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  • 151.
    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.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    An analysis of the nonlinear behavior of lignin-based flax composites2013In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 49, no 2, p. 139-154Article in journal (Refereed)
    Abstract [en]

    A lignin composite reinforced with 30% flax fibers at two levels of relative humidity, 34 and 66%, was used in this study. The nonlinearity of the composite was analyzed by studying the degradation of its modulus and the development of viscoelastic and viscoplastic strains. The reduction in the modulus of lignin-based composites in tension starts before the maximum in the stress-strain curve is reached and can be as large as 50%. With increasing relative humidity, these effects are slightly magnified. The time-dependent phenomena in tension were examined in short-term creep and strain recovery tests, demonstrating a rather high viscoplastic strain in lignin composites. Both viscoelastic and viscoplastic strains are larger at a higher relative humidity

  • 152.
    Pupurs, Andrejs
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Goutianos, Stergios
    Technical University of Denmark, Risø Campus.
    Brondsted, Povl
    Technical University of Denmark, Risø Campus.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Interface debond crack growth in tension-tension cyclic loading of single fiber polymer composites2013In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 44, no 1, p. 86-94Article in journal (Refereed)
    Abstract [en]

    Fiber/matrix interface debond crack growth from a fiber break is defined as one of the key mechanisms of fatigue damage in unidirectional composites. Considering debond as an interface crack its growth in cyclic loading is analyzed utilizing a power law, where the debond growth rate is a power function of the change of the strain energy release rate in the cycle. To obtain values of two parameters in the power law cyclic loading of fragmented single fiber specimen is suggested. Measurements of the debond length increase with the number of load cycles in tension-tension fatigue are performed for glass fiber/epoxy single fiber composites. Analytical method in the steady-state growth region and FEM for short debonds are combined for calculating the strain energy release rate of the growing debond crack. Interface failure parameters in fatigue are determined by fitting the modeling and experimental results. The determined parameters for interface fatigue are validated at different stress levels.

  • 153.
    Pupurs, Andrejs
    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.
    3-D fem modeling of fiber/matrix interface debonding in UD composites including surface effects2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: IOP Publishing Ltd , 2012Conference paper (Refereed)
    Abstract [en]

    Fiber/matrix interface debond growth is one of the main mechanisms of damage evolution in unidirectional (UD) polymer composites. Because for polymer composites the fiber strain to failure is smaller than for the matrix multiple fiber breaks occur at random positions when high mechanical stress is applied to the composite. The energy released due to each fiber break is usually larger than necessary for the creation of a fiber break therefore a partial debonding of fiber/matrix interface is typically observed. Thus the stiffness reduction of UD composite is contributed both from the fiber breaks and from the interface debonds. The aim of this paper is to analyze the debond growth in carbon fiber/epoxy and glass fiber/epoxy UD composites using fracture mechanics principles by calculation of energy release rate GII. A 3-D FEM model is developed for calculation of energy release rate for fiber/matrix interface debonds at different locations in the composite including the composite surface region where the stress state differs from the one in the bulk composite. In the model individual partially debonded fiber is surrounded by matrix region and embedded in a homogenized composite.

  • 154.
    Pupurs, Andrejs
    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.
    Analiz rasprostranenia poteri sceplenia v kompozite s odinochnym voloknom pri ciklicheskom nagruzhenii metodom mekhaniki razrushenia2011In: Mechanika kompozitnych materialov, ISSN 0203-1272, Vol. 47, no 1, p. 151-174Article in journal (Refereed)
    Abstract [en]

    A model is developed to analyze fiber/matrix debond growth along the broken fiber interface in a single fiber composite subjected to tension-tension fatigue. Paris law expressed in terms of debond length growth rate and strain energy release rate is used. Analytical solution for Mode II energy release rate GII is obtained for long debonds where the interface crack growth is self-similar. For short debonds the interface crack interacts with the fiber break and therefore FEM modeling in combination with the virtual crack closure technique was performed to calculate the increase of GII. Finally, the calculated GII dependences are summarized in simple expressions that are used to simulate debond growth in fatigue. A parametric study of the effect of Paris law parameters on debond growth is performed.

  • 155.
    Pupurs, Andrejs
    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.
    Energy release rate based fiber/matrix debond growth in fatigue: Part I: Self-similar crack growth2013In: Mechanics of Advanced Materials and Structures, ISSN 1537-6494, E-ISSN 1537-6532, Vol. 20, no 4, p. 276-287Article in journal (Refereed)
    Abstract [en]

    The strain energy release rate related to debond crack growth along the fiber/matrix interface in a unidirectional (UD) composite with a broken fiber is analyzed. The UD composite is represented by a model with axial symmetry consisting of three concentric cylinders: broken and partially debonded fiber in the middle surrounded by matrix, which is embedded in a large block of effective composite. Analytical solution for Mode II energy release rate is found and parametric analysis performed in the self-similar debond crack propagation region. It is shown that many anisotropic elastic constants of the fiber, which are usually not known, have a small effect on .

  • 156.
    Pupurs, Andrejs
    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.
    FEM modeling of fiber/matrix debond growth in tension-tension cyclic loading of unidirectional composites2013In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 22, no 8, p. 1144-1160Article in journal (Refereed)
    Abstract [en]

    The fiber/matrix interface crack (debond) growth from fiber break in unidirectional composite subjected to high stress tension–tension cyclic loading is analyzed. The debond growth is simulated calculating the strain energy release rate GII by FEM in three-dimensional formulation and using power law with respect to the GII change to describe the debond growth rate. Two models were applied. In Model 1 the partially debonded fiber/matrix cylindrical unit with a fiber break is surrounded from all sides by an effective composite phase. In Model 2 the effective composite phase was used around the fiber/matrix unit except the region between the unit and the specimen surface, which contains neat matrix only. Calculations show that the average GII is slightly larger, when the analyzed fiber is closer to the specimen surface. The debond growth was simulated using interface fatigue parameters obtained from single fiber composite specimen tests in the literature. Simulation results show that debonds from fiber breaks close to the specimen surface grow faster than from fiber breaks inside the composite specimen.

  • 157.
    Pupurs, Andrejs
    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.
    Fracture mechanics analysis of debond growth in a single-fiber composite under cyclic loading2011In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 47, no 1, p. 109-124Article in journal (Refereed)
    Abstract [en]

    A model is developed to analyze the growth of a fiber/matrix debond along a broken fiber interface in a single-fiber composite subjected to tension-tension fatigue. The Paris law expressed in terms of debond growth and strain energy release rates is used. An analytical solution for the Mode II energy release rate G II is obtained for long debonds, where the interface crack growth is self-similar. For short debonds, the interface crack interacts with the fiber break, and therefore a FEM modeling in combination with the virtual crack closure technique was performed to calculate the increase in G II . Finally, the calculated G II dependences are summarized in simple expressions that are used to simulate the debond growth in fatigue. A parametric study of the effect of Paris law parameters on the debond growth is performed.

  • 158.
    Pupurs, Andrejs
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP, Piteå.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Micro-crack development in carbon fiber battery in cyclic charge/discharge2013In: Proceedings of the 19th International Conference on Composite Materials: ICCM 2013, Montreal (Canada), ICCM , 2013, p. 1519-1528Conference paper (Refereed)
  • 159. Pupurs, Andrejs
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modeling mechanical fatigue of UD composite: multiple fiber breaks and debond growth2009In: 5th International EEIGM/AMASE/FORGEMAT Conference on Advanced Materials Research, Bristol: IOP Publishing Ltd , 2009Conference paper (Refereed)
    Abstract [en]

    The objective of this paper is to analyze fiber/matrix debond crack growth in unidirectional (UD) composites during high stress cyclic tension-tension loading. High stress loading means that fiber breaks and consecutive fiber/matrix interface debond growth are expected. Fracture mechanics concepts are applied to analyze damage evolution

  • 160.
    Pupurs, Andrejs
    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.
    Modeling mechanical stress and exfoliation damage in carbon fiber electrodes subjected to cyclic intercalation/deintercalation of lithium ions2014In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 65, p. 69-79Article in journal (Refereed)
    Abstract [en]

    Gradients in lithium ion concentration distribution in carbon fiber are accompanied by non-uniform fiber swelling leading to development of mechanical stresses. During lithium deintercalation these stresses may lead to initiation and growth of radial cracks in the fiber. The subsequent cycle of intercalation may result in arc-shaped cracks deviating from the tip of the radial cracks. These phenomena decrease the mechanical properties of fibers if used in structural batteries and reduce the charging properties of the battery by decreased diffusivity of lithium ions and by exfoliating layers on the fiber surface. The crack propagation and possible damage evolution scenarios are analyzed using linear elastic fracture mechanics. The crack geometry dependent ion concentration distributions and the elastic stress distributions were found using finite element software ANSYS.

  • 161.
    Pupurs, Andrejs
    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.
    Modelling mechanical stresses due to intercalation and de-intercalation of lithium-ions in a carbon fiber battery2012In: Proceedings of the 15th European Conference on Composite Materials, Venice, 24-28 June 2012 / [ed] Marino Quaresimin; Laszlo Kollar; Leif Asp, Venice, 2012Conference paper (Refereed)
    Abstract [en]

    Gradients in Li+ ion concentration distribution in a carbon fiber are accompanied by nonuniform fiber swelling leading to development of mechanical stresses. During lithium deintercalation these stresses may lead to initiation and growth of radial cracks in the fiber. This phenomenon decreases the mechanical properties of fibers if used in structural batteries and reduces the charging properties of the battery by initiating exfoliation of layers on the fiber surface. The radial crack propagation and possible damage evolution scenarios are analyzed using linear elastic fracture mechanics.

  • 162.
    Pupurs, Andrejs
    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.
    Steady-state energy release rate for fiber/matrix interface debond growth in unidirectional composites2017In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921, Vol. 26, no 4, p. 560-587Article in journal (Refereed)
    Abstract [en]

    Analytical closed form solution is derived for energy release rate due to steady-state debond growth along a broken fiber in unidirectional composite. The problem is represented by a three concentric cylinder model, where the broken and partially debonded fiber is embedded in resin and this unit is surrounded by homogenized effective composite. Parametric analysis shows that parameters in the very simple quadratic expression for energy release rate dependence on temperature change and mechanical strain are rather insensitive with respect to changes in constituent elastic properties and fiber content. The importance to have a large outer radius of the effective composite phase in the concentric cylinder model is demonstrated.

  • 163. Pupurs, Andrejs
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    UD composite in mechanical fatigue: modelling multiple fiber breaks and debond growth2009In: ICCM 17, Edinburgh: 17th International Conference on Composite Materials ; 27 Jul 2009 - 31 Jul 2009, Edinburgh International Convention Centre, Edinburgh, UK, London: IOM Communications , 2009Conference paper (Refereed)
    Abstract [en]

    When unidirectional fiber reinforced polymer composites are loaded in tension-tension fatigue multiple fiber breaks occur in random positions during the first cycle. Assuming that fibers do not experience fatigue, the further damage evolution with increasing number of cycles is in form of debonds growing along the fiber/matrix interface. Fracture mechanics analysis is performed to compare this mechanism with an alternative one where debonds initiate from transverse matrix cracks.

  • 164. Pupurs, Andrejs
    et al.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Unidirectional composite in mechanical fatigue: modelling debond growth from fibre breaks2010In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 39, no 3-5, p. 128-136Article in journal (Refereed)
    Abstract [en]

    The aim of this paper is to analyse fibre/matrix debond crack growth during high stress cyclic tension-tension loading of unidirectional composites. The debond crack evolution analysis is based on fracture mechanics concepts that mode II energy release rate calculations are performed analytically for long debonds, where crack growth is self-similar, and numerically for short debonds by finite element method in combination with virtual crack closure technique. From the calculation results simple expressions are derived for an arbitrary mechanical and thermal loading case. Finally, the obtained expressions are applied in Paris law for debond growth simulations in cyclic tension-tension loading.

  • 165.
    Pupurs, Andrejs
    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.
    Krasnikovs, A.
    Riga Technical University, Latvia.
    Energy release rate based fiber/matrix debond growth in fatigue: Part II: Debond growth analysis using Paris law2013In: Mechanics of Advanced Materials and Structures, ISSN 1537-6494, E-ISSN 1537-6532, Vol. 20, no 4, p. 288-296Article in journal (Refereed)
    Abstract [en]

    The strain energy release rate related to debond crack growth along the fiber/matrix interface in a unidirectional composite with a broken and partially debonded fiber is analyzed. The focus in this paper (Part II) in contrast to the self-similar crack growth analysis in Part I [1] is on growth of short debonds near the fiber break. Since self-similarity condition is not valid for interactive cracks, numerical FEM simulations were used to calculate magnification of previously described coefficients in the strain energy release rate expression. The findings from these studies are used in simulation of the debond growth in tension-tension fatigue using Paris law.

  • 166.
    Pupurs, Andrejs
    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.
    Loukil, M.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea.
    Mattsson, David
    Swerea.
    Damage development and stiffness reduction in laminates in out-of-plane loading2014In: 16th European Conference on Composite Materials, ECCM 2014: Seville, Spain, 22 - 26 June 2014, European Conference on Composite Materials, ECCM , 2014Conference paper (Refereed)
    Abstract [en]

    Simple approach based on Classical Laminate Theory (CLT) and effective stiffness of damaged layer is suggested for bending stiffness determination of laminate with intralaminar cracks in surface 90-layers. The effective stiffness of layer with cracks as a function of crack density is back-calculated comparing in-plane stiffness of laminates with and without damage. The accuracy of the CLT and effective stiffness approach is demonstrated comparing with bending stiffness results from FEM simulated 4-point bending test on laminate with damage. Analytical model for damaged laminate stiffness is presented which gives similar values for effective stiffness as FEM calculations for unit cell. Effect of local delaminations initiated from transverse cracks is analyzed.

  • 167.
    Pupurs, Andrejs
    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.
    Loukil, Mohamed Sahbi
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Kahla, Hiba Ben
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mattsson, D.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers2016In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 82, p. 244-252Article in journal (Refereed)
    Abstract [en]

    Simple approach based on Classical Laminate Theory (CLT) and effective stiffness of damaged layer is suggested for bending stiffness determination of laminate with intralaminar cracks in surface 90-layers and delaminations initiated from intralaminar cracks. The effective stiffness of a layer with damage is back-calculated comparing the in-plane stiffness of a symmetric reference cross-ply laminate with and without damage. The in-plane stiffness of the damaged reference cross-ply laminate was calculated in two ways: 1) using FEM model of representative volume element (RVE) and 2) using the analytical GLOB-LOC model. The obtained effective stiffness of a layer at varying crack density and delamination length was used to calculