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  • 1.
    Ahmed, Sheikh Ali
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Morén, Tom
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Hagman, Olle
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Cloutier, Alain
    Wood Research Center (CRB), Department of Wood and Forest Sciences, Faculty of Forestry, Geography and Geomatics, Laval University, Quebec.
    Fang, Chang-Hua
    Wood Research Center (CRB), Department of Wood and Forest Sciences, Faculty of Forestry, Geography and Geomatics, Laval University, Quebec.
    Elustondo, Diego
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Anatomical properties and process parameters affecting blister/blow formation in densified European aspen and downy birch sapwood boards by thermo-hygro-mechanical compression2013In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 48, no 24, p. 8571-8579Article in journal (Refereed)
    Abstract [en]

    Approximately, 13.5 % of the standing volume of productive forest land in Sweden is covered by birch and aspen, which provides the vast potential to produce value-added products such as densified wood. This study shows whether it is possible to densify those species with a thermo-hygro-mechanical (THM) process using heat, steam, and pressure. In this process, transverse compression on thin European aspen (Populus tremula) and downy birch (Betula pubescens) boards was performed at 200 ºC with a maximum steam pressure of 550 kPa. To obtain a theoretical 50 % compression set, the press’s maximum hydraulic pressure ranged from 1.5 to 7.3 MPa. Preliminary tests showed that ~75 % of the birch boards produced defects (blisters/blows) while only 25 % of the aspen boards did. Mainly, radial delamination associated with internal checks in intrawall and transwall fractures caused small cracks (termed blisters) while blows are characterized by relatively larger areas of delamination visible as a bumpy surface on the panel. Anatomical investigations revealed that birch was more prone to those defects than aspen. However, those defects could be minimized by increasing the pre-treatment time during the THM processing.

  • 2.
    Andersons, J.
    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
    Institute of Polymer Mechanics, University of Latvia.
    Weichert, D.
    Institute of General Mechanics, RWTH-Aachen University.
    Modeling the effect of reinforcement discontinuity on the tensile strength of UD flax fiber composites2011In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 46, no 15, p. 5104-5110Article in journal (Refereed)
    Abstract [en]

    To exploit the potential of natural fibers as reinforcement of polymer matrix composites, aligned bast fiber composite materials are being produced and studied. Bast fiber reinforcement is discontinuous due to the limited length of natural fibers, which needs to be reflected in predictive models of mechanical properties of composites. The strength in tension in the fiber direction of an aligned flax fiber-reinforced composite is modeled assuming that a cluster of adjacent fiber discontinuities is the origin of fracture. A probabilistic model of tensile strength, developed for UD composites containing a microdefect, is applied. It follows from the theoretical analysis that the experimental tensile strength as a function the fiber volume fraction can be described with acceptable accuracy assuming the presence of a cluster of ca. 4 × 4 elementary fiber discontinuities

  • 3.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Uniformity of filament strength within a flax fiber batch2009In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 44, no 2, p. 685-687Article in journal (Refereed)
  • 4.
    Backman, A.C.
    et al.
    Luleå tekniska universitet.
    Lindberg, Henrik
    Luleå tekniska universitet.
    Differences in wood material responses for radial and tangential direction as measured by dynamic mechanical thermal analysis2001In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 36, no 15, p. 3777-3783Article in journal (Refereed)
    Abstract [en]

    Wood is a complex cellular structure with different properties in the radial and tangential direction. Many researchers have measured dynamic properties in the longitudinal direction and a few in the radial direction but very little data can be found in the literature on dynamic mechanical properties in the tangential direction. The purpose of the work presented in this paper was to investigate the dynamic mechanical behaviour in the radial and tangential directions of wood (Pinus sylvestris). Testing was done in tension at 1 Hz with a Dynamic Mechanical Thermal Analyser. Properties in radial and tangential direction were different. The radial direction showed a higher elastic modulus and lower loss factor levels at temperatures between -120°C and 80°C. The tangential direction had on average a higher peak temperature than the radial direction for a loss factor peak around -80°C. It is the opposite of synthetic composites where the stiffer direction has a higher peak temperature. A loss factor peak at around 0°C was seen, most significantly in the tangential direction. This peak has scarcely been reported in the literature before. The distance between annual rings did not significantly affect the dynamic behaviour in the tangential direction.

  • 5.
    Easterling, K.E.
    Luleå tekniska universitet.
    Metallurgical applications of scanning transmission electron microscopy1977In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 12, no 5, p. 857-868Article in journal (Refereed)
    Abstract [en]

    It is shown that scanning transmission microscopy (STEM) has followed two main lines of development, the pure STEM based upon a field emission electron source in which the emphasis is given to high resolution, and a combined system in which STEM is an attachment to a conventional transmission microscope (TEM+STEM). When used in combination with an energy dispersive X-ray spectrometer, the combined TEM+STEM system is shown to be extremely versatile and possibly the more useful for the applied metallurgist. The high vacuum requirements of pure STEM, however, make this system suitable to be used in conjunction with an Auger spectrometer. Examples of the various microanalysis facilities of STEM are given in the article, including micro-diffraction, rocking-beam channelling patterns, qualitative and quantitative X-ray spectroscopy analysis, particle analysis, and in situ experimentation. The controversial subject of whether thicker specimens can be studied in STEM compared with conventional TEM is also discussed

  • 6.
    Ericson, M.L.
    et al.
    Luleå tekniska universitet.
    Lindberg, Henrik
    Luleå tekniska universitet.
    Method of measuring energy dissipation during crack propagation in polymers with an instrumented ultramicrotome1996In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 13, no 3, p. 655-662Article in journal (Refereed)
    Abstract [en]

    In order to characterize very local energy dissipation during crack propagation in polymers, an ultramicrotome was instrumented to measure the energy dissipated during sectioning. The work to section per unit area, W s, was measured for five different amorphous polymers [polymethyl methacrylate (PMMA), polystyerene (PS), polycarbonate (PC) and two epoxy resins] in the glassy state. When the section thickness was varied between 60 and 250 nm, W s varied between 15 and 100 Jm-2, depending on the material and section thickness. The method and the results are compared with other methods used for determining the energy dissipation at a local level as well as at a macroscopic level in polymers. The differences between different polymers were found to be contradictory to macroscopic fracture toughness, G lc, measurements. The material that showed the highest W s had the lowest G lc values reported. Possible mechanisms for energy dissipation during sectioning are also discussed.

  • 7.
    Gamstedt, E.K.
    et al.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Fatigue damage mechanisms in unidirectional carbon-fibre-reinforced plastics1999In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 34, no 11, p. 2535-2546Article in journal (Refereed)
    Abstract [en]

    The fatigue life behaviour and the underlying micromechanisms have been studied in two different Types of unidirectional carbon-fibre-reinforced plastics loaded in tension-tension along the fibre direction. The carbon fibres (AS4) were the same in the two composite systems. One thermoplastic matrix (polyetheretherketone, PEEK) and one thermosetting matrix (epoxy toughened with a thermoplastic additive) were used. The macroscopic fatigue behaviour was characterised by fatigue life diagrams. Surface replicas were taken intermittently during the course of the fatigue tests to monitor the active fatigue damage micromechanisms. The thermoset based composite showed a higher fatigue resistance with few microcracks initiated at distributed fibre breaks growing at a decelerating rate. The thermoplastic composite had a more pronounced fatigue degradation with a steeper fatigue life curve, which was caused by widespread propagating debonds and matrix cracks. The use of a tougher and more ductile matrix results in an inferior fatigue life performance, due to a more widely distributed accumulation of damage that propagates at a higher rate.

  • 8.
    Gibson, L.J.
    et al.
    Department of Civil Engineering, Massachusetts Institute of Technology.
    Ashby, M.F.
    Engineering Department, Cambridge University.
    Easterling, Kenneth E.
    Luleå tekniska universitet.
    Structure and mechanics of the iris leaf1988In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 23, no 9, p. 3041-3048Article in journal (Refereed)
    Abstract [en]

    The structure of the iris leaf resembles that of a sandwich beam with fibre composite faces separated by a low-density foam core. Such structures have a high specific stiffness because the separation of the faces by the lightweight core increases the moment of inertia of the section with little increase in weight. In this paper we examine the structure of the leaf of the bearded iris and show that its flexural stiffness can be explained in terms of the mechanics of sandwich beams

  • 9.
    Guo, S.J.
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Dou, S.X.
    University of New South Wales.
    Liu, H.K.
    University of New South Wales.
    Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors1992In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 27, no 11, p. 3043-3049Article in journal (Refereed)
    Abstract [en]

    Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J c, drops 80% from 124 (OT) to 29 A cm-2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm-2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm-2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ c=(α/B 0)/[(1+B/B 0)] n can be used to describe the field dependence of the critical current density, Jc, in the field range 0.2-1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB c2 for bismuth-based bulk highT c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.

  • 10.
    Hassan, Mohammad L.
    et al.
    Cellulose and Paper Department, National Research Center, Dokki.
    Hassan, Enas A.
    Cellulose and Paper Department, National Research Center, Dokki.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of pretreatment of bagasse fibers on the properties of chitosan/microfibrillated cellulose nanocomposites2011In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 46, no 6, p. 1732-1740Article in journal (Refereed)
    Abstract [en]

    Bleached bagasse pulp was pretreated with dilute alkali and xylanase enzymes before isolation of microfibrillated cellulose using ultra-high friction grinding and high-pressure homogenization. The isolated nanofibers were used with chitosan polymer to prepare chitosan nanocomposites by solution casting at nanofiber loading from 2.5 to 20%. The effect of nanofibers loading on moisture sorption, dry and wet tensile strength, crystallinity, thermal stability, and dynamic mechanical thermal properties was studied using tensile testing, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and dynamic mechanical thermal analysis (DMTA). Nanocomposites with good transparency were obtained at the different nanofibers loadings. Chitosan nanocomposites made using nanofibers isolated from bagasse fibers treated with xylanase or alkali showed higher dry and wet tensile strength than those made using nanofibers isolated from untreated bagasse pulp. DMTA results showed higher storage modulus and indicated higher glass transition temperature for the chitosan nanocomposites than that of neat chitosan. XRD patterns showed that, at low nanofibers loading, addition of bagasse nanofibers to chitosan matrix increased ordering of chitosan chains upon drying the nanocomposites films.

  • 11.
    He, Ming
    et al.
    Nanjing University of Technology.
    Feng, Xin
    Nanjing University of Technology.
    Lu, Xiaohua
    Nanjing University of Technology.
    Ji, Xiaoyan
    Liu, Chang
    Nanjing University of Technology.
    Bao, Ningzhong
    Nanjing University of Technology.
    Xie, Jingwei
    Nanjing University of Technology.
    A controllable approach for the synthesis of titanate derivatives of potassium tetratitanate fiber2004In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 39, p. 3745-3750Article in journal (Refereed)
    Abstract [en]

    Three types titanate derivatives, K2Ti6O13 fiber, K2Ti8O17 fiber and anatase TiO2 fiber, were synthesized by ion-exchange reaction from potassium tetratitanate fiber (K2Ti4O9) based on the hydrate conditions predicted by a thermodynamic model. These products were formed by a heat treatment of the corresponding hydrate intermediates K1.33H0.67Ti4O9·H2O, KHTi4O9·0.5H2O and H2Ti4O9·1.2H2O which were quantitatively obtained by controlling the pH value and the equilibrium concentration of potassium ion. The mole ratio of Ti/K in solid phase (R) of the target products was taken as the controlling aim in the hydrate process. The temperature for heat treatment of hydrate intermediates was determined by thermogravimetry (TG) and differential scanning calorimetry (DSC). All products retained fibrous morphology similar to that of K2Ti4O9 used as the starting material.

  • 12. Ion, John
    et al.
    Moisio, T.
    Lappeenranta University of Technology.
    Pedersen, T.F.
    Danish Corrosion Centre.
    Sørensen, B.
    Danish Corrosion Centre.
    Hansson, C.M.
    Danish Corrosion Centre.
    Laser surface modification of a 13.5% Cr, 0.6% C steel1991In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 26, no 1, p. 43-48Article in journal (Refereed)
    Abstract [en]

    A 13.5% Cr, 0.6% C steel, with an initial microstructure of chromium carbides in a ferrite matrix, was heat-treated by scanning a high-power laser beam over the surface. The aim was to compare the physical and chemical properties produced by this type of selective surface treatment with those resulting from a conventional furnace desensitization and quench-hardening heat treatment. Surface heating homogenized the carbon originally bound in the carbides sufficiently to produce martensite, giving hardening to levels comparable with a conventional heat treatment. Chromium-rich zones, carbides and retained austenite were also detected in the heated microstructure. Surface melting produced complete homogenization of both carbon and chromium, which resulted in the retention of large amounts of austenite in the microstructure on cooling to room temperature. Subsequent refrigeration at - 196 °C transformed some of the austenite to martensite. Pitting corrosion and local reductions in hardness were observed adjacent to treated areas under certain conditions, due to precipitation of secondary carbides and elevated tempering, respectively.

  • 13.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Andersons, J.
    Institute of Polymer Mechanics, University of Latvia.
    Wallström, Lennart
    Interfacial shear strength of flax fiber/thermoset polymers estimated by fiber fragmentation tests2005In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 40, no 9-10, p. 2721-2722Article in journal (Refereed)
    Abstract [en]

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

  • 14.
    Kvien, Ingvild
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Sugiyama, Junji
    Research Institute for Sustainable Humanosphere, Kyoto University.
    Votrubec, Martin
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Characterization of starch based nanocomposites2007In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 42, no 19, p. 8163-8171Article in journal (Refereed)
    Abstract [en]

    The goal of this study was to characterize the nanostructure and the properties of starch based nanocomposites with either cellulose nano whiskers (CNW) or layered silicates (LS) (synthetic hectorite) as reinforcements. Modified potato starch was used as matrix with water and sorbitol as plasticizers and with 5 wt.% of either of the reinforcements. Two methods were explored to prepare samples for transmission electron microscopy (TEM) examination; chemical fixation and freeze etching. It was possible to characterize the nanostructure both parallel and perpendicular to the nanocomposite surface by the freeze etching technique. Both nanocomposites showed well-distributed reinforcements in the starch matrix. Dynamic mechanical thermal analysis showed that the storage modulus was significantly improved at elevated temperatures, especially for the layered silicate nanocomposite. Both nanocomposites showed a significant improvement in tensile properties compared to the pure matrix.

  • 15.
    Larker, Rickard
    et al.
    Luleå tekniska universitet.
    Wei, L.-Y
    Luleå tekniska universitet.
    Loberg, Bengt
    Olsson, M.
    Department of Materials Science, Uppsala University.
    Johansson, S.
    Department of Materials Science, Uppsala University.
    AEM investigation of ceramic/incology 909 diffusional reactions after joining by HIP1994In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 29, no 16, p. 4404-4414Article in journal (Refereed)
    Abstract [en]

    Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si3N4 and either 60 vol% TiN or 50 vol% TiB2, while three were monolithic materials, namely Si3N4 with 2.5 wt% Y2O3 as a sintering additive, Si3N4 without additives, and Si2 N2O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 μm in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (≤100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium.

  • 16.
    Lindberg, Henrik
    et al.
    Luleå tekniska universitet.
    Bertilsson, H.
    Luleå tekniska universitet.
    Morphology of polyamide 6-polybutadiene multiblock copolymers1991In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 26, no 16, p. 4383-4388Article in journal (Refereed)
    Abstract [en]

    The morphology of polyamide 6-polybutadiene multiblock copolymers, commonly used for reaction injection moulding, have been investigated using transmission electron microscopy, dynamic mechanical techniques, calorimetry and wide angle X-ray diffraction. Phase separation is found to be almost complete and the crystallization of the PA6 blocks is slightly higher than in pure PA6. The morphology shows similarities to what has been reported for segmented polyether-esters. The PA6 lamellar dimensions found in the micrographs agree with dimensions calculated from melting point depression and X-ray data.

  • 17.
    Lindberg, Henrik
    et al.
    Luleå tekniska universitet.
    Vesely, D.
    Brunel University.
    Bertilsson, H.E.
    Luleå tekniska universitet.
    Chlorine loss and mass loss from Polyvinylchloride and polyvinylidenchloride under the electron beam1985In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 20, no 6, p. 2225-2232Article in journal (Refereed)
    Abstract [en]

    The loss of chlorine during the irradiation of PVC and PVDC in the electron microscope has been measured by the decay of the X-ray chlorine Kα signal. A number of factors affecting the measured beam damage curves have been considered and the experimental errors reduced to ±10%. The results show that the chlorine decay curves can be best described by the sum of two exponentials, corresponding to the two different chlorine decay processes, these being: the dehydrochlorination of the polymer molecules and the dehydrochlorination of the polyene structure formed by the beam damage. The higher initial chlorine content of PVDC compared to PVC will result in a larger amount of chlorine atoms reacting with the polyene structure, which is more stable in the electron beam than the undamaged polymer. The chlorine loss, measured by X-ray analysis, has been compared to the mass loss, measured by energy loss analysis, and also with the volume changes of isolated spherical PVC particles. It has been concluded that the mass loss is almost entirely due to chlorine loss and that the residual structure has a density similar to the undamaged PVC.

  • 18.
    Lindberg, Henrik
    et al.
    Luleå tekniska universitet.
    Vesely, D.
    Brunel University.
    Bertilsson, H.E.
    Chalmers University of Technology.
    Electron beam damage of some chlorine containing polymers and PVC blends1989In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 24, no 8, p. 2825-2832Article in journal (Refereed)
    Abstract [en]

    The electron beam damage of PVC, PVDC, chlorinated PE, chlorosulphonated PE (Hypalon) and chloroendic acid polyester has been studied by chlorine loss and mass loss measurements. The mass loss decay curves have been recalculated for true mass loss and correlated with the X-ray measurements of chlorine loss. It is shown that the major mechanism of degradation in these polymers is dehydrochlorination. The rate of dehydrochlorination is dependent on the chemical structure of the polymer, and in particular on the availability of hydrogen for HCl formation. However, in case of chloroendic acid polyester, more complicated volatile chlorinated compounds are also formed. The dehydrochlorination rate and mass loss of miscible PVC blends, namely PVC with organotin stabilizer, PMMA, PCL and SAN have been studied. The rate of chlorine loss from PVC was found to be unchanged for all blends except for PMMA and PCL, where it slightly increased due to the chemical reaction between the molecular fragments of both polymers. The differences between the thermal degradation and electron beam damage are also discussed.

  • 19.
    Lindhagen, Johan.E.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Temperature changes in polymer composites during tensile loading1997In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 32, no 15, p. 4071-4076Article in journal (Refereed)
    Abstract [en]

    Temperature changes in polymeric composite laminates subjected to uniaxial monotonic tensile loading were studied. The laminates were transverse and longitudinal unidirectional glass fibre-epoxy and carbon fibre-epoxy laminates, and hybrid crossply laminates with longitudinal glass fibre-epoxy and transverse carbon fibre-epoxy layers. The temperature decreased linearly with increased tensile stress in the elastic region, except for longitudinal carbon fibre specimens (where the fibres have a small but negative coefficient of thermal expansion), which exhibited a small temperature increase. The occurrence of non-linear stress-strain behaviour in transverse carbon fibre specimens altered the rate of temperature change. When cracks appeared in laminates, the temperature immediately started to rise. The temperature changes in crossply laminates were interpreted from measurements on unidirectional specimens and knowledge of the damage mechanisms.

  • 20.
    Lingois, Philippe
    et al.
    Luleå tekniska universitet.
    Berglund, Lars
    Luleå tekniska universitet.
    Modeling elastic properties and volume change in dental composites2002In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 37, no 21, p. 4573-4579Article in journal (Refereed)
    Abstract [en]

    A modeling approach was applied to study elastic properties and volume change in dental composites. Mechanics modeling results were compared with experimental data in model materials of known composition where the filler content was varied. Composite behavior was predicted based on polymer and filler properties in order to improve basic understanding. Model predictions agree well with data. The models were used to discuss effects of resin properties, filler volume fraction and microstructure (particle shape and filler size distribution).

  • 21.
    Lingois, Philippe
    et al.
    Luleå tekniska universitet.
    Berglund, Lars
    Luleå tekniska universitet.
    Greco, A.
    Kungliga tekniska högskolan, KTH.
    Maffezoli, A.
    Department of Materials Science, University of Lecce.
    Chemically induced residual stresses in dental composites2003In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 38, no 6, p. 1321-1331Article in journal (Refereed)
    Abstract [en]

    In several European countries, dental composites are replacing mercury-containing amalgams as the most common restorative materials. One problem with dental composites is residual stresses which may lead to poor performance of the restoration. In the present study, a combined modeling and materials characterization approach is presented and predictions compare well with experimental data on residual stresses. The model takes stress relaxation into account through the complete relaxation time spectrum of the resin. The approach allows for detailed parametric studies where resin and composite composition as well as cure conditions may be tailored with respect to residual stress generation.

  • 22.
    Manessis, Dionissios
    et al.
    Stevens Institute of Technology, Hoboken, NJ.
    Du, Honghua
    Stevens Institute of Technology, Hoboken, NJ.
    Larker, Richard
    Luleå tekniska universitet.
    Oxide and interface characteristics of oxidized silicon oxynitride ceramics: an investigation by electron microscopy1998In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 33, no 17, p. 4447-4453Article in journal (Refereed)
    Abstract [en]

    Hot-isostatically pressed silicon oxynitride (Si2N2O) ceramics free from sintering aids were oxidized in 1 atm dry oxygen at 1100 and 1300°C. The structural and chemical characteristics of the oxide and the nature of the oxide-Si2N2O interface were determined using cross-sectional transmission electron microscopy in conjunction with small-probe energy dispersive X-ray analysis and selected-area electron diffraction. Oxidation of Si2N2O resulted in the formation of amorphous SiO2. The oxide-Si2N2O interface was chemically abrupt. The interface was very flat when parallel to low-index, high atomic density Si2N2O crystal planes but became notably undulated if oriented to high index, low atomic density planes. About 6 vol% residual SiO2 phase was present in the bulk of the Si2N2O ceramics. Current results have provided an important baseline for the understanding of the oxidation behaviour of Si2N2O.

  • 23.
    Mannberg, Peter
    et al.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Nyström, Birgitha
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Service life assessment and moisture influence on bio-based thermosetting resins2014In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 49, no 10, p. 3687-3693Article in journal (Refereed)
    Abstract [en]

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

  • 24.
    Mannberg, Peter
    et al.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Nyström, Birgitha
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Wallström, Lennart
    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.
    Service life assessment and moisture influence on bio-based composites2014In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 49, no 15, p. 5265-5270Article in journal (Refereed)
    Abstract [en]

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

  • 25.
    Mouzon, Johanne
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Glowacki, Emily
    Luleå tekniska universitet.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Comparison between slip-casting and uniaxial pressing for the fabrication of translucent yttria ceramics2008In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 43, no 8, p. 2849-2856Article in journal (Refereed)
    Abstract [en]

    Slip casting and uniaxial pressing were compared as first consolidation stages prior to cold isostatic pressing (CIP) to produce translucent yttria ceramics. In the first step, yttria slurries suitable for slip casting were prepared. The viscosity was optimized with respect to the starting agglomeration state, amount of dispersant, milling time, and number of milling balls. Secondly, pellets were prepared either by slip casting or uniaxial pressing and then cold-isostatically pressed. Finally, the pellets were made translucent by a combination of pre-sintering and hot isostatic pressing (HIP). Although slip-cast and pressed samples exhibited similar green-body densities after CIP and pre-sintering, the samples prepared by slip casting were more homogeneous in terms of translucency and microstructure throughout their bodies. This was attributed to the ability of slip casting to minimize density gradients during packing, and to the beneficial effect of ball-milling to remove larger agglomerates before casting. Therefore, slip casting as a first consolidation stage prior to CIP appears to be more suitable than uniaxial pressing in order to prepare homogeneous optical ceramics.

  • 26.
    Sun, Z.
    et al.
    Lappeenranta University of Technology.
    Ion, John
    Laser welding of dissimilar metal combinations1995In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 30, no 17, p. 4205-4214Article in journal (Refereed)
    Abstract [en]

    The ability to manufacture a product using a number of different metals and alloys greatly increases flexibility in design and production. Properties such as heat, wear and corrosion resistance can be optimized, and benefits in terms of production economics are often gained. Joining of dissimilar metal combinations is, however, a challenging task owing to the large differences in physical and chemical properties which may be present. Laser welding, a high power density but low energy-input process, provides solutions to a number of problems commonly encountered with conventional joining techniques. Accurate positioning of the weld bead, rapid heating and cooling, low distortion, process flexibility, and opportunities for product redesign are its principal characteristics. The review describes the principles underlying laser welding of dissimilar metal combinations and highlights the above benefits in a number of practical applications. It is concluded that there is potential for its application in many industrial sectors.

  • 27. Talreja, Ramesh
    Multi-scale modeling in damage mechanics of composite materials2006In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 41, no 20, p. 6800-6812Article in journal (Refereed)
    Abstract [en]

    This paper addresses the multi-scale modeling aspects of damage in composite materials. The multiplicity of the scales of the operating mechanisms is discussed and clarified by taking examples of damage in a unidirectional ceramic matrix composite and in a cross ply polymer matrix composite laminate. Two multi-scale modeling strategies - the hierarchical and the synergistic - are reviewed in the context of deformational response. Finally, the "big picture" as it relates to the cost-effective manufacturing of composite structures intended for long-term performance is outlined and desired future direction in multi-scale modeling is discussed

  • 28.
    Thuvander, Fredrik
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    In situ observations of fracture mechanisms for radial cracks in wood2000In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 35, no 24, p. 6277-6283Article in journal (Refereed)
    Abstract [en]

    This paper presents the findings of work carried out to describe the micromechanisms of radial crack growth in wood. TR and TL cracks are both radial cracks but TR grows radially and TL longitudinally. TR cracks are known to show higher fracture toughness than TL cracks. The TR fracture surfaces also indicate a more tortuous crack path. Since the reason for this is unclear, details of the TR crack growth mechanisms in green Pinus sylvestris L were studied. This was done by in-situ optical microscopy as the crack was cutting through alternating layers of soft earlywood and stiff latewood. At the scale of individual cells, the crack tip advanced by separating cell walls at the middle lamella in a splitting or peeling mode. At the scale of growth rings, stick-slip type of crack growth was observed and new crack planes were often formed. The stress distribution in a material with alternating stiff and soft layers is causing this. This stress distribution also contributes to the tendency for inclined cracks to deviate in the radial direction. For interpretation of fracture mechanisms, the importance of scale interaction and the combined influences of microstructure and stress state are emphasized.

  • 29.
    Thuvander, Fredrik
    et al.
    Luleå tekniska universitet.
    Jernkvist, Lars Olof
    Luleå tekniska universitet.
    Gunnars, Jens
    Luleå tekniska universitet.
    Influence of repetitive stiffness variation on crack growth behaviour in wood2000In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 35, no 24, p. 6259-6266Article in journal (Refereed)
    Abstract [en]

    Softwoods have a repetitive variation in stiffness over their growth rings, which is due to the difference in cellular structure between the latewood and earlywood. In this paper, the influence of the repetitive stiffness variation on radially growing cracks is studied by detailed finite element analyses, in which the wood material is represented by a layered orthotropic continuum. The distribution of stress around the crack is found to be very different from crack tip stress fields in homogenous isotropic materials. The latewood layer ahead of the crack experiences a significant tensile stress, which indicates that formation of new secondary cracks ahead of the primary crack front is a likely mechanism for crack propagation. This mechanism is also favoured by the fact that the primary crack is subjected to a significant shielding from the stiff latewood, which tends to arrest the primary crack in the soft earlywood layer. Analyses are performed for materials with various growth ring widths, and the calculated results are compared with reported experimental observations.

  • 30.
    Thuvander, Fredrik
    et al.
    Luleå tekniska universitet.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Measurements of crack tip strain field in wood at the scale of growth rings2000In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 35, no 24, p. 6267-6275Article in journal (Refereed)
    Abstract [en]

    The fracture mechanisms of wood have often been interpreted on the scale of cell walls. Although this scale is important, the scale of growth rings needs to be considered in the same context. In the present study, the crack tip strain field of radial TR cracks at the scale of growth rings is measured by electronic speckle photography. The methodology is discussed in detail as well as the data reduction scheme. The tip is in the earlywood layer and the crack plane of the TR crack is perpendicular to the stiffer latewood layer. Increasing opening mode load is applied in-situ as the crack is observed by reflected light optical microscopy. Strains are measured on direct images of the microstructure. In contrast to some other methodologies, this allows direct correlation between strain field and microstructure. In the softer earlywood, tangential strains extend considerable distances in the tangential direction. Due to the stiff latewood, the strain is heavily constrained in the radial direction. This nature of the local strain field has been largely neglected, despite its obvious significance to TR crack growth mechanisms.

  • 31.
    Wang, G.X.
    et al.
    School of Materials Science and Engineering, University of New South Wales.
    Lu, G.Q.
    Department of Chemical Engineering, University of Queensland.
    Pei, Benyan
    Luleå tekniska universitet.
    Yu, A.B.
    School of Materials Science and Engineering, University of New South Wales.
    Oxidation mechanism of Si3N4-bonded SiC ceramics by CO, CO2 and steam1998In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 33, no 5, p. 1309-1317Article in journal (Refereed)
    Abstract [en]

    This paper presents a theoretical and experimental investigation into the oxidation reactions of Si3N4-bonded SiC ceramics. Such ceramics which contain a small amount of silicon offer increased oxidation and wear resistance and are widely used as lining refractories in blast furnaces. The thermodynamics of oxidation reactions were studied using the JANAF tables. The weight gain was measured using a thermogravimetric analysis technique to study the kinetics. The temperature range of oxidation measurements is from 1073 to 1573 K and the oxidation atmosphere is water vapour, pure CO and CO-CO2 gas mixtures with various CO-to-CO2 ratios. Thermodynamic simulations showed that the oxidation mechanism of Si3N4-bonded SiC ceramics is passive oxidation and all components contribute to the formation of a silica film. The activated energies of the reactions follow the sequence Si3N4 > SiC > Si. The kinetic study revealed that the oxidation of Si3N4-bonded SiC ceramics occurred in a mixed regime controlled by both interface reaction and diffusion through the silica film. Under the atmosphere conditions prevailing in the blast furnace, this ceramic is predicted to be passively oxidized with the chemical reaction rate becoming more dominant as the CO concentration increases

  • 32.
    Wei, Liu-Ying
    et al.
    Luleå tekniska universitet.
    Dunlop, G.L.
    CRC for Alloy and Solidification Technology (CAST), Department of Mining and Metallurgical Engineering, The University of Queensland.
    Westengen, H.
    Norsk Hydro a.s., Research Centre Porsgrunn.
    Solidification behaviour and phase constituents of cast Mg-Zn-misch metal alloys1997In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 32, no 12, p. 3335-3340Article in journal (Refereed)
    Abstract [en]

    The solidification path and phase constituents of alloys in the magnesium-rich corner of the Mg-Zn-misch metal (MM) pseudo-ternary system have been investigated by a combination of differential thermal analysis, analytical electron microscopy and X-ray diffraction. The solidification behaviour diagram for this system was found to be dominated by a large two-phase ( -Mg plus T-phase) field in which the lowest eutectic temperature was 500°C. T-phase has a c-centred orthorhombic crystal structure and exhibits a wide range of stoichiometry. The interdendritic eutectic phase in the pseudo-binary Mg-MM system is Mg12MM which has the same b c t structure as Mg12Ce. The eutectic temperature here is 593°C.

  • 33.
    Wei, L.Y.
    et al.
    Luleå tekniska universitet.
    Dunlop, G.L.
    University of Queensland.
    Westengen, H.
    Norsk hydro.
    Age hardening and precipitation in a cast magnesium-rare-earth alloy1996In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 31, no 2, p. 387-397Article in journal (Refereed)
    Abstract [en]

    The precipitation sequence responsible for the age-hardening behaviour of a cast Mg-1.3 wt% rare-earth alloy has been investigated by analytical electron microscopy. Very fine intermediate precipitates formed at an early stage of ageing. Plate-shaped Mg3MM β precipitates (MM = misch metal) and hexagonal prism-shaped Mg12MM β precipitates were primarily responsible for age hardening. Precipitate morphologies, crystal structures and crystallographic orientation relationship were determined for the various types of precipitates that formed during ageing at different temperatures.

  • 34.
    Westman, Anna-Karin
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Wei, L.-Y
    Luleå tekniska universitet.
    TEM study of the interface between HIPed silicon nitride and encapsulation borosilicate glass2000In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 35, no 11, p. 2847-2854Article in journal (Refereed)
    Abstract [en]

    A transmission electron microscope study has been made of a silicon nitride component with 6 w/o yttrium oxide as a sintering aid hot isostatically pressed (HIP) with an encapsulation glass of borosilicate. The TEM study concentrated on the interface region between ceramic and glass. Two different types of hexagonal boron nitride were formed near the interface. One, with a textured structure, seemed to nucleate heterogeneously on the surfaces of silicon oxynitride grains. The (001) planes of the crystals extended outwards, giving a thickness of approximately 0.5 microns. The other type formed as hexagonally shaped grains separate from the first type and appeared to have grown as several segments in different directions around a nucleus. In each segment BN layers are parallel to each other and perpendicular to their common [001]BM direction. This second type of BN crystal was also detected a little further from the surface within the silicon nitride. The volume fraction of additive glassy phase tended to be lower in this surface region than in the bulk. Possible mechanisms of prevention of encapsulation glass penetration into the porous ceramic component during HIP were discussed

1 - 34 of 34
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