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  • 201.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Material models for damaged composite laminates2006In: Fracture of Nano and Engineering Materials and Structures: Proceedings of the 16th European Conference of Fracture, Alexandroupolis, Greece, July 3-7, 2006 / [ed] E. E. Gdoutos, Encyclopedia of Global Archaeology/Springer Verlag, 2006, Vol. paper 504, p. 1275-Conference paper (Refereed)
  • 202.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanics of damaged fiber reinforced composite laminates2001In: Advances in fracture and damage mechanics II: : [International Conference on Fracture and Damage Mechanics, 18 - 20 September 2001, Politecnico di Milano, Milan, Italy] / [ed] M. Guagliano, Geneva: Hoggar , 2001, p. 359-364Conference paper (Refereed)
    Abstract [en]

    The classical laminate theory is generalized for a case when layers contain damage in form of intralaminar cracks. The homogenized constitutive relationships for damaged laminate have a similar form as in the classical theory of not damaged laminates. Closed form expressions for extensional stiffness matrix are derived. Apart from elastic properties of layers they contain only laminate geometrical parameters and average relative displacements of crack surfaces. Simple power law type of formulas for these quantities, obtained using FEM based parametric analysis render accurate predictions for the elastic properties degradation of laminates.

  • 203.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microdamage Modeling in Laminates2015In: Damage Growth in Aerospace Composites, Cham: Encyclopedia of Global Archaeology/Springer Verlag, 2015, p. 141-173Chapter in book (Refereed)
    Abstract [en]

    During service life structures made of laminated composites are subjected to complex combinations of thermo-mechanical and environmental loads. The final macroscopic failure of composite laminate is preceded by initiation and evolution of several microdamage modes in layers. This is because the transverse tensile strain to failure of unidirectional composites is lower than other failure strain components. Therefore transverse cracking of layers with off-axis orientation with respect to the main load direction, caused by combined action of transverse tensile stress and shear stress, is usually the first mode of damage (Parvizi and Bailey, J. Mater. Sci. 13:2131–2136, 1978 [1]; Jamison et al., ASTM STP 836:21–55, 1984 [2]).

  • 204.
    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.

  • 205.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    On effective properties of layers in damaged composite laminates2011In: Advances in Fracture and Damage Mechanics X: selected, peer reviewed papers from the 10th International Conference on Fracture and Damage Mechanics (FDM 2011), September 19-21, 2011, Dubrovnik, Croatia / [ed] Z. Tonković ; M.H. Aliabadi, Durnten-Zurich: Trans Tech Publications Inc., 2011, p. 490-493Conference paper (Refereed)
    Abstract [en]

    Methodology is suggested for calculation of effective elastic constants of layer containing intralaminar cracks. Trends are presented for layers with low and medium high crack densities, where the interaction between cracks can be neglected. The effective properties of the damaged layer are determined by back- calculation using previously developed GLOB-LOC model for thermo-mechanical properties of damaged laminate [1]. It is shown that the effective transverse modulus and shear modulus of a layer decrease linearly with increasing crack density, whereas longitudinal modulus and major Poisson's ratio do not change at all. Experimental data for multiple cracking are analyzed showing that linear approximation of crack density versus applied strain may have sufficient accuracy. These two linear trends are used to calculate the effective elastic properties of a layer as a function of strain

  • 206.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Physical interpretation of parameters in synergistic continuum damage mechanics model for laminates2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 13, p. 2592-2600Article in journal (Refereed)
    Abstract [en]

    The continuum damage mechanics (CDM) model by Talreja [Talreja R. Damage characterization by internal variables. In: Pipes RB, Talreja R, editors. Comput Mater Series, vol. 9. Dam Mech Comp Mater, Elsevier, Amsterdam; 1994, p. 53-78], which is one of the most efficient for damaged laminate stiffness determination, is analyzed using earlier developed theoretical framework which links the macro-response of a damaged laminate to the opening and sliding displacements of crack surfaces in an exact form. The physical meaning of the material parameters in CDM is revealed. Exact expressions for these parameters are derived and transformation rules with changing layer orientation angle presented. These parameters were previously considered as the most troublesome part of the CDM model with need for additional tests for their determination. The error introduced by the two main assumptions in the CDM model ((a) stiffness change is linear with respect to damage tensor, (b) crack face sliding effect can be neglected) is evaluated and found that the linearization is often justified but sliding cannot be neglected in most of the lay-ups.

  • 207.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Quantification of damage and evolution modeling in multidirectional laminates2006In: Polymer Composite Materials for Wind Power Turbines: proceedings of the 27th Risø International Symposium on Materials Science, 4 - 7 September 2006, Risø National Laboratory, Roskilde, Denmark / [ed] Hans Lilholt, Roskilde: Risø National Laboratory , 2006, p. 349-356Conference paper (Refereed)
  • 208.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Strategies for stiffness analysis of laminates with microdamage: combining average stress and crack face displacement based methods2015In: Zeitschrift für angewandte Mathematik und Mechanik, ISSN 0044-2267, E-ISSN 1521-4001, Vol. 95, no 10, p. 1081-1097Article in journal (Refereed)
    Abstract [en]

    Many approximate analytical models have been developed to calculate stress state between intralaminar cracks with the aim to predict the degradation of certain elastic property (most often axial modulus or shear modulus) of cross-ply laminate. Often they are plane stress solutions and laminate constants like Poisson's ratios cannot be considered. On the other hand the so called GLOB-LOC approach, presented in WWFE III, allows calculation of any thermo-elastic property of a general symmetric laminate with an arbitrary number of cracks in each layer provided that two local parameters – average and normalized crack opening displacement (COD) and crack face sliding displacement (CSD) are known. In this paper relationships are derived expressing these two parameters (COD and CSD) with average value of transverse stress and in-plane shear stress perturbation between cracks. Expressions are exact and independent on the approximations in the stress model. As examples, average perturbation functions for two shear lag models and Hashin's variational model are used to calculate damaged laminate properties that would not be available in original formulation: Poisson's ratio and thermal expansion coefficients. Predictions are compared with test data for GF/EP laminates and with more accurate predictions based on FEM calculations

  • 209.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Thermoelastic constants of damaged laminates: COD- and CSD-based methods2016In: Modeling damage, fatigue and failure of composite materials, Cambridge: Woodhead Publishing Limited, 2016, p. 243-287Chapter in book (Refereed)
    Abstract [en]

    The effect of intralaminar cracking in layers of laminates on laminate macroscopic thermoelastic constants is the subject of this chapter. General expressions are derived for arbitrary symmetric laminates with any number of cracks in different layers. In addition to layer properties and laminate lay-up, the expressions contain two local parameters characterizing the presence of the crack: its average values of crack opening displacement (COD) and crack sliding displacement (CSD) corresponding to a unit stress in the layer.It is shown that these two parameters are proportional to the corresponding average perturbations of the axial and shear stresses due to cracks. This allows using any of the simple analytical stress solutions (shear lag models, variational models, etc.) to calculate all thermoelastic constants. As an alternative, which is more accurate but less mechanism based, COD and CSD are obtained by fitting simple functions to the finite-element method-based parametric analysis results. Predictions are compared with experimental data for cross-ply laminates.

  • 210.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Akshantala, Nagendra
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Stiffness changes in laminates caused by intralaminar cracks and shear deformation1997In: Composites and functionally graded materials, American Society of Mechanical Engineers , 1997, p. 165-168Conference paper (Refereed)
    Abstract [en]

    An experimental study has been conducted to study stiffness changes in laminates containing inclined (off-axis) plies. The class of laminates studied is glass-epoxy (0, ±θ4, 01/2)s with θ = 25, 40, 55 and 70. It is found that for 0>40 multiple cracks occur in the θ-plies while for θ<40 no cracks are found. However, for all θ angles the laminate moduli change. A continuum damage mechanics approach is used to calculate stiffness changes due to cracks. In the absence of cracks the shear modulus reduction in the θ-plies, determined from independent tests on a (±45)s laminate, is found to provide good prediction of the observed stiffness changes.

  • 211.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Akshantala, Nagendra V.
    School of Aerospace Engineering, Georgia Institute of Technology, Atlanta.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Crack opening displacement and the associated response of laminates with varying constraints1999In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 8, no 2, p. 174-193Article in journal (Refereed)
    Abstract [en]

    This paper reports an experimental study of the opening displacement behavior of 90-ply cracks in a class of composite laminates where the constraint to cracking is varied systematically. The study is motivated by a synergistic approach to damage mechanics proposed by Talreja (1996) where the crack opening displacement forms a key micromechanically determined parameter in a continuum damage formulation of the composite response. It is found that although the opening displacement shows scatter from point to point along a crack length, its average over the crack length forms a robust parameter. This parameter, when introduced as the damage influence coefficient in a continuum description of damage, allows satisfactory predictions of the elastic moduli changes due to transverse cracks in the considered class of laminates with varying constraints.

  • 212.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif E.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Microdamage in composite laminates: Experiments and observation2014In: Experimental and Applied Mechanics: Selected, peer reviewed papers from the 2014 International Conference on Experimental and Applied Mechanics (EAM 2014), January 20-21, 2014, Miami, USA / [ed] Tan Jin, Trans Tech Publications Inc., 2014, p. 84-89Conference paper (Refereed)
    Abstract [en]

    The first mode of damage in fiber reinforced composite laminates is usually intralaminar cracking in layers with off-axis orientation regarding the main loading direction. This papers analyzes the most typical methods used for the characterization of the damage state: edge replicas, optical microscopy, x-ray images, acoustic emission, speckle interferometry and Raman spectroscopy

  • 213.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Cold climate effects on composite materials: preliminary results1994In: Proceedings: Polartech '94 - International Conference on Development and Commercial Utilization of Technologies in Polar Regions, March 22-25, 1994 Luleå, Sweden, Högskolan i Luleå , 1994, p. 397-406Conference paper (Refereed)
  • 214.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Effect of transverse cracking on thermo-mechanical properties of cross-ply laminates1992In: FRC '92: 5th International conference on fibre reinforced composites : Papers and programme, Plastics and rubber institute , 1992Conference paper (Refereed)
  • 215.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    On transverse strength in polymer composites specimen thickness effects1993In: Deformation and fracture of composites: [2nd International Conference on Deformation and Fracture of Composites] ; 29 -31 March 1993, The Manchester Conference Centre, UMIST, Manchester, UK ; [papers], London: Institute of Materials , 1993Conference paper (Refereed)
  • 216.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Toughness of carbon fiber/epoxy laminates based on intermediate modulus fibers1993In: Proceedings of the Ninth International Conference on Composite Materials (ICCM/9) : Madrid, 12 - 16 July, 1993 / [ed] Antonio Miravete, Zaragoza: Univ. of Zaragoza , 1993, p. 888-894Conference paper (Refereed)
    Abstract [en]

    The transverse mechanical behaviour of two composites with the same matrix, an AS4/8552 carbon fiber/epoxy composite and an IM8 /8552 intermediate modulus composite, was studied in unidirectional and cross-ply laminates. The influence of the chosen fibers on failure strain and strength as well as on the multiple cracking in cross-ply laminates was analysed. The transverse cracking toughness was not affected by the different fibers.

  • 217.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    A Model for Prediction of the Transverse Cracking Strain in Cross-Ply Laminates1992In: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 11, no 7, p. 708-728Article in journal (Refereed)
    Abstract [en]

    A modified variational analysis based on the principle of minimum com plementary energy, is used to describe the stress state around a through-the-thickness transverse crack in [0 m/90n]s cross-ply laminates. The analysis takes into account nonlinear stress distribution in the thickness direction of the 0°-plies and contains a thermoelastic consideration. A failure criterion based on linear elastic fracture mechanics is used to pre dict the onset of transverse matrix cracking in cross-ply laminates. The predictions are compared with experimental data and with previous analyses developed in the framework of the shear lag theory or the variational approach suggested by Hashin.

  • 218.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A:
    Testing transverse strain to failure in laminates1994In: 50 Years of Progress in Materials and Science Technology, 1994, p. 727-733Conference paper (Refereed)
    Abstract [en]

    Unidirectional transverse tension data are unrelated to cross-ply laminate cracking strains. Instead, a new hybrid laminate test is proposed. Five toughened carbon fiber/epoxy materials were sandwiched between longitudinal glass fiber/epoxy layers. It was then possible to register transverse cracks by acoustic emission. The first twenty cracks of [02/904]s hybrid laminates were used to determine Weibull distribution functions. Predictions of cracking strains in regular cross-ply laminates showed good agreement with experimental data. The hybrid laminate test for transverse strain to failure therefore offers promise as a new test method.

  • 219.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Thermo-elastic properties of composite laminates with transverse cracks1994In: Journal of composites technology & research, ISSN 0884-6804, E-ISSN 1945-7537, Vol. 16, no 1, p. 77-87Article in journal (Refereed)
    Abstract [en]

    Different models based on variational analysis are compared with respect to the thermo-elastic properties of cross-ply laminates with transverse cracks in the 90°-layer. This approach, pioneered by Hashin, applies the principle of minimum complementary energy to describe the stress state in a cross-ply laminate with transverse cracks. Apart from material data, only laminate geometry data and stress distribution assumptions are required. The model developed in the present study contains stress distribution assumptions that are closer to reality than previous models. Non-uniform stress distributions are included through-the-thickness of both the 0°- and the 90°-layers. An important result is the improved agreement with stiffness reduction data. As expected from the nature of the model, predictions generally provide lower bounds to the stiffness of the cracked laminate. However, at high crack densities, this is not the case. This additional stiffness reduction at high crack densities indicates the presence of new damage types, such as branched cracks and local delamination.

  • 220.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Two-dimensional transverse cracking in [Om/90n]s cross-ply laminates1993In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 12, no 5, p. 699-723Article in journal (Refereed)
  • 221.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Ericson, Mats L.
    Linköpings universitet.
    Specimen size effects on modulus of GMT and other inhomogeneous composites1992In: Journal of Thermoplastic Composite Materials, ISSN 0892-7057, E-ISSN 1530-7980, Vol. 5, no 2, p. 105-114Article in journal (Refereed)
    Abstract [en]

    Mean values and standard deviations are often used to characterize certain mechanical properties. It is widely known that some properties, e.g., tensile strength of brittle solids and their standard deviations, are affected by the volume studied. In the pres ent study, the mean value and the standard deviation for the longitudinal elastic modulus of a cuboid specimen are treated. A model to predict the mean values and standard devia tions for varying lengths and widths is presented. Experimental data from the literature for glass mat reinforced polypropylene is used to show that the model shows excellent ac curacy with experimental data

  • 222.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Ericson, M.L.
    Transverse single-fibre test for interfacial debonding in composites: 2. Modelling1997In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 28, no 4, p. 317-326Article in journal (Refereed)
    Abstract [en]

    A method was developed for determination of interfacial toughness parameters from a new transverse single-fibre test. In previous work, experimental procedures were developed. Interfacial debond growth in the arc and fibre direction was measured for transparent matrix polymer composites. In the present study, an earlier stress analysis by Toya was modified and combined with a critical energy release rate criterion, taking mode mixity effects into account. The developed model was used to determine interfacial toughness parameters. The model was able to describe and explain the main experimental observations of the debonding process. Treatment of the contact problem present for large debond angles is of interest for future work

  • 223.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Chihalenko, A.
    Riga Technical University, Latvia.
    Crack opening geometry in cracked composite laminates1997In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 6, no 1, p. 96-118Article in journal (Refereed)
    Abstract [en]

    Talreja and coworkers have reported brittle and tough polymer matrix composites to show different cross-ply laminate stiffness reductions due to transverse cracks, despite very similar elastic properties. Studies of crack opening displacement (COD) in [0,,,90,] laminates may help in solution of this problem. Approximate analytical models based on the variational approach pioneered by Hashin were therefore applied in addition to FEM-calculations. Approximate models were concluded not to be reliable for future analyses of this problem. Experimental COD for brittle GF/EP laminates at low crack densities was fairly well predicted by FEM-calculations. None of the models were able to predict experimental data at high crack densities. Residual plastic strains are suggested as an explanation and may also be responsible for reported matrix-related differences in stiffness reduction.

  • 224.
    Varna, Janis
    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.
    Jakovics, A.
    Department of Physics & Mathematics Latvia University.
    A study of the opening displacement of transverse cracks in cross-ply laminates1993In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 2, no 3, p. 272-289Article in journal (Refereed)
    Abstract [en]

    Transverse cracks in cross-ply laminates are investigated experimentally to reveal the essential characteristics of their opening displacement under tensile loads. The average crack opening displacement is studied as a function of the longitudinal overall strain and the effects of matrix toughness and transverse ply thickness on this parameter are examined. The interactive effects between closely spaced transverse cracks are also ex amined and found to be significant. Implications of the experimentally observed features on the micromechanics and continuum damage type models are discussed.

  • 225.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Björnberg, H.
    On the determination of transverse strain to failure in composites1994In: Composites testing and standardisation: ECCM-CTS 2, European Conference on Composites Testing and Standardisation, September 13 - 15, 1994, Hamburg, Germany / [ed] P.J. Hogg; K. Schultze, Luleå tekniska universitet, 1994Conference paper (Refereed)
  • 226.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Effects of voids on damage mechanism in RTM laminates under tensile loading1994In: Conference papers: 6th International Conference on Fibre Reinforced Composites, University of Newcastle upon Tyne, UK, 29 - 31 March 1994, Institute of Materials , 1994, p. 8.1-8.5Conference paper (Refereed)
    Abstract [en]

    The effect of voids on the transverse mechanical behavior was investigated for unidirectional resin transfer molded laminates of glass fiber fabric/vinylester. Laminates with the highest average content of voids had a transverse strain to failure as high as 2 percent whereas low void content laminates failed at 0.3 percent. Only a few large and well-defined transverse cracks formed in low void content laminates before final failure. Multiple transverse cracks with irregular shape as well as numerous smaller cracks formed in the high void content laminates. A simple model was developed in order to explain the high strain to failure of high void content laminates

  • 227.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Eitzenberger, Johannes
    Modeling UD composite stiffness reduction due to multiple fiber breaks and interface debonding2007In: Proceedings of COMP07: 6th International Symposium on Advanced Composites 16-18 May, 2007, Corfu, Greece, 2007, Vol. Paper COMP07-073Conference paper (Refereed)
    Abstract [en]

    Stiffness of a unidirectional composite (UD) containing fiber breaks with partial debonding is analyzed. Using divergence theorem exact relationships are obtained which link the entire stiffness matrix of the damaged UD composite with two robust parameters from the local solution: average opening displacement (COD) of the fiber break and its sliding displacement (CSD). Both are normalized with respect to the size of the fiber crack and to the far field stress in the fiber. The effect of the partial fiber/matrix debonding at the interface is included in the model through increasing crack opening. Using parametric inspection of obtained expressions it is shown that CSD does not affect the longitudinal stiffness and the COD effect on transverse and shear modulus and Poisson's ratio of the damaged composite is negligible. It motivated the decision to focus on COD and the longitudinal modulus. In this paper cracks are considered as non-interactive which makes the stiffness predictions conservative in a large fiber crack density region.The dependence of COD on fiber and matrix properties, fiber content, debond length etc is described by simple fitting functions obtained from extensive FEM based parametric analysis. FEM analysis is performed using a model which consists of three concentric cylinders: a) broken fiber; b) matrix cylinder; c) large cylinder of effective composite cylinder. The observed trends are described by simple fitting functions which with a high accuracy describe COD's of perfectly bonded and partially debonded cracks.Simulations performed for carbon and glass fiber polymer composites show the significance of debond length on the stiffness reduction.

  • 228.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Eitzenberger, Johannes
    Megnis, Modris
    Damage evolution in composite laminats with matrix cracks and fiber breaks2006In: Proceedings of the 3rd International Conference on Composites Testing and Model Identification: CompTest 2006, Faculty of Engineering, University of Porto, Portugal , 2006, p. 98-99Conference paper (Other academic)
  • 229.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Eitzenberger, Johannes
    Pupurs, Andrejs
    Models for UD composite behaviour in tensile fatigue: fiber breaks, matrix cracks and debonds2008In: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Conference paper (Other academic)
  • 230.
    Varna, Janis
    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.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Viscoelastic and viscoplastic behavior of GF/VE [±45]s laminates2013In: Challenges in Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials: Proceedings of the 2012 Annual Conference on Experimental and Applied Mechanics / [ed] Bonnie Antoun; H. Jerry Qi; Richard Hall; G.P. Tandon; Hongbing Lu; Charles Lu, New York: Encyclopedia of Global Archaeology/Springer Verlag, 2013, Vol. 2, p. 149-160Conference paper (Refereed)
    Abstract [en]

    The nonlinear and time dependent stress-strain response of non-crimp fabric (NCF) [±45] s laminates in tension is studied. Testing methodology is suggested to separate and quantify the effect on the inelastic response of damage development, nonlinear viscoelastic effects and viscoplasticity. Material model based on Schapery’s work on viscoelasticity and Zapas viscoplastic functional with added damage terms is used. Simulation is performed and validated with constant stress rate tensile tests, identifying the nonlinear viscoelasticity and viscoplasticity as the major sources of the nonlinear response.

  • 231.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hansson, T.
    Volvo Aero Corporation.
    Failure Mechanisms and the Macro-Response of Carbon Fiber Reinforced SiC Matrix Composites1999In: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 164-165, p. 209-212Article in journal (Refereed)
    Abstract [en]

    Inelastic response of ceramic matrix woven 2.5 -D C/SiC composites have been analyzed using quasi-static test data. The changes in composite stiffness properties (elastic modulus, in-plane and out-of-plane Poisson's ratio) are determined experimentally. The increase of residual strains with the applied stress is analyzed and described by simple functions. Micromechanical damage mechanisms observed and quantified using optical microscopy.

  • 232.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Effect of thermal aging and thermal fatigue on intralaminar cracking in laminates loaded in tension2009In: ICCM 17, Edinburgh: 17th International Conference on Composite Materials ; 27 Jul 2009 - 31 Jul 2009, Edinburgh International Convention Centre, Edinburgh, UK, Lindon: IOM Communications , 2009Conference paper (Refereed)
    Abstract [en]

    Laminated composites subjected to a large number of thermal cycles between cryogenic to elevated temperatures close to glass transition, experience formation of intralaminar cracks, causing degradation of thermo-mechanical properties. The objective of this paper is to improve the understanding of the microdamage initiation and growth mechanisms in thermomechanical fatigue.

  • 233. Varna, Janis
    et al.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Thermal aging and fatigue effect on resistance to transverse cracking2008Conference paper (Other academic)
  • 234.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Akshantala, N.V.
    Georgia Institute of Technology.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Damage in composite laminates with off-axis plies1999In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, no 14, p. 2139-2147Article in journal (Refereed)
    Abstract [en]

    Damage in off-axis plies of composite laminates is studied by examining the configuration [0/±θ4/01/2]s with θ=25, 40, 55, 70 and 90 subjected to tensile loading in the axial direction. It is found that for the values of θ, where the stress in the off-axis plies normal to the fibers is tensile, ply cracks lying along fibers initiate and increase in number, while for other θ values the plies do not undergo this damage, as expected. However, the overall laminate elastic moduli are also found to change for the θ values where no ply cracks exist. It is postulated that a shear-induced degradation of the off-axis plies is responsible for the observed laminate moduli changes. The prediction of changes in these moduli by using the ply shear modulus measured on [±θ4]s appears to support this postulate. For the case of moduli changes caused by ply cracks the recently proposed synergistic damage-mechanics approach [1] is applied. The implications of the findings of this work on a class of continuum damage-mechanics formulations proposed in the literature are discussed.

  • 235.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Berglund, Lars A.
    Luleå tekniska universitet.
    Interfacial toughness evaluation from the single-fiber fragmentation test1996In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 56, no 9, p. 1105-1109Article in journal (Refereed)
    Abstract [en]

    The single-fiber fragmentation test is widely used to determine the interfacial shear strength of polymer composites. In the present study, HTA carbon fibers in an epoxy matrix are studied for the two cases of strong and weak interfaces. The average fiber length is reported as a function of strain, both plotted on logarithmic scales. As deviation from linearity is observed, interfacial debond cracks extend to significant length. Debond crack lengths at given strains are determined from experimental data. A finite-element stress analysis in combination with a fracture mechanics criterion is used to determine values of Gc, the interfacial fracture toughness. Gc is independent of debond length, supporting the idea that Gc is a valid fracture criterion.

  • 236.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Berglund, Lars A.
    Luleå tekniska universitet.
    Material issues in large composite ship structures2001In: Cancom 2001: Third Canadian International Composites Conference, 2001Conference paper (Refereed)
    Abstract [en]

    In Sweden, six 72 meter navy ships are produced using sandwich structures. The skins are carbon fiber/vinylester and the core is primarily cross-linked PVC foam. The production technique is vacuum injection molding of large sandwich panels. Several material issues have been studied during the course of ship development. Compression strength is one important problem where the present carbon fiber non-crimp fabrics (NCF) and vinylester resins seem to give much lower data than prepreg-based composites used in aerospace applications. Compression testing and microscopy techniques are used to study this effect. Lower matrix yield strength, higher fiber waviness and poor testing techniques were shown to be the reasons for low compressive strength. The improved understanding of compressive failure in NCF composites also increased confidence in ship structure performance.

  • 237.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Berglund, Lars A.
    Luleå tekniska universitet.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Effect of voids on failure mechanisms in RTM laminates1995In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 53, no 2, p. 241-249Article in journal (Refereed)
    Abstract [en]

    Most studies on the effects of voids aim for determination of the relationship between void content and the fracture property of interest. The effect of voids on the mechanisms of transverse failure was therefore investigated for predominantly unidirectional resin transfer moulded laminates of glass-fibre-fabric/vinylester. Laminates with the highest average content of voids had a transverse strain to failure as high as 2% whereas low void content laminates failed at 0.3%. Only a few large and well-defined transverse cracks formed in low void content laminates before final failure. Multiple transverse cracks with irregular shape as well as numerous smaller cracks formed in the high void content laminates. The irregularity of these cracks resulted in lower stress concentration and stress level in the small amount of weft bundles orientated in the loading direction. A simple model was developed in order to demonstrate this and explain the high strain to failure of high void content laminates.

  • 238.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Loukil, Mohamed Sahbi
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microdamage initiation and growth in laminated composites2009Conference paper (Refereed)
  • 239.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    A synergistic damage-mechanics analysis of transverse cracking in [±θ/904]s laminates2001In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, no 5, p. 657-665Article in journal (Refereed)
    Abstract [en]

    The deformational response of [±θ/904]s laminates under longitudinal tensile loading is treated by a synergistic approach that combines continuum damage mechanics (CDM) and micromechanics. The constraint of θ° plies on transverse cracks in 90° plies is represented by a crack-opening displacement (COD) parameter in the CDM model and is expressed in terms of the ply properties and ply thickness ratios on the basis of a finite-element calculation. A methodology is proposed for predicting the laminate stiffness variations with crack density and applied strain (or stress) for various values of ply orientation θ and is found to give good results.

  • 240. Varna, Janis
    et al.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Intralaminar cracking evolution in symmetric laminates1999Conference paper (Refereed)
  • 241.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Mixed micromechanics and continuum damage mechanics approach to transverse cracking in [S, 90n ]s , laminates2001In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 37, no 2, p. 115-126Article in journal (Refereed)
    Abstract [en]

    The stiffness reduction in [S, 90 n ] slaminates due to transverse cracking in 90-layers is analyzed using the synergistic continuum damage mechanics (SCDM) and a micromechanics approach. The material constants involved in the SCDM model are determined using the stiffness reduction data for a reference cross-ply laminate. The constraint efficiency factor, which depends on the stiffness and geometry of neighboring layers, is assumed to be proportional to the average crack opening displacement (COD). The COD as a function of the constraint effect of adjacent layers and crack spacing is described by a simple power law. The crack closure technique and Monte Carlo simulations are used to model the damage evolution: the 90-layer is divided into a large number of elements and the critical strain energy rate G c having the Weibull distribution is randomly assigned to each element. The crack density data for a [02/904] s cross-ply laminate are used to determine the Weibull parameters. The simulated crack density curves are combined with the CDM stiffness reduction predictions to obtain the stiffness versus strain. The methodology developed is successfully used to predict the stiffness reduction as a function of crack density in [±Θ/904] s laminates.

  • 242.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mixed micromechanics and continuum damage mechanics approach to transverse cracking in [S,90°n]s laminates2001In: Mechanika kompozitnych materialov, ISSN 0203-1272, Vol. 37, no 2, p. 193-209Article in journal (Refereed)
  • 243.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Mixed micromechanics and continuum damage mechanics approach to transverse cracking in (s,90)s laminates2000Conference paper (Refereed)
  • 244.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Karnikovs, Andrejs
    Stiffness degradation in cracked cross-ply laminates1998In: Science, technologies and applications: ECCM-8, European Conference on Composite Materials, 3 - 6 June 1998, Naples, Italy / [ed] Ignazio Crivelli Visconti, Cambridge: Woodhead Publishing Materials , 1998, p. 573-580Conference paper (Refereed)
    Abstract [en]

    model for stiffness reduction in cross-ply laminates based on generalized plane strain assumptions is developed. Simple analytical expressions are obtained for longitudinal modulus and Poisson's ratio reduction as a function of transverse crack density. Additionally to the crack density, these expressions contain only elastic and geometrical properties of constituent laminae and the average crack opening displacement (COD) normalized with respect to the far-field strain. Calculations of COD are performed using FEM and several analytical models based on shear lag assumptions and variational analysis. Their predictive capabilities are verified using comparison with experimental data and FEM results. A simple procedure is suggested for COD determination, requiring for a given material only one FEM calculation using 'an average laminate' with 'an average crack spacing'. The obtained COD is used with good results for stiffness prediction for a wide variety of laminates of this material

  • 245.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Katerelos, Dionisios
    Galiotis, Costas
    Secondary damage modes at the transverse crack TIP caused by strain magnification2007In: Proceedings of COMP07: 6th International Symposium on Advanced Composites 16-18 May, 2007, Corfu, Greece, 2007Conference paper (Refereed)
  • 246.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Transverse cracks in cross-ply laminates 2. Stiffness degradation1998In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 34, no 2, p. 153-170Article in journal (Refereed)
  • 247.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Crack opening displacement as a basic parameter for stiffness reduction modeling in laminated composites1999In: ICCM-12 Europe: 12th International Conference on Composite Materials ; Palais des Congrès, Paris, Tours: ICCM , 1999Conference paper (Refereed)
  • 248.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Krasnikovs, Andrejs
    Riga Technical University, Latvia.
    Kumar, Rajesh A.
    Georgia Institute of Technology.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    A synergistic damage mechanics approach to viscoelastic response of cracked cross-ply laminates2004In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 13, no 4, p. 301-334Article in journal (Refereed)
    Abstract [en]

    A synergistic damage mechanics approach, i.e., a combined continuum damage mechanics (CDM) and micromechanics approach, is developed to characterize the viscoelastic response of cross-ply laminates with transverse cracks. The approach relies upon a second-order tensor-based description of damage wherein the crack opening displacement appears as an explicit function. The time variation of this function is calculated by micromechanics and is inserted into the continuum damage formulation. The relaxation moduli calculated for fixed states of damage (transverse crack density) by this approach agree well with independently calculated values by a CDM approach and finite element model. (Example materials: carbon-epoxy and glass-epoxy composites.)

  • 249.
    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

  • 250.
    Varna, Janis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Loukil, Mohamed Sahbi
    Swerea SICOMP.
    Stiffness prediction of damaged laminates based on the concept of laminate effective stiffness2016In: ECCM 2016: Proceeding of the 17th European Conference on Composite Materials, European Conference on Composite Materials , 2016Conference paper (Refereed)
    Abstract [en]

    IThe concept of the effective stiffness of a unidirectional layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates with damaged 90-layer. The effective stiffness of the damaged layer is obtained from the difference between damaged and undamaged laminate stiffness. The effective longitudinal modulus and Poisson's ratio of the layer are equal to their initial values. A very simple expression for the effective transverse modulus change with normalized crack density has sufficient accuracy and generality to be used in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different UD composites.

23456 201 - 250 of 295
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