the storage modulus increased from 2.8 to 4.2 GPa with 13 vol% cellulosenanofibers. The mechanical properties also showed anisotropy, as the flexural and storage moduli were approximately25% higher in the longitudinal direction, indicating that the nanofiber network inside the epoxymatrix had an oriented nature.

  • 34. Nordin, Lars-Olof
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nonlinear viscoplastic and nonlinear viscoelastic material model for paper fiber composites in compression2006In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 37, no 2, p. 344-355Article in journal (Refereed)
    Abstract [en]

    Compressive behavior of phenol-formaldehyde impregnated paper composites is studied in creep and strain recovery tests observing large nonlinear viscoelastic strains and irreversible strains, describing the latter as viscoplasticity. Stiffness reduction was not observed in experiments and therefore is not included in the material model. Schapery's nonlinear viscoelastic and nonlinear viscoplastic constitutive law is used as a material model and the stress dependent non-linearity functions are determined. First, the time and stress dependence of viscoplastic strains is described by Zapas et al. model and identified measuring the irreversible strains after creep tests of different length at the same stress and doing the same for creep tests of a fixed length but at different stress. Then, the determination of nonlinear viscoelastic stress dependent parameters is performed.

  • 35. Nordlund, Markus
    et al.
    Fernberg, S.P.
    SICOMP AB, Swedish Institute of Composites.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Particle deposition mechanisms during processing of advanced composite materials2007In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 38, no 10, p. 2182-2193Article in journal (Refereed)
    Abstract [en]

    Liquid composite moulding of advanced composite materials often comprises infiltration of a particle-filled resin into a multi-scale porous fabric. These injections/infusions are subject to severe particle depositions inside the reinforcement, leading to undesired inhomogeneous mechanical and functional properties. Hence, the mechanisms for particle depositions are investigated by detailed meso-scale experiments, analysed by microscopic imaging and micro-particle image velocimetry, and macroscopic infusions of a biaxial non-crimp fabric. It is shown that two main particle deposition mechanisms are filtration during fibre bundle impregnation and filtration induced by stationary flow through fibre bundles. It is also clarified where in the reinforcement the particles will deposit. Finally, a number of suggestions on how to process advanced composite materials with a more homogeneous particle distribution are launched.

  • 36. Nordlund, Markus
    et al.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Frishfelds, Vilnis
    Jakovics, A.
    University of Latvia.
    Permeability network model for non-crimp fabrics2006In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 37, no 6, p. 826-835Article in journal (Refereed)
    Abstract [en]

    In this work a global permeability model is developed and applied to a biaxial Non-crimp stitched fabric (NCF). This model focuses on a detailed meso-scale description of the fabric geometry, which takes into account the local permeability distribution in a fabric due to perturbations of the geometry as well as the geometrical features which arise from the stitching process. It is shown in this work that these features significantly affects the global permeability. The influence of the amount and type of perturbation of a fabric is also studied in this work. It is shown that perturbation of the fabric geometry affect the global permeability but not as much as the stitching process. The model developed is finally validated with experimental permeability data and it is suggested how to use the model for an arbitrary lay-up.

  • 37. Odenberger, Torbjörn
    et al.
    Andersson, Magnus
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Experimental flow-front visualisation in compression moulding of SMC2004In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 35, no 10, p. 1125-1134Article in journal (Refereed)
    Abstract [en]

    This work is primarily focused on experimental visualisation of the flow during mould closure in compression moulding of sheet moulding compound. Circular plates are manufactured with industry scale equipment at close to production conditions. Special attention is given to the advancing flow front, for which the full complexity is captured by means of continuous high resolution close-up monitoring. From the experimental visualisation of the flow front, three phases are defined, namely squish, flow, and boiling. During the initial phase, squish, outer layers do not remain outer layers, the actual flow is very complex and air is likely to be entrapped. The governing process parameters during this phase are mould temperature, mould closing speed and amount of preheating in the mould. During the second phase, flow, the flow is stable and seemingly viscous. During the last phase, boiling, bubbles are observed in the low pressure region at the flow front, favouring the void content both internally and on the surface. Based on a chemical analysis including mass spectrometry and thermogravimetry, the gas is probably styrene.

