Change search
Refine search result
123456 101 - 150 of 296
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 101.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubrir
    Caractérisation de l’endommagement des matériaux composites pour l’aéronautique à partir des mesures plein champ des déplacements et modélisation2012Conference paper (Refereed)
  • 102.
    Loukil, M.S.
    et al.
    RISE SICOMP, Linköping. Department of Management and Engineering, Linköping University, Linköping..
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Crack face sliding displacement (CSD) as an input in exact GLOB-LOC expressions for in-plane elastic constants of symmetric damaged laminates2019In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921Article in journal (Refereed)
    Abstract [en]

    The crack opening and crack sliding displacements of both faces of an intralaminar crack are the main parameters defining the significance of each crack in laminate stiffness degradation, according to the previously published GLOB-LOC approach for symmetric laminates with an arbitrary number of cracks in all plies. In the exact stiffness expressions of this approach, the crack density is always multiplied by crack opening displacement and crack sliding displacement. The dependence of crack opening displacement on geometrical and elastic parameters of adjacent plies was studied previously and described by simple fitting functions. The crack sliding displacement has been analyzed for low-crack densities only and the proposed finite element method-based fitting expressions are oversimplified not including the out-of-plane ply stiffness effects. Based on finite element method analysis, more accurate expressions for so-called non-interactive cracks are suggested in the presented article. For the first time the shear stress perturbations are analyzed and interaction functions are presented with the feature that they always lead to slightly conservative predictions. The presented simple fitting functions, when used in the GLOB-LOC model, give predictions that are in a good agreement with finite element method results and with experimental data for laminates with damaged off-axis plies in cases when crack face sliding is of importance. The significance of including crack sliding displacement in stiffness predictions is demonstrated.

  • 103.
    Lundmark, P.
    et al.
    Luleå tekniska universitet.
    Katerelos, D.T.G.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Galiotis, C.
    Raman spectroscopy assesment of matrix cracking results in GFRP laminates2006In: Fracture of Nano and Engineering Materials and Structures: Proceedings of the 16th European Conference of Fracture, Alexandroupolis, Greece, July 3-7, 2006 / [ed] E. E. Gdoutos, Encyclopedia of Global Archaeology/Springer Verlag, 2006, Vol. Paper 333, p. 1277-1285Conference paper (Refereed)
  • 104.
    Lundmark, P.
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Constitutive relationships for laminates with ply cracks in in-plane loading2005In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 14, no 3, p. 235-259Article in journal (Refereed)
    Abstract [en]

    A theoretical framework which allows determining the whole set of 2-D thermomechanical constants of a damaged laminate as a function of crack density in different layers is presented. In this approach, closed-form expressions, which contain thermoelastic ply properties, laminate layup, and crack density as the input information are obtained. It is shown that the crack opening displacement (COD) and crack face sliding displacement, normalized with respect to a load variable, are important parameters in these expressions influencing the level of the properties degradation. They are determined in this paper using generalized plain strain FEM analysis results for noninteractive cracks. The strong dependence of the COD on the relative stiffness and thickness of the surrounding layers, found in this study, is described by a power law. The methodology is validated and the possible error introduced by the noninteractive crack assumption is estimated by comparing with the 3-D FEM solution for a cross-ply laminate with two orthogonal systems of ply cracks. Experimental data and comparison with other models are used for further verification.

  • 105.
    Lundmark, P.
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modeling thermo-mechanical properties of damaged laminates2003In: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 251-252, p. 381-387Article in journal (Refereed)
    Abstract [en]

    In this paper we present a theoretical framework which allows us to determine the whole set of thermo-mechanical constants of the damaged laminates a function of crack density in different layers. In this approach closed form expressions are obtained which contains the ply properties, laminate lay-up and crack density as the input information. It is shown that the crack face opening and crack face sliding displacement normalized with respect to load are important parameters in these expressions affecting the level of properties degradation. They are determined in this paper using generalized plain strain FEM analysis results for non-interactive cracks. The methodology is validated and the possible error introduced by the non-interactive crack approach is estimated comparing with the 3-D FEM solution for cross-ply laminate with two systems of orthogonal cracks in plies. A limited number of experimental data are presented and used for further verification. [Material: GF/EP.]

  • 106.
    Lundmark, Peter
    et al.
    Swerea SICOMP AB.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage evolution and characterisation of crack types in CF/EP laminates loaded at low temperatures2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 9, p. 2631-2641Article in journal (Refereed)
    Abstract [en]

    Tensile testing of CF/EP AS4/8552 cross-ply laminates at room (RT) and cryogenic (around -150 deg C) temperatures has been performed to study the effect of temperature on damage (intralaminar cracking) evolution. Microscopy studies of the specimen edges showed a significant difference in damage pattern for the two different temperatures. At the low temperature (LT), more complex crack types were obtained that could not be found in specimens tested at the RT. The effect of these crack types on the laminate tensile modulus was studied by FEM. In analytical stiffness modelling complex shape crack was replaced by an 'effective' normal (straight) crack with an 'effective' crack opening displacement (COD) that leads to the same reduction in laminate stiffness. A crack efficiency factor was introduced to characterize the significance of complex crack shapes for stiffness reduction. The reduction of tensile modulus for a laminate damaged at low temperature was measured and compared with model predictions.

  • 107.
    Lundmark, Peter
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stiffness reduction in laminates at high intralaminar crack density: effect of crack interaction2011In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 20, no 2, p. 279-297Article in journal (Refereed)
    Abstract [en]

    The previously developed closed form expressions for thermo-elastic properties of laminates with intralaminar cracks (Lundmark and Varna, 2005) contain crack surface opening and sliding as main local parameters. The dependence of these parameters on various material and geometrical characteristics was in (Lundmark and Varna, 2005) described by power functions valid only for noninteractive cracks in a given layer (low crack density). In this article the 90-layer crack interaction in terms of its effect on the crack opening displacement (COD) is discussed. The effect on COD is described by the introduced ‘interaction function' which is determined fitting results of finite element (FE) analysis for cross-ply laminates. To simplify the application in stiffness predictions, the numerically found weak dependence of the interaction on geometrical and material parameters is neglected and the interaction function is presented as a function of crack density only. Using the interaction function to determine the COD, the previously developed calculation scheme (Lundmark and Varna, 2005) has been used to predict stiffness reduction in the entire crack density region. The results are validated comparing with tests and FE simulations.

  • 108.
    Mannberg, Peter
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Giannadakis, Konstantinos
    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.
    Jakovics, Andris
    University of Latvia.
    Moisture absorption and degradation of Glass fiber/Vinylester composites2010In: Selected papers of the International Scientific Colloquium "Modelling for Material Processing": Riga, Latvia, 16 - 17 September, 2010 / [ed] A. Jakovics, Salaspils: University of Latvia Press, 2010, p. 157-162Conference paper (Refereed)
    Abstract [en]

    Vacuum Infusion is a closed mould manufacturing process that can replace open mould processes. To improve resin flow it is common to add flow enhancement layers to the reinforcement stack. This paper is aimed to analyze water uptake in these materials and its effect on mechanical performance. Ageing data for 3000 h in 90ºC water are presented for GF NCF/VE composite without flow layer and three composites with it. Tensile tests demonstrate degradation of mechanical properties and increasing viscoelastic and viscoplastic strains in wet composites.

  • 109. 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.

  • 110. 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.

  • 111.
    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.

  • 112. 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.

  • 113.
    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.

  • 114.
    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)
  • 115.
    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)
  • 116.
    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.

  • 117. 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.

  • 118. 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.

  • 119. Mattsson, David
    et al.
    Joffe, Roberts
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage in NCF composites under tension: effect of layer stacking sequence2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 9, p. 2666-2682Article in journal (Refereed)
    Abstract [en]

    Non-crimp fabric (NCF) cross-ply composites response to tensile loading is investigated showing large effect of the fabric layer stacking sequence: much larger elastic modulus reduction was observed in [0/90/0/90]S than in [90/0/90/0]S case. Since transverse cracks in 90°-bundles may give modulus decrease about 5%, the observed 40% stiffness reduction is attributed to failure and delamination of bundles oriented in the direction of the applied load. Analysis of micrographs shows extensive delaminations and 0°-bundle breaks. FE calculations showed that failure of 0°-bundles at the surface is energetically more favorable. However, the fracture resistance of surface bundles is higher due to smaller bundle waviness and the density of bundle cracks on the surface was not larger than inside. Two possible reasons for the higher stiffness reduction in the [0/90/0/90]S NCF composite were suggested: (a) If two imperfect 0°-bundle layers are separated by a 90°-bundle layer their resistance to failure is lower than when they are situated next to each other; (b) the effect of each surface 0°-bundle break on the composite stiffness is larger (due to less constraint from the surrounding material the opening of surface bundle breaks is much larger).

