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  • 51.
    Ansari, Farhan
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Salajkova, Michaela
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Zhou, Qi
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Berglund, Lars
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Fiber- och polymerteknologi.
    Cellulose nanocomposites - Controlling dispersion and material properties through nanocellulose surface modification2015Inngår i: ICCM International Conferences on Composite Materials, International Committee on Composite Materials , 2015Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The use of cellulosic nanofibers as reinforcement in polymer composites offers great advantages over their petroleum counterparts. Apart from being strong, stiff and low density; they are obtained from naturally occurring resources and as such are favorable from an environmental point of view. A major problem while studying nanomaterials is their tendency to agglomerate, thus leading to inhomogeneous distribution within the polymer matrix. This often results in stress concentrations in the matrix rich regions when the material is subjected to load and therefore, limits the potential application of these materials. A common approach to circumvent this is by surface modification, which facilitates the dispersion in non-polar matrices. An environmental friendly approach, inspired by clay chemistry, was used to functionalize the CNC surface. It was shown that the CNC could be modified in a rather convenient way to attach a variety of functional groups on the surface. Primarily, the problem of cellulose nanocrystal (CNC) distribution in a hydrophobic polymer matrix is investigated. Composites prepared from modified CNC were studied and compared with unmodified CNC. The distribution of the CNC is carefully monitored at different stages via UV-Vis spectroscopy and scanning electron microscopy (SEM). The mechanical properties of the resulting materials were characterized by dynamic mechanical as well as uniaxial tensile tests. It was shown that a homogeneous distribution of the CNC exposes a tremendous amount of surface area to interact with the matrix. In such a case, the stress transfer is much more efficient and perhaps, the matrix behavior is modified, which leads to significant improvements in the mechanical properties.

  • 52. Arab, A.
    et al.
    Stommel, M.
    Wallström, Lennart
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Rack, A.
    Investigation of fibre degradation in natural fibre reinforced biocomposites2013Inngår i: Proceedings of the 5th International Conference on Sustainable Materials, Polymers and Composites: Ecocomp 2013, 2013, s. 174-185Konferansepaper (Fagfellevurdert)
  • 53. Arab, A.
    et al.
    Stommel, M.
    Wallström, Lennart
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Varna, Janis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Stiffness prediction in green composites using homogenization techniques2013Inngår i: Proceedings of the 19th International Conference on Composite Materials: ICCM 2013, Montreal (Canada), 2013, s. 1214-1222Konferansepaper (Fagfellevurdert)
  • 54.
    Arab, Asghar
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Stommel, L.
    Saarland University.
    Wallström, Lennart
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Fibre Orientation Investigation in Short Natural Fibre Reinforced Composites Using Synchrotron Imaging2013Konferansepaper (Fagfellevurdert)
  • 55.
    Asfaw, Habtom Desta
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Kemiska sektionen, Institutionen för kemi - Ångström, Strukturkemi.
    Multifunctional Carbon Foams by Emulsion Templating: Synthesis, Microstructure, and 3D Li-ion Microbatteries2017Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Carbon foams are among the existing electrode designs proposed for use in 3D Li-ion microbatteries. For such electrodes to find applications in practical microbatteries, however, their void sizes, specific surface areas and pore volumes need be optimized. This thesis concerns the synthesis of highly porous carbon foams and their multifunctional applications in 3D microbatteries. The carbon foams are derived from polymers that are obtained by polymerizing high internal phase water-in-oil emulsions (HIPEs).

    In general, the carbonization of the sulfonated polymers yielded hierarchically porous structures with void sizes ranging from 2 to 35 µm and a BET specific surface area as high as 630 m2 g-1. Thermogravimetric and spectroscopic evidence indicated that the sulfonic acid groups, introduced during sulfonation, transformed above 250 oC to thioether (-C-S-) crosslinks which were responsible for the thermal stability and charring tendency of the polymer precursors. Depending on the preparation of the HIPEs, the specific surface areas and void-size distributions were observed to vary considerably. In addition, the pyrolysis temperature could also affect the microstructures, the degree of graphitization, and the surface chemistry of the carbon foams.

    Various potential applications were explored for the bespoke carbon foams. First, their use as freestanding active materials in 3D microbatteries was studied. The carbon foams obtained at 700 to 1500 oC suffered from significant irreversible capacity loss during the initial discharge. In an effort to alleviate this drawback, the pyrolysis temperature was raised to 2200 oC. The resulting carbon foams were observed to deliver high, stable areal capacities over several cycles. Secondly, the possibility of using these structures as 3D current collectors for various active materials was investigated in-depth. As a proof-of-concept demonstration, positive active materials like polyaniline and LiFePO4 were deposited on the 3D architectures by means of electrodeposition and sol-gel approach, respectively. In both cases, the composite electrodes exhibited reasonably high cyclability and rate performance at different current densities. The syntheses of niobium and molybdenum oxides and their potential application as electrodes in microbatteries were also studied. In such applications, the carbon foams served dual purposes as 3D scaffolds and as reducing reactants in the carbothermal reduction process. Finally, a facile method of coating carbon substrates with oxide nanosheets was developed. The approach involved the exfoliation of crystalline VO2 to prepare dispersions of hydrated V2O5, which were subsequently cast onto CNT paper to form oxide films of different thicknesses.

  • 56.
    Asfaw, Habtom
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Kemiska sektionen, Institutionen för kemi - Ångström.
    Tai, Cheuk-Wai
    Stockholm University.
    Nyholm, Leif
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Kemiska sektionen, Institutionen för kemi - Ångström, Oorganisk kemi.
    Edström, Kristina
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Kemiska sektionen, Institutionen för kemi - Ångström, Strukturkemi.
    Surface-oxidized NbO2 nanoparticles for high performance lithium microbatteriesManuskript (preprint) (Annet vitenskapelig)
  • 57.
    Asp, Leif
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Finite element delamination study of a notched composite plate under flexural loads2010Inngår i: Journal of Materials Science and Engineering. A, ISSN 1934-8959, Vol. 4, nr 8, s. 66-73Artikkel i tidsskrift (Fagfellevurdert)
  • 58.
    Asp, Leif
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Fracture and yield predictions for epoxies in composite-like stress state1994Licentiatavhandling, med artikler (Annet vitenskapelig)
  • 59. Asp, Leif
    Multifunctional composite materials for energy storage in structural load paths2013Inngår i: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, nr 4, s. 144-149Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper presents an overview of the research performed to date by a Swedish interdisciplinaryteam of scientists striving to develop multifunctional composite materials for storage of electric energy in mechanical load paths. To realise structural batteries from polymer composites, research pursued on carbon fibres for use as negative electrode in the battery as well as on polymer electrolytes for use as polymer matrix in the composite is reported. The work on carbon fibres comprises characterisation of the electrochemical capacity of commercial carbon fibre grades and how this is affected by mechanical load. Co-polymers are studied for their multifunctional performance with respect to lithium ion conductivity and stiffness. Also, rational processing of these polymer electrolytes and the effect of processing on their properties are addressed

  • 60.
    Asp, Leif
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Structural battery materials2012Inngår i: Proceedings of the 15th European Conference on Composite Materials / [ed] Marino Quaresimin; Laszlo Kollar; Leif Asp, Venice, 2012Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Since four years Swerea SICOMP has been leading a team of Swedish researchers developing structural battery materials from polymer composites. The research performed in the Swedish project KOMBATT (Lightweight structural energy storage materials) is funded by the Swedish foundation for strategic research (SSF). The research addresses two technical challenges in particular. Firstly, solid polymer electrolytes that efficiently transfer loads in the composite and simultaneously transports lithium ions, while being electrically insulating, must be developed. Secondly, the ability of the reinforcement, i.e. The carbon fibres, to intercalate lithium ions as part of the chemical redox reactions, while maintaining its mechanical properties must be assured. This paper is the first in a series of papers at this conference from the KOMBATT project team and presents background and overview of the project.

  • 61.
    Asp, Leif
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Transverse failure initiation in polymer composites1995Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Transverse failure is one of the most important failure modes in polymer composites. The phenomenon often causes the first deviations from nonlinear laminate behavior. Also, in pressure vessels and pipes, fluid leakage through a path of transverse cracks is often the limiting design criterion. In the present work, experimental and theoretical studies focused on the micromechanical level have been carried out. The objective was to investigate transverse failure initiation in the matrix. The other major mechanism of failure initiation, fiber/matrix debonding, was not considered. The triaxial nature of the matrix stress state in glass fiber/epoxy was confirmed by finite element analysis. Experimental results for glassy epoxies subjected to composite-like stress states demonstrated large reductions in strain to failure as compared with uniaxial loading. The triaxial stress state is therefore by itself a sufficient explanation for the low transverse strain to failure in polymer composites. Plastic yielding in the matrix was demonstrated not to be the cause of failure initiation. Instead cavity induced cracking was suggested as a failure mechanism. A criterion was proposed based on a critical value for the dilatational energy density. Comparison with experimental results for epoxies subjected to a variety of multiaxial load-cases supported the criterion. Additional support was obtained from comparison with experimental results in the literature for transverse failure of glass fiber/epoxy at different fiber contents. Although the epoxy matrix was different from those in the present study, general trends in data were supported by predictions based on the criterion and finite element analysis. Thermal residual stresses were found to be important for high fiber contents. Based on the criterion, a conservative estimate of composite strain to failure was obtained. This is reasonable since the criterion predicts initiation, not final failure. Based on the model, effects from changes in constituent properties were examined in a parametric finite element analysis. Fiber modulus was found to strongly influence transverse failure. Introduction of a third phase interphase between fiber and matrix was also investigated. Beneficial results on transverse failure strain caused by matrix initiation was observed for thin rubbery interphases.

  • 62.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    A biaxial thermomechanical disk test for glassy polymers1997Inngår i: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 37, nr 1, s. 96-101Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Failure criteria for polymers need to include effects from the stress state. For this reason, biaxial test results are of interest. However, biaxial test methods usually require expensive equipment. In the test method presented here, a disk of epoxy is bonded between a steel ring and a steel disk. The temperature is then lowered until fracture is observed. Experiments were performed on three different glassy epoxy polymers. The biaxial stress state was analyzed by finite element analysis and by an approximate analytical model. Experimental observations support the ability of the method to provide material property data. An approximate analytical model was found sufficiently accurate for stress analysis and determination of the stress state at failure

  • 63.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Gudmundsson, Peter
    Department of Solid Mechanics, Royal Institute of Technology.
    Effects of a composite-like stress state on the fracture of epoxies1995Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 53, nr 1, s. 27-37Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The strain to failure of a transversely loaded composite is much lower than for the pure matrix in uniaxial tension. Several studies of composites suggest the triaxial matrix stress state as one of the explanations. In order to investigate this experimentally, a triaxial tensile test previously used for rubbers (the poker-chip test) was successfully applied to four epoxies in the glassy state. The chosen specimen geometry mimicked the most severe stress state in the matrix as determined by finite element analysis of a transversely loaded glass-fiber/epoxy composite. The poker-chip strains to failure in the primary loading direction were 0.5-0.8%, whereas uniaxial strains to failure were 1.8-7%. The triaxial stress state in composite matrices may therefore by itself be a sufficient explanation for low values of transverse composite strains to failure

  • 64.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Criterion for crack initiation in glassy polymers subjected to a composite-like stress state1996Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 11, s. 1291-1301Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three epoxy systems of interest as composite matrix materials are examined for their yielding and failure behavior under uniaxial, biaxial and triaxial stress states. Yield criteria applicable to glassy polymers, i.e. accounting for the hydrostatic stress effect on the deviatoric stress to yielding, are assessed. It is found that under stress states resembling those in matrix constrained between fibers, e.g. equibiaxial and equitriaxial tension, yielding is suppressed while brittle failure, presumably caused by crack growth from cavitation, occurs. A criterion for this mode of failure is proposed as the critical dilatational strain energy density. Experimental data are found to support this criterion.

  • 65.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Effects of fiber and interphase on matrix-initiated transverse failure in polymer composites1996Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 45, nr 6, s. 657-665Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Failure initiation in polymer-matrix composites loaded transverse to the fibers is investigated by a numerical parametric study where the effects of constituent properties, interphase properties and thickness are examined. Failure initiation in the matrix only is studied, interfacial debonding not being considered. Two modes of failure - yielding and cavitation-induced brittle failure - are examined. A criterion for the cavitation-induced brittle failure has been proposed previously and failure prediction based on this criterion was found to agree with experimental data for a glass-fiber-reinforced epoxy. The present study shows that the elastic modulus of fibers has a large effect on the stress and strain to failure initiation. A rubbery interphase material is found in most cases to have a beneficial effect. The site at which failure initiates and the governing mode of failure initiation are also affected by the fiber modulus and the interphase properties

  • 66.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Prediction of matrix-initiated transverse failure in polymer composites1996Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 56, nr 9, s. 1089-1097Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A study has been conducted of failure in unidirectionally-reinforced fiber composites loaded in tension normal to the fibers. The case considered is when this failure is governed by failure of the matrix rather than fiber/matrix debonding. Both yielding and cavitation-induced brittle failure of the matrix are considered. The latter mode of failure was suggested previously as the likely mode to occur in epoxies under stress states that are purely or nearly hydrostatic tension. Three fiber packing arrangements (square, hexagonal and square-diagonal) with different fiber volume fractions were studied numerically by a finite element method to determine the local stress states. It is found that cavitation-induced brittle failure occurs much before yielding in all cases. Experimental data taken from the literature support this finding.

  • 67.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars
    Luleå tekniska universitet.
    Gudmundson, P.
    Mechanical behavior of epoxy resins in uniaxial and triaxial loading1993Inngår i: High temperature ceramic matrix composites: 6th EuropeanConference on Composite Materials, 20 - 24 September 1993, Bordeaux; HT-CMC / [ed] M. Neitzel; J.C. Lambert, Cambridge: Woodhead Publishing Materials , 1993, s. 323-328Konferansepaper (Fagfellevurdert)
  • 68.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Brandt, F.
    Effects of pores and voids on the interlaminar delamination toughness of a carbon/epoxy composite1997Inngår i: Proceedings / Eleventh International Conference on Composite Materials, Gold Coast, Queensland, Australia, 14th - 18th July 1997: ICCM-11 / [ed] Murray L Scott, Melbourne: Woodhead Publishing Materials , 1997, s. 322-331Konferansepaper (Fagfellevurdert)
  • 69.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Edgren, Fredrik
    Sjögren, A.
    Effects of stitch pattern on the mechanical properties of non-crimp fabric composites2004Inngår i: From nano-scale interactions to engineering structures: ECCM 11, 11th European Conference on Composite Materials ; May 31 - June 3, 2004, Rhodes, Greece / [ed] Costas Galiotis, Rhodos: European Society for Composite Materials , 2004Konferansepaper (Fagfellevurdert)
  • 70.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Juntikka, R.
    Swerea SICOMP AB, Mölndal.
    High velocity impact on NCF reinforced composites2006Inngår i: From the science of composites to engineering applications: the dawning future of composites: ECCM 12, 12th European Conference on Composite Materials, Biarritz, 29th August - 1st September 2006 ; [conference proceedings] / [ed] Jacques Lamon, Biarritz, 2006Konferansepaper (Fagfellevurdert)
  • 71.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Marklund, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Varna, Janis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Modelling stiffness and strength of non-crimp fabric composites: semi-laminar analysis2011Inngår i: Non-crimp fabrics composites: manufacturing, properties and applications, Cambridge: Woodhead Publishing Materials , 2011, s. 402-438Kapittel i bok, del av antologi (Fagfellevurdert)
  • 72.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Marklund, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Varna, Janis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Olsson, Robin
    Swerea SICOMP AB, Mölndal.
    Multiscale modelling of non-crimp fabric composites2012Inngår i: Proceedings of the ASME International Mechanical Engineering Congress and Exposition--2012: presented at ASME 2012 International Mechanical Engineering Congress and Exposition, November 9-15, 2012 Houston, Texas USA, New York: American Society of Mechanical Engineers , 2012, Vol. 3, s. 581-590Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Damage initiation and evolution in NCF composites leading to final failure includes a multitude of mechanisms and phenomena on several length scales. From an engineering point-of-view a computational scheme where all mechanisms would be explicitly addressed is too complex and time consuming. Hence, methods for macroscopic performance prediction of NCF composites, with limited input regarding micro- And mesoscale details, are requested. In this paper, multi-scale modelling approaches for in-plane transverse strength of NCF composites are outlined and discussed. In addition a simplistic method to predict transverse tensile and compressive strength for textile composites featuring low or no fibre waviness is presented

  • 73.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Nilsson, Karl-Fredrik
    SICOMP AB, Swedish Institute of Composites.
    Delamination criticality in slender compression-loaded composite panels2002Inngår i: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 221-222, s. 3-16Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, a damage tolerance model based on the assumption of delamination criticality in compression loaded slender composite panels is outlined. In particular, the verification of the model by comparison between numerical predictions and experimental results is reviewed. Growth of shallow delaminations in slender panels is shown to be promoted by the global buckling of the panel. Consequently, care must be taken if structures with delaminations are to be allowed to buckle. In the paper, application of the model for aircraft design is briefly discussed. The overall predicted panel behaviour agrees with observations for test coupons. However, very small geometrical changes are shown to have tremendous effects on the predicted behaviour. Consequently, in structural design one must consider the sensitivity of geometrical conditions on the predicted behaviour. Therefore, reduction of the structural item into a design element is suggested. To generate conservative designs the suggested design element is to represent the worst case

  • 74.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Sjögren, B.A.
    Berglund, Lars A.
    Prediction of failure initiation in polypropylene with glass beads1997Inngår i: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 18, nr 1, s. 9-15Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 75.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Szpieg, Magdalena
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Wysocki, Maciej
    Swerea SICOMP AB.
    Mechanical performance and modelling of a fully recycled modified CF/PP composite2012Inngår i: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 46, nr 12, s. 1503-1517Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A fully recycled carbon fiber reinforced maleic anhydride grafted polypropylene (MAPP)-modified polypropylene (rCF/rPP) composite material has been developed and characterized. This new composite was manufactured employing papermaking principles, dispersing the recycled carbon fibers (rCF) in water, and forming them into mats. Two layers of the recycled polypropylene (rPP) films manufactured using press-forming were sandwiched between three rCF preform layers in a stack. The stack was heated and press-formed resulting in a composite plate with a nominal thickness of 1.20 mm and a fiber volume fraction of 40%. A series of tensile tests using rectangular specimens cut in four different directions (0°, 90°, ± 45°) in the composite plate were performed to confirm in-plane material isotropy. Models to predict stiffness and strength of the short fiber rCF/rPP composite were also employed and validated using experiments. The models were found to be in good agreement with experimental results. Fiber length distribution measurements were performed before (unprocessed) and after (processed) composite manufacturing to investigate the influence of processing on fiber degradation. The results revealed a significant reduction in fiber length by the press-forming operation. To model the viscoelastic and viscoplastic responses of the composite an inelastic material model was employed and characterized using a series of creep and recovery tests. From the creep tests, it was found that the time and stress dependence of viscoplastic strains follows a power law. The viscoelastic response of the composite was found to be linear in the investigated stress range. The material model was validated in constant stress rate tensile tests and the agreement was good, even close to the rupture stress.

  • 76.
    Aulin, C.
    et al.
    RISE, Innventia.
    Salazar-Alvarez, G.
    Lindström, T.
    RISE, Innventia.
    High strength flexible and transparent nanofibrillated cellulose-nanoclay biohybrid films with tunable oxygen and water vapor permeability2012Inngår i: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, nr 20, s. 6622-6628Artikkel i tidsskrift (Fagfellevurdert)
  • 77.
    Bachinger, A.
    et al.
    Composite Structures, Swerea SICOMP AB.
    Marklund, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap. Composite Structures, Swerea SICOMP AB.
    Rössler, J.
    Hellström, Pär
    Composite Structures, Swerea SICOMP AB.
    Asp, Leif
    Composite Structures, Swerea SICOMP AB.
    Stiffness-modifiable composite for pedestrian protection2014Inngår i: 16th European Conference on Composite Materials, ECCM 2014: Seville, Spain, 22 - 26 June 2014, European Conference on Composite Materials, ECCM , 2014Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A novel functional material allowing stiffness-reduction upon external stimulation was developed. Implementation of such technology in the design of a car front has high potential to result in increased protection of vulnerable road users (VRUs). The composite material is obtained by coating carbon fibres with a thermoplastic polymer in a continuous process, followed by infusion with an epoxy resin. The process is scalable for industrial use. The coating process was optimized regarding coating efficiency, energy consumption, risks involved for operating personnel and environment, and tailored to gain the optimal coating thickness obtained from numerical calculations. A drastic decrease in transversal stiffness could be detected for the composite material by dynamic mechanical thermal analysis (DMTA), when the temperature was increased above the glass transition temperature of the thermoplastic interphase. The ability of the material to achieve such temperature and associated reduction in stiffness by the application of current was verified using a special 3-point bending setup developed for this task.

  • 78.
    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 analysis2001Inngår i: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 36, nr 15, s. 3777-3783Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 79.
    Backman, A.C.
    et al.
    Luleå tekniska universitet.
    Lindberg, K.A.H.
    Luleå tekniska universitet.
    Interaction between wood and polyurethane-alkyd lacquer resulting in a decrease in the glass transition temperature2002Inngår i: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 85, nr 2, s. 595-605Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The long-term properties of paint and glue are of great interest to both manufacturers and users of these materials. If a good bond is achieved, the surface between the wood and the paint or glue will be less susceptible to degradation. Thus, the wood and polymer must be compatible and develop some kind of bonding force between them. A high degree of interaction between wood and commercial polyurethane-alkyd lacquer was shown as a decrease by 10°C of the glass transition temperature (Tg) for the lacquer on wood compared to the pure lacquer. The lacquer also demonstrated good adhesion to wood at a microscale. The interaction was investigated with dynamic mechanical thermal analysis and scanning electron microscopy fractography. The reason for the decrease in Tg is probably because of the lacquer having a higher free volume when applied to the wood, most likely due to it being subjected to tensile forces developed during the drying of the lacquer. Results from investigations of wood impregnated with two different acrylates, a polymethylmethacrylate and a more hydrophilic acrylate, support the suggestion that a decrease in Tg will occur if the polymer adheres to wood, but that poor interaction with little or no adhesion will result in no decrease in Tg. This article also presents results of the dynamic mechanical behavior of Scots Pine in the tangential direction

  • 80.
    Bader, Thomas K.
    Vienna University of Technology, Austria.
    Mechanical properties of sound and of deteriorated softwood at different length scales: Poromicromechanical modeling and experimental investigations2011Doktoravhandling, monografi (Annet vitenskapelig)
    Abstract [en]

    Due to its natural origin and its inherent heterogeneities, mechanical properties of wood are highly anisotropic and show a broad variability, not only between different wood species, but also within a tree. Similar to other biological materials, the wood mi- crostructure is well organized and hierarchically structured from the annual rings visible to the naked eye down to the wood polymers cellulose, hemicellulose, and lignin at the nanometer-scale. This thesis aims at a deeper understanding of the role of different hi- erarchical levels and their corresponding physical and chemical characteristics in relation to mechanical properties of sound wood and of deteriorated wood. This is achieved by means of micromechanical modeling and experimental analyses.

    This thesis starts with the re-formulation of an existing micromechanical model for the elastic behavior and elastic limit states of wood in the framework of poromechanics. The mechanical role of cell wall water at different hierarchical levels is investigated by means of this model. In a broader sense, the developed model allows to investigate the transition of eigenstresses from the cell wall to the softwood level. Moreover, this poromicromechanical model forms the basis for subsequent consideration of a microscopic failure criterion for lignin for the derivation of softwood failure stresses. The suitability of the modeling approach is underlined by a satisfactory agreement of the model-predicted failure stresses with experimental results of biaxial strength tests on Norway spruce.

    As a result of partly considerably different microstructural characteristics, Common yew exhibits exceptional mechanical properties compared to other softwood species. The re- lationship between microstructure and stiffness properties of Common yew and Norway spruce is investigated by means of the poromicromechanical model and mechanical tests across various length scales. Moreover, this offers the opportunity of a broader model validation. The influence of differences in microfibril angle of the S2 cell wall layer and in mass density between yew and spruce is found to be more dominant than the influence of differences in the annual ring characteristics.

    The suitability of the poromicromechanical model to predict changes in mechanical prop- erties upon fungal decay is demonstrated. For this purpose, relationships between mi- crostructure and mechanical properties of deteriorated wood are experimentally explored. Changes in mechanical properties and in the microstructure, measured at pine wood samples after standard wood durability tests using one brown rot fungus (Gloeophyl- lum trabeum) and one white rot fungus (Trametes versicolor), are presented. Transverse stiffnesses are revealed to be more sensitive to degradation than longitudinal stiffness, particularly as a result of pronounced degradation of hemicelluloses. Moreover, ultrason- ically derived anisotropy ratios of elastic stiffnesses allow to identify certain degradation mechanisms of the two considered fungi. The experimental campaign is complemented by micromechanical modeling. For this purpose, the micromechanical model is extended to take into account degradation-specific microstructural characteristics. 

  • 81. Bader, Thomas K.
    et al.
    Braovac, Susan
    Fackler, Karin
    Hofstetter, Karin
    Stiffness Properties of the Archaeological Oak Wood from the Oseberg Ship2011Inngår i: Cultural Heritage Preservation.EWCHP - 2011: Proceedings of the European Workshop on Cultural Heritage Preservation. Berlin, Germany, September 26 to 28, 2011, Fraunhofer IRB Verlag, 2011, s. 164-170Konferansepaper (Fagfellevurdert)
  • 82.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Braovac, Susan
    University of Oslo, Norway.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Microstructure-Stiffness Relations of the Ancient Oak Wood from the Oseberg Ship2010Inngår i: International Workshop on "Modeling Mechanical Behavior of Wooden Cultural Objects", Krakow, 2010, s. 22-23Konferansepaper (Annet vitenskapelig)
  • 83.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Dastoorian, Foroogh
    Vienna University of Technology, Austria ; University of Tehran, Iran.
    Ebrahimi, Ghanbar
    University of Tehran, Iran.
    Unger, Gerhard
    Vienna University of Technology, Austria.
    Lahayne, Olaf
    Vienna University of Technology, Austria.
    Hellmich, Christian
    Vienna University of Technology, Austria.
    Pichler, Bernhard
    Vienna University of Technology, Austria.
    Combined ultrasonic-mechanical characterization of orthotropic elastic properties of an unrefined bagasse fiber-polypropylene composite2016Inngår i: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 95, s. 96-104Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Use of wood-fiber plastics for construction purposes calls for comprehensive understanding of their anisotropic mechanical properties. As a respective contribution, we here report the first-ever complete elasticity characterization of an orthotropic bagasse fiber polypropylene composite, requiring identification of nine independent constants. For this purpose, we carry out characterization in principal material directions. Six diagonal stiffness tensor components are quantified based on ultrasonic longitudinal and shear wave velocity measurements; and three diagonal compliance tensor components are identified as the inverses of three Young’s moduli derived from unloading regimes of quasi-static uniaxial compression tests. Combination of all measurement data in the framework of orthotropic linear elasticity provides access to all off-diagonal stiffness and compliance tensor components, opening the door to quantifying six Poisson’s ratios. 

  • 84.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    de Borst, Karin
    University of Glasgow, UK.
    Poroelastic properties of hardwood at different length scales2013Inngår i: Poromechanics V: proceedings of the fifth Biot Conference on Poromechanics, July 10-12, 2013, Vienna, Austria / [ed] Christian Hellmich, Bernhard Pichler, Dietmar Adam, Reston: American Society of Civil Engineers (ASCE), 2013, s. 1830-1836Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Hardwoods show a very complex, hierarchically organized microstructure. Slight structural differences at various length scales bring about a huge variety of hardwood species. This motivates the development of a micromechanical model for hardwood. Since differences in the microstructure of the material can be considered in the model, it offers the opportunity to explain the variability of mechanical properties of the whole class of hardwood. The micromechanical model is formulated in the framework of poroelasticity. In this contribution, poroelastic properties at different length scales of the material are discussed. Validation of the micromechanical model is based on an extensive experimental database covering elastic properties and microstructural characteristics of different temperate and tropical hardwood species. Exemplary parameter studies demonstrate the ability of the model to study the contribution of specific microstructural characteristics to the load transfer and the deformation characteristics of wood. 

  • 85.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    de Borst, Karin
    University of Glasgow, UK.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Micromechanical modeling of Common yew and Norway spruce2013Inngår i: Proceedings in Applied Mathematics and Mechanics: PAMM, ISSN 1617-7061, E-ISSN 1617-7061, Vol. 13, nr 1, s. 185-186Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this contribution, a micromechanical modeling approach in the framework of poromechanics is adopted to study structure-stiffness relations of two quite different species, namely spruce and yew, in detail. In particular, microstructural specialties of yew and spruce are assessed. A dominant influence of the cellulose content and its orientation on the stiffness of the cell wall is revealed, while on the macroscopic scale, density is found to be the governing microstructural characteristic for elastic properties.

  • 86.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    de Borst, Karin
    University of Glasgow, UK.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Micromechanical Modeling of Wood: Multiscale Modeling and Model Validation2013Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Due to its natural origin and its inherent heterogeneities, mechanical properties of wood are highlyanisotropic and show a broad variability, not only between different wood species, but also within a tree [1].Similar to other biological materials, the wood microstructure is well organized and hierarchically structuredfrom the annual rings visible to the naked eye down to the wood polymers cellulose, hemicellulose, andlignin at the nanometer-scale. The aim of the research conducted at the Institute for Mechanics of Materialsand Structures is a deeper understanding of the role of different hierarchical levels and their correspondingphysical and chemical characteristics in relation to mechanical properties of softwood and hardwood. This isachieved by means of micromechanical modeling and experimental analyses at various length scales.

    A micromechanical model provides the opportunity to predict poroelastic properties of softwood andhardwood tissues at different hierarchical levels from microstructural and compositional data [1,2]. Thehierarchical organization of wood is mathematically represented in a multiscale model. Effective poroelasticproperties are predicted by means of continuum micromechanical approaches (self-consistent method andMori-Tanaka method), the unit cell method, and laminate theory. These approaches are extended to accountfor water-induced eigenstresses within representative volume elements and repetitive unit cells, which aresubsequently upscaled to the macroscopic wood level.

    Verification of the micromechanical model for softwood and hardwood with a comprehensive experimentaldataset, shows that it suitably predicts elastic properties at different length scales under the assumption ofundrained conditions [3,4]. Moreover, Biot tensors, expressing how much of the cell wall water-induced porepressure is transferred to the boundary of an overall deformation-free representative volume element (RVE),and Biot moduli, expressing the porosity changes invoked by a pore pressure within such an RVE can bestudied at different length scales. Consequently, the relevance and the contribution of specificmicrostructural characteristics to the load transfer and the deformation characteristics in case of moisturechanges in wood can be studied. Besides the scientific interest in structure-function-relationships, theseinvestigations are motivated by the growing importance of wood as building material.

  • 87.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    de Borst, Karin
    University of Glasgow, UK.
    Shear stiffness and its relation to the microstructure of 10 European and tropical hardwood species2017Inngår i: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 12, nr 2, s. 82-91Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this study, shear stiffness properties of 10 different hardwood species and their relation to the corresponding species-specific microstructure are investigated. For this purpose, shear stiffness of 10 different hardwood species is experimentally measured by means of ultrasonic testing. In addition, a micromechanical model for hardwood is applied in order to illustrate the influence of certain microstructural characteristics such as mass density and volume fractions of vessels and ray cells on the shear stiffness. Comprehensive microstructural and mechanical data from previous investigations of the same hardwood material support the interpretation of the microstructure–shear stiffness relationships. Mass density was confirmed to be the dominant microstructural characteristic for shear stiffness. Also, ultrasound shear wave propagation velocity increases with density, particularly in the radial-tangential (RT) plane. In addition to density, comparably higher shear stiffness GLR can be explained by comparably higher ray content and lower vessel content. As for GLT, a ring porous structure seems to lead to higher shear stiffness as compared to a diffuse porous structure. For this shear stiffness, vessel and ray content were found to have a less impact. Also, the rolling shear stiffness GRT was found to be higher for a diffuse porous structure than for a ring porous one. Moreover, the data supports that ray cells act as reinforcements in the RT plane and lead to higher GRT

  • 88. Bader, Thomas K.
    et al.
    Hofstetter, Karin
    Technische Universität, Austria.
    Pilzabbau von Holz: Quantifizierung des Steifigkeitsverlusts auf Basis von mikromechanischen Überlegungen2010Inngår i: Wiener Holzschutztage 2010: 25. - 26. November 2010, Wien, Wien: Wiener Holzschutztage , 2010, Vol. 28, s. 50-55Konferansepaper (Annet vitenskapelig)
    Abstract [de]

    Pilzbefall bewirkt eine Zersetzung des Materials durch Mikroorganismen und damit unweigerlich auch eine Veränderung des mechanischen Verhaltens von Holz. Die Auswirkung der mikrostrukturellen Änderungen auf makroskopisch beobachtbare mechanische Materialkennwerte wie Steifigkeit und Festigkeit lassen sich mittels Mehrskalenmodellierung abschätzen und quantifizieren. Die Eignung solcher Mehrskalenmodelle als Prognosewerkzeuge für Dauerhaftigkeitsbetrachtungen wurde im WoodWisdom-Netzwerk „WoodExter“ im Rahmen eines umfangreichen Testprogramms untersucht. Es wurde dabei sowohl ein Braunfäule verursachender Pilz (Gloephyllum trabeum) als auch ein Weißfäule verursachender Pilz (Trametes versicolor) verwendet. Die Vorgehensweise sowie die gewonnenen Einblicke und Erkenntnisse sind in diesem Beitrag zusammengefasst. Nach einer Kurzbeschreibung der hierarchischen Struktur von Holz und deren Modifikation durch Pilze folgt ein Abriss über die verwendeten Verfahren der Mehrskalenmodellierung. Der Schwerpunkt liegt in der Präsentation der Ergebnisse sowie der experimentellen Validierung des Modells durch Vergleich von Messwerten mit zugehörigen Modellvorhersagen. Schließlich werden im Ausblick mögliche Anwendungen der Modellierung skizziert. 

  • 89.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Alfredsen, Gry
    Norwegian Forest and Landscape Institute, Norway.
    Bollmus, Susanne
    Georg-August-University of Göttingen, Germany.
    Changes in microstructure and stiffness of Scots pine (Pinus sylvestris L) sapwood degraded by Gloeophyllum trabeum and Trametes versicolor Part II: Anisotropic stiffness properties2012Inngår i: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 66, nr 2, s. 199-206Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Fungal decay considerably affects the macroscopic mechanical properties of wood as a result of modifications and degradations in its microscopic structure. While effects on mechanical properties related to the stem direction are fairly well understood, effects on radial and tangential directions (transverse properties) are less well investigated. In the present study, changes of longitudinal elastic moduli and stiffness data in all anatomical directions of Scots pine (Pinus sylvestris) sapwood which was degraded by Gloeophyllum trabeum (brown rot) and Trametes versicolor (white rot) for up to 28 weeks have been investigated. Transverse properties were found to be much more deteriorated than the longitudinal ones. This is because of the degradation of the polymer matrix between the cellulose microfibrils, which has a strong effect on transverse stiffness. Longitudinal stiffness, on the other hand, is mainly governed by cellulose microfibrils, which are more stable agains fungal decay. G. trabeum (more active in earlywood) strongly weakens radial stiffness, whereas T. versicolor (more active in latewood) strongly reduces tangential stiffness. The data in terms of radial and tangential stiffnesses, as well as the corresponding anisotropy ratios, seem to be suitable as durability indicators of wood and even allow conclusions to be made on the degradation mechanisms of fungi.

  • 90.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Alfredsen, Gry
    Norwegian Forest and Landscape Institute, Norway.
    Bollmus, Susanne
    Georg-August-University of Göttingen, Germany.
    Microstructure and stiffness of Scots pine (Pinus sylvestris L) sapwood degraded by Gloeophyllum trabeum and Trametes versicolor Part I: Changes in chemical composition, density and equilibrium moisture content2012Inngår i: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 66, nr 2, s. 191-198Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Fungal degradation alters the microstructure of wood and its physical and chemical properties are also changed. While these changes are well investigated as a function of mass loss, mass density loss and changes in equilibrium moisture content are not well elucidated. The physical and chemical alterations are crucial when linking microstructural characteristics with macroscopic mechanical properties. In the present article, a consistent set of physical, chemical and mechanical characteristics is presented, which were measured on the same sample before and after fungal degradation. In the first part of this two-part contribution, elucidating microstructure/stiffness-relationships of degraded wood, changes in physical and chemical data are presented, which were collected from specimens of Scots pine (Pinus sylvestris) sapwood degraded by Gloeophyllum trabeum (brown rot) and Trametes versicolor (white rot) for up to 28 weeks degradation time. A comparison of mass loss with corresponding mass density loss demonstrated that mass loss entails two effects: firstly, a decrease in sample size (more pronounced for G. trabeum), and secondly, a decrease of mass density within the sample (more pronounced for T. versicolor). These two concurrent effects are interrelated with sample size and shape. Hemicelluloses and cellulose are degraded by G. trabeum, while T. versicolor was additionally able to degrade lignin. In particular because of the breakdown of hemicelluloses and paracrystalline parts of cellulose, the equilibrium moisture content of degraded samples is lower than that in the initial state.

  • 91.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Keunecke, Daniel
    ETH Zürich, Switzerland.
    Microstructure–Stiffness Relationships of Common Yew and Norway Spruce2012Inngår i: Strain, ISSN 0039-2103, E-ISSN 1475-1305, Vol. 48, nr 4, s. 306-316Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Yew (Taxus baccata L.) exhibits among conifers a unique macroscopic elastic behaviour. For example, it shows a comparatively low longitudinal elastic modulus related to its comparatively high density. We herein explore the microstructural origin of these peculiarities, aiming at the derivation of microstructure–stiffness relationships. We measure stiffness properties of yew at different hierarchical levels and compare them to corresponding stiffnesses of Norway spruce (Picea abies [L.] Karsten). Cell wall stiffness is investigated experimentally by means of nanoindentation in combination with microscopy and thermogravimetric analysis. On the macroscopic level, we perform uniaxial tension and ultrasonic tests. Having at hand, together with previously reported stiffnesses, a consistent data set of mechanical, chemical and physical properties across hierarchical levels of wood, we discuss influences of microstructural characteristics at different scales of observation. Moreover, a micromechanical model is applied to predict trends of effects of the microstructure on the investigated stiffness properties. On the cell wall level, particularly, the amount of cellulose and its orientation – which was earlier reported to be distinctly different for yew and spruce – result in differences between the two considered species. On the macroscopic scale, model predicted effects of the annual ring structure on transverse stiffness and shear stiffness are found to be smaller than effects of the microfibril angle and mass density.

  • 92.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Hellmich, Christian
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    From lignin to spruce: Poromechanical upscaling of wood strength2011Inngår i: 2010 MRS Fall Meeting: Symposium V/NN/OO/PP – Soft Matter, Biological Materials and Biomedical Materials—Synthesis, Characterization and Applications / [ed] A.J. Nolte, K. Shiba, R. Narayan, D. Nolte, Warrendale, Pennsylvania, USA: Materials Research Society, 2011, Vol. 1301, s. 75-80Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Wood strength is highly anisotropic, due to the inherent structural hierarchy of the material. In the framework of a combined random-periodic multiscale poro-micromechanics model, we here translate compositional information throughout this hierarchy into the resulting anisotropic strength at the softwood level, based on “universal” elastic properties of cellulose, hemicelluloses, and lignin, and on the shear strength of the latter elementary constituent. Therefore, derivation of the elastic energy in a piece (representative volume element – RVE) of softwood, stemming from homogeneous macroscopic strains prescribed in terms of displacements at the boundary of the RVE and from pressure exerted by water filling the nanoporous space between the hemicelluloses-lignin network within the cell walls, with respect to the shear stiffness of lignin, yields higher order strains in the lignin phase, approximating micro-stress peaks leading to local lignin failure. Relating this (quasi-brittle) failure to overall softwood failure (or strictly speaking, elastic limit of softwood) results in a macroscopic microstructure-dependent failure criterion for softwood. The latter satisfactorily predicts the biaxial strength of spruce at various loading angles with respect to the grain direction. The model also predicts the experimentally well-established fact that uniaxial tensile and compressive strengths, as well as the shear strength of wood, depend quasi-linearly on the cell water content, but highly nonlinearly on the lumen porosity. 

  • 93.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Hellmich, Christian
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Homogenization and Localization in a Multiscale Microporomechanical Model for Wood Strength2009Konferansepaper (Annet vitenskapelig)
  • 94.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Hellmich, Christian
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Multiscale Microporomechanics Model for Estimation of Elastic Limit States of Softwood Materials2009Konferansepaper (Annet vitenskapelig)
  • 95.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Hellmich, Christian
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Multiscale Microporomechanics of Softwood: Applications and Experimental Model Validation2010Inngår i: IV European Conference on Computational Mechanics (ECCM 2010), Paris, France: European Community on Computional Methods in Applied Sciences (ECCOMAS), 2010Konferansepaper (Annet vitenskapelig)
  • 96.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Hellmich, Christian
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    On the Relevance of Lignin Failure for Softwood Strength: a Poromicromechanical Approach2011Inngår i: XI International Conference on Computational Plasticity - Fundamentals and Applications, Barcelona: International Center for Numerical Methods in Engineering (CIMNE), 2011Konferansepaper (Annet vitenskapelig)
  • 97.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Hellmich, Christian
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Poromechanical scale transitions of failure stresses in wood: from the lignin to the spruce level2010Inngår i: Zeitschrift für angewandte Mathematik und Mechanik, ISSN 0044-2267, E-ISSN 1521-4001, Vol. 90, nr 10-11, s. 750-767Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Wood strength is highly anisotropic, due to the inherent structural hierarchy of the material. In the framework of a combined random-periodic multiscale poro-micromechanics model, we here translate compositional information throughout this hierarchy into the resulting anisotropic strength at the softwood level, based on “universal” elastic properties of cellulose, hemicellulose, and lignin, and on the shear strength of the latter elementary constituent. Relating, through elastic energy-derived higher-order strains in a poromechanical representative volume element, the (quasi-)brittle failure of lignin to overall softwood failure, results in a macroscopic microstructure-dependent failure criterion for softwood. The latter satisfactorily predicts the biaxial strength of spruce at various loading angles with respect to the grain direction. The model also predicts the experimentally well-established fact that uniaxial tensile and compressive strengths, as well as the shear strength of softwood, depend quasi-linearly on the cell water content, but highly nonlinearly on the lumen porosity.

  • 98.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Hofstetter, Karin
    Vienna University of Technology, Austria.
    Hellmich, Christian
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    The poroelastic role of water in cell walls of the hierarchical composite “softwood”2010Inngår i: Acta Mechanica, ISSN 0001-5970, E-ISSN 1619-6937, Vol. 217, nr 1, s. 75-100Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Wood is an anisotropic, hierarchically organized material, and the question how the hierarchical organization governs the anisotropy of its mechanical properties (such as stiffness and strength) has kept researchers busy for decades. While the honeycomb structure of softwood or the chemical composition of the cell wall has been fairly well established, the mechanical role of the cell wall water is less understood. The question arises how its capability to carry compressive loads (but not tensile loads) and its pressurization state affect mechanical deformations of the hierarchical composite “wood”. By extending the framework of poro-micromechanics to more than two material phases, we here provide corresponding answers from a novel hierarchical set of matrix-inclusion problems with eigenstresses: (i) Biot tensors, expressing how much of the cell wall water-induced pore pressure is transferred to the boundary of an overall deformation-free representative volume element (RVE), and (ii) Biot moduli, expressing the porosity changes invoked by a pore pressure within such an RVE, are reported as functions of the material’s composition, in particular of its water content and its lumen space. At the level of softwood, where we transform a periodic homogenization scheme into an equivalent matrix-inclusion problem, all Biot tensor components are found to increase with decreasing lumen volume fraction. A further research finding concerns the strong anisotropy of the Biot tensor with respect to the water content: Transverse components increase with increasing water content, while the relationship “longitudinal Biot tensor component versus volume fraction of water within the wood cell wall” exhibits a maximum, representing a trade-off between pore pressure increase (increasing the longitudinal Biot tensor component, dominantly at low water content) and softening of the cell wall (reducing this component, dominantly at high water contents). Soft cell wall matrices reinforced with very stiff cellulose fibers may even result in negative longitudinal Biot tensor components. The aforementioned maximum effect is also noted for the Biot modulus.

  • 99.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Wikete, Christoph
    de Borst, Karin
    Elastic Properties of Hardwood at Different Length Scales Predicted by Means of a Micromechanical Model2012Inngår i: Proceedings of the 6th European Congress on Computational Methods in Applied Sciences and Engineering (ECCOMAS 2012), Vienna University of Technology, Vienna, Austria, Vienna, Austria, 2012Konferansepaper (Annet vitenskapelig)
  • 100.
    Bader, Thomas K.
    et al.
    Vienna University of Technology, Austria.
    Wikete, Christoph
    Jäger, Andreas
    Hofstetter, Karin
    Eberhardsteiner, Josef
    Mechanical Properties and Microstructural Characteristics of Hardwood2010Inngår i: COST Action FP 0802 Workshop: Wood Structure/Function-Relationships, 5-8 October, 2010, Hamburg, Germany, Hamburg, Germany, 2010, s. 61-Konferansepaper (Annet vitenskapelig)
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