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  • 1.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modelling stiffness and strength of non-crimp fabric composites: semi-laminar analysis2011In: Non-crimp fabrics composites: manufacturing, properties and applications, Cambridge: Woodhead Publishing Materials , 2011, p. 402-438Chapter in book (Refereed)
  • 2.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Olsson, Robin
    Swerea SICOMP AB, Mölndal.
    Multiscale modelling of non-crimp fabric composites2012In: 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, p. 581-590Conference paper (Refereed)
    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

  • 3.
    Bachinger, A.
    et al.
    Composite Structures, Swerea SICOMP AB.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. 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 protection2014In: 16th European Conference on Composite Materials, ECCM 2014: Seville, Spain, 22 - 26 June 2014, European Conference on Composite Materials, ECCM , 2014Conference paper (Refereed)
    Abstract [en]

    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.

  • 4.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: COST Action FP0802 Experimental and computational methods in wood micromechanics2009Conference paper (Other (popular science, discussion, etc.))
  • 5.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Micromechanism based material models for natural fiber composites2005Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The objective of Paper A was to develop a material model which includes all nonlinear viscoelastic phenomena observed in compressive tests on vinyl ester specimens using experimental data. The constitutive model developed by Schapery in the particular form previously presented by Megnis and Varna is used. The observed elastic behavior of thin specimens is explained based on parametric FEM analysis. The objective of Paper B was to use Schapery's model for nonlinear viscoelasticity and a power law for viscoelastic compliance to characterize the observed behavior for the material. The developed model has accuracy sufficient for practical applications. However, at high stresses the attempts to describe the viscoelastic compliance by a power law with a stress independent exponent were unsatisfactory and therefore stress dependence of this exponent was included in the data analysis. The objective of Paper C was to analyze the effect of geometrical parameters and constituent properties on effective properties of natural fiber composites. In order to do that we first develop an analytical model valid for orthotropic phase materials and for an arbitrary number of phases. This model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self- consistent approach.

  • 6.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mikromekanisk modell för beräkning av fiberegenskaper i träfiberkompositer2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 118-Conference paper (Other academic)
  • 7.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modeling the mechanical performance of natural fiber composites2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Due to environmental concerns the interest in use of renewable and recyclable materials has dramatically increased over the recent years. Wood and other lignocellulosic fiber reinforced polymers have large potential as structural materials due to the high specific stiffness, high specific strength and high aspect ratio of the fibers. Composites made from wood fiber mats from paper production are also interesting from an economical point of view. In present time the limited use of cellulosic fiber composites in structural design is predominantly associated with disadvantages such as dimensional instability in humid environments, lack of well defined fiber properties and the fibers low ability to adhere to common matrix materials for efficient stress transfer. A better understanding of dimensional stability and both long term and short term mechanical performance of cellulose fiber composites is necessary if these materials are to reach their full potential. The objective of the work presented in this thesis is twofold: (i) to present material models and suitable data reduction methodology with the ambition to characterize these materials very complex time dependent behavior (Paper A and B) and (ii) to develop micromechanical models that can be used in parametric studies of fiber properties and their influence on composite properties (Paper C-E). In Paper A the nonlinear viscoelastic behavior of flax/polypropylene composites was characterized using different forms of the creep compliance. The viscoplastic behavior was described using a nonlinear function with respect to time and stress. In Paper B hemp/lignin composites were characterized in terms of nonlinear viscoelastic behavior using Prony series form of creep compliance. The viscoplastic behavior was described using the same nonlinear function as in Paper A. The presented material model also included a stiffness degradation function based on previous strain history. An incremental form of the constitutive model was used to simulate the material behavior in loading and unloading ramps and validated through experiments. In Paper C the effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness was analyzed. An analytical model for an N-phased concentric cylinder assembly with orthotropic properties of constituents was developed and used. The model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. In Paper D the same concentric cylinder model was used and extended to include also free hygroexpansion terms in the elastic stress-strain relationship. The hygroelastic properties on three levels were calculated. Using material data for the wood polymers available from literature the swelling characteristics on the (i) ultrastructural level, i.e. the microfibril unit cell was determined; (ii) the hygroexpansion coefficients of the fiber cell wall layers were determined and finally (iii) the hygroexpansion coefficients of an aligned wood fiber composite were calculated. In Paper E the influence of helical fiber structure on composite properties was evaluated. The fibers helical structure leads to an extension-twist coupling and thus a free fiber will deform axially and also rotate upon loading in longitudinal fiber axis direction. Within the composite the fiber rotation will be restricted however. Therefore, the decision was to compare the elastic properties in two extreme cases on both fiber- and composite level: (i) free rotation and (ii) no rotation of the layers in the cylinder assembly.