  • 120. Mattsson, David
    et al.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Methodology for characterization of internal structure parameters governing performance in NCF composites2007In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 38, no 1, p. 44-57Article in journal (Refereed)
    Abstract [en]

    Mechanical performance of non-crimp fabric composites is very dependent on their internal meso- and micro-structure which is defined by the manufacturing process of the fabric and composite processing conditions. This paper identifies the most important parameters which control mechanical properties of these materials. The identification is based on experimental observations and available theoretical findings. Characteristics of the internal structure of non-crimp fabric composites are analyzed in context of their significance for in-plane elastic and failure properties. Methodology for determination of most typical geometrical parameters of composites using optical observations of cross-sections of manufactured laminates is described. The methodology is applied to characterize cross-ply and quasi-isotropic composite laminates. These results are analyzed and a comparison between the laminates is performed.

  • 121. Mattsson, David
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Average strain in fiber bundles and its effect on NCF composite stiffness2007In: Journal of engineering materials and technology, ISSN 0094-4289, E-ISSN 1528-8889, Vol. 129, no 2, p. 211-219Article in journal (Refereed)
    Abstract [en]

    Transverse strain in bundles governs transverse cracking in noncrimp fabric (NCF) composites. Finite element (FE) analysis shows that this strain may be significantly lower than the applied macroscopic strain component in the same direction. This feature is important for damage evolution modeling. The isostrain assumption which in different combinations is widely used in stiffness models is inadequate because the strain in different mesoelements (bundles of different orientation and matrix regions) is assumed the same. Analyzing by FEM the importance of media surrounding the bundle on average transverse strain it was found that an increasing ratio of the bundle transverse stiffness to the matrix stiffness leads to a decrease of the strain in the bundle. An increase of the stiffness in the same direction in adjacent layers leads to an increase of the transverse strain in the bundle. Higher bundle volume fraction in the layer leads to larger transverse strain in the bundle. These trends are described by a power law and used to predict the average strain in bundles. The calculated H matrix which establishes the relationship between strains in the mesoelement and representative volume element strains is used to calculate the "effective stiffness" of the bundle. This effective stiffness is the main element in simple but exact expressions derived to calculate the stiffness matrix of NCF composites. Considering the three-dimensional (3D) FE model as the reference, it was found that all homogenization methods used in this study have sufficient accuracy for stiffness calculations, but only the presented method gives reliable predictions of strains in bundles.

  • 122. Mattsson, David
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Formation of damage and its Effect on Non-crimp Fabric Reinforced Composites Loaded in Tension2004In: MACM 2004 conference proceedings: Proceedings of the Tenth International Conference on Marine Applications of Composite Materials, Composites education association , 2004Conference paper (Refereed)
  • 123. Mattsson, David
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, A.
    Stiffness degradation in Non-Crimp fabric composite in tension due to damage2004In: 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 , 2004Conference paper (Refereed)
  • 124.
    Megnis, Modris
    et al.
    Luleå tekniska universitet.
    Olsson, Tomas
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lindberg, Henrik
    Luleå tekniska universitet.
    Mechanical performance of linseed oil impregnated pine as correlated to the take-up level2002In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 36, no 1, p. 1-18Article in journal (Refereed)
    Abstract [en]

    The mechanical performance of pine sapwood (pinus sylvestris), impregnated with linseed oil to different take-up levels, is evaluated using several test methods. SEM is used to study morphological changes following the impregnation procedure. The reduction of mechanical properties is attributed to a) localized cell wall damage in the ray region that facilitates longitudinal inter-cell split in L-R plane (macrocrack) initiation and propagation; b) submicroscopical cracking in the S1 sublayer that reduces the resistance to Mode I and Mode II inter-cell splitting at any location where the oil front has passed. Mechanical testing shows the following effect of the impregnation on failure a) the Mode I fracture toughness GIc in L-T and L-R planes, determined in DCB test, is significantly lowered with no significant difference in fracture resistance reduction in between planes; b) 3-point flexural test for specimen geometry leading to cracking in R-L and T-L planes show that the flexural strength as well as flexural modulus are reduced due to impregnation; c) 3-point flexural tests on longitudinal specimens used to determine the impregnation effect on longitudinal modulus EL and shear moduli GLT and GLR, reveal only minor changes. Fracture surfaces in mechanical tests are analyzed using SEM, and differences are explained by described microdamage mechanisms.

  • 125.
    Megnis, Modris
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Micromechanics based modeling of nonlinear viscoplastic response of unidirectional composite2003In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 63, no 1, p. 19-31Article in journal (Refereed)
    Abstract [en]

    Schapery's nonlinear viscoelastic, viscoplastic material model is used to characterize the inelastic response of glass fiber epoxy composite (Vicotex NVE 913/28%/192/EC9756 300MM produced by HEXCEL Composites Ltd.). Since the viscoelastic response is found to be linear, the nonlinearity is included only in viscoplastic terms. The micromechanical Hashin's Concentric Cylinder Assembly model in viscoelastic formulation is used to predict the viscoplastic behavior of the composite using matrix viscoelasticity. The time and stress dependence of viscoplastic strains is obtained from off-axis creep tests as difference between experimental measured creep strains and predicted linear viscoelastic strains. Viscoplastic shear strain in material symmetry axes in identified as the main source of viscoplasticity. Two master curves are determined that describe the viscoplastic shear strain dependence on time and stress correspondingly. The developed material model is validated in creep tests on [0 sub n 90 sub m ] sub s and [plus /minus45 sub n ] sub s specimens.

  • 126.
    Megnis, Modris
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nonlinear viscoelastic, viscoplastic characterization of unidirectional GF/EP composite2003In: Mechanics of time-dependant materials, ISSN 1385-2000, E-ISSN 1573-2738, Vol. 7, no 3-4, p. 269-290Article in journal (Refereed)
    Abstract [en]

    Viscoplastic strains of unidirectional continuous fiber composite (HEXCEL GF/EPprepreg system) are studied experimentally and theoretically. Creep and strainrecovery tests are used. Schapery's nonlinear viscoelastic viscoplasticconstitutive equations are used and generalized to describe inelastic behavior ofunidirectional composite under isothermal creep and strain recovery conditions. Themethodology to quantify the viscoplastic strains with respect to applied stress isproposed. Viscoplastic strains of composite are described by plastic shear strain inmaterial symmetry axis. Assumptions has been used and validated that the functiondescribing the stress and time dependence of viscoplastic strain can be presented asa product of two, time and correspondingly stress dependent, master curves.

  • 127.
    Megnis, Modris
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Allen, David A.
    Texas A&M University.
    Holmberg, Anders
    Swedish Institute of Composites.
    Micromechanical modeling of viscoelastic response of GMT composite2001In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 35, no 10, p. 849-882Article in journal (Refereed)
    Abstract [en]

    Experimental studies have been performed to obtain creep compliance functions of polypropylene (PP) and Glass Mat reinforced Thermoplastics (GMT) with PP matrix. It was found that both GMT and PP in the considered loading region may be considered as linear viscoelastic materials. The obtained viscoelastic compliance functions were successfully used to describe material behavior in the stress relaxation test. A micromechanical model based on the correspondence principle in the Laplace domain was developed to describe the viscoelastic behavior of GMT. This model considers the GMT composite with a given fiber orientation distribution function as consisting of an infinite number of unidirectional layers with orientations corresponding to this distribution function. The viscoelastic properties of the unidirectional layer are calculated using Hashin's concentric cylinder model that uses the experimentally determined viscoelastic properties of PP matrix. The predictions for GMT have been compared with experimental data. The model predicts rather good initial properties of GMT but it gives slightly less time dependence than compared to experimental data for both relaxation functions and compliance. The cause of the difference (debonding) between matrix and fiber, nonuniform fiber spatial distribution, stress concentrations etc.) is discussed.

  • 128.
    Mujika, F.
    et al.
    Departamento de Ingeniería Mecánica, Escuela de Ingeniería Técnica Industrial, Universidad del País Vasco/Euskal Herriko Unibertsitatea.
    Mondragon, I.
    Departamento de Ingeniería Química y del Medio Ambiente Escuela de Ingeniería Técnica Industrial, Universidad del País Vasco/Euskal Herriko Unibertsitatea.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    45° flexure test for measurement of in-plane shear modulus2002In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 36, no 20, p. 2313-2337Article in journal (Refereed)
    Abstract [en]

    A new method to obtain the in-plane shear modulus GLT for unidirectional oriented composite materials is proposed. The method is based on an original analytic way for calculating middle point displacement in a 3-point flexure test. The bending-twisting coupling effects in such a test induce the lift-off of the specimen at the fixture supports for some geometrical conditions. Thereby, contact points are located at two opposite points of the specimen. Consequently, new bending moments along the width of the specimen and twisting moments appear. By supposing resultant moments and shear forces per unit length are uniformly distributed, these distributions are calculated for static conditions along longitudinal and transverse cross sections of the specimen. After having expressed strain energy as a function of resultant moments and resultant shear forces per unit length, Second Castigliano's theorem is applied in order to calculate the middle point displacement. No similar analytic way has been encountered in classical laminated beams theory or in classical laminated plates theory. The displacement obtained in this work and the one obtained from classical laminated beams theory are particularised to the case of 45 fibre orientation. GLT expressions have been derived from those displacement expressions in three ways: two of them from the solution of this work, not considering and considering shear effects, respectively, and the third one from displacement obtained from classical laminated beams theory. Experiments have been made for different geometric conditions in order to test the influence of geometric parameters in experimental results. For span-to-width ratios up to two, the values obtained are quite constant and agree well with the in-plane shear modulus value obtained by the material manufacturer using 45 tensile test.

  • 129.
    Niklas, Karl J.
    et al.
    Cornell University, Ithaca.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Non-parametric Statistical Formulas for Factors of Safety of Plant Stems1999In: Journal of Theoretical Biology, ISSN 0022-5193, E-ISSN 1095-8541, Vol. 197, no 2, p. 135-147Article in journal (Refereed)
    Abstract [en]

    A previously proposed statistical approach for computing factors of safety (i.e. numerical measures of mechanical reliability) for any load bearing structure, like a vertical plant stem, is here extended to cope with organic structures whose morphological or mechanical properties have Weibull frequency distributions. This approach is illustrated using the actual lengthLand critical buckling lengthLerof flower stalks (peduncles) collected from isogenic garlic (Allium sativum) populations grown under windy field and protected glasshouse conditions. Our analyses of the data indicate thatLandLerof peduncles harvested from both populations have Weibull frequency distributions, that the factor of safety for glasshouse grown peduncles is very near unity (i.e.S=1.03), and that the factor of safety of field grown peduncles is 73% higher than that of glasshouse grown plants (i.e.S=1.73). Comparisons between theS-values computed on the basis of our formulas and on the basis of the quotient of the mean values ofLerandLfor each of the two populations indicate that the statistical method gives biologically realisticS-values and that the difference in theS-values for stems grown under protected and unprotected environmental conditions likely reflects the effects of chronic mechanical perturbation (due to wind-induced drag) on normal stem growth and development.

  • 130. 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.

  • 131. Nordin, Lars-Olof
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Methodology for parameter identification in nonlinear viscoelastic material model2005In: Mechanics of time-dependant materials, ISSN 1385-2000, E-ISSN 1573-2738, Vol. 9, no 4, p. 259-280Article in journal (Refereed)
    Abstract [en]

    Two methodologies for identification of material functions in Schapery's nonlinear viscoelastic material model are compared in context to their ability to deal with deviations from Heaviside stepwise load application and unloading in real test conditions where the time intervals for load increase to plateau value and to unloading to zero are finite. In the first method the description of the whole loading, creep, unloading and recovery process is given by one-step load application and one-step unloading whereas in the second method the load increase and decrease intervals are approximated by two-step load application with 0.5 of the load applied in the increase region. Vinyl ester with known viscoelastic properties and incremental form of Schapery's constitutive equation is used to simulate "experimental data" for several length of load application and unloading. The two data reduction methodologies are applied to these "data" and the accuracy of identified material functions is compared with the true values (input data).

  • 132. Nordin, Lars-Olof
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nonlinear viscoelastic behavior of paper fiber composites2005In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 65, no 10, p. 1609-1625Article in journal (Refereed)
    Abstract [en]

    Anisotropic material model for composites made from phenol-formaldehyde impregnated paper is developed starting from Schapery's nonlinear viscoelastic and nonlinear viscoplastic constitutive law derived from thermodynamics. Methodology for determination of the included stress-dependent functions has been developed and used to determine properties of two composites in two directions related to material symmetry. Creep with following strain recovery tests have been performed at several levels of stress. Data reduction is based on two-step load application methodology to include effect of finite time of load increase to the plateau value and its removal. No stiffness reducing micro damage was detected and the plastic deformations were considered negligible or accounted for in a simplified manner. The material model was used to simulate a constant stress rate loading and unloading test. Model is validated comparing simulations with results from test.

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

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

  • 134.
    Nättinen, Kalle
    et al.
    VTT.
    Lampinen, Johanna
    VTT.
    Madsen, Bo
    Risø, DTU.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Wallström, Lennart
    Varna, Janis
    Starch acetate N-fibre composites2008In: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Conference paper (Refereed)
    Abstract [en]

    Cellulosic fibre composite compounds based on hemp and flax fibres in triethyl citrate plasticised starch acetate were prepared by melt processing. Composites were prepared with fibre contents up to 50w%. A stiffness of 8.3 GPa and stress at maximum load of 51 MPa were obtained with 40w% flax fibre in a plasticised starch acetate with 20w% triethyl citrate. An almost linear relation between fibre content and the tensile properties was found. The impact strength of the composites was increased with the plasticiser content. The porosity content was low even at high fibre content.

