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  • 1.
    Achi, Elif
    et al.
    Eurocopter Deutschland GmbH (ECD), Stress and Dynamic System Department.
    Talreja, Ramesh
    Characterization of viscoelasticity and damage in high temperature polymer matrix composites2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, no 14, p. 2506-2519Article in journal (Refereed)
    Abstract [en]

    This paper describes a thermodynamics based model for viscoelastic composites with damage and illustrates its use in characterization of viscoelastic response of polymer matrix woven fabric composites subjected to loading at high temperatures. The characterization is conducted by an experimental method aided by finite element (FE) modeling. The experimental characterization is based on creep data obtained under constant loads of different magnitudes and at different temperatures, and on recovery data collected after unloading. A carbon fiber/polyamide resin woven composite with glass transition temperature (Tg) of around 380 °C was used in the experimental program. A FE model was developed to determine the non-linear viscoelastic response by implementing incremental constitutive relations into an ABAQUS® code. The laminate viscoelastic properties were obtained by finite element micromechanics analysis using the neat resin data as input. Comparing its results with creep-recovery test data at different temperature and stress levels validated the FE model. There are several factors affecting the viscoelastic behavior of polymer matrix composites such as temperature, moisture and stress level. Accordingly, a large number of tests need to be performed to characterize the viscoelastic response experimentally for each fiber-matrix combination. For this purpose an efficient and systematic experimental procedure was used to understand the effects of temperature and stress level on the viscoelastic response, to clarify the damage-viscoelasticity coupling and to determine the viscoelastic properties of the material system

  • 2.
    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, p. 131-145Article 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.

  • 3.
    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, p. 693-702Article 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.

  • 4.
    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, p. 2011-2028Article 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.

  • 5. Asp, Leif
    The effects of moisture and temperature on the interlaminar delamination toughness of a carbon/epoxy composite1998In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 58, no 6, p. 967-977Article in journal (Refereed)
    Abstract [en]

    HTA/6376C composite has been investigated for influence of temperature and moisture content on the interlaminar delamination toughness in mode I, mode II and mixed mode conditions. Dry and moisture-saturated specimens were tested over the temperature range - 50 °C to 100 °C. Evaluation methods based on load/displacement and load measurements were employed. In pure mode II the critical strain-energy release rate drops with moisture content and increase in temperature. In mixed mode the critical strainenergy release rate also decreases with moisture content, but no general trends in the dependence on temperature is observed. The critical strain-energy release rate in pure mode I is unaffected by changes in moisture content and was found to increase slightly at elevated temperatures. During crack propagation, enhanced fiber bridging due to increases in temperature and moisture content promotes R-curve behavior in the pure mode I tests. The resulting mode mixity of the mixed-mode bending (MMB) tests is found to be severely affected by the evaluation methods. Methods based on load measurements only are considered to give unreliable strain-energy release rates as the measured compliance/displacement relationships were found to be non-linear even prior to crack growth. Further studies are needed to assess the mixed-mode ratio in the MMB test.

  • 6.
    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, p. 27-37Article 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

  • 7.
    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, p. 1291-1301Article 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.

  • 8.
    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, p. 657-665Article 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

  • 9.
    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, p. 1089-1097Article 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.

  • 10. Asp, Leif
    et al.
    Juntikka, Rickard
    Swerea SICOMP AB, Mölndal.
    High velocity impact on NCF reinforced composites2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 9, p. 1478-1482Article in journal (Refereed)
    Abstract [en]

    In the current paper, a series of high velocity impact tests using Φ50 and Φ25 mm ice spheres and 0.32 g granite stones on non-crimp fabric (NCF) composite plates are reported. The impact tests were performed using an air gun and velocities between 100 m/s and 199 m/s. The impact events were monitored using a high-speed camera, with a 20 million frames per second capacity, as well as by a displacement transducer for out-of-plane displacement measurements of the impacted plates. NCF composite plates of two different thicknesses were impacted. The composites were manufactured from carbon fibre and epoxy resin by vacuum infusion.Engineering type models were employed to predict impact response and impact damage formation. Comparison between predicted and resulting damage for the impact test validates the application of a semi-empirical model for predicting impact velocity thresholds for damage formation. Analytical models relying on the assumption of solid impact bodies cannot be employed for these types of impact.

  • 11.
    Bengtsson, Magnus
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Gatenholm, Paul
    CTH.
    Oksman, Kristiina
    The effect of crosslinking on the properties of polyethylene/wood flour composites2005In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 65, no 10, p. 1468-1479Article in journal (Refereed)
    Abstract [en]

    In this study, the possibility of using silane technology in crosslinking composites of wood flour and polyethylene has been investigated. Composites of vinyltrimethoxy silane grafted high density polyethylene and wood flour were produced by compounding in a twin-screw extruder. Gel content analysis with p-xylene extraction revealed higher gel content in the samples where wood flour was added compared to neat crosslinked matrix. Mechanical analysis of the crosslinked composites showed increased tensile strength with increasing amount of wood flour, which might be an indication of improved adhesion between the matrix and the wood flour. The stiffness increased with increasing amount of wood flour with accompanied decrease in strain at break. Dynamic mechanical thermal analysis of crosslinked plastics and composites showed no significant shift in the γ-transition towards higher temperature for the composites compared to neat plastic. Short-term creep experiments showed reduced creep deformation with increasing amount of wood flour. Crosslinking of the composites reduced the creep deformation further. A boiling test in water followed by tensile testing showed that the crosslinked composites were less susceptible to water uptake compared to the non-crosslinked. Moreover, the decrease in tensile strength of the crosslinked composites was not as significant as for the non-crosslinked composites. Scanning electron microscopy revealed good compatibility and adhesion between the plastic and the wood flour for crosslinked composites.

  • 12.
    Bengtsson, Magnus
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Oksman, Kristiina
    Silane crosslinked wood plastic composites: Processing and properties2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, no 13, p. 2177-2186Article in journal (Refereed)
    Abstract [en]

    The focus of the study has been to produce silane crosslinked wood plastic composites in a compounding process. Silane crosslinking is one way to improve the mechanical and long-term properties of wood plastic composites. Silane crosslinked composites with different amounts of vinyltrimethoxy silane were produced in a compounding process using a co-rotating twin-screw extruder. The composites were stored in a sauna and at room temperature to study the effect of humidity on the degree of crosslinking. Gel content and swelling experiments showed that the highest degree of crosslinking was found in the composites stored in a sauna. The crosslinked composites showed toughness, impact strength and creep properties superior to those composites to which no silane was added. The flexural modulus, on the other hand, was lower in the crosslinked samples than in the non-crosslinked ones. Differential scanning calorimetry measurements of the composites showed a lower crystallinity in the crosslinked samples than in the non-crosslinked.

  • 13.
    Bengtsson, Magnus
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Stark, Nicole
    Forest Products Laboratory.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Durability and mechanical properties of silane cross-linkedwood thermoplastic composites2007In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 67, no 13, p. 2728-2738Article in journal (Refereed)
    Abstract [en]

    In this study, silane cross-linked wood-polyethylene composite profiles were manufactured by reactive extrusion. These composites were evaluated regarding their durability and mechanical properties in comparison with two non-cross-linked wood-polyethylene composites. An addition of only 2% w/w of silane solution during manufacturing was enough to achieve almost 60% degree of cross-linking after curing. The cross-linked composites showed flexural toughness superior to the non-cross-linked composites. The cross-linked composites also absorbed less moisture during a boiling test in water and this was an indirect evidence of improved interfacial adhesion. After accelerated weathering for 1000-3000 h the general trend was a decrease in flexural modulus and strength of both the non-cross-linked and cross-linked composites. The decrease in modulus seemed to be lower for the cross-linked composites while the decrease in strength seemed to be higher compared to the non-cross-linked composites. Weathering also resulted in a considerable colour fading of the composites. Water absorption-freeze-thaw cycling decreased the flexural modulus of non-cross-linked composites considerably while there was no statistical decrease in modulus for the cross-linked composites. There was only an insignificant decrease in strength for the composites after the water absorption-freeze-thaw cycling.

  • 14.
    Bulsara, V.N.
    et al.
    School of Mechanical Engineering, Georgia Institute of Technology, Atlanta.
    Talreja, Ramesh
    Qu, JianMin
    School of Mechanical Engineering, Georgia Institute of Technology, Atlanta.
    Damage initiation under transverse loading of unidirectional composites with arbitrarily distributed fibers1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 5, p. 673-682Article in journal (Refereed)
    Abstract [en]

    A limiting property governing the thermomechanical behavior of composites is the strength transverse to the fibers. The present study investigates the dependence of this property on the distribution of fibers in the cross-section of a unidirectional composite. The approach taken is to consider actual distributions, which are arbitrary and are not necessarily described by random or periodic distributions. A fundamental issue in studying non-uniform distributions is the size of a representative volume element (RVE). By the use of an actual radial distribution function obtained for a ceramic-matrix composite by quantitative stereology in conjunction with a simulation technique developed in this study, the RVE size is investigated with respect to initiation of debonding and radial matrix cracking - two basic mechanisms governing the transverse strength of composites. Tensile loading transverse to the fibers and residual stresses induced by thermal cooldown are considered separately as loading modes for transverse failure. The results provide some useful insight into the importance of non-uniformity of fiber spatial distribution with regard to the transverse failure of composites. A limiting property governing the thermomechanical behavior of composites is the strength transverse to the fibers. The present study investigates the dependence of this property on the distribution of fibers in the cross-section of a unidirectional composite. The approach taken is to consider actual distributions, which are arbitrary and are not necessarily described by random or periodic distributions. A fundamental issue in studying non-uniform distributions is the size of a representative volume element (RVE). By the use of an actual radial distribution function obtained for a ceramic-matrix composite by quantitative stereology in conjunction with a simulation technique developed in this study, the RVE size is investigated with respect to initiation of debonding and radial matrix cracking - two basic mechanisms governing the transverse strength of composites. Tensile loading transverse to the fibers and residual stresses induced by thermal cooldown are considered separately as loading modes for transverse failure. The results provide some useful insight into the importance of non-uniformity of fiber spatial distribution with regard to the transverse failure composites.

  • 15.
    Carlson, Tony
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ordéus, Daniel
    Swerea SICOMP AB.
    Wysocki, Maciej
    Swerea SICOMP AB.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Structural capacitor materials made from carbon fibre epoxy composites2010In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 70, no 7, p. 1135-1140Article in journal (Refereed)
    Abstract [en]

    In this paper an approach towards realising novel multifunctional polymer composites is presented. A series of structural capacitor materials made from carbon fibre reinforced polymers have been developed, manufactured and tested. The structural capacitor materials were made from carbon fibre epoxy pre-preg woven laminae separated by a paper or polymer film dielectric separator. The structural capacitor multifunctional performance was characterised measuring capacitance, dielectric strength and interlaminar shear strength. The developed structural CFRP capacitor designs employing polymer film dielectrics (PA, PC and PET) offer remarkable multifunctional potential.

  • 16.
    Carlstedt, David
    et al.
    Department of Industrial and Materials Science, Chalmers University of Technology, SE-412 96, Gothenburg,.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics. RISE SICOMP AB, SE-431 22, Mölndal.
    Asp, Leif E.
    Department of Industrial and Materials Science, Chalmers University of Technology, SE-412 96, Gothenburg,.
    Effects of state of charge on elastic properties of 3D structural battery composites2019In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 169, p. 26-33Article in journal (Refereed)
    Abstract [en]

    The effects of state of charge (SOC) on the elastic properties of 3D structural battery composites are studied. An analytical model based on micromechanical models is developed to estimate the effective elastic properties of 3D structural battery composite laminae at different SOC. A parametric study is performed to evaluate how different design parameters such as volume fraction of active materials, stiffness of constituents, type of positive electrode material, etc. affect the moduli of the composite lamina for extremes in SOC. Critical parameters and configurations resulting in large variations in elastic properties due to change in SOC are identified. As the extreme cases are of primary interest in structural design, the effective elastic properties are only estimated for the electrochemical states corresponding to discharged (SOC = 0) and fully charged (SOC = 1) battery. The change in SOC is simulated by varying the volume and elastic properties of the constituents based on data from literature. Parametric finite element (FE) models for square and hexagonal fibre packing arrangements are also analysed in the commercial FE software COMSOL and used to validate the analytical model. The present study shows that the transverse elastic properties E2">E2 and G23">G23 and the in-plane shear modulus G12">G12 are strongly affected by the SOC while the longitudinal stiffness E1">E1 is not. Fibre volume fraction and the properties of the coating (such as stiffness and Poisson's ratio) are identified as critical parameters that have significant impact on the effect of SOC on the effective elastic properties of the composite lamina. For configurations with fibre volume fraction Vf">Vf ≥ 0.4 and Young's modulus of the coating of 1 GPa or higher, the transverse properties E2">E2 and G23">G23 change more than 30% between extremes in SOC. Furthermore, for configurations with high volume fractions of electrode materials and coating properties approaching those of rubber the predicted change in transverse stiffness E2">E2 is as high as +43%. This shows that it is crucial to take effects of SOC on the elastic properties into account when designing 3D structural battery composite components.

  • 17.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Asp, Leif
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Failure of NCF composites subjected to combined compression and shear loading2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, no 15, p. 2865-2877Article in journal (Refereed)
    Abstract [en]

    Earlier studies have shown that formation of kink bands is the mechanism that is likely to govern failure of compression loaded non-crimp fabric (NCF) composite laminates. Because of this, a failure criterion for prediction of failure caused by kinking under multiaxial (axial compression and shear) loading has been adapted to a NCF composite system. The criterion has been validated for compression tests of quasi-isotropic laminates tested in uniaxial compression. By performing compression tests of the laminate at different off-axis angles, it was possible to vary the ratio of compressive axial stress/shear stress in the specimens. The test results proved that the criterion works well for predictions of kinking governed failure for the present material system. Detailed fractographic studies confirmed that formation of kink bands was the mechanism responsible for specimen failure. Kink bands were also found to develop at loads significantly lower than load at specimen failure.

  • 18.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Mattsson, David
    Asp, Leif
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Formation of damage and its effects on non-crimp fabric reinforced composites loaded in tension2004In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 64, no 5, p. 675-692Article in journal (Refereed)
    Abstract [en]

    Non-crimp fabric (NCF) composites, manufactured by resin infusion techniques are one of the most promising next generation composite materials. They offer large potential for application in primary structures as they give excellent performance at low production costs. However, before NCF composites will be efficiently used in design, detailed understanding of governing micro mechanisms must be accumulated and described by predictive models. In the present study, NCF cross-ply laminates have been tested in tension. Intralaminar cracks caused in the 90° fibre bundle layers and their effect on laminate mechanical properties have been monitored. Occurrence of ‘novel' type of cracks propagating in the load direction (longitudinal cracks) is explained by a thorough FE analysis using an Representative Volume Element (RVE) approach, revealing stress concentrations caused by 0° fibre bundle waviness. Effects of damage on mechanical properties are modelled using modified micro mechanical models developed for analysis of conventional laminated composites. The analysis reveals mechanical degradation to be ruled by the crack opening displacement (COD). However, unlike traditional composites, transverse cracks do not generally extend through the entire thickness of the 90° layer, but are rather contained in single fibre bundles, limiting the COD

  • 19.
    Edgren, Fredrik
    et al.
    Swerea SICOMP AB, Mölndal.
    Soutis, C.
    University of Sheffield.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage tolerance analysis of NCF composite sandwich panels2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 13, p. 2635-2645Article in journal (Refereed)
    Abstract [en]

    This paper concerns development and validation of impact damage representations in carbon fibre non-crimp fabric reinforced face sheets for damage tolerance analysis of sandwich panels loaded in compression. For this purpose, experimental data accompanied by fractographic observations have been employed to scrutinize numerical predictions by state-of-the-art notch strength models. As a result, equivalent hole representations of visible impact damage (VID) and, more surprisingly, of the subtle barely visible impact damage (BVID) are recommended for reliable damage tolerance prediction of the compression after impact (CAI) load case for the investigated panels. This recommendation relies on the identification of the mechanisms controlling failure resulting in reliable damage tolerance predictions employing a linear cohesive zone model.

  • 20.
    Elnekhaily, Sarah A.
    et al.
    Department of Materials Science and Engineering, Texas A&M University, College Station TX; Department of Metallurgical and Materials Eng., Faculty of Petroleum and Mining Eng., Suez University, Suez, Egypt.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Department of Materials Science and Engineering, Texas A&M University, College Station, TX; Department of Aerospace Engineering, Texas A&M University, College Station, TX .
    Damage initiation in unidirectional fiber composites with different degrees of nonuniform fiber distribution2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 155, p. 22-32Article in journal (Refereed)
    Abstract [en]

    This paper reports a study of the initiation of the first failure event in unidirectional composites subjected to transverse tension. Two energy based point failure criteria – critical dilatational energy density and critical distortional energy density – are considered. The manufacturing induced disorder in the fiber distribution in the composite cross section is described in terms of the degree of nonuniformity, which is quantified and for which an algorithm is developed. The nonuniformity is captured in a representative volume element (RVE) whose minimum size is determined based on statistics of nearest fiber distance distribution. Several realizations of the RVE for three fiber volume fractions and three degrees of nonuniformity are analyzed using a finite element model. A parametric study of the effect of matrix/fiber stiffness ratio on the damage initiation is also conducted. Significant effects of the fiber distribution nonuniformity on the strain to onset of damage are found.

  • 21.
    Ericson, Mats L.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars
    Luleå tekniska universitet.
    Deformation and fracture of glass-mat-reinforced polypropylene1992In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 43, no 3, p. 269-281Article in journal (Refereed)
    Abstract [en]

    The mechanical properties of glass-mat-reinforced thermoplastics, GMT, can be varied within wide limits through the choice of glass mat, fibre content, and thermoplastic matrix. A great variety of fibre configurations and fibre contents is commercially available. In order to understand how the fibre structure of GMT materials controls the mechanical properties, two structurally different GMT materials were studied. Several grades with different fibre contents were used for each material. One material had short fibres (12 mm long), discretely dispersed with in-plane random orientation. The other material had in-plane continuous looped bundles of fibres. The deformation and fracture of these materials have been studied as a function of fibre content. The present work shows that the material with discretely dispersed fibres has higher values of tensile creep modulus, tensile strength, elongation at fracture, and work of fracture (area under the stress/strain curve). The results are discussed and related to the structural differences between the two materials.

  • 22.
    Ericson, Mats L.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Processing and mechanical properties of orientated preformed glass-mat-reinforced thermoplastics1993In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 49, no 2, p. 121-130Article in journal (Refereed)
    Abstract [en]

    The stiffness and strength of moulded, glass-mat-reinforced thermoplastics (GMT) components would be increased by the presence of highly orientated fibres at critical locations. A previously described method to produce preformed GMT materials was therefore further developed to make orientation of the fibres in the preform possible. Two ‘orientation plates' were used to orientate the fibres during spray-up of a glass-fibre/polyethylene preform. The preform was then heated by hot gas and compression moulded. The ratio of the highest and lowest stiffnesses of a given plate was in the range of 27 to 38. Micromechanics equations were used with classical lamination theory to design a model laminate with stiffness properties in close agreement with experimental data. Reasons for the relatively wide fibre orientation distribution and low fibre length efficiency factor obtained for the model laminate are discussed and improvements suggested.

  • 23.
    Farge, L.
    et al.
    Nancy Université-INPL.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Z.
    Nancy Université-INPL.
    Damage characterization of a cross-ply carbon fiber/epoxy laminate by an optical measurement of the displacement field2010In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 70, no 1, p. 94-101Article in journal (Refereed)
    Abstract [en]

    Using Electronic Speckle Pattern Interferometry (ESPI), full-field displacement measurement was performed on the edge of a cracked cross-ply graphite/epoxy laminate subjected to a tensile loading. The displacement jumps corresponding to cracks are clearly visible and can be used to determine the Crack Opening Displacement (COD) values along the cracks. The main objective of this study is to determine if the application of successive loads of increasing magnitude may have modified the existing cracks and thereby changed the COD dependence on the applied stress. Moreover, we have tested the applicability of the assumed linear elastic COD behavior in the presence of very high stress concentration at the crack tips. The profile of the opening along the crack was also studied.