  • 8.
    Marklund, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Multiscale methodology for matrix failure prediction in non-crimp fabric composites2010In: ECCM 14: 14th European Conference on Composite Materials ; June 7 - 10, 2010, Budapest, Hungary, Budapest: BUTE Dep. of Polymer Engineering , 2010Conference paper (Refereed)
    Abstract [en]

    This paper examines the possibility of performing a combined micro-meso approach to model transverse matrix failure within the bundle structure for non-crimp fabric composites. Failure initiation sites are predicted considering the discrete bundle structure, its volume fraction and fibre volume fraction within bundles, as well as the triaxial stress state built up from the combined contributions of mechanical loading and thermal/chemical shrinkage due to curing at elevated temperature. An analytical expression for average strain in a bundle with simplified geometry is used to link the micro- and meso length scales. The work presented here is intended to give further guidelines in the creation of a black-box modelling tool, which utilises traditional laminar analyses for non-crimp fabric composites consisting of layers having “effective” ply properties.

  • 9.
    Marklund, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Olsson, Robin
    Swerea SICOMP AB, Mölndal.
    Transverse strength of unidirectional non-crimp: multiscale Modelling2014In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 65, p. 47-56Article in journal (Refereed)
    Abstract [en]

    A multiscale approach is used to predict transverse tensile and transverse compressive strength of unidirectional non-crimp fabric (NCF) composites. Numerical analysis on fibre/matrix scale is performed to obtain the transverse strength of the fibre bundle to be further used in an analytical mesoscale model to predict the strength of the unidirectional NCF composite. Design of unidirectional layer composites with the same fibres, interface, matrix and volume fractions as in the bundle is suggested as an alternative method for bundle strength determination. Good agreement of both methods for bundle transverse strength determination is demonstrated. The simple analytical model used on mesoscale gives accurate predictions of the tensile transverse strength whereas the compressive strength is underestimated. The necessity of including bundle waviness in models when bidirectional NCF composites are analyzed is demonstrated by FEM stress analysis and by experimental data showing differences in transverse cracking pattern due to bundle waviness.

  • 10. Marklund, Erik
    et al.
    Eitzenberger, Johannes
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nonlinear viscoelastic viscoplastic material model including stiffness degradation for hemp/lignin composites2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 9, p. 2156-2162Article in journal (Refereed)
    Abstract [en]

    In repeating tensile tests with increasing maximum strain for every loading cycle the hemp/lignin composites clearly showed a nonlinear behavior and hysteresis loops in loading and unloading. The explanation for this behavior is the inherent viscoelastic nature for this type of material, but also noticeable stiffness degradation with increasing strain level. Creep tests performed at different stress levels revealed a nonlinear viscoelastic response and after recovery viscoplastic strain was detected for high stress levels. It is demonstrated that Schapery's model is suitable to model nonlinear viscoelasticity whereas viscoplastic strain may be described by a nonlinear functional presented by Zapas and Crissman. In a creep test this functional leads to a power law with respect to time and stress. In order to include stiffness reduction due to damage Schapery's model has been modified by incorporating a maximum strain-state dependent function reflecting the elastic modulus reduction with increasing strain measured in tensile tests. A generalized incremental model of the constitutive equation for viscoelastic case has been used to validate the developed material model in a linear stress controlled loading and unloading ramp. The model successfully describes the main features for the investigated material and shows good agreement with test data within the considered stress range.

  • 11.
    Marklund, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nilsson, Sören
    Swerea SICOMP AB.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Matrix failure prediction in polymer composites including effects of temperature and degree of cure2011In: Composites 2011: ECCOMAS Thematic Conference, 3rd International Conference on the Mechanical Response of Composites, Hannover, 21 - 23 September, 2011 / [ed] R. Rolfes; E.L. Jansen, Hannover: LUH, Institut für Statik und Dynamik , 2011, p. 579-586Conference paper (Other academic)
  • 12. Marklund, Erik
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Micromechanical modeling of wood fiber composites2008In: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Conference paper (Refereed)
    Abstract [en]

    An analytical concentric cylinder model for an N-phase composite with orthotropic properties of constituents was previously presented by the authors. The model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. With only minor modifications the model allows for including also free hygroexpansion terms in the elastic stress-strain relationship to deal with orthotropic phase swelling. Thus the effect of wood fiber ultrastructure and cell wall hygroelastic properties on wood fiber composite hygroexpansion can be analyzed. Using properties available from literature on the three main wood polymers, cellulose, hemicellulose and lignin multiscale modeling was performed to calculate the hygroexpansion coefficients of the fiber cell wall and an aligned wood fiber composite. The fiber cell wall was modeled regarding each individual layer S1, S2 and S3 as a balanced and symmetric laminate since it was assumed that the fiber will be restricted from bending and rotation within the composite. Regarding the fiber cell wall as a balanced and symmetric laminate enables us to calculate "apparent" fiber properties when rotation is not allowed. In reality the fiber's helical structure leads to an extension-twist coupling and thus a free fiber will deform axially and also rotate upon loading in longitudinal fiber direction making the response more compliant. Within the composite the fiber rotation will be restricted however. Therefore, the decision was also to compare the two extreme cases (i) free rotation and (ii) no rotation of the fiber in the composite.

  • 13.
    Marklund, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Micromechanical modelling of wood fibre composites2009In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 38, no 2, p. 118-123Article in journal (Refereed)
    Abstract [en]

    A concentric cylinder model for an N-phase composite with orthotropic properties of constituents was previously presented by the authors. With only minor modifications the model allows for including also free hygroexpansion terms in the elastic stress-strain relationship in order to deal with orthotropic phase swelling. Thus the effect of wood fibre ultrastructure and cell wall hygroelastic properties on wood fibre composite hygroexpansion may be analysed. Multiscale modelling was performed to calculate the hygroexpansion coefficients of both the fibre cell wall and the aligned wood fibre composite. Furthermore, the fibre's helical structure leads to an extension-twist coupling and thus a free fibre will deform axially and also rotate upon loading in longitudinal fibre direction. Within the composite, however, the fibre rotation will be restricted. Therefore, the decision was to compare the composite performance in the two extreme cases (i) free rotation (ii) no rotation of the fibre in the composite.

  • 14. Marklund, Erik
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modeling the effect of helical fiber structure on wood fiber composite elastic properties2009In: Applied Composite Materials, ISSN 0929-189X, E-ISSN 1573-4897, Vol. 16, no 4, p. 245-262Article in journal (Refereed)
    Abstract [en]

    The effect of the helical wood fiber structure on in-plane composite properties has been analyzed. The used analytical concentric cylinder model is valid for an arbitrary number of phases with monoclinic material properties in a global coordinate system. The wood fiber was modeled as a three concentric cylinder assembly with lumen in the middle followed by the S3, S2 and S1 layers. Due to its helical structure the fiber tends to rotate upon loading in axial direction. In most studies on the mechanical behavior of wood fiber composites this extension-twist coupling is overlooked since it is assumed that the fiber will be restricted from rotation within the composite. Therefore, two extreme cases, first modeling fiber then modeling composite were examined: (i) free rotation and (ii) no rotation of the cylinder assembly. It was found that longitudinal fiber modulus depending on the microfibril angle in S2 layer is very sensitive with respect to restrictions for fiber rotation. In-plane Poisson’s ratio was also shown to be greatly influenced. The results were compared to a model representing the fiber by its cell wall and using classical laminate theory to model the fiber. It was found that longitudinal fiber modulus correlates quite well with results obtained with the concentric cylinder model, whereas Poisson’s ratio gave unsatisfactory matching. Finally using typical thermoset resin properties the longitudinal modulus and Poisson’s ratio of an aligned softwood fiber composite with varying fiber content were calculated for various microfibril angles in the S2 layer.

  • 15.
    Marklund, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Modeling the hygroexpansion of aligned wood fiber composites2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 7-8, p. 1108-1114Article in journal (Refereed)
    Abstract [en]

    The effect of wood fiber ultrastructure and cell wall hygroelastic properties on wood fiber composite hygroexpansion has been analyzed. An analytical concentric cylinder model extended to include also free hygroexpansion of orthotropic phase materials has been used on several length scales. Using properties of the three main wood polymers, cellulose, hemicellulose and lignin the longitudinal and transverse hygroexpansion coefficients for the microfibril unit cell were obtained and the volume fraction change of the wood polymers in the microfibril unit cell depending on relative humidity was calculated. The fiber cell wall was modeled regarding each individual S1, S2 and S3 layer and the cell wall longitudinal hygroexpansion coefficient was determined depending on microfibril angle in the S2 layer. A homogenization procedure replacing the S1, S2 and S3 layers with one single layer was found not to influence the results significantly for low microfibril angles. Finally the hygroexpansion coefficients of an aligned softwood fiber composite were calculated.