  • 135.
    Oldenbo, Magnus
    et al.
    Volvo Car Corporation.
    Mattsson, David
    Volvo Car Corporation, Exterior Engineering, Dept 93610 PV3C.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Global stiffness of a SMC panel considering process induced fiber orientation2004In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 23, no 1, p. 37-49Article in journal (Refereed)
    Abstract [en]

    A material model, that translates into a stiffness matrix, the second order fiber orientation tensor, described by Advani and Tucker, and the stiffness matrix of a composite with aligned ellipsoidal inclusions, has been implemented in a FE programme and validated. The stiffness of a SMC panel with known state of fiber orientation is calculated using FEM. The influence of process induced fiber orientation is analysed. The fiber orientation for a realistic charge pattern for the panel has been obtained through mould filling simulation in a separate project. It is found that the fiber orientation has a rather small impact on the global stiffness. Only 0.8% lower stiffness compared to isotropic material model is obtained taking into account the fiber orientation distribution. The main reason for the low impact of the process induced fiber orientation is that the charge is symmetrically placed in the mould leading to a symmetric fiber orientation distribution.

  • 136.
    Oldenbo, Magnus
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A constitutive model for non-Linear behavior of SMC accounting for linear viscoelasticity and micro-damage2005In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 26, no 1, p. 84-97Article in journal (Refereed)
    Abstract [en]

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

  • 137.
    Oldenbo, Magnus
    et al.
    Volvo, Göteborg.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Constitutive model for non-linear behaviour of SMC2003In: Annual Technical Conference - ANTEC, Conference Proceedings, Society of Plastics Engineers, 2003, Vol. Vol. 3, p. 2967-2973Conference paper (Refereed)
    Abstract [en]

    In the present paper an approach for modelling SMC material as visco-elastic damageable is presented. Damage theory by Chow and Wang is used in combination with linear visco-elasticity. The model is exemplified through application to uniaxial creep test with material of different damage states. Simultaneous testing and simulations on a modern SMC material is performed with good accuracy, especially at moderate damage levels.

  • 138. Olsson, Tomas
    et al.
    Megnis, Modris
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lindberg, Henrik
    Luleå tekniska universitet.
    Measurement of the uptake of linseed oil in pine by the use of an X-ray microdensitometry technique2001In: Journal of Wood Science, ISSN 1435-0211, E-ISSN 1611-4663, Vol. 47, no 4, p. 275-281Article in journal (Refereed)
    Abstract [en]

    Pine sapwood (Pinus sylvestris) was impregnated with linseed oil to three levels of uptake. The distribution of the penetrant was found by taking microdensity measurements of an impregnated sample and then using an ethanol extraction procedure to remove the linseed oil. A second set of X-ray measurements at identical locations in the same sample allowed the linseed oil to be indirectly mapped. An uneven distribution of linseed oil in the specimens with the lowest uptake (25% increase in weight) was seen as sharp gradients in the densitometry curves. With increased filling by the linseed oil, these gradients were gradually smoothed. Microstructural changes in specimens with high uptake were revealed using scanning electron microscopy. Through a combination of X-ray microdensitometry investigation and changes observed in the wood''s mechanical properties and morphology, it was concluded that liquid flow during impregnation results in significant damage to the cell structure.

  • 139.
    Olsson, Tomas
    et al.
    Luleå tekniska universitet.
    Megnis, Modris
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lindberg, Henrik
    Luleå tekniska universitet.
    Study of the transverse liquid flow paths in pine and spruce using scanning electron microscopy2001In: Journal of Wood Science, ISSN 1435-0211, E-ISSN 1611-4663, Vol. 47, no 4, p. 282-288Article in journal (Refereed)
    Abstract [en]

    Samples of pine (Pinus sylvestris) and spruce (Picea abies) were impregnated with a low-viscous epoxy resin using a vacuum process. The epoxy was cured in situ and the specimens sectioned. Deposits of the cured epoxy was then observed in the wood cavities using a scanning electron microscope. The investigation concentrated on tracing the transverse movements of a viscous liquid in the wood, and special attention was therefore given to the cross-field area between ray cells and longitudinal tracheids. A damage hypothesis is proposed based on the results obtained in the present investigation in combination with those from earlier studies on linseed oil-impregnated pine: In addition to the morphology of the bordered pits, viscous liquid flow in wood is dependent on damage that occurs during the impregnation procedure. For pine sapwood, liquid flow is enabled through disrupted window pit membranes, which divide the longitudinal tracheids and the ray parenchyma cells. A mechanism accounting for the reduced permeability of pine heartwood is believed to be deposits of higher-molecular-weight substances (extractives) in the ray parenchyma cells and on the cell walls. In spruce the thicker ray cells in combination with the smaller pits, which are connected to the longitudinal tracheids, reduce permeability considerably.

  • 140.
    Paris, F.
    et al.
    ESII Sevilla.
    Cano, J.C.
    ETSII Valladolid.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    The fiber-matrix interface crack: A numerical analysis using Boundary Elements1990In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 82, no 1, p. 11-29Article in journal (Refereed)
    Abstract [en]

    The calculation of the energy release rate of a debonding interface crack between fiber and matrix in plane strain requires a knowledge of the stress state and displacements at the neighbourhood of the crack tip. Analytical solutions which involve interpenetrations of the free boundaries of the fiber and matrix have classically been used to predict the variation of the energy release rate with the debonding angle. In this paper a numerical study of this problem as a contact problem, using Boundary Elements, is carried out. The singular stress components are modelled by means of singular discontinuous elements, a particular contact algorithm, which is very appropriate for cases like that under consideration where the fiber is much stiffer than the matrix, having been proposed. The numerical procedure shows the appearance of a contact zone with physical meaning starting at a certain value of the debonding angle, which implies the presence of a pure mode II of fracture. The values of G found by BEM are compared with those predicted by the use of an analytical solution showing clear discrepancies when the contact zone starts to develop, BEM always predicting smaller values of the energy release rate, in accordance with the absence of mode I in the presence of a contact zone.

  • 141.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Identification Of Parameters For Direct And Inverted Model To Predict Performance Of Materials Exhibiting Non-Linear Viscoelastic Behavior2015In: Proceedings of 7th International Conference on Composites Testing and Model Identification / [ed] C. González; C. López; J. LLorca, Madrid, Spain: IMDEA, Madrid (SPAIN) , 2015Conference paper (Refereed)
  • 142.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nyström, Birgitha
    Development of constitutive model for composites exhibiting time dependent properties2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, IOP Publishing Ltd , 2013, article id 12007Conference paper (Refereed)
    Abstract [en]

    Regenerated cellulose fibres and their composites exhibit highly nonlinearbehaviour. The mechanical response of these materials can be successfully described by themodel developed by Schapery for time-dependent materials. However, this model requiresinput parameters that are experimentally determined via large number of time-consuming testson the studied composite material. If, for example, the volume fraction of fibres is changed wehave a different material and new series of experiments on this new material are required.Therefore the ultimate objective of our studies is to develop model which determines thecomposite behaviour based on behaviour of constituents of the composite. This paper gives anoverview of problems and difficulties, associated with development, implementation andverification of such model.

  • 143.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Saseendran, Sibin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP, Piteå, Sweden.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Basso, Margherita
    Department of Chemistry, Materials and Chemical Engineering, Politecnico di Milano, Italy. The Research Hub by Electrolux Professional, Pordenone, Italy.
    Effect of degree of cure on viscoplastic shear strain development in layers of [45/−45]s glass fibre/ epoxy resin composites2018In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 24, p. 3277-3288Article in journal (Refereed)
    Abstract [en]

    Effect of degree of cure on irreversible (viscoplastic) shear strain development in layers of glass fibre/ epoxy resin (LY5052 epoxy resin) [+45 °/−45 °]s laminate is studied performing a sequence of constant stress creep and viscoelastic strain recovery tests. For fixed values of degree of cure in range from 79.7% to 100%, the viscoplastic strains were measured as dependent on time and stress and Zapa's integral representation was used to characterize the observed behaviour. It is shown that at all degrees of cure the viscoplastic behaviour can be described by Zapa's model with parameters dependent on degree of cure. It is shown that for degree of cure lower than 80% the viscoplastic strains grow much faster and are much more sensitive to the increase of the applied shear stress. These irreversible strains developing in the final phase of the curing can significantly alter the residual stress state in the composite structure.

  • 144.
    Pupure, Liva
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Applications and limitations of non-linear viscoelastic model for simulation of behaviour of polymer composites2015Conference paper (Refereed)
    Abstract [en]

    There are two alternative formulation of non-linear viscoelastic model to describe strain and stress controlled tests. Both models for non-linear viscoelastic materials are not compatible, and cannot be directly inverted if so required in certain cases. In order to do it numerical procedures has to be employed. Methodology for simulating nonlinear stress-strain response in iso-strain situations of fiber composites based on properties on constituents is presented.

  • 145.
    Pupure, Liva
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Methodology for macro-modeling of bio-based composites with inelastic constituents2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 163, p. 41-48Article in journal (Refereed)
    Abstract [en]

    Methodology for development of a macro-scale model (with strain as an input) for Regenerated Cellulose fiber (RCF) composites with highly non-linear (viscoelastic (VE) and viscoplastic (VP)) constituents is presented and demonstrated. The VE is described by Schapery's models and Zapas' model is used for VP. For a purely VE constituent the model can be identified from stress relaxation in constant strain tests. In the presence of VP the constant strain test does not render VE stress relaxation functions, because part of the applied strain is VP and the VE strain is changing. As an alternative creep and strain recovery tests are suggested to find the plasticity law and also the nonlinear creep compliances to identify the VE model where stress is an input. The incremental form of this model is then inverted and used to simulate the VE relaxation tests and the simulated relaxation functions are used to identify the VE model with VE strain as an input.

    Models for constituents are used in micromechanics simulations of the composite behavior in arbitrary ramps including the composite VE relaxation test. Using the latter, a macro-model is developed and its validity and accuracy are demonstrated.

  • 146.
    Pupure, Liva
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modeling of Natural Fiber Composites2015In: Natural Fiber Composites, CRC Press, Taylor & Francis Group, , 2015, p. 221-253Chapter in book (Refereed)
  • 147.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Division of Materials Science, Composite Centre Sweden, Luleå University of Technology.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Division of Materials Science, Composite Centre Sweden, Luleå University of Technology.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Division of Materials Science, Composite Centre Sweden, Luleå University of Technology.
    Modelling of mechanical behaviour of polymeric composites with nonlinear constituents2014In: 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]

    There is need to model behaviour of time-dependent non-linear material in stress or/and strain controlled experiments. This study explores possibility to apply model in both forms developed by Schapery for viscoelastic materials. Viscoelasticity has been analysed using experimental data from creep and relaxation tests. Incremental simulation procedure, which inverts model, where strains are expressed through stresses, is used to simulate relaxation curves for bio-based polymer. Comparison of viscoelastic parameters obtained from simulated and experimental relaxation curves has been performed.

  • 148.
    Pupure, Liva
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Natural fiber composite: Challenges simulating inelastic response in strain-controlled tensile tests2016In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 50, no 5, p. 575-587Article in journal (Refereed)
    Abstract [en]

    Problems occurring, when nonlinear time-dependent material model with parameters identified in creep tests is applied to simulate high-strain response in strain-controlled tests, are described and analyzed. Reasons for discrepancies with experimental loading curves are revealed. Presented numerical/experimental examples deal with three bio-based composites showing highly nonlinear behavior due to damage, nonlinear viscoelasticity and viscoplasticity. Schapery's approach for viscoelasticity and Zapas' model for viscoplasticity are used. The model is generalized to include microdamage effect. It is shown that the main problem in simulations at high stresses is the reliability of data from creep test for model identification in this region because creep rupture limits the available data region and extrapolation to higher stresses is rather uncertain. Alternative solution is to employ relaxation tests at high strains to obtain the missing information. However, it would work only in absence of viscoplastic strains: viscoelastic relaxation functions cannot be determined by maintaining constant total strain if viscoplastic-strain is developing. Based on sensitivity analysis of composite response to variations of the elastic modulus, damage, viscoelastic and viscoplastic parameters, suggestions are made for improving (further “tuning”) the model in high stress region by using tensile stress–strain curves in quasi-static loading.

  • 149.
    Pupure, Liva
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    On viscoplasticity characterization of natural fibres with high variability2015In: Advanced Composites Letters, ISSN 0963-6935, Vol. 24, no 6, p. 125-129Article in journal (Refereed)
    Abstract [en]

    Zapas model has shown good results in characterizing viscoplasticity in polymers and composites. Typically all viscoplastic parameters for this model are obtained in creep and strain recovery tests at different stress levels and at different test length. Obtaining parameters for highly variable bio-based materials is more challenging: trends are hidden in the scatter and data fitting is complicated. This paper suggests "single specimen methodology" for complete viscoplastic characterization and identification

  • 150.
    Pupure, Liva
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP, Piteå.
    Berthold, Fredrik
    Rise Bioeconomy/Innventia AB, Stockholm.
    Miettinen, Arttu
    Department of Physics, University of Jyväskylä.
    Mechanical properties of natural fiber composites produced using dynamic sheet former2018In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280Article in journal (Refereed)
    Abstract [en]

    Composites formed from wood fibers and man-made cellulosic fibers in PLA (polylactic acid) matrix, manufactured using sheet forming technique and hot pressing, are studied. The composites have very low density (due to high porosity) and rather good elastic modulus and tensile strength. As expected, these properties for the four types of wood fiber composites studied here improve with increasing weight fraction of fibers, even if porosity is also increasing. On the contrary, for man-made cellulosic fiber composites with circular fiber cross-section, the increasing fiber weight fraction (accompanied by increasing void content) has detrimental effect on stiffness and strength. The differences in behavior are discussed attributing them to fiber/ fiber interaction in wood fiber composites which does not happen in man-made fiber composites, and by rather weak fiber/matrix interface for man-made fibers leading to macro-crack formation in large porosity regions.

123456 101 - 150 of 296
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf