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  • 1. 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, p. 351-357Article 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.

  • 2.
    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, p. 1503-1517Article 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.

  • 3.
    Di Stasio, Luca
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. IJL, EEIGM, Université de Lorraine.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    IJL, EEIGM, Université de Lorraine.
    Effect of the proximity to the 0°/90° interface on Energy Release Rate of fiber/matrix interface crack growth in the 90°-ply of a cross-ply laminate under tensile loading2019In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793XArticle in journal (Refereed)
    Abstract [en]

    Models of Representative Volume Elements (RVEs) of cross-ply laminates with different geometric configurations and damage states are studied. Debond growth is characterized by the estimation of the Mode I and Mode II Energy Release Rate (ERR) using the Virtual Crack Closure Technique (VCCT). It is found that the presence of the 0°/90° interface and the thickness of the 0° layer have no effect, apart from laminates with ultra-thin 90° plies where it is however modest. The present analysis support the claim that debond growth is not affected by the plythickness effect.

  • 4.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Asp, Leif
    Bull, Peter H.
    Kungliga tekniska högskolan, KTH.
    Compressive failure of impacted NCF composite sandwich panels: characterisation of the failure process2004In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 38, no 6, p. 495-514Article in journal (Refereed)
    Abstract [en]

    In the present study, non-crimp fabric (NCF) composite face sheet sandwich panels have been tested in compression after impact (CAI). Damage in the face sheets was characterised by fractography. Compression after impact loaded panels were found to fail by plastic fibre microbuckling (kinking) in the damaged face sheet. Studies of panels for which loading was interrupted prior to failure revealed extensive stable kink band formation at several positions and in numerous plies. Kink bands initiated and propagated within a wide region close to the point of impact. In addition, kink bands initiated in zones with high shear stresses, away from the impact centre line. Consequently, the fractographic results from this investigation do not support the assumption of modelling the impact damage as an equivalent hole. To achieve accurate predictions of kink band initiation, the stress field must be known. The results from this study imply that bending effects caused by remaining dent or material eccentricities in the damaged region must be considered.

  • 5.
    Fadel, Shaimaa M
    et al.
    Cellulose and Paper Department, National Research Center, Cairo.
    Hassan, Mohammad L
    Cellulose and Paper Department, National Research Center, Cairo.
    Oksman, Kristiina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Improving tensile strength and moisture barrier properties of gelatin using microfibrillated cellulose2013In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 16, p. 1977-1985Article in journal (Refereed)
    Abstract [en]

    Microfibrillated cellulose isolated from bagasse was used to prepare novel nanocomposites using cross-linked gelatin as a biodegradable polymer matrix. Microfibrillated cellulose loadings up to 25% (weight percent) were used. The prepared nanocomposites were characterized regarding their wet and dry tensile strength, water sorption, and water vapor permeability. Nanocomposites’ surfaces were examined by scanning electron microscopy; the scanning electron microscopy images indicated homogeneous distribution of microfibrillated cellulose in the gelatin matrix. Microfibrillated cellulose improved wet and dry maximum tensile stress and modulus of cross-linked gelatin but resulted in a decrease of its strain at break. Microfibrillated cellulose did not affect the water absorption of cross-linked gelatin but significantly improved its moisture barrier property.

  • 6.
    Fernberg, Patrik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gong, Guan
    Swerea SICOMP AB, Piteå.
    Mannberg, Peter
    Swerea SICOMP AB, Piteå.
    Tsampas, Spyros
    Swerea SICOMP AB, Mölndal.
    Development of novel high Tg polyimide-based composites: Part I: RTM processing properties2018In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 2, p. 253-260Article in journal (Refereed)
    Abstract [en]

    In this study, an assessment of the composite processing-related properties of a newly developed 6-FDA-based phenylethynyl-terminated polyimide (available under the tradename NEXIMID®MHT-R) is presented. Processing schemes, used for preparing high quality carbon fibre-reinforced composites by the use of conventional resin transfer moulding are developed and presented. The influences of manufacturing parameters on glass transition temperature of the composites are presented. The results confirm that composites with exceptionally high Tg, in the range between 350 and 460℃ can be achieved. A manufacturing scheme that yields in composites with Tg of 370℃ is presented and proposed as a good candidate to serve as baseline for further studies.

  • 7. Gebart, Rikard
    Permeability of unidirectional reinforcements for RTM1992In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 26, no 6, p. 1100-1133Article in journal (Refereed)
    Abstract [en]

    The permeability of an idealized unidirectional reinforcement consisting of regularly ordered, parallel fibres is derived starting from first principles (Navier-Stokes equations) both for flow along and for flow perpendicular to the fibres. First, an approximate analytical solution for transverse flow is derived which differs from the Kozeny-Carman equation for the permeability of a porous medium in that the transverse flow stops when the maximum fibre volume fraction is reached. The solution for flow along the fibres has the same form as the Kozeny-Carman equation. A comparison shows excellent agreement between a numerical solution of the full flow equations and the approximate one at medium to high fibre volume fractions (Vf > 0.35). The theoretical predictions of permeability were tested in a specially designed mould. The results from the experiments with an unsaturated polyester resin (Jotun PO-2454) and the unidirectional reinforcement did in all cases show excellent agreement with results predicted by Darcy's law (the square of the flow front position increases linearly with time if the injection pressure is kept constant). The theoretical model could be fitted to the experimental data both for flow along the fibres and for cross flow based on data for flow along the fibres only. The fitting is obtained by adjusting one parameter in the model, the effective fibre radius, to a value about four times larger than the real fibre radius (15 μm). Scanning electron microscopy shows that the fibres are arranged in bundles looking like cylinders with ellipsoidal cross section which may be the explanation for the effective fibre radius in the fitted model equation being larger than the real fibre radius.

  • 8.
    Giannadakis, Konstantinos
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Potential of a simple variational analysis in predicting shear modulus of laminates with cracks in 90-layers2014In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 48, no 15, p. 1843-1856Article in journal (Refereed)
    Abstract [en]

    The potential of accurate modelling of the shear modulus reduction of laminates with cracked 90-layers using models based on the minimization of the complementary energy with improved stress description in the constraint layers is evaluated. This group of models refine Hashin’s model by introducing shape functions with unknown parameters to represent the out-of-plane shear stress distribution across the constraint layer thickness. The Hashin’s model becomes a particular case of the presented when the shape parameter approaches to zero. The most accurate shape parameters are found in the result of minimization. Three models are compared: the present variational model, Hashin’s model and the shear lag model introduced by Soutis which assumes linear out-of-plane shear stress distribution over an unknown part of the layer. It is shown in this paper that the size of this part may be determined by minimization of the complementary energy. The present model is the most accurate amongst the three, whereas Soutis’ model is more accurate than the Hashin’s model for laminates with constraint layer, thicker than the cracked layer. The comparison with finite element method results shows reasonable agreement. Agreement can be improved developing models with better description of the stress state in the cracked layer.

  • 9.
    Hajikazemi, Mohammad
    et al.
    Aerospace Engineering Department and Centre of Excellence in Computational Aerospace Engineering, Amirkabir University of Technology, Iran.
    Sadr, Mohammad Homayoun
    Aerospace Engineering Department and Centre of Excellence in Computational Aerospace Engineering, Amirkabir University of Technology, Iran.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Analysis of cracked general cross-ply laminates under general bending loads: A variational approach2017In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 22, p. 3089-3109Article in journal (Refereed)
    Abstract [en]

    A novel variational model is developed that can predict the stress transfer between plies in cracked general cross-ply laminates subject to general in-plane (Nxx, Nyy, Nxy) and out-of-plane bending (Mxx, Myy, Mxy) loading. The effects of thermal residual stresses are taken into account. Admissible stress systems, which satisfy equilibrium and all boundary and interface conditions, are constructed and the principle of minimum complementary energy is employed to find the optimal solution. The approach yields rigorous lower bounds for stiffness matrices. A methodology based on Levin's theorem is developed to evaluate the effective thermal expansion coefficients of non-symmetric cracked laminates. A ply-refinement technique is used in order that through-thickness variations of the stress components can be precisely taken into account. It is found that the developed method, when used in conjunction with ply refinement technique, results in stress fields and thermo-mechanical properties comparable in accuracy to refined finite and boundary element solutions.

  • 10.
    Joffe, Roberts
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Madsen, Bo
    Technical University of Denmark, Risø Campus.
    Nättinen, Kalle
    VTT Technical Research Centre of Finland, Espoo.
    Miettinen, Arttu
    University of Jyväskylä.
    Strength of cellulosic fiber/starch acetate composites with variable fiber and plasticizer content2015In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 49, no 8, p. 1007-1017Article in journal (Refereed)
    Abstract [en]

    In this experimental study, the performance of injection-molded short flaxand hemp fibers in plasticized starch acetate were analyzed in terms ofstrength. Parameters involved in the analysis are a variable fiber andplasticizer content. The measured strength of the composites varies in therange of 12–51 MPa for flax fibers and 11-42 MPa for hemp fibers, which issignificantly higher than the properties of the unreinforced starch acetatematrix. The micro-structural parameters used in modeling of compositestrength were obtained from optical observations and indirectmeasurements. Some of these parameters were qualitatively verified by Xraymicrotomography.

  • 11.
    Loukil, Mohamed Sahbi
    et al.
    RISE SICOMP, Sweden. Department of Management and Engineering, Linköping University, Sweden.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effective shear modulus of a damaged ply in laminate stiffness analysis: Determination and validation2019In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793XArticle in journal (Refereed)
    Abstract [en]

    The concept of the “effective stiffness” for plies in laminates containing intralaminar cracks is revisited presenting rather accurate fitting expressions for the effective stiffness dependence on crack density in the ply. In this article, the effective stiffness at certain crack density is back-calculated from the stiffness difference between the undamaged and damaged laminate. Earlier finite element method analysis of laminates with cracked 90-plies showed that the effective longitudinal modulus and Poisson’s ratio of the ply do not change during cracking, whereas the transverse modulus reduction can be described by a simple crack density dependent function. In this article, focus is on the remaining effective constant: in-plane shear modulus. Finite element method parametric analysis shows that the dependence on crack density is exponential and the fitting function is almost independent of geometrical and elastic parameters of the surrounding plies. The above independence justifies using the effective ply stiffness in expressions of the classical laminate theory to predict the intralaminar cracking caused stiffness reduction in laminates with off-axis plies. Results are in a very good agreement with (a) finite element method calculations; (b) experimental data, and (c) with the GLOB-LOC model, which gives a very accurate solution in cases where the crack face opening and sliding displacements are accurately described.

  • 12.
    Loukil, Mohamed Sahbi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ayadi, Zoubir
    Institut Jean Lamour.
    Applicability of solutions for periodic intralaminar crack distributions to non-uniformly damaged laminates2013In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 3, p. 287-301Article in journal (Refereed)
    Abstract [en]

    Stiffness reduction simulation in laminates with intralaminar cracks is usually performed assuming that cracks are equidistant and crack density is the only parameter needed. However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In this article, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model, in which the normalized average crack-opening and crack-sliding displacements are the main characteristics of the crack, is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis, the crack-opening displacement and crack-sliding displacement are calculated using finite element method, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution, we obtain lower bond to elastic modulus. A ‘double-periodic’ approach presented to calculate the crack-opening displacement of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers of cross-ply laminates, whereas for high crack density in surface layers, it underestimates the modulus reduction.

  • 13.
    Lundström, Staffan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Frishfelds, Vilnis
    Jakovics, Andris
    University of Latvia.
    A statistical approach to permeability of clustered fibre reinforcements2004In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 38, no 13, p. 1137-1149Article in journal (Refereed)
    Abstract [en]

    The focus is set on mesoscale modelling of permeability of real fabrics used in composite manufacturing. Of particular interest is the effect of expected perturbations from perfect geometries, such as fibre bundle crimp, on the permeability. To start with, variational methods are used to calculate the perme-ability of individual gaps between fibre bundles. Based on this study a network of unit cells is formed enabling studies of two- and three-dimensional flow through the structure. From such an analysis the overall permeability of an arbitrary distribution of unit cell permeabilities can be calculated. Here random and controlled distributions are simulated. The former is an approximate representation of a continuous strand mat and the latter may describe Non-Crimp Fabrics. The result is that for random distributions, the permeability decreases with the maximum variation in unit cell while for a controlled permeability distribution the overall permeability can as well increase as decrease depending on the type of perturbation. In both cases the type of flow: one-, two- or three-dimensional strongly influence on the quantitative result. Hence, for the type of fabrics studied, it is necessary to model the full 3D-flow through to get a correct permeability value.

  • 14. Lundström, Staffan
    et al.
    Gebart, Rikard
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Effect of perturbation of fibre architecture on permeability inside fibre flows1995In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 29, no 4, p. 424-443Article in journal (Refereed)
    Abstract [en]

    The influence on the permeability inside fibre tows from fibre packing arrangement and non-uniform fibre diameter is investigated. The analysis mainly consider flow transverse to the fibres and is based on the fibre geometry. Perturbations from perfect quadratic and hexagonal arrangements of fibres at constant fibre volume fraction are considered. The importance of microgeometry on permeability is pointed out and it is indicated how to deal with this problem. The analysis shows that the fibre transverse permeability is strongly dependent on the fibre size distribution. When an arrangement of equally sized fibres is continuously changed from quadratic to hexagonal it is furthermore found that the transverse permeability is highest just before the packing gets hexagonal. The analysis is also applied to a simple FORTRAN code to simulate removal of fibers from a quadratic arrangement. The simulations shows that the transverse permeability decreases with increased number of removed fibres under constant fibre volume fraction.

  • 15.
    Lundström, Staffan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sundlöf, Henrik
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Holmberg, J. Anders
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Modeling of power-law fluid flow through fiber beds2006In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 40, no 3, p. 283-296Article in journal (Refereed)
    Abstract [en]

    An apparent permeability of an idealized unidirectional reinforcement is derived for power-law fluid flow perpendicular to the fibers. The reinforcement consists of regularly ordered, parallel fibers arranged in a quadratic or hexagonal pattern. The expression is obtained starting from first principles for low Reynolds number, incompressible and isothermal flow and is expressed in terms of the radius of the fibers, the fiber volume fraction, and the power-law index. The model is successfully verified with computational fluid dynamic simulations.

  • 16.
    Lundström, T. Staffan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Frishfelds, Vilnis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Modeling filtration of particulate flow during impregnation of dual-scale fabrics2013In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 15, p. 1907-1915Article in journal (Refereed)
    Abstract [en]

    Filtration of particles during impregnation of dual-scale fabrics is studied numerically for a number of geometries and initial positions of the particles by improving a previously derived model for a high density of the particles. The initial position and size of the particles are varied. The main result is that structural composites can be tailor-made as to additional properties by such an approach

  • 17.
    Madsen, Bo
    et al.
    Risø, DTU.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Peltola, Heidi
    VTT.
    Nättinen, Kalle
    VTT.
    Short cellulosic fiber/starch acetate composites: micromechanical modeling of Young’s modulus2011In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 45, no 20, p. 2119-2131Article in journal (Refereed)
    Abstract [en]

    This study is presented to predict the Young’s modulus of injection-molded short cellulosic fiber/plasticized starch acetate composites with variable fiber and plasticizer content. A modified rule of mixtures model is applied where the effect of porosity is included, and where the fiber weight fraction is used as the basic independent variable. The values of the input model parameters are derived from experimental studies of the configuration of the composites (volumetric composition, dimensions, and orientation of fibers), as well as the properties of the constituent fiber and matrix phases (density and Young’s modulus). The measured Young’s modulus of the composites varies in the range 1.1–8.3 GPa, and this is well predicted by the model calculations. A property diagram is presented to be used for the tailor-making of composites with Young’s modulus in the range 0.2–10 GPa.

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

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

  • 19.
    Megnis, Modris
    et al.
    Luleå tekniska universitet.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Allen, David A.
    Texas A&M University.
    Holmberg, Anders
    Swedish Institute of Composites.
    Micromechanical modeling of viscoelastic response of GMT composite2001In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 35, no 10, p. 849-882Article in journal (Refereed)
    Abstract [en]

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

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

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

  • 21. Nordlund, Markus
    et al.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical study of the local permeability of noncrimp fabrics2005In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 39, no 10, p. 929-947Article in journal (Refereed)
    Abstract [en]

    Noncrimp stitched fabrics (NCFs) are often used as reinforcing materials in high-performance composite materials. Prediction models of the processing stage of the manufacturing are highly desirable in order to enhance the control of the process and enable the production of materials with higher quality. In NCFs, layers of parallel fiber bundles consisting of a large number of fibers are stitched together with other layers to form a network of interbundle channels in different directions. In earlier works, numerical simulations on unit cells had been performed in order to predict the global permeability of NCFs. It was shown that features like the thread influence the local permeability of the unit cells and therefore, the local permeability distribution of a fabric also. Furthermore, this influences the global permeability of the entire fabric. In the present paper, different geometrical features are therefore studied in order to investigate their influence on the local permeability within an NCF. The stitching process in addition to the interbundle channels, gives rise to two geometrical features, the thread which penetrates the channels and the crossing of fibers between two neighboring fiber bundles. The influences of these two features on the local permeability are studied together with variations of other geometrical parameters of the fabric. Computational Fluid Dynamics are used for the flow simulations in order to calculate the local permeability for the different unit cells. To ensure quality and trust, the numerical accuracy of the simulations is also studied. This work proves that the thread and the crossings, as well as the variations of the width and the height of the interbundle channels, have great influence on the local permeability. Prediction models therefore, have to take these features as well as geometry distortions, which influence the local permeability distribution, into account in order to make accurate predictions of the global permeability of a fabric.

  • 22.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Saseendran, Sibin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP, Piteå, Sweden.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Basso, Margherita
    Department of Chemistry, Materials and Chemical Engineering, Politecnico di Milano, Italy. The Research Hub by Electrolux Professional, Pordenone, Italy.
    Effect of degree of cure on viscoplastic shear strain development in layers of [45/−45]s glass fibre/ epoxy resin composites2018In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 24, p. 3277-3288Article in journal (Refereed)
    Abstract [en]

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

  • 23.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Natural fiber composite: Challenges simulating inelastic response in strain-controlled tensile tests2016In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 50, no 5, p. 575-587Article in journal (Refereed)
    Abstract [en]

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

  • 24.
    Saseendran, Sibin
    et al.
    Materials and Production, RISE SICOMP AB, Sweden.
    Berglund, Daniel
    Materials and Production, RISE SICOMP AB, Sweden.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stress relaxation and strain recovery phenomena during curing and thermomechanical loading: Thermorheologically simple viscoelastic analysis2019In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 53, no 26-27, p. 3841-3859Article in journal (Refereed)
    Abstract [en]

    Stress relaxation and strain recovery phenomena during curing and changed thermal conditions are analyzed using a viscoelastic model developed for thermorheologically complex materials (VisCoR). By making several simplifying assumptions regarding the material behavior, the incremental form of the VisCoR model is reformulated to a version describing thermorheologically simple material and presented in one-dimension for simplicity. The model (called VisCoR-simple) is used to analyze material behavior under various conditions, including stress relaxation behavior at varying temperatures and time scales; tensile loading and unloading tests at high temperatures; stress build up and “frozen-in” strains during curing and following cool-down and strain recovery during the next step of heating. Furthermore, the differences between the so-called “path-dependent” model, which is a linear elastic model with different elastic properties in glassy and rubbery regions, and the presented viscoelastic model are studied. The path-dependent model is an extreme case of the viscoelastic model presented. The importance of considering viscoelasticity when considering temperature and curing effects on polymers and the shortcomings of the path-dependent model are revealed and discussed.

  • 25.
    Shakeri, Alireza
    et al.
    Golestan University, Gorgan.
    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.
    Self-reinforced nanocomposite by partial dissolution of cellulose microfibrils in ionic liquid2012In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 46, no 11, p. 1305-1311Article in journal (Refereed)
    Abstract [en]

    All-cellulose nanocomposite films with different ratios of cellulose I and II were produced by partial dissolution of microfibrillated cellulose using ionic liquid and subsequent film casting. The films were isotropic, transparent to visible light, highly crystalline, and contained different amounts of undissolved cellulose I crystallites in a matrix of dissolved cellulose. X-ray diffraction confirmed that cellulose I, i.e., the major polymorphic modification of cellulose in these nanocomposites, is rearranged to cellulose II crystal packing after the partial dissolution. The all-cellulose nanocomposite showed enhanced thermal properties, with thermal degradation temperature increased by 22% compared with thedissolved cellulose. The SEM and AFM studies verified that the nano-sized cellulose crystallites were well dispersed in the matrix. Results from DMA showed that the storage modulus was increased from 270 MPa for the dissolved cellulose to 1104 MPa for the nanocomposite with lower dissolution grade. This indicates that the all-cellulose nanocomposite films contained undissolved cellulose fragments – possibly cellulose I crystallites or aggregates of crystallites in a matrix of regenerated cellulose.

  • 26.
    Singh, Chandra Veer
    et al.
    University of Toronto.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A synergistic damage mechanics approach to mechanical response of composite laminates with ply cracks2013In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 20-21, p. 2475-2501Article in journal (Refereed)
    Abstract [en]

    We treat selected test cases in the third world wide failure exercise by the approach described as synergistic damage mechanics. This approach utilizes micromechanics and continuum damage mechanics to predict the overall mechanical response of composite laminates with ply cracking in multiple orientations. The material constants needed in the continuum damage mechanic formulation are calculated from stiffness property changes incurred in a reference laminate. For other laminate configurations, the stiffness changes are derived using a relative constraint parameter which is calculated from the constraint on the opening displacement of ply cracks within the given cracked laminate evaluated numerically by a finite element analysis of appropriately constructed representative unit cell. The number density of ply cracks (cracks per unit length normal to the crack planes) under quasi-static loading is calculated by an energy-based approach. Finally, the stress–strain response of a laminate is determined by combining stiffness property changes and evolution of crack number density.

  • 27.
    Sisodia, Samjay
    et al.
    Department of Engineering Sciences, Division of Applied Mechanics, Uppsala University.
    Gamstedt, E. Kristofer
    Department of Engineering Sciences, Division of Applied Mechanics, Uppsala University.
    Edgren, Fredrik
    GKN Aerospace, Trollhättan.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effects of voids on quasi-static and tension fatigue behaviour of carbon-fibre composite laminates2015In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 49, no 17, p. 2137-2148Article in journal (Refereed)
    Abstract [en]

    The effect of voids on quasi-isotropic carbon-fibre reinforced plastic laminates under quasi-static loading is compared with that under cyclic tension loading. Emphasis is placed on following damage development at the non-crimp fabric ply-level by investigating the influence of voids on damage accumulation, most notably transverse cracking and delamination. Details from experiments include micrographs of voids taken in both scanning-electron and light microscopy, measurements of void content and crack density using light microscopy, and stiffness plots from both quasi-static and cyclic tests. The stiffness results are compared with theoretical predictions accounting for transverse cracks. Voids have a significantly more detrimental effect on the mechanical properties in cyclic loading compared with quasi-static loading. Specifically, the stiffness reduction development, the underlying transverse cracking in layers and the number of cycles to failure are affected. Quality control by only quasi-static testing for void-containing composite materials to be used in components subjected to fatigue cannot therefore be recommended

  • 28.
    Szpieg, Magdalena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Giannadakis, Konstantinos
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Viscoelastic and viscoplastic behavior of a fully recycled carbon fibre-reinforced maleic anhydride grafted polypropylene modified polypropylene composite2012In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 46, no 13, p. 1633-1646Article in journal (Refereed)
    Abstract [en]

    The effect of maleic anhydride grafted polypropylene (MAPP) coupling agents on properties of a new composite made of recycled carbon fibers and recycled polypropylene (rCF/[rPP + MAPP]) was studied experimentally. This new material presented significantly improved properties, compared to the previous generation, without the addition of MAPP (Giannadakis K, Szpieg M and Varna J. Mechanical performance of recycled carbon fibre/PP. Exp Mech 2010; published online.). This was mostly attributed to improvement of the fiber/matrix interface. The inelastic and time-dependent behavior of the MAPP modified composite material in tension was analyzed. A series of quasi-static tensile and creep tests were performed to identify the material model, which accounts for: (a) damage-related stiffness reduction, (b)development of stress and time-dependent irreversible strains described as viscoplasticity, (c) nonlinear viscoelastic behavior. The damage-related stiffness reduction was found to be less than 10%. Although damage-dependent stiffness was not the main source of nonlinearity, it was included in the inelastic material model. In creep tests, it was found that the time and stress dependence of viscoplastic strains follows a power law, which makes the determination of the parameters in the viscoplasticity model relatively simple. The viscoelastic response of the composite was found to be linear in the investigated stress domain. The material model was validated in constant stress rate tensile tests.

  • 29. Talreja, Ramesh
    Transverse cracking and stiffness reduction in composite laminates1985In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 19, no 4, p. 355-375Article in journal (Refereed)
    Abstract [en]

    A systematic classification of the effects of transverse cracking on the stress-strain response of composite laminates is presented. Stiffness reductions resulting from transverse cracking in glass/epoxy and graphite/epoxy laminates from crack initiation to crack saturation are predicted using the stiffness-damage relationships developed by the author in a previous work. Good agreement with the experimental results is found. An assessment of the ply discount method for predicting stiffness reductions at crack saturation is also made.

  • 30. Talreja, Ramesh
    et al.
    Yalvaç, Selim
    Georgia Institute of Technology, Atlanta.
    Yats, Larry
    Georgia Institute of Technology, Atlanta.
    Wetters, Derrick
    Georgia Institute of Technology, Atlanta.
    Transverse cracking and stiffness reduction in cross ply laminates of different matrix toughness1992In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 26, no 11, p. 1644-1663Article in journal (Refereed)
    Abstract [en]

    Cross ply laminates of (02,902)s configuration having AS4 graphite fibers in three epoxy resins of different toughness: 3501-6, Tactix 556 and Tactix 695, have been tested to determine their transverse cracking behavior and the associated mechanical response under longitudinal tensile loading. The test data are analyzed using Talreja's continuum damage model [1-3]. The material constants needed in the model to predict the inplane stiffness changes are determined. The measured Poisson's ratio, which shows significant change, is compared with the prediction of the model. The constants for the three materials are found to increase with the increase in their fracture toughness.

  • 31.
    Tamus, V.
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Andersons, J.
    Institute of Polymer Mechanics, University of Latvia.
    Sparnins, Edgars
    Institute of Polymer Mechanics, University of Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Response of cross-ply composite to off-axis loading2002In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 36, no 18, p. 2125-2134Article in journal (Refereed)
    Abstract [en]

    Polymer composites are known to exhibit nonlinear stress-strain response due to nonlinearly elastic or plastic deformation of the matrix and damage accumulation. Mechanistic modeling of material response explicitly accounting for these interacting factors often leads to complex theories. Plasticity theory formalism provides an alternative for nonlinear deformation description of composite material. We examine the applicability of an orthotropic plasticity model, developed by Sun et al. for unidirectionally reinforced composite, to composite laminate. The response of a symmetric and balanced cross-ply glass/epoxy laminate is studied under uniaxial tensile loading at different angles to the material orthotropy axis. It is found that the associated flow rule and a quadratic approximation of the orthotropic potential function provide satisfactory description for the nonlinear strain component under monotonic loading for 15-45° off-axis angle range. The nonlinearity in on-axis loading (modulus degradation)is well described by stiffness reduction due to the cracks in transverse plies. Meanwhile the change of elastic modulus due to intralaminar cracking can be neglected in off-axis loading, but the possible intensification of nonlinear deformation in off-axis loading caused by the presence of intralaminar cracks agrees well with orthotropic potential formalism.

  • 32.
    Tsampas, Spyros
    et al.
    Swerea SICOMP AB, Mölndal.
    Fernberg, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Development of novel high Tg polyimide-based composites: Part II: Mechanical characterisation2018In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 2, p. 261-274Article in journal (Refereed)
    Abstract [en]

    In this study, the mechanical performance assessment of a newly developed carbon fibre-reinforced polyimide composite system T650/NEXIMID® MHT-R is presented. This system was subjected to a series of mechanical tests at ambient and elevated temperature (320℃) to determine basic material properties. Moreover, an additional test was conducted, using a T650/NEXIMID® MHT-R laminate in which the fibre sizing was thermally removed prior to laminate manufacturing, to investigate the effect of fibre treatment on mechanical performance. The experimental results indicated that the T650/NEXIMID® MHT-R composites along with exceptionally high Tg (360–420℃) exhibited competitive mechanical properties to other commercially available polyimide and epoxy-based systems. At elevated temperature, the fibre-dominated properties were not affected whilst the properties defined by matrix and fibre/matrix interface were degraded by approximately 20–30%. Finally, the fibre sizing removal did not affect the tensile and compressive strength, however, the shear strength obtained from short-beam shear test was deteriorated by approximately 15%. to serve as baseline for further studies.

  • 33.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modelling mechanical performance of damaged laminates2013In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 47, no 20-21, p. 2443-2474Article in journal (Refereed)
    Abstract [en]

    This article is the author’s contribution to the third World-Wide Failure Exercise which aims at benchmarking current damage models for composites. Reduction of thermo-elastic constants of laminates and their nonlinear behaviour due to intralaminar cracking and nonlinear shear response of the composite are analysed using global–local approach. The macroscopic properties of damaged laminates are expressed in simple forms containing density of intralaminar cracks and their surface displacement features obtained from local solutions. The initiation and evolution of the intralaminar damage is analysed using strength-based approach for laminates with thick layers and fracture mechanics approach for thin layers. Due to a lack of information, certain characteristics, such as statistical failure properties distribution parameters and transition point (thickness) from strength to fracture mechanics applicability, were assumed. All calculations are based on analytical expressions, some of which were developed previously through numerical analysis. The present method was applied to solve 9 out of the 13 test cases of the third World-Wide Failure Exercise and that was sufficient to illustrate the capability of the damage model.

  • 34.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Loukil, Mohamed Sahbi
    Swerea SICOMP.
    Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation2017In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 14, p. 1945-1959Article in journal (Refereed)
    Abstract [en]

    The old concept of the effective stiffness of a 90-layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates. It is shown, focusing on the effective transverse modulus, that the expected dependence of this property on composite elastic properties and laminate lay-up is very weak and follows very simple rules. Calculations show that the effective longitudinal modulus and Poisson’s ratio of the layer are not affected at all by intralaminar cracking. Simple fitting curve for effective transverse modulus change with normalized crack density is obtained from analysis of GF/EP cross-ply laminate. It is shown, comparing with FEM results and experimental data, that this expression can be used as a ‘master curve’ in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different unidirectional composites

  • 35.
    Xu, Johanna
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lindbergh, Göran
    School of Chemical Science and Engineering, KTH Royal Institute of Technology.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Carbon fiber composites with battery function: Stresses and dimensional changes due to Li-ion diffusion2018In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 20, p. 2729-2742Article in journal (Refereed)
    Abstract [en]

    Structural composite materials that simultaneously carry mechanical loads, while storing electrical energy offers the potential of significantly reduced total component weight owing to the multifunctionality. In the suggested micro-battery, the carbon fiber is employed as a negative electrode of the battery and also as a composite reinforcement material. It is coated with a solid polymer electrolyte working as an ion conductor and separator while transferring mechanical loads. The coated fiber is surrounded by a conductive positive electrode material matrix. This paper demonstrates a computational methodology for addressing mechanical stresses arising in a conceptualized micro-battery and dimensional changes of the cell during electrochemical cycling, caused by time-dependent gradients in lithium ion concentration distribution.

  • 36.
    Xu, Johanna
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Matrix and interface microcracking in carbon fiber/polymer structural micro-battery2019In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 53, no 25, p. 3615-3628Article in journal (Refereed)
    Abstract [en]

    In this paper, the propagation of radial matrix cracks and debond cracks at the coating/matrix interface in unidirectional carbon fiber structural micro-battery composite are studied numerically. The micro battery consists of a solid electrolyte-coated carbon fiber embedded in an electrochemically active polymer matrix. Stress analysis shows that high hoop stress in the matrix during charging may initiate radial matrix cracks at the coating/matrix interface. Several 2-D finite element models of the transverse plane with different arrangements of fibers and other matrix cracks were used to analyze the radial matrix crack growth from the coating/matrix interface of the central fiber in a composite with a square packing of fibers. Energy release rates of radial cracks along two potential propagation paths are calculated under pure electrochemical loading. The presence of a radial matrix crack imposes changes in the stress distribution along the coating/matrix interface, making debonding relevant for consideration. Results for energy release rates show that the debond crack growth is governed by mode II.

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