  • 38.
    Okabe, T.
    et al.
    Department of Aerospace Engineering, Tohoku University.
    Imamura, H.
    Department of Aerospace Engineering, Tohoku University.
    Sato, Y.
    Department of Aerospace Engineering, Tohoku University.
    Higuchi, R.
    Department of Aerospace Engineering, Tohoku University.
    Koyanagi, J.
    Department of Materials Science and Technology, Tokyo University of Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Experimental and Numerical Studies of Initial Cracking in CFRP Cross-Ply Laminates2015In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 68, p. 81-89Article in journal (Refereed)
    Abstract [en]

    This work is concerned with the conditions for formation of the first (initial) cracks in composite laminates with cutouts or ply drop-offs subjected to in-plane loading. We study here the crack formation on the free edge of CFRP cross-ply laminates experimentally and by numerical stress and failure analysis. The free-edge surface strains are measured by the digital image correlation (DIC) technique. The numerical analysis consists of a two-scale approach, where the macro-level analysis is performed with a three-dimensional finite-element method (3D FEM) and the micro-level analysis uses a periodic unit-cell (PUC) in the transverse plies. The constitutive assumption made for the macro-level analysis is an orthotropic linear thermo-elastic solid for the unidirectional plies with a thin isotropic viscoplastic layer between the longitudinal and transverse plies. In the PUC, the fibers are assumed linear elastic, while the matrix is modeled as an elastic-viscoplastic solid. Crack formation is assumed to occur in the matrix by the dilatation induced brittle failure mechanism for which the dilatation energy density criterion is used.

  • 39.
    Oksman, Kristiina
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Aitomäki, Yvonne
    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.
    Siquiera, Gilberto
    Applied Wood Materials Laboratory, Swiss Federal Laboratories for Materials Science and Technology (EMPA), CH-8600 Dübendorf.
    Zhou, Qi
    School of Biotechnology, Royal Institute of Technology (KTH), AlbaNova University Centre, Stockholm.
    Butylina, Svetlana
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Tanpichai, Supachok
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Zhou, Xiaojian
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hooshmand, Saleh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Review of the recent developments in cellulose nanocomposite processing2016In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 83, p. 2-18Article in journal (Refereed)
    Abstract [en]

    This review addresses the recent developments of the processing of cellulose nanocomposites, focusing on the most used techniques, including solution casting, melt-processing of thermoplastic cellulose nanocomposites and resin impregnation of cellulose nanopapers using thermoset resins. Important techniques, such as partially dissolved cellulose nanocomposites, nanocomposite foams reinforced with nanocellulose, as well as long continuous fibers or filaments, are also addressed. It is shown how the research on cellulose nanocomposites has rapidly increased during the last 10 years, and manufacturing techniques have been developed from simple casting to these more sophisticated methods. To produce cellulose nanocomposites for commercial use, the processing of these materials must be developed from laboratory to industrially viable methods.

  • 40.
    Oldenbo, Magnus
    et al.
    Volvo Car Corporation.
    Fernberg, S.P.
    SICOMP AB, Swedish Institute of Composites.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Mechanical behaviour of SMC composites with toughening and low density additives2003In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 34, no 9, p. 875-885Article in journal (Refereed)
    Abstract [en]

    A new type of SMC material (Flex-SMC) developed for automotive exterior body panels has been investigated. Flex-SMC contains hollow glass micro-spheres and thermoplastic toughening additives. A conventional SMC (Std-SMC) was used as a reference material. Materials were tested in monotonic tension and compression. Stiffness degradation with strain as well as fracture toughness was determined. In situ SEM was used to study failure mechanisms. Flex-SMC has a density almost 20% lower than Std-SMC and has higher impact resistance. The damage threshold strain of the Flex-SMCs is higher than for Std-SMC. Flex-SMCs have more than twice the fracture toughness of Std-SMC. The major reason identified is that Flex-SMCs shows extensive fibre pullout

  • 41.
    PM, Visakh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Thomas, Sabu
    Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala.
    Oksman, Kristiina
    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.
    Crosslinked natural rubber nanocomposites reinforced with cellulose whiskers isolated from bamboo waste: processing and mechanical/thermal properties2012In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 43, no 4, p. 735-741Article in journal (Refereed)
    Abstract [en]

    Crosslinked natural rubber (NR) nanocomposites were prepared using cellulose nanowhiskers (CNWs) that were extracted from bamboo pulp residue of newspaper production, as the reinforcing phase. The coagulated NR latex containing bamboo nanowhiskers (master batch) was compounded with solid NR and vulcanizing agents using a two-roll mill and subsequently cured to introduce crosslinks in the NR phase. No evidence of micro-scaled aggregates of cellulose nanowhiskers in NR matrix was observed in scanning electron microscopy (SEM) images. The addition of CNWs had a positive impact on the tensile strength, E-modulus, storage modulus, tan delta peak position and thermal stability of the crosslinked NR. Theoretical modeling of the mechanical properties showed a lower performance than predicated and therefore further process optimization and/or compatibilization are required to reach the maximum potential of these nanocomposites.

  • 42.
    Pupurs, Andrejs
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Goutianos, Stergios
    Technical University of Denmark, Risø Campus.
    Brondsted, Povl
    Technical University of Denmark, Risø Campus.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Interface debond crack growth in tension-tension cyclic loading of single fiber polymer composites2013In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 44, no 1, p. 86-94Article in journal (Refereed)
    Abstract [en]

    Fiber/matrix interface debond crack growth from a fiber break is defined as one of the key mechanisms of fatigue damage in unidirectional composites. Considering debond as an interface crack its growth in cyclic loading is analyzed utilizing a power law, where the debond growth rate is a power function of the change of the strain energy release rate in the cycle. To obtain values of two parameters in the power law cyclic loading of fragmented single fiber specimen is suggested. Measurements of the debond length increase with the number of load cycles in tension-tension fatigue are performed for glass fiber/epoxy single fiber composites. Analytical method in the steady-state growth region and FEM for short debonds are combined for calculating the strain energy release rate of the growing debond crack. Interface failure parameters in fatigue are determined by fitting the modeling and experimental results. The determined parameters for interface fatigue are validated at different stress levels.

  • 43.
    Pupurs, Andrejs
    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.
    Loukil, Mohamed Sahbi
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Kahla, Hiba Ben
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mattsson, D.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers2016In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 82, p. 244-252Article in journal (Refereed)
    Abstract [en]

    Simple approach based on Classical Laminate Theory (CLT) and effective stiffness of damaged layer is suggested for bending stiffness determination of laminate with intralaminar cracks in surface 90-layers and delaminations initiated from intralaminar cracks. The effective stiffness of a layer with damage is back-calculated comparing the in-plane stiffness of a symmetric reference cross-ply laminate with and without damage. The in-plane stiffness of the damaged reference cross-ply laminate was calculated in two ways: 1) using FEM model of representative volume element (RVE) and 2) using the analytical GLOB-LOC model. The obtained effective stiffness of a layer at varying crack density and delamination length was used to calculate the A, B and D matrices in the unsymmetrically damaged laminate. The applicability of the effective stiffness in CLT to solve bending problems was validated analyzing bending of the damaged laminate in 4-point bending test which was also simulated by 3-D FEM.

  • 44.
    Ranganathan, Nalini
    et al.
    Faculty of Forestry, University of Toronto.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nayak, Sanjay K
    Central Institute of Plastics Engineering & Technology, (CIPET), Guindy.
    Sain, Mohini
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Structure property relation of hybrid biocomposites based on jute, viscose and polypropylene: The effect of the fibre content and the length on the fracture toughness and the fatigue properties2016In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 83, p. 169-175Article in journal (Refereed)
    Abstract [en]

    In the present study, the extent of jute and viscose fibre breakage during the extrusion process on the fracture toughness and the fatigue properties was investigated. The composite materials were manufactured using direct long fibre thermoplastic (D-LFT) extrusion, followed by compression moulding. The fracture toughness (KIC) and the fracture energy (GIC) of the PP-J30 composites were significantly improved (133% and 514%, respectively) with the addition of 10 wt% viscose fibres, indicating hindered crack propagation. The addition of viscose fibres resulted in three times higher fatigue life compared with that of the unmodified jute composites. Further, with the addition of (2 wt%) MAPP, the PP-J30-V10 resulted in a higher average viscose fibre length of 8.1 mm, and the fracture toughness and fracture energy increased from 9.1 to 10.0 MPa.m1/2 and 28.9 to 31.2kJ/m2, respectively. Similarly, the fatigue life increased 51% compared with the PP-J30-V10, thus demonstrating the increased work energy due to hindrance of the propagation of cracks.

  • 45.
    Singh, Anshu A.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Geng, Shiyu
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Herrera Vargas, Natalia
    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.
    Aligned plasticized polylactic acid cellulose nanocomposite tapes: Effect of drawing conditions2018In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 104, p. 101-107Article in journal (Refereed)
    Abstract [en]

    Aligned nanocomposite tapes based on plasticized polylactic acid (PLA) and 1 wt.% cellulose nanofibers (CNF) were prepared using uniaxial solid-state drawing, and the effects of drawing conditions including temperature, speed and draw ratio on the material were studied. Microscopy studies confirmed alignment and the formation of ‘shish-kebab’ morphology in the drawn tape. Mechanical properties demonstrate that the solid-state drawing is a very effective way to produce stronger and tougher PLA nanocomposites, and the toughness can be improved 60 times compared to the undrawn tape. Additionally, the thermal properties, i.e. storage modulus, glass transition temperature and degree of crystallinity were improved. These improvements are expected due to the synergistic effect of CNF in the nanocomposite and orientations induced by the solid-state drawing.

  • 46.
    Sjögren, A.
    et al.
    Luleå tekniska universitet.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Mäder, E.
    Institute of Polymer Research, Dresden.
    Effects of fibre coating (size) on properties of glass fibre/vinyl ester composites1999In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 30, no 8, p. 1009-1015Article in journal (Refereed)
    Abstract [en]

    Effects of fibre coating (size) on transverse cracking has been investigated. Two glass fibre/vinyl ester model composites were studied, denoted CA and NoCA and based on different size compositions. Various single fibre tests were not able to quantify the interfacial failure of CA as the interface never failed. The CA size consisted of a film former and a methacrylsilane-coupling agent whereas the NoCA size did not contain any coupling agent. The study reveals limitations with single fibre composite tests for fibre/matrix combinations with high interfacial toughness. Cross-ply laminates based on NoCA demonstrated significant inferior transverse cracking toughness as compared with CA laminates. Composites based on commercially sized glass fibre were also investigated and they performed almost as poorly as the NoCA material, demonstrating large potential for improvement in commercial composites. Results further indicate that the remarkable transverse cracking toughness of the CA material stems partly from strong fibre/matrix adhesion but also from high ductility of the matrix region close to the fibre surface.

  • 47.
    Sjögren, A.
    et al.
    Structures Department, The Aeronautical Research Institute of Sweden.
    Krasnikovs, Andrejs
    Department of Strength of Materials, Riga Technical University.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Experimental determination of elastic properties of impact damage in carbon fibre/epoxy laminates2001In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 32, no 9, p. 1237-1242Article in journal (Refereed)
    Abstract [en]

    This paper describes an investigation of in-plane elastic properties of impact damaged regions in composite laminates. Quasi-isotropic carbon fibre/epoxy laminates were impacted and the impact damage examined by ultrasonic C-scanning, optical microscopy and thermal deplying. After impact damage observations, specimens were cut from the laminates and tested in tension and compression. The elastic modulus of the impact damage was, in both tension and compression, mainly controlled by the amount of fibre breakage. Interestingly, layers with broken fibres could sustain some load in compression, which led to higher elastic modulus in compression than in tension. The effect of delaminations on the elastic modulus was quite small in both tension and compression. The through-the-thickness variation of in-plane stiffness was studied by successively removing plies. The variation in stiffness was negligible, probably as a result of the very uniform distribution of delaminations and fibre breakage through the thickness of the laminates.

  • 48.
    Svanberg, J. Magnus
    et al.
    Luleå tekniska universitet.
    Holmberg, J. Anders
    SICOMP AB, Swedish Institute of Composites.
    Prediction of shape distortions: Part I. FE-implementation of a path dependent constitutive model2004In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 36, no 5, p. 711-721Article in journal (Refereed)
    Abstract [en]

    There is a great interest, especially from the aircraft industry, to increase the ability to understand and predict development of shape distortions and residual stresses during manufacture of polymer composite components. An increased ability to predict shape distortions will result in more cost efficient development, improved performance and optimised manufacturing of composites. To be able to predict residual stresses and shape distortions a model is needed that accounts for all important mechanisms involved. In a previous work by the authors, it was demonstrated that such-models must account for thermal expansion (different in glassy and rubbery state), chemical shrinkage due to the crosslinking reaction and finally frozen-in deformations. The present paper presents a simple mechanical constitutive model that accounts for the mechanisms mentioned above. The model is a limiting case of linear visco-elasticity that permits us to replace the rate dependence by a path dependence on the state variables: strain, degree of cure and temperature. This means significant savings in computational time, memory requirements and costs for material characterisation as compared to conventional visco-elastic models. This is the first of two papers, the second paper deals with experimental validation and analysis of mechanical boundary conditions during prediction of shape distortion.

  • 49.
    Svanberg, J. Magnus
    et al.
    Luleå tekniska universitet.
    Holmberg, J. Anders
    SICOMP AB, Swedish Institute of Composites.
    Prediction of shape distortions: Part II. Experimental validation and analysis of boundary conditions2004In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 35, no 6, p. 723-734Article in journal (Refereed)
    Abstract [en]

    During cure of thermoset composite structures residual stresses and/or shape distortions are always present. Residual stresses can cause apparent strength reduction or failure, even prior demoulding, respectively, shape distortions can deform a component so that the component becomes useless. For this reason a lot of effort has been spent to develop simulation tools to be able to foresee problems related to residual stresses and/or shape distortions and make the first part right. In a companion paper a process model for shape distortion predictions was developed and implemented into ABAQUS. In the present paper the model is validated for a material and cure schedule typical for RTM and autoclave processes. Comparisons between predicted and experimental shape distortion shows that the model and simulation approach used capture both effects from different cure schedules as well as the mechanical interaction between composite and tooling during in-mould cure. The results show that changing the mechanical boundary conditions significantly affects the shape distortion prediction. Therefore accurate modelling of the composite-tooling interaction is an important part of a shape distortion analysis

  • 50.
    Svanberg, J. Magnus
    et al.
    Luleå tekniska universitet.
    Holmberg, J.A.
    SICOMP AB, Swedish Institute of Composites.
    An experimental investigation on mechanisms for manufacturing induced shape distortions in homogeneous and balanced laminates2001In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 32, no 6, p. 827-838Article in journal (Refereed)
    Abstract [en]

    Manufacturing induced shape distortions is a common problem for composite manufacturers. For single curved geometries the phenomenon is known as spring-in. Today a lot of effort is spent to develop modelling tools for prediction of spring-in and shape distortions in general. However, good experimental data is rare in the literature and there are no established constitutive models capable of account for the effect from different cure schedules. In this paper, experimental data for spring-in of glass-fibre epoxy composites are presented. The experiments were performed with angle brackets manufactured by RTM, in a steel mould with accurate temperature control. Different in-mould temperature have been used to point out and separate different mechanisms responsible for spring-in.

12 1 - 50 of 60
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