  • 24.
    Fernberg, Patrik
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A
    Luleå tekniska universitet.
    Bridging law and toughness characterisation of CSM and SMC composites2001In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, no 16, p. 2445-2454Article in journal (Refereed)
    Abstract [en]

    This work presents an experimental investigation of the fracture properties of three different short-fiber-reinforced composites [one chopped strand mat (CSM) and two sheet molding compound (SMC) materials]. Fracture tests are performed on double-cantilever beam (DCB) specimens loaded with pure bending moments. In this experimental configuration, the bridging law for the material can be derived directly from measurements. No significant dependency on specimen height was observed in our results. The bridging laws determined can, therefore, be considered as material properties. The coupling between microstructure and fracture behaviour is discussed through the measured bridging laws. The beneficial effect (in terms of fracture energy) of increasing tendency for pull-out is confirmed for one SMC, referred to as Flex-SMC, which shows remarkably high fracture energy, Jc=56.0 kJ/m2, compared to a standard SMC, termed Std-SMC, Jc=25.9 kJ/m2. This increasing tendency for pull-out is observed to shift the bridging law towards larger crack openings. On the basis of our observations we find the concept of characterising the failure behaviour in terms of bridging laws attractive since it can be used as a tool for the tailoring of the microstructure towards desired fracture behaviour.

  • 25.
    Fernberg, Patrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Jekabsons, Normunds
    Luleå tekniska universitet.
    Determination of bridging laws for SMC materials from DENT tests2003In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 63, no 15Article in journal (Refereed)
    Abstract [en]

    bridging law (or cohesive zone law) approach is employed to evaluate the fracture of double edge notched tensile (DENT) specimen from two SMC materials (Std-SMC and Flex-SMC) with considerable difference in fracture characteristics. Linear- and non-linear FEM was used to separate volumetric body contributions and true crack opening from measured displacements (measured with extensometer) over the cracked region. We found that extrinsic non-linear material response gave a significant contribution to measured displacements. The paper also considers the influence of specimen dimensions on the characteristics of fracture. This is of great importance since stable crack growth is required in order to allow a complete determination of the bridging law in a single experiment. By applying corrections for volumetric displacements in the data reduction scheme we were able to estimate the bridging laws of the two SMCs respectively. They were found to be of decreasing nature and two distinct regions were identified. For small δ, a steeply decreasing part where debonding and fiber fracture are major micro-scale failure mechanism, is identified. At larger δ, a transition to a less steep bridging law response is observed. Pullout friction governs the bridging law at that stage

  • 26.
    Gamstedt, E.K.
    et al.
    Luleå tekniska universitet.
    Sjögren, B.A.
    Luleå tekniska universitet.
    Micromechanisms in tension-compression fatigue of composite laminates containing transverse plies1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 49, no 2, p. 167-178Article in journal (Refereed)
    Abstract [en]

    For both monotonic and fatigue loading conditions, debonding is the subcritical micromechanism which leads to transverse cracking and ultimately influences final failure of the composite structure. Previous studies show that tension-compression fatigue is more detrimental than tension-tension fatigue to transverse and multidirectional laminates. By analysing the debonding mechanisms and modelling thereof, the macroscopic fatigue behaviour can be better understood. Also, the dominant crack-propagation mode can be identified which may be of use in selection of constituent material properties to optimise fatigue resistance. In this study, glass-fibre-reinforced vinyl-ester was used. The adverse effect of compressive load excursions was verified by counting the transverse cracks in cross-ply laminates. The mechanisms were studied in low-cycle fatigue of a specimen containing a single transverse fibre. Compressive load cycles led to significantly increased debond growth. In tension, contact zones developed at the crack tips for sufficiently large debonds. Because of the mismatch in elastic properties, an opening zone appeared at the tips of the interfacial crack when the same debond was subjected to a compressive load. Since debond propagation is more susceptible to mode I loading, the sensitivity to tension-compression fatigue is explained by the effective crack-tip opening in compressive loading for sufficiently large debond cracks. This has also been verified by finite-element analysis.

  • 27.
    Gamstedt, E.Kristofer
    et al.
    Luleå tekniska universitet.
    Berglund, Lars. A.
    Luleå tekniska universitet.
    Peijs, Ton
    Eindhoven University of Technology.
    Fatigue mechanisms in unidirectional glass-fibre-reinforced polypropylene1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 5, p. 759-768Article in journal (Refereed)
    Abstract [en]

    Polypropylene (PP) and polypropylene modified with maleic anhydride (MA-PP) reinforced by continuous longitudinal glass fibres have been investigated. The most prominent effect of the modification with maleic anhydride in the composite is a stronger fibre/matrix interface. The effects of interfacial strength on fatigue performance and on the underlying micromechanisms have been studied for these composite systems. Tension-tension fatigue tests (R=0.1) were carried out on 0 glass-fibre/PP and glass-fibre/MA-PP coupons. The macroscopic fatigue behaviour was characterized in terms of stiffness reduction and fatigue-life curves. The results showed that the longitudinal Young's modulus degraded more rapidly for glass-fibre/PP, which was caused by a higher degree of damage growth and accumulation. The improvement in monotonic strength was negligible, but the fatigue life was prolonged by about one decade for the composite with the stronger interface by use of the maleic-anhydride grafted polypropylene matrix. During the fatigue testing, the microscopic mechanisms were monitored intermittently by a surface replication technique. From microscopic observations, it could be concluded that the better fatigue resistance of glass-fibre/MA-PP can be attributed to the greater interfacial strength and the resistance to debond propagation

  • 28.
    Geng, Shiyu
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Haque, MD Minhaz Ul
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Crosslinked polyvinyl acetate (PVAc) reinforced with cellulose nanocrystals (CNC) – structure and mechanical properties2016In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 126, p. 35-42Article in journal (Refereed)
    Abstract [en]

    The structure of cellulose-based nanocomposites significantly influences their final mechanical properties. However, obtaining a good dispersion of hydrophilic nanocellulose materials in a hydrophobic polymer matrix is challenging. In this study, two unique methods were developed to improve the dispersion of cellulose nanocrystals (CNC) in a poly(vinyl acetate) (PVAc) matrix. One method was the crosslinking of PVAc by sodium tetraborate (borax), which is expected to prevent agglomeration of CNCs during the drying process, and the other method was the in-situ polymerization of vinyl acetate in the presence of CNCs to generate good compatibility between CNC and PVAc. The results showed that the crosslinking degree of PVAc could be varied by tuning the pH. The atomic force microscopy images illustrate that after drying, the in-situ polymerized PVAc/CNC composite was much better dispersed than the composite produced using mechanical mixing. The mechanical and thermo-mechanical characterizations indicate that the in-situ nanocomposite with 10 wt% of CNC had a higher strength and storage modulus compared with the mixed composite with the same CNC concentration. Further investigations of the restriction effect caused by the crosslinker are required.

  • 29.
    Graciani, Enrique
    et al.
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Mantič, Vladislav
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Paris, Federico
    Escuela Superior de Ingenieros, Universidad de Sevilla.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Numerical analysis of debond propagation in the single fibre fragmentation test2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 15-16, p. 2514-2520Article in journal (Refereed)
    Abstract [en]

    A numerical analysis, using the Boundary Element Method, of the stress state within the specimen in the Single Fibre Fragmentation Test is presented first. Thermal residual stresses and fibre-matrix interfacial friction along the debonding crack faces have been considered in the study. Special attention has been paid to the axial stresses along the fibre and the interfacial tractions and relative displacements in the neighbourhood closest to the debonding crack tips. In order to analyze the debond propagation, the associated Energy Release Rate has been evaluated from the near-tip elastic solution. Numerical results show that both the effects of thermal residual stresses and of fibre-matrix interfacial friction are opposed to the debond propagation. Additionally, the effect of the debond propagation on the load transfer through the interface has been studied, showing that fibre-matrix interfacial friction has a weak influence on the distance needed to re-establish the nominal axial load within the fragment.

  • 30. Grubbström, Göran
    et al.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Influence of wood flour moisture content on the degree of silane-crosslinking and its relationship to structure property relations of wood-thermoplastic composites2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 7-8, p. 1045-1050Article in journal (Refereed)
    Abstract [en]

    The objective of this work was to examine how the moisture content of wood flour affects the degree of crosslinking when producing silane-crosslinked wood-thermoplastic composites. Crosslinked composites were produced by adding a silane solution to the compounding process of wood flour and polyethylene. Crosslinked composites of pre-dried as well as non-dried wood flour were prepared and their degree of crosslinking at various storage conditions was determined. Mechanical properties and the creep response of the crosslinked composites were tested in order to establish structure-properties relations. The results showed that all crosslinked composites displayed higher strengths and lower creep responses compared with non-crosslinked control samples. However, the degree and rate of crosslinking proved to be lower when a larger amount of moisture was present in the compounding process. It was concluded that the silane-grafting yield was lower when wood flour of a higher moisture content was used.

  • 31.
    Hagberg, Johan
    et al.
    Applied Electrochemistry, Department of Chemical Engineering, KTH Royal Institute of Technology.
    Maples, Henry A.
    Polymer and Composite Engineering (PaCE) Group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna.
    Alvim, Kayne S.P.
    Polymer and Composite Engineering (PaCE) Group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna.
    Xu, Johanna
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Johannisson, Wilhelm
    Lightweight Structures, Department of Aeronautical and Vehicle Engineering, KTH Royal Institute of Technology.
    Bismarck, Alexander
    Polymer and Composite Engineering (PaCE) Group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna.
    Zenkert, Dan
    Lightweight Structures, Department of Aeronautical and Vehicle Engineering, KTH Royal Institute of Technology.
    Lindbergh, Göran
    Applied Electrochemistry, Department of Chemical Engineering, KTH Royal Institute of Technology.
    Lithium iron phosphate coated carbon fiber electrodes for structural lithium ion batteries2018In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 162, p. 235-243Article in journal (Refereed)
    Abstract [en]

    A structural lithium ion battery is a material that can carry load and simultaneously be used to store electrical energy. We describe a path to manufacture structural positive electrodes via electrophoretic deposition (EPD) of LiFePO4 (LFP), carbon black and polyvinylidene fluoride (PVDF) onto carbon fibers. The carbon fibers act as load-bearers as well as current collectors. The quality of the coating was studied using scanning electron microscopy and energy dispersive X-ray spectroscopy. The active electrode material (LFP particles), conductive additive (carbon black) and binder (PVDF) were found to be well dispersed on the surface of the carbon fibers. Electrochemical characterization revealed a specific capacity of around 60–110 mAh g−1 with good rate performance and high coulombic efficiency. The cell was stable during cycling, with a capacity retention of around 0.5 after 1000 cycles, which indicates that the coating remained well adhered to the fibers. To investigate the adhesion of the coating, the carbon fibers were made into composite laminae in epoxy resin, and then tested using 3-point bending and double cantilever beam (DCB) tests. The former showed a small difference between coated and uncoated carbon fibers, suggesting good adhesion. The latter showed a critical strain energy release rate of ∼200–600 J m−2 for coated carbon fibers and ∼500 J m−2 for uncoated fibers, which also indicates good adhesion. This study shows that EPD can be used to produce viable structural positive electrodes.

  • 32.
    Hooshmand, Saleh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Aitomäki, Yvonne
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Linn
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mathew, Aji P.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Enhanced alignment and mechanical properties through the use of hydroxyethyl cellulose in solvent-free native cellulose spun filaments2017In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 150, p. 79-86Article in journal (Refereed)
    Abstract [en]

    In this study, the addition of hydroxyethyl cellulose (HEC) in cellulose nanofiber filaments is shown to improve the solvent-free processing and mechanical properties of these biobased fibers as well as their compatibility with epoxy. An aqueous dope of cellulose nanofiber (CNF) with HEC was spun and the resulting filaments cold-drawn. The HEC increased the wet strength of the dope allowing stable spinning of low concentrations of CNF. These lower concentrations promote nanofiber alignment which is further improved by cold-drawing. Alignment improves the modulus and strength and an increase of over 70% compared to the as-spun CNF only filaments was achieved. HEC also decreases hydrophilicity thus increasing slightly the interfacial shear strength of the filaments with epoxy resin. The result is continuous biobased fibers with improved epoxy compatibility that can be prepared in an upscalable and environmentally friendly way. Further optimization is expected to increase draw ratio and consequently mechanical properties.

  • 33.
    Huang, Hansong
    et al.
    Department of Aerospace Engineering, Texas A&M University.
    Talreja, Ramesh
    Effects of void geometry on elastic properties of unidirectional fiber reinforced composites2005In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 65, no 13, p. 1964-1981Article in journal (Refereed)
    Abstract [en]

    Voids are one of the most common types of manufacturing process induced defects in composite materials that have detrimental effect on the material properties. The void content can be reduced by carefully chosen process parameters, such as pressure and temperature, but often at the price of higher cost. A quantitative relationship between void characteristics and material properties would allow the trade-off between the cost and the desired product performance. The characteristics of interest include volume fraction, size, shape, and spatial distribution. In this paper, methods for determining effective elastic constants of unidirectional continuous fiber reinforced composites containing voids of various characteristics are presented. Finite element analysis (FEA) is performed on a representative volume cell based on observed void microstructure to determine the effective elastic constants. Analytical method based on Mori-Tanaka theory is also utilized for comparison. The predictions by FEA and the analytical method are compared with each other as well as with available experimental data. Overall good agreement is found. A parametric study reveals that the void content has severe impact on the out-of-plane modulus, while the in-plane properties are less significantly affected. For a given void content, the shape of the voids has different effect on different moduli. Flat voids are benign for in-plane moduli but undesirable for out-of-plane stiffness. Long voids reduce significantly the out-of-plane shear modulus, but have little effect on the in-plane properties

  • 34.
    Huang, Hansong
    et al.
    Department of Aerospace Engineering, Texas A&M University.
    Talreja, Ramesh
    Numerical simulation of matrix micro-cracking in short fiber reinforced polymer composites: Initiation and propagation2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, no 15, p. 2743-2757Article in journal (Refereed)
    Abstract [en]

    This paper presents a numerical simulation based analysis of micro-cracking in short fiber reinforced polymer composites. For this case, the conventional linear elastic fracture mechanics approach is shown not to be useful; instead, the Rice-Tracey ductile fracture model is shown to work well in the framework of the local approach to fracture. The model is first applied to the case of matrix cracking from the broken fiber end in a fiber fragmentation test of a single-fiber reinforced composite. The model predicts the measured conical crack path successfully, including the crack initiation angle and the kink formation as the crack propagates away from the fiber. Furthermore, the predicted dependence of the crack length on the nominal strain is found to be in qualitative agreement with measured data. Next the model is applied to micro-cracking in an aligned short fiber composite. The analysis predicts propagation of a matrix crack from the debonded fiber end towards the neighboring fiber at an oblique angle to the fiber axis. Before reaching the neighboring fiber, the crack is found to divert gradually towards the fiber axis. This behavior explains the so-called fiber-avoidance cracking mode reported in the literature. A parametric study is performed to reveal the dependence of the locally-averaged failure stress/strain on the fiber length and volume fraction

  • 35.
    Huang, Yongxin
    et al.
    Wind R&D Center, Siemens Energy.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Statistical Methodology for Assessing Manufacturing Quality Related to Transverse Cracking in Cross Ply Laminates2014In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 95, p. 100-106Article in journal (Refereed)
    Abstract [en]

    We present a statistical analysis based methodology for making assessment of the manufacturing quality of cross ply composite laminates as it relates to its effect on transverse cracking evolution. Assuming a two-parameter Weibull distribution of tensile strength of the transverse plies to represent randomly distributed manufacturing defects, multiple crack formation in the plies is simulated in the non-interactive and interactive regimes of cracking using the local stress fields calculated by a variational analysis. The statistical methodology is demonstrated on crack density evolution in cross ply laminates manufactured by four different processing routes and loaded in monotonic tension in the axial direction. The differences in the crack density evolution, supposedly due to different defect population induced by the four manufacturing conditions, could be described by the proposed statistical simulation method.

  • 36.
    Hui, C.Y.
    et al.
    Cornell University, Ithaca.
    Shia, D.
    Cornell University, Ithaca.
    Berglund, Lars
    Luleå tekniska universitet.
    Estimation of interfacial shear strength: an application of a new statistical theory for single fiber composite test1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 13, p. 2037-2046Article in journal (Refereed)
    Abstract [en]

    A simple procedure is proposed for estimating the effect of different sizings on the strength of fiber/matrix interface. This procedure is based on a newly developed statistical theory for single-fiber composite tests. This procedure is used to evaluate the interfacial shear strength of HTA carbon fiber in an epoxy matrix with two different sizings. It is shown that if the experiment data are presented in the form of the normalized variables, and σ/σc, the two sets of data corresponding to the two different sizings can be collapsed into a single curve which can be obtained theoretically. Excellent agreement between experiments and theory is found. An argument is presented for the use of the interfacial shear strength as an engineering parameter to characterize interface adhesion. We also investigate the effect of residual stresses due to thermal-expansion coefficient mismatch between the fiber and the matrix on the interpretation of single-fiber composite tests. We found that the experimentally determined Weibull parameters are very sensitive to the effect of these residual stresses and must be taken into consideration when interpreting single-filament composite tests.

  • 37.
    Jekabsons, Normunds
    et al.
    Luleå tekniska universitet.
    Fernberg, Patrik
    Luleå tekniska universitet.
    Prediction of progressive fracture of SMC by application of bridging laws2003In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 63, no 15, p. 2133-2142Article in journal (Refereed)
    Abstract [en]

    Experimentally obtained load vs. displacement curves from compact tension tests (CT) of two different SMC materials are analyzed in this paper. Three different CT specimen geometries are considered. Progressive fracture is attained in all tests. This gives rise to a long post-peak tail part in the load vs. displacement curve. By implementing bridging laws and volumetric stiffness degradation of bulk SMC in an FEM model we are able to reproduce the two larger geometries considered (50 mm×50 mm and 100 mm×100 mm) with high accuracy. Discrepancy between model predictions and experiments for the smallest geometry considered (20×20 mm) was observed. This was due to premature compressive failure on the side opposite to the CT specimen precrack. The successful use of bridging laws strongly suggests that they are intrinsic properties governing fracture behavior of SMC materials. The fact that our bridging laws were determined based on independent tests on different specimen geometry is adding strong arguments to preceding conclusion.

  • 38.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Varna, Janis
    COD-based simulation of transverse cracking and stiffness reduction in [S/90n]s laminates2001In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, no 5, p. 637-656Article in journal (Refereed)
    Abstract [en]

    Closed-form expressions for the thermo-elastic properties of [S/90n]s laminates with transverse cracks in the 90° layers are derived. Provided the normalised average crack-opening displacement (COD) is known, expressions contain only crack density, geometrical parameters and elastic constants of layers. The average COD dependence on the crack spacing and constraint effect of adjacent sub-laminates is analysed by using finite-element method in plane stress formulation. It is found that the out-of-plane elastic constants have an insignificant effect on COD. A simple power law relating average COD to elastic and geometrical parameters of constituents is derived. The obtained power law and the developed methodology are successfully used to predict the reduction of thermo-elastic properties and damage evolution of [±θ/904]s laminates. The crack-closure technique and Monte-Carlo simulations are used to model the damage development. The 90° layer is divided in to a large number of elements and Gc values are assigned to each element according to a Weibull distribution. Parameters in the Weibull distribution are determined by using experimental crack density versus strain curve for glass-fibre/epoxy [02/904]s cross-ply laminates. Damage development in [S/904]s laminates of the same material, containing sub-laminates with ±θ layers only, is modelled by using these Weibull parameters and the results are in good agreement with test data. The effect of the thickness of the 90° layer on damage development is discussed in strength and fracture mechanics formulation.

  • 39.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Analytical modeling of stiffness reduction in symmetric and balanced laminates due to cracks in 90° layers1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 11, p. 1641-1652Article in journal (Refereed)
    Abstract [en]

    Stiffness reduction in [S,90n]s symmetric laminates, containing orthotropic sub-laminates (S) and cracked 90° layer, is analyzed. Closed-form expressions relating stiffness changes to the transverse crack density are derived. They contain only material properties, laminate geometry and a stress-perturbation function that is proportional to the normalized average crack-opening displacement. Stress-distribution models [shear lag, based on variational approach, and finite-element analysis (FEA)] are adopted for the [S,90n]s configurations and used to calculate the stress-perturbation function. Predictions are compared with experimental data for [θ, 904]s θ=0, 15, 30, 40 glass-fiber/epoxy-resin laminates. Generally the FEA model slightly underestimates stiffness reduction whereas both of the variational models used lead to similar results, slightly lower than experimental. Even the shear-lag model may be successfully used if the shear-lag parameter is first obtained from fitting test results for cross-ply laminate of the same material.

  • 40.
    Jonoobi, Mehdi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Harun, Jalaluddin
    University Putra Malaysia.
    Mathew, Aji P.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanical properties of cellulose nanofiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion2010In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 70, no 12, p. 1742-1747Article in journal (Refereed)
    Abstract [en]

    The aim of this study was to develop cellulose nanofiber (CNF) reinforced polylactic acid (PLA) by twin screw extrusion. Nanocomposites were prepared by premixing a master batch with high concentration of CNFs in PLA and diluting to final concentrations (1, 3, 5 wt%) during the extrusion. Morphology, mechanical and dynamic mechanical properties (DMA) were studied theoretically and experimentally to see how different CNF concentrations affected the composites' properties. The tensile modulus and strength increased from 2.9 GPa to 3.6 GPa and from 58 MPa to 71 MPa, respectively, for nanocomposites with 5 wt% CNF. The DMA results were also positive; the storage modulus increased for all nanocomposites compared to PLA; being more significant in the high temperature region (70°C). The addition of nanofibers shifted the tan delta peak towards higher temperatures. The tan delta peak of the PLA shifted from 70°C to 76°C for composites with 5 wt% CNF.

  • 41.
    Katerelos, D.T.G.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Kashtalyan, M.
    University of Aberdeen.
    Soutis, C.
    University of Sheffield.
    Galiotis, C.
    FORTH/ICE-HT, Patras.
    Matrix cracking in polymeric composites laminates: modelling and experiments2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 12, p. 2310-2317Article in journal (Refereed)
    Abstract [en]

    Composites ability to retain functionality in the presence of damage is a crucial safety and economic issue. Generally the first damage mode in composite laminates is matrix cracking, which affects the mechanical properties of the structure long before its load-bearing capacity is exhausted. In this paper, a detailed analysis of the effect of matrix cracking on the behaviour of cross-ply [0/90]s and unbalanced symmetric [0/45]s glass/epoxy laminates loaded statically in tension is performed. Theoretical predictions of stiffness reduction due to damage are based on the Equivalent Constraint Model (ECM), which takes into account concurrent matrix cracking in all plies of the laminate, although matrix cracking under consideration is developing only within the off-axis ply of the laminates. The longitudinal Young's modulus predictions are compared to experimentally derived data obtained using laser Raman spectroscopy (LRS). The good agreement between predicted and measured values of the reduced longitudinal Young's modulus validates the ECM model and proves that its basic assumptions are accurate. Thus, the predictions for all the mechanical properties by the ECM model are within a realistic range, while experimental evidence is required for further validation.

  • 42.
    Katerelos, D.T.G.
    et al.
    Institute of Chemical Engineering and High Temperature Processes.
    Lundmark, P.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Galiotisa, C.
    University of Patras.
    Analysis of matrix cracking in GFRP laminates using Raman spectroscopy2007In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 67, no 9, p. 1946-1954Article in journal (Refereed)
    Abstract [en]

    A Raman spectroscopic technique was used to investigate the elastic modulus degradation and the residual strain development in cross-ply and [0/452/0]T glass fibre/epoxy laminates. Glass has a poor Raman signal and, therefore, embedded aramid fibres were used as strain sensors for mapping of the internal strain development within the 0° ply due to matrix cracking in the off-axis ply. The elastic modulus and the residual strain were derived from the changes of the distance between well identified strain "peaks" on the aramid fibres. The elastic modulus reduction and the residual strain development are described by a closed form model. The modulus reduction predictions are accurate, whereas the experimental residual strain is higher than that predicted. Since both phenomena are governed by the same elastic parameters, the discrepancy cannot be explained by purely elastic analysis. Development of inelastic strains in the off-axis layer is suggested as a possible cause

  • 43.
    Katerelos, D.T.G.
    et al.
    FORTH/ICE-HT, Patras.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Galiotis, C.
    FORTH/ICE-HT, Patras.
    Energy criterion for modelling damage evolution in cross-ply composite laminates2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 12, p. 2318-2324Article in journal (Refereed)
    Abstract [en]

    The energy dissipated in cross-ply laminates during loading-unloading loops is obtained from stress-strain curves for cross-ply laminates and used in an energy based approach to predict the development of matrix cracking. The dissipated energy is correlated to the crack density growth data recorded for a reference laminate. The critical strain energy release rate, Gc obtained in this way is increasing with the applied strain. This phenomenon reflects the statistical nature of Gc distribution in the 90-layer: the first cracks (lower strain) develop in positions with lower fracture toughness. The obtained Gc data are in a good agreement with fracture toughness data obtained using LEFM based "compliance calibration" model in which the stiffness change with increasing strain is used. Finally, the matrix cracking development is successfully simulated using in the LEFM model, the data for critical strain energy release rate and an earlier derived stiffness-crack density relationship. It has been demonstrated that knowing the laminates geometry and measuring the laminate stiffness reduction with strain or (alternatively measuring the dissipated energy) the damage evolution may be simulated, thus reducing the necessity for optical observations to validation only.

  • 44.
    Kim, A.S.
    et al.
    Chalmers University of Technology.
    Bengtsson, S.
    Warren, Richard
    Luleå tekniska universitet.
    Fracture strength testing of δ-alumina fibres with variable diameters and lengths1993In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 47, no 4, p. 331-333Article in journal (Refereed)
    Abstract [en]

    Tensile tests were performed on individual δ-alumina fibres (Saffil, RF grade). The results revealed a large scatter in strengths and a clear dependence of the fracture strength on the specimen volume. The tests were evaluated on the basis of the Weibull probability function, a special modification of the Weibull analysis being developed that successfully copes with the problem of testing fibres with various diameters and test lengths. For the sample studied the Weibull modulus, m, was found to be 2·2, with a scaling constant δ0 = 6·0 MPa (units of volume mm3; i.e. V0 = 1 mm3).

  • 45.
    Lee, Koon-Yang
    et al.
    Department of Chemical Engineering, University College London.
    Aitomäki, Yvonne
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Wallenberg Wood Science Centre, Royal Institute of Technology.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Bismarck, Alexander
    Vienna University of Technology, Polymer & Composite Engineering (PaCE) Group, Department of Chemical Engineering, Imperial College London, South Kensington Campus.
    On the use of nanocellulose as reinforcement in polymer matrix composites2014In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 105, p. 15-27Article in journal (Refereed)
    Abstract [en]

    Nanocellulose is often being regarded as the next generation renewable reinforcement for the production of high performance biocomposites. This feature article reviews the various nanocellulose reinforced polymer composites reported in literature and discusses the potential of nanocellulose as reinforcement for the production of renewable high performance polymer nanocomposites. The theoretical and experimentally determined tensile properties of nanocellulose are also reviewed. In addition to this, the reinforcing ability of BC and NFC is juxtaposed. In order to analyse the various cellulose-reinforced polymer nanocomposites reported in literature, Cox-Krenchel and rule-of-mixture models have been used to elucidate the potential of nanocellulose in composite applications. There may be potential for improvement since the tensile modulus and strength of most cellulose nanocomposites reported in literature scale linearly with the tensile modulus and strength of the cellulose nanopaper structures. Better dispersion of individual cellulose nanofibres in the polymer matrix may improve composite properties

  • 46.
    Leijonmarck, Simon
    et al.
    Division of Applied Electrochemistry, Department of Chemical Engineering and Technology, KTH Royal Institute of Technology.
    Carlson, Tony
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lindbergh, Göran
    Division of Applied Electrochemistry, Department of Chemical Engineering and Technology, KTH Royal Institute of Technology.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Maples, Henry A.
    Polymer & Composite Engineering (PaCE) Group, Department of Chemical Engineering, Imperial College London, South Kensington Campus.
    Bismarck, Alexander
    Polymer & Composite Engineering (PaCE) Group, Department of Chemical Engineering, Imperial College London, South Kensington Campus.
    Solid polymer electrolyte-coated carbon fibres for structural and novel micro batteries2013In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 89, p. 149-157Article in journal (Refereed)
    Abstract [en]

    We report a method to deposit a thin solid polymer electrolyte (SPE) coating around individual carbon fibres for the realisation of novel battery designs. In this study an electrocoating method is used to coat methacrylate-based solid polymer electrolytes on to carbon fibres. By this approach a dense uniform, apparently pinhole-free, poly(methoxy polyethylene glycol (350) monomethacrylate) coating with an average coating thickness of 470 nm was deposited around carbon fibres. Li-triflate, used as supporting electrolyte remained in the coating after the electrocoating operation. The Li-ion content in the solid polymer coating was found to be sufficiently high for battery applications. A battery device was built employing the SPE coated carbon fibres as negative electrode demonstrating reversible specific capacity of 260 mAh/g at low currents (C/10), suggesting that these coated carbon fibres can be employed in future structural composite batteries.

  • 47.
    Lindhagen, Johan E.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Application of bridging-law concepts to short-fibre composites: 1. DCB test procedures for bridging law and fracture energy2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 6, p. 871-883Article in journal (Refereed)
    Abstract [en]

    This is the first paper in a series of four where notch sensitivities, fracture energies and bridging laws of short-fibre polymer composites are investigated. In the context of crack-bridging, the bridging law is an important material parameter. The bridging law can be used in combination with a stress analysis to address failure problems, for instance large-scale bridging, where linear elastic fracture mechanics is not valid. The bridging law of the material is sensitive to material composition and fibre architecture. Owing to the lack of established procedures, it is of interest to develop experimental and analytical methods for determination of the bridging law and fracture energy of short-fibre polymer composites. A method based on a large DCB specimen loaded by pure bending moments is used. Commercial GMT and SMC materials are investigated in addition to chopped-strand-mat laminates based on glass fibres of two different lengths and two thermoset matrices of different ductility. Fracture energies and bridging law data are successfully determined. All materials demonstrate softening bridging laws and this is discussed on the basis of observed mechanisms of failure and existing micromechanical models.

  • 48.
    Lindhagen, Johan E.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Application of bridging-law concepts to short-fibre composites: 2. Notch sensitivity2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 6, p. 885-893Article in journal (Refereed)
    Abstract [en]

    This is the second paper in a series of four where notch sensitivities, fracture energies and bridging laws are studied in short-fibre polymer composites. Estimates based on an order-of-magnitude criterion indicate that previous notch sensitivity studies on short-fibre composites are limited to small notch sizes in the ductile region. For this reason, centre-hole notch sensitivity is studied experimentally as a function of relatively large notch diameters in the range 15-60 mm. The materials have different matrices, glass-fibre content and fibre lengths. The onset of notch sensitivity is observed for all materials (glass-mat thermoplastics, sheet-moulding compounds and chopped-strand-mat laminates), although large notch sizes are required. The reasons for this are discussed as well as the influence of different material parameters. On the basis of the material bridging law and laminate Young's modulus, it was possible to rank different short-fibre composites with respect to notch sensitivity.

  • 49.
    Lindhagen, Johan E.
    et al.
    Luleå tekniska universitet.
    Gamstedt, E.K.
    Risø National Laboratory, Roskilde.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Application of bridging-law concepts to short-fibre composites: 3. Bridging law derivation from experimental crack profiles2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 16, p. 2883-2894Article in journal (Refereed)
    Abstract [en]

    This is the third paper in a series of four where notch sensitivity, fracture energy and bridging laws are studied in short-fibre polymer composites. Here, bridging laws are derived from experimental crack-opening profiles in centre-hole notched tensile specimens. The materials studied are three types of commercial glass-mat composites with different reinforcement structures and matrices. The materials have softening bridging laws and the calculated fracture energies from bridging laws are in good agreement with values determined directly by experiment. The calculated maximum local bridging stress is found to be higher than the uniaxial tensile strength. An outline of a failure criterion for notched specimens based on the crack-bridging approach is presented.

  • 50.
    Lindhagen, Johan E.
    et al.
    Luleå tekniska universitet.
    Jekabsons, Normunds
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Application of bridging-law concepts to short-fibre composites: 4. FEM analysis of notched tensile specimens2000In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 60, no 16, p. 2895-2901Article in journal (Refereed)
    Abstract [en]

    This is the fourth paper in a series of four where notch sensitivities, fracture energies and bridging laws in short-fibre polymer composites are investigated. In this paper finite-element modelling (FEM) of centre-hole-notched tensile specimens is performed, with different bridging laws governing crack growth. Crack lengths, crack profiles and stress distributions are predicted. The results are compared with experimentally determined crack shapes from an earlier investigation. Only with softening bridging laws can the experimental results be matched. The predicted crack lengths are sensitive to bridging-law parameters. When bridging laws determined by the double cantilever beam (DCB) method are applied, the predicted crack lengths and profiles show good correlation with the experimental results. The results support the validity of the DCB method to determine bridging laws in short-fibre composites.

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