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  • 1.
    Aitomäki, Yvonne
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Westin, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. University of Jyvaskyla, Department of Physics.
    Korpimäki, Jani
    CSI Composites.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nanofibre distribution in composites manufactured with epoxy reinforced with nanofibrillated cellulose: model prediction and verification2016In: IOP Conference Series: Materials Science and Engineering, ISSN 1757-8981, E-ISSN 1757-899X, Vol. 139, 012011Article in journal (Refereed)
    Abstract [en]

    In this study a model based on simple scattering is developed and used to predict the distribution of nanofibrillated cellulose in composites manufactured by resin transfer moulding (RTM) where the resin contains nanofibres. The model is a Monte Carlo based simulation where nanofibres are randomly chosen from probability density functions for length, diameter and orientation. Their movements are then tracked as they advance through a random arrangement of fibres in defined fibre bundles. The results of the model show that the fabric filters the nanofibres within the first 20 µm unless clear inter-bundle channels are available. The volume fraction of the fabric fibres, flow velocity and size of nanofibre influence this to some extent. To verify the model, an epoxy with 0.5 wt.% Kraft Birch nanofibres was made through a solvent exchange route and stained with a colouring agent. This was infused into a glass fibre fabric using an RTM process. The experimental results confirmed the filtering of the nanofibres by the fibre bundles and their penetration in the fabric via the inter-bundle channels. Hence, the model is a useful tool for visualising the distribution of the nanofibres in composites in this manufacturing process.

  • 2.
    Almgren, Karin M
    et al.
    STFI-Packforsk AB, Box 5604, SE-114 86 Stockholm.
    Gamstedt, Kristofer
    Department of Fiber and Polymer Technology, Royal Institute of Technology - KTH.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Contribution of wood fiber hygroexpansion to moisture induced thickness swelling of composite plates2010In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 31, no 5, 762-771 p.Article in journal (Refereed)
    Abstract [en]

    One of the main drawbacks of wood fiber-based composite materials is their propensity to swell due to moisture uptake. Because the wood fibers are usually the main contributor to hygroexpansion, it is of interest to quantify the hygroexpansion coefficient of wood fibers, to compare and rank different types of fibers. This investigation outlines an inverse method to estimate the transverse hygroexpansion coefficient of wood fibers based on measurements of moisture induced thickness swelling of composite plates. The model is based on composite micromechanics and laminate theory. Thickness swelling has been measured on polylactide matrix composites with either bleached reference fibers or crosslinked fibers. The crosslinking modification reduced the transverse hygroexpansion of the composites and the transverse coefficient of hygroexpansion of the fibers was reduced from 0.28 strain per relative humidity for reference fibers to 0.12 for cross-linked fibers.

  • 3.
    Al-Ramahi, Nawres
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, RobertsLuleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.Varna, JanisLuleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Model for numerical simulation and parametric analysis of composite adhesivejoints under thermo-mechanical loading2017Conference proceedings (editor) (Refereed)
    Abstract [en]

    Abstract: The current investigation focuses on development and verification of a modelfor numerical simulation of performance of adhesive joints under tensile loading. Differentcombination of materials in joints is considered: metal-metal, composite-composite andcomposite-metal. The objective of this paper is to present simulation results of joints usingan accurate finite element model including non-linear behaviour and large deformation.Moreover, several loading scenarios are analysed, including simultaneous application oftemperature and mechanical load. Not only the effect of temperature on mechanicalperformance of materials (adhesive as well as adherents) is analysed but also built up ofresidual thermal stresses during the manufacturing of joints are taken into account. Thisapproach is demonstrated by simulation of tensile tests of joints at several temperatures.Two scenarios of application of temperature and mechanical load using large deformationtheory are considered: 1) the thermal and mechanical loads are applied simultaneously (theproperties of the materials are adjusted accordingly to their performance at differenttemperatures); 2) temperature is applied on specimen which is not macroscopicallyconstrained and the obtained stress distribution is used as initial state for the nextsimulation of mechanical loaded joint. The influence of edge effects (due to limited widthof the joint) on the stress distribution within the joint are studied. In order to eliminatethese effects the periodic boundary conditions (BC) are used in the numerical model.These BC are adjusted to optimize numerical model and obtain efficient calculation routinefor analysis of stresses within interior part of the structure. The validity of these BCs isevaluated and verified by analysing number of case studies. The comparison between full3D FEM model and simplified 2D model is carried out. The resulting stress distributions inthe overlap region of joints are presented for different joints (the parameters are: materialcombinations, material models, geometry of adhesive layer, constraints and BCs) withcomprehensive analysis and recommendations for optimal numerical model that can beused in joint design.

  • 4.
    Andersons, J.
    et al.
    University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Estimation of the tensile strength of an oriented flax fiber-reinforced polymer composite2011In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 42, no 9, 1229-1235 p.Article in journal (Refereed)
    Abstract [en]

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

  • 5.
    Andersons, J.
    et al.
    University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hojo, M.
    Ochiai, S.
    Kyoto University.
    Fibre fragment distribution in a single-fibre composite tension test2001In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 32, no 4, 323-332 p.Article in journal (Refereed)
    Abstract [en]

    Single fibre fragmentation tests are performed for brittle fibres with Weibull strength distribution and different surface treatments. The fragmentation process is modelled and closed-form expressions for break spacing distribution are obtained. The model accounts for the effect of finite fibre length on the initial fragmentation as well as for break interaction on the advanced fragmentation stage. It is assumed that the exclusion zone due to fibre-matrix interface failure and stress recovery in the fibre is linearly dependent on the applied load. This assumption is validated experimentally. The derived theoretical average fragment length dependence on applied load is used to determine the fibre strength distribution parameters and the effective interfacial shear stress for carbon/epoxy single fibre composites with different fibre surface treatment and for glass/vinylester single fibre composite. Fragment length distribution is predicted for several load levels. Predictions are in good agreement with experimental data

  • 6.
    Andersons, J.
    et al.
    University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hojo, M.
    Mesoscopic Materials Research Center, Kyoto University.
    Ochiai, S.
    Mesoscopic Materials Research Center, Kyoto University.
    Glass fibre strength distribution determined by common experimental methods2002In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 62, no 1, 131-145 p.Article in journal (Refereed)
    Abstract [en]

    The tensile strength of brittle fibres is routinely described by the Weibull distribution. The parameters of the distribution can be obtained from tests on single fibres and fibre bundles or from model composite tests. However, there is growing evidence that the distribution parameters obtained by different experimental techniques differ systematically. In order to investigate the possible causes of such discrepancies, single-fibre tension, fibre bundle, and single-fibre fragmentation tests are employed in this study to obtain strength distribution of commercial E-glass fibres. The results reveal parameter dependence on the approach used to extract the distribution parameters from experimental data. Particularly, in the case of single-fibre tension tests, the shape parameter obtained from average fibre strength vs. length data is larger than that obtained at a fixed gauge length. It is assumed that the apparent fibre strength scatter is caused by both the inherent flaw structure along a fibre and by the fibre-to-fibre strength variability within a batch, due to slightly differing processing and handling history of the fibres. Fibre fragmentation test results are used to derive the Weibull distribution parameters applicable to the fibre batch. The strength distribution obtained is compared with strength data for the single fibres, and reasonably good agreement is observed.

  • 7.
    Andersons, J.
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Sandmark, R.
    Materials division, SINTEF SI.
    Constrained fragmentation of composites under uniaxial loading1995In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 31, no 1, 26-33 p.Article in journal (Refereed)
  • 8.
    Andersons, J.
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sparnins, Edgars
    Institute of Polymer Mechanics, University of Latvia.
    Weichert, D.
    Institute of General Mechanics, RWTH-Aachen University.
    Modeling the effect of reinforcement discontinuity on the tensile strength of UD flax fiber composites2011In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 46, no 15, 5104-5110 p.Article in journal (Refereed)
    Abstract [en]

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

  • 9.
    Andersons, J.
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Leterrier, Y.
    École Polytechnique Fédérale de Lausanne.
    Joffe, Roberts
    Statistical model of coating fragmentation under equibiaxial load1998In: Materials and Manufacturing Processes, ISSN 1042-6914, E-ISSN 1532-2475, Vol. 13, no 4, 597-602 p.Article in journal (Refereed)
    Abstract [en]

    A statistical model of coating cracking under equibiaxial tension is proposed based on a Weibull strength distribution for the coating. Crack length and spacing distributions are derived assuming that cracks initiate in random locations and propagate straight till stopping upon encountering a geometiical obstacle (another crack). The theoretical distributions are verified by comparing with simulated cracking patterns obtained by the Monte-Carlo method. An analysis of crack patterns of SiO2 coatings on a PET film under biaxial tension is performed. Qualitative agreement with the theoretical crack spacing distribution is observed.

  • 10.
    Andersons, J.
    et al.
    University of Latvia.
    Sparnins, Edgars
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Prediction of crack onset strain in composite laminates at mixed mode cracking2009In: 5th International EEIGM/AMASE/FORGEMAT Conference on Advanced Materials Research, Bristol: IOP Publishing Ltd , 2009, Vol. 5Conference paper (Refereed)
    Abstract [en]

    Failure process of continuous fiber reinforced composite laminates in tension usually starts with appearance of intralaminar cracks. In composite laminates with complex lay-ups and/or under combined loading, intralaminar cracks may develop in plies with different reinforcement directions. A necessary part of mixed mode cracking models is the criterion of failure. For propagation-controlled fracture it is usually formulated in terms of energy release rates and their critical values of the particular composite material. Intralaminar fracture toughness of unidirectionally reinforced glass/epoxy composite was experimentally determined at several mode I and mode II ratios. It is found that the crack propagation criterion, linear in terms of the energy release rates, reasonably well approximates the test results. The determined mixed mode cracking criterion was applied to predict intralaminar crack onset in cross-ply glass/epoxy composite under tensile loading. The predicted crack onset strain values agree with test results at small off-axes angles of the cracking ply (on-axis and 15° off-axis loading), but underestimate crack onset at larger reinforcement angles with respect to the loading direction. The discrepancy is likely to be caused by the deviation of linearity in laminate response before cracking onset in these laminates, related to non-linear shear characteristics of unidirectional plies. The applicability of strength-based fracture criterion for initiation-controlled cracking is discussed.

  • 11.
    Andersons, J.
    et al.
    University of Latvia, Riga.
    Sparnins, Edgars
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stiffness and strength of flax fiber/polymer matrix composites2006In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 27, no 2, 221-229 p.Article in journal (Refereed)
    Abstract [en]

    Flax fiber composites with thermoset and thermoplastic polymer matrices have been manufactured and tested for stiffness and strength under uniaxial tension. Flax/polypropylene and flax/maleic anhydride grafted polypropylene composites are produced from compound obtained by coextrusion of granulated polypropylene and flax fibers, while flax fiber mat/vinylester and modified acrylic resin composites are manufactured by resin transfer molding. The applicability of rule-of-mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers is considered.

  • 12.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Initiation and propagation controlled intralaminar cracking in cross-ply laminates: Chapter 102009In: Encyclopedia of Polymer Composites: Properties, Performance and Applications, New York: Nova Science Publishers, Inc., 2009Chapter in book (Other academic)
  • 13.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanical damage characteristics of elementary hemp fibers and scale effect of fiber strength2012In: High Performance Structure and Materials VI: papers presented at the 6th International Conference on High Performance Structures and Materials held at the Wessex Institute of Technology in the New Forest, UK] / [ed] W.P. De Wilde; C.A. Brebbia; S. Hernandez, Southampton: WIT Press, 2012, 157-167 p.Conference paper (Refereed)
    Abstract [en]

    Ecological and economical considerations foster replacement of man-made fibers by natural renewable fibers in various industrial applications. Bast fibers of such plants as, e.g., flax, hemp, jute etc., are particularly attractive as a reinforcement of polymer-matrix composites due to their high specific stiffness and strength in the axial direction. The elementary bast fibers exhibit pronounced scatter of strength. It necessitates probabilistic description of their strength via a distribution function that reflects damage morphology and severity in fibers. Fiber fracture is shown to originate from mechanical defects of the bast cell wall, the most prominent of them being kink bands. While the number of kink bands in a fiber is easily determined by optical microscopy, direct experimental measurement of their strength is complicated. Therefore, alternative approaches are sought, enabling extraction of strength characteristics of the kink bands from fiber tests via appropriate probabilistic models. Analytical distribution function of bast fiber strength has been derived, allowing for the effect of mechanical damage in the form of kink bands. The fiber characteristics measured have been used to evaluate the kink band density and strength distributions. The theoretical distribution is verified against experimental tensile strength data of elementary hemp fibers at several gauge lengths and found to provide acceptable accuracy in predicting the scale effect of strength.

  • 14. Andersons, Janis
    et al.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sparnins, Edgars
    Institute of Polymer Mechanics, University of Latvia.
    Evaluation of interfacial shear strength by tensile tests of impregnated flax fiber yarns2012In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 46, no 3, 351-357 p.Article in journal (Refereed)
    Abstract [en]

    Adhesion of flax fibers and polymer matrix as well as mutual bonding of elementary fibers in a technical fiber are among the principal factors governing the mechanical response of flax fiber reinforced polymer-matrix composites. A method for evaluation of adhesion is proposed based on tension tests of impregnated fiber yarns, with subsequent characterization by optical microscopy of length distribution of fibers pulled out of the yarn fracture surfaces. An elementary probabilistic model is derived relating aspect ratio distribution of the pulled out fibers to the fiber tensile strength distribution and the effective interfacial shear strength. The method was applied to flax fiber/vinylester resin yarns and an estimate of interfacial shear strength at 17 MPa was obtained.

  • 15.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sparnins, Edgars
    Statistical model of the transverse ply cracking in cross-ply laminates by strength and fracture toughness based failure criteria2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 9, 2651-2665 p.Article in journal (Refereed)
    Abstract [en]

    Cross-ply laminate subjected to tensile loading provides a relatively well understood and widely used model system for studying progressive cracking of the transverse ply. This test allows to identify material strength and/or toughness characteristics as well as to establish relation between damage level and the composite stiffness reduction. The transverse ply cracking is an inherently stochastic process due to the random variability of local material properties of the plies. The variability affects both crack initiation (governed by the local strength) and propagation (governed by the local fracture toughness). The primary aim of the present study is elucidation of the relative importance of these phenomena in the fragmentation process at different transverse and longitudinal ply thickness ratios. The effect of the random crack distribution on the mechanical properties reduction of the laminate is also considered. Transverse ply cracking in glass fiber/epoxy cross-ply laminates of the lay-ups [02/902]s, [0/902]s, and [0/904]s is studied. Several specimens of each lay-up were subjected to uniaxial quasistatic tension to obtain crack density as a function of applied strain. Crack spacing distributions at the edge of the specimen also were determined at a predefined applied strain. Statistical model of the cracking process is derived, calibrated using crack density vs. strain data, and verified against the measured crack spacing distributions.

  • 16.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sparnins, Edgars
    Rubenis, Oskars
    Institute of Polymer Mechanics, University of Latvia.
    Progressive cracking mastercurves of the transverse ply in a laminate2009In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 30, no 8, 1175-1182 p.Article in journal (Refereed)
    Abstract [en]

    In this study, progressive cracking of a transverse layer in a cross-ply composite laminate subjected to tensile loading is considered. Using the results of a probabilistic cracking model, approximate relations for crack density as a function of stress are derived for initiation-controlled and propagation-controlled cracking. It is shown that the crack density evolution in the transverse ply can be represented by a mastercurve in suitably normalized coordinates. The mastercurve approach is applied to progressive cracking in glass/epoxy laminates

  • 17.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Modniks, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    An Improved Method For Identification Of The Interfacial Shear Strength By Tensile Tests Of Short-Fiber Composites2015In: 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)
  • 18.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Modniks, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Madsen, Bo
    Technical University of Denmark, Risø Campus, Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark.
    Nättinen, Kalle
    Bemis Flexible Packaging Europe, Bemis Valkeakoski Oy.
    Apparent interfacial shear strength of short-flax-fiber/starch acetate composites2016In: International Journal of Adhesion and Adhesives, ISSN 0143-7496, E-ISSN 1879-0127, Vol. 64, 78-85 p.Article in journal (Refereed)
    Abstract [en]

    The paper deals with an indirect industry-friendly method for identification of the interfacial shear strength (IFSS) in a fully bio-based composite. The IFSS of flax fiber/starch acetate is evaluated by a modified Bowyer and Bader method based on an analysis of the stress-strain curve of a short-fiber-reinforced composite in tension. A shear lag model is developed for the tensile stress-strain response of short-fiber-reinforced composites allowing for an elasticperfectly plastic stress transfer. Composites with different fiber volume fractions and a variable content of plasticizer have been analyzed. The apparent IFSS of flax /starch acetate is within the range of 5.5 to 20.5 MPa, depending on composition of the material. The IFSS is found to be greater for composites with a higher fiber loading and to decrease with increasing content of plasticizer. The IFSS is equal or greater than the yield strength of the neat polymer, suggesting good adhesion, as expected for the chemically compatible constituents.

  • 19.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    The onset of mixed mode intralaminar cracking in a cross-ply composite laminate2008In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 44, no 6, 549-556 p.Article in journal (Refereed)
    Abstract [en]

    The intralaminar fracture toughness of a unidirectionally reinforced glass/epoxy composite is determined experimentally at several mode I and mode II loading ratios. The crack propagation criterion, expressed as a quadratic form in terms of single-mode stress intensity factors (alternatively, linear in terms of energy release rates), approximates the test results reasonably well. The mixed-mode cracking criterion obtained is used to predict the intralaminar crack on set in a cross-ply glass/epoxy composite under off-axis tensile loading.

  • 20.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Uniformity of filament strength within a flax fiber batch2009In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 44, no 2, 685-687 p.Article in journal (Refereed)
  • 21.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Wallström, Lennart
    Strength distribution of elementary flax fibres2005In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 65, no 3-4, 693-702 p.Article in journal (Refereed)
    Abstract [en]

    Flax fibres, along with a number of other natural fibres, are being considered as an environmentally friendly alternative of synthetic fibres in fibre-reinforced polymer composites. A common feature of natural fibres is a much higher variability of mechanical properties. This necessitates study of the flax fibre strength distribution and efficient experimental methods for its determination. Elementary flax fibres of different gauge lengths are tested by single fibre tension in order to obtain the stress-strain response and strength and failure strain distributions. The applicability of single fibre fragmentation test for flax fibre failure strain and strength characterization is considered. It is shown that fibre fragmentation test can be used to determine the fibre length effect on mean fibre strength and limit strain.

  • 22.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Nyström, Birgitha
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Scale effect of the tensile strength of flax-fabric-reinforced polymer composites2011In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 30, no 23, 1969-1974 p.Article in journal (Refereed)
    Abstract [en]

    The development of UD natural fiber composites, considered for application as structural materials, necessitates evaluation of the scale effect of their strength. Alignment of the fibers in flax bast fiber composites can be achieved by employing textile reinforcement, such as yarns and fabrics. Cutting specimens for mechanical tests out of such textile-reinforced composite plates results in a complex non-uniform reinforcement structure at their edges, which may affect the strength of specimens. Scale effect of the tensile strength in the fiber direction of flax fabric reinforced composites is studied in the current work. A model accounting for both volume and edge effect of the specimens on their tensile strength is proposed.

  • 23.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Porike, Evija
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Strength distribution of elementary flax fibres due to mechanical defects2008In: 11th International Inorganic-Bonded Fiber Composites Conference (IIBCC): Madrid, Nov. 4 - 7, 2008. Proceedings / [ed] Blanco Suárez; Maria Ángeles, Madrid: Universidad Complutense de Madrid , 2008Conference paper (Refereed)
  • 24.
    Andersons, Janis
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Rubenis, Ojars
    Institute of Polymer Mechanics, University of Latvia.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Estimation of laminate stiffness reduction due to cracking of a transverse ply by employing crack initiation- and propagation-based master curves2008In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 44, no 5, 441-450 p.Article in journal (Refereed)
    Abstract [en]

    The applicability range of toughness-and strength-based criteria for progressive cracking of a transverse layer in a cross-ply composite laminate subjected to tensile loading is considered. Using a deterministic cracking model, approximate relations for the crack density as a function of stress are derived for initiation-and propagation-controlled types of cracking. The master-curve approach is applied to progressive cracking in glass/epoxy laminates. The accuracy of estimation of laminate stiffness reduction by using crack density master curves is evaluated.

  • 25.
    Andersson, Börje
    et al.
    Aeronautical Research Institute of Sweden.
    Sjögren, Anders
    Aeronautical Research Institute of Sweden.
    Berglund, Lars
    Luleå tekniska universitet.
    Micro- and meso-level residual stresses in glass-fiber/vinyl-ester composites2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 10, 2011-2028 p.Article in journal (Refereed)
    Abstract [en]

    Residual stresses in glass-fiber composites were studied on the micro and meso scales by computational and experimental methods. Transmitted polarized light images of thin sections were compared with 3D finite-element solutions of a sample containing 1410 fibers. Calculated point-wise stresses were derived from a linear thermoelastic model with negligibly small numerical errors. Regions with calculated maximum compressive stresses showed good agreement with experimentally observed optical bands. A material with poor interfacial adhesion showed weaker optical effects indicating fiber/matrix debonding. On the basis of these results it seems likely that irreversible matrix deformation and debonding can take place in the curing phase.

  • 26.
    André, Alann
    et al.
    Swerea SICOMP AB, Mölndal.
    Kliger, Robert
    Division of Structural Engineering, Chalmers University of Technology.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Compression failure mechanism in small scale timber specimens2014In: Construction and Building Materials, ISSN 0950-0618, E-ISSN 1879-0526, Vol. 50, 130-139 p.Article in journal (Refereed)
    Abstract [en]

    Understanding the failure mechanism of wood loaded in compression parallel to the grain has been shown to be an important parameter in the design of timber beams strengthened with fibre-reinforced plastics (FRP). In this paper, a constitutive relationship for wood under uniaxial compression load parallel to the grain was determined experimentally. Several parameters, such as silviculture, moisture content and radial position in the log in relation to the pith from where the specimen was sawn, were considered. Small clear-wood specimens were used. The strain localisation in the failure region (kinkband) was monitored using the digital image correlation method. The results show that silviculture and moisture content are two very important parameters which influence the compression failure mechanism. Furthermore, there is a significant difference in behaviour between specimens from the juvenile region of the log and specimens from mature wood. Based on experimental results, two numerical models were built, considering either a global or a local constitutive relationship. It was demonstrated that both numerical models yield accurate results and that, depending on the experimental equipment available, a constitutive relationship could be extracted and used as input in these numerical models.

  • 27.
    André, Alann
    et al.
    Swerea SICOMP AB.
    Nilsson, Sören
    Swerea SICOMP AB.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Finite element delamination study of a notched composite plate under flexural loads2009In: ICCM 17, Edinburgh: 17th International Conference on Composite Materials ; 27 Jul 2009 - 31 Jul 2009, Edinburgh International Convention Centre, Edinburgh, UK, London: IOM Communications , 2009Conference paper (Refereed)
    Abstract [en]

    The delamination process in notched composite plates under flexural loading has been investigated using finite element analysis. Cohesive elements implemented in the commercial finite element package ABAQUS have been used in the region around the drilled-hole, and positioned between layers where delamination was observed during experiments presented in an accompanying paper. The delamination initiation and subsequent propagation was studied between the layers at the tension side separately and simultaneously. For all FE models, the load displacement curve was in good agreement with the one from experiments. However, the amount of damage reported from the fractography study was more extensive than that predicted by the models. Finally, it was shown that the models with only one cohesive layer show significantly different results to that of the model with four cohesive layers in terms of size of the degradation area.

  • 28.
    Ansari, Farhan
    et al.
    Department of Fiber and Polymer Technology, KTH Royal Institute of Technology.
    Granda, Luis A.
    Laboratory of Paper Engineering and Polymer Materials (LEPAMAP) Group, Department of Chemical Engineering, University of Girona.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Department of Fiber and Polymer Technology, KTH Royal Institute of Technology.
    Vilaseca, Fabiola
    Department of Fiber and Polymer Technology, KTH Royal Institute of Technology.
    Experimental evaluation of anisotropy in injection molded polypropylene/wood fiber biocomposites2017In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 96, 147-154 p.Article in journal (Refereed)
    Abstract [en]

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

  • 29. Arab, A.
    et al.
    Stommel, M.
    Wallström, Lennart
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Rack, A.
    Investigation of fibre degradation in natural fibre reinforced biocomposites2013In: Proceedings of the 5th International Conference on Sustainable Materials, Polymers and Composites: Ecocomp 2013, 2013, 174-185 p.Conference paper (Refereed)
  • 30. Arab, A.
    et al.
    Stommel, M.
    Wallström, Lennart
    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.
    Stiffness prediction in green composites using homogenization techniques2013In: Proceedings of the 19th International Conference on Composite Materials: ICCM 2013, Montreal (Canada), 2013, 1214-1222 p.Conference paper (Refereed)
  • 31.
    Arab, Asghar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stommel, L.
    Saarland University.
    Wallström, Lennart
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fibre Orientation Investigation in Short Natural Fibre Reinforced Composites Using Synchrotron Imaging2013Conference paper (Refereed)
  • 32.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Finite element delamination study of a notched composite plate under flexural loads2010In: Journal of Materials Science and Engineering. A, ISSN 1934-8959, Vol. 4, no 8, 66-73 p.Article in journal (Refereed)
  • 33.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fracture and yield predictions for epoxies in composite-like stress state1994Licentiate thesis, comprehensive summary (Other academic)
  • 34. Asp, Leif
    Multifunctional composite materials for energy storage in structural load paths2013In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, no 4, 144-149 p.Article in journal (Refereed)
    Abstract [en]

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

  • 35.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Structural battery materials2012In: Proceedings of the 15th European Conference on Composite Materials / [ed] Marino Quaresimin; Laszlo Kollar; Leif Asp, Venice, 2012Conference paper (Refereed)
    Abstract [en]

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

  • 36.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Transverse failure initiation in polymer composites1995Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

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

  • 37.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    A biaxial thermomechanical disk test for glassy polymers1997In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 37, no 1, 96-101 p.Article in journal (Refereed)
    Abstract [en]

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

  • 38.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Gudmundsson, Peter
    Department of Solid Mechanics, Royal Institute of Technology.
    Effects of a composite-like stress state on the fracture of epoxies1995In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 53, no 1, 27-37 p.Article in journal (Refereed)
    Abstract [en]

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

  • 39.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Criterion for crack initiation in glassy polymers subjected to a composite-like stress state1996In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 11, 1291-1301 p.Article in journal (Refereed)
    Abstract [en]

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

  • 40.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Effects of fiber and interphase on matrix-initiated transverse failure in polymer composites1996In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 45, no 6, 657-665 p.Article in journal (Refereed)
    Abstract [en]

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

  • 41.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Prediction of matrix-initiated transverse failure in polymer composites1996In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 56, no 9, 1089-1097 p.Article in journal (Refereed)
    Abstract [en]

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

  • 42.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Gudmundson, P.
    Mechanical behavior of epoxy resins in uniaxial and triaxial loading1993In: High temperature ceramic matrix composites: 6th EuropeanConference on Composite Materials, 20 - 24 September 1993, Bordeaux; HT-CMC / [ed] M. Neitzel; J.C. Lambert, Cambridge: Woodhead Publishing Materials , 1993, 323-328 p.Conference paper (Refereed)
  • 43.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Brandt, F.
    Effects of pores and voids on the interlaminar delamination toughness of a carbon/epoxy composite1997In: Proceedings / Eleventh International Conference on Composite Materials, Gold Coast, Queensland, Australia, 14th - 18th July 1997: ICCM-11 / [ed] Murray L Scott, Melbourne: Woodhead Publishing Materials , 1997, 322-331 p.Conference paper (Refereed)
  • 44.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Edgren, Fredrik
    Sjögren, A.
    Effects of stitch pattern on the mechanical properties of non-crimp fabric composites2004In: 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)
  • 45.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Juntikka, R.
    Swerea SICOMP AB, Mölndal.
    High velocity impact on NCF reinforced composites2006In: From the science of composites to engineering applications: the dawning future of composites: ECCM 12, 12th European Conference on Composite Materials, Biarritz, 29th August - 1st September 2006 ; [conference proceedings] / [ed] Jacques Lamon, Biarritz, 2006Conference paper (Refereed)
  • 46.
    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, 402-438 p.Chapter in book (Refereed)
  • 47.
    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, 581-590 p.Conference 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

  • 48.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nilsson, Karl-Fredrik
    SICOMP AB, Swedish Institute of Composites.
    Delamination criticality in slender compression-loaded composite panels2002In: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 221-222, 3-16 p.Article in journal (Refereed)
    Abstract [en]

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

  • 49.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sjögren, B.A.
    Berglund, Lars A.
    Prediction of failure initiation in polypropylene with glass beads1997In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 18, no 1, 9-15 p.Article in journal (Refereed)
    Abstract [en]

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

  • 50.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Szpieg, Magdalena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Wysocki, Maciej
    Swerea SICOMP AB.
    Mechanical performance and modelling of a fully recycled modified CF/PP composite2012In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 46, no 12, 1503-1517 p.Article in journal (Refereed)
    Abstract [en]

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

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