  • 16.
    Marklund, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Nonlinear viscoelasticity and viscoplasticity of polypropylene/flax fibre composites2005In: Theplac 2005: International Workshop on Thermoplastic Matrix Composites, 2005Conference paper (Refereed)
  • 17.
    Marklund, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage progression in non-crimp fabric composites2011In: Non-crimp fabrics composites: manufacturing, properties and applications, Cambridge: Woodhead Publishing Materials , 2011Chapter in book (Refereed)
  • 18.
    Marklund, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stiffness and strength modelling of non-crimp fabric composites2011In: 52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 2011, Red Hook: Curran Associates, Inc., 2011, Vol. 1, p. 679-695Conference paper (Refereed)
    Abstract [en]

    This work comprises methodologies for micro-meso stiffness modelling and limited analysis for strength prediction. It is intended to formulate some guidelines and recommendations when modelling NCF composites mechanical performance. For both micro- and meso stiffness modelling, analytical models are compared to FE investigations considering idealised structures. Important aspects when modelling matrix failure of NCF composite bundles are presented to highlight some of the challenges in future modelling. Possible failure criteria for modelling matrix failure within fibre bundles have been investigated; their strength, problems and weaknesses are revealed.

  • 19. Marklund, Erik
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Neagu, R. Cristian
    Laboratoire de Technologie des Composites et Polymères (LTC) Ecole Polytechnique Fédérale de Lausanne.
    Gamstedt, E. Kristofer
    STFI-Packforsk AB.
    Stiffness of aligned wood fiber composites: effect of microstructure and phase properties2008In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 42, no 22, p. 2377-2405Article in journal (Refereed)
    Abstract [en]

    The effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness is analyzed. An analytical model for an N-phase composite with orthotropic properties of constituents is developed and used. This model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. It was found that most macro-properties are governed by only one property of the cell wall which is very important in attempts to back-calculate the fiber properties. The role of lumen (whether it filled by resin or not) has a very large effect on the composite shear properties. It is shown that several of the unknown anisotropic constants characterizing wood fiber are not affecting the stiffness significantly and rough assumptions regarding their value would suffice. The errors introduced by application of the Hashin's model and neglecting the orthotropic nature of the material behavior in cylindrical axes are evaluated. The effect of geometrical deviations from circular cross-section, representing, for example, collapsed fibers, is analyzed using the finite element method (FEM) and the observed trends are discussed.

  • 20. Marklund, Erik
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Wallström, Lennart
    Nonlinear viscoelasticity and viscoplasticity of flax/polypropylene composites2006In: Journal of engineering materials and technology, ISSN 0094-4289, E-ISSN 1528-8889, Vol. 128, no 4, p. 527-536Article in journal (Refereed)
    Abstract [en]

    In tensile tests the flax/polypropylene composites clearly show nonlinear behavior in loading and hysteresis loops in unloading. In creep tests performed at different load levels the response was nonlinear viscoelastic, and after recovery, viscoplastic strains were detected. No degradation in stiffness could be seen and thus nonlinear viscoelasticity and viscoplasticity were assumed to be the main cause for the observed behavior. The fracture surface of a specimen that experienced creep rupture at 24 MPa was investigated using a scanning electron microscope. The viscoplastic response was studied experimentally and described by a power law with respect to time and stress level in the creep test. The nonlinear viscoelasticity was described using Schapery's model. The application of Prony series and a power law to approximate the viscoelastic compliance was investigated. Both descriptions have accuracy sufficient for practical applications. However, at high stresses the attempts to describe the viscoelastic compliance by a power law with a stress-independent exponent failed and therefore stress dependence of this exponent was included in the data analysis. The accuracy within the considered stress range is good, but the thermodynamic consistency of this procedure has to be proven.

  • 21. Nordin, Lars-Olof
    et al.
    Marklund, Erik
    Ståhlberg, Daniel
    Royal Institute of Technology, Fibre & Polymer Technology, Stockholm.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mats, Johansson
    Royal Institute of Technology, Fibre & Polymer Technology, Stockholm.
    Mechanical response of thermoset polymers under high compressive loads, 22005In: Macromolecular materials and engineering (Print), ISSN 1438-7492, E-ISSN 1439-2054, Vol. 290, no 11, p. 1073-1082Article in journal (Refereed)
    Abstract [en]

    A nonlinear viscoelastic material model was used to describe the experimental behaviour of thin vinyl ester specimens subjected to compression in thickness direction. The stress-dependent material functions in the model were found in creep and strain recovery tests on thick cylindrical specimens. The elastic and creep response of thin thermoset polymer specimens subjected to compressive loads was simulated while varying the geometry of the test set samples. The calculated increase in the apparent elastic modulus and decrease of the creep-strain rate due to reduced thickness-to-width ratio is in a good qualitative correlation with experimental results for corresponding geometries. The constraint due to friction and interaction with the material outside the loaded surface area were identified as the cause for high apparent stiffness, which converges with decreasing thickness to an asymptotic value dependent on the modulus and Poisson's ratio of the material.

  • 22.
    Olsson, Robin
    et al.
    Swerea SICOMP AB.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Jansson, Niklas
    Volvo Aero Corporation.
    Strength of NCF composite bundles under biaxial stress2011In: Composite materials for structural performance: towards higher limits: of the 32nd Risø International Symposium on Materials Science ; 5 - 9 September 2011, Roskilde, Denmark / [ed] S. Fæster, Roskilde: Risø National Laboratory for Sustainable Energy, Danmarks Tekniske Universitet , 2011Conference paper (Refereed)
  • 23.
    Olsson, Robin
    et al.
    Swerea SICOMP AB, Mölndal.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Jansson, N.
    Cold Structures, 9641NJ, Volvo Aero Corporation.
    Testing of carbon/epoxy NCF strength under mixed in-plane loading2012In: Proceedings of the 15th European Conference on Composite Materials / [ed] Marino Quaresimin; Laszlo Kollar; Leif Asp, Venice, 2012Conference paper (Refereed)
    Abstract [en]

    The measured stiffness and strength of a carbon/epoxy unidirectional NCF system in shear, tension and compression are compared with test results for the pure resin and for impregnated bundle material under various combinations of in-plane compressive and tensile loading. The study is a part of a project to develop mesomechanics models to predict failure of NCF materials under triaxial loading by use of data for the pure resin and for bundles impregnated by resin. A simplified analytical rule-of-mixtures model is suggested for stiffness and strength of the NCF material. Good agreement is shown for shear and tension along and transverse to the bundles. Compressive strengths are significantly underestimated, apparently due to deficiencies in the compressive test method used for the bundle material.

  • 24.
    Zrida, Hana
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, Nancy Universite, Science et Ingénierie des Matériaux et Métallurgie (SI2M), Institut Jean Lamour, Nancy, Laboratoire de Science et Génie des Surfaces, EEIGM, Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy, Institut Jean Lamour, University of Lorraine, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effective stiffness of curved 0°-layers for stiffness determination of cross-ply non-crimp fabric composites2014In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 33, no 14, p. 1339-1352Article in journal (Refereed)
    Abstract [en]

    The effect of the 0°-tow waviness on axial stiffness of cross-ply non-crimp fabric composites is analysed using multiscale approach. The curved 0°- and 90°-layers are represented by flat layers with effective stiffness properties and classical laminate theory is used to calculate the macroscopic stiffness. The effective 0°-layer stiffness is calculated analysing isolated curved 0°-layers subjected not only to end loading, but also to surface loads. The surface loads are identified in a detailed finite element analysis and approximated by a sinus shaped function with amplitude depending on the waves parameters. The sinus shaped surface loads are then applied to an isolated curved 0°-layer finite element model together with end loading to calculate the effective stiffness of the layer. Finally, the effective 0°-layer stiffness was successfully used to calculate the macroscopic stiffness of the composite proving validity of the approach being used and showing that, without losing accuracy, elastic properties in the 90°-layers with bundle structure can be replaced by the transverse stiffness of the homogenised 90°-layer material.

  • 25.
    Zrida, Hana
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour, SI2M, EEIGM 6 Rue Bastien Lepage, F-54010 Nancy.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Master curve approach to axial stiffness calculation for non-crimp fabric biaxial composites with out-of-plane waviness2014In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 64, p. 214-221Article in journal (Refereed)
    Abstract [en]

    The effect of 0∘-tow out-of-plane waviness on the biaxial Non-Crimp-Fabric (NCF) composite axial stiffness is investigated. Homogenizing, the bundle mesostructure of the NCF composite is replaced by layers. Then the composite is represented by a laminate with flat layers with effective stiffness properties representing the curved 0∘-layer and the 90∘-layer with varying thickness. It is shown that the NCF composite knock-down factor characterizing the stiffness degradation has almost the same dependence on wave parameters as the knock-down factor for the curved 0∘-layer. Numerical analysis showed that 90∘-layer knock-down factor versus amplitude curves for different wavelength can be reduced to one master curve which can be described by a one-parameter expression with the parameter dependent on the used material. This observation is used to obtain high accuracy for analytical predictions for knock-down factors for cases with different wavelength and amplitudes based on two FE calculations only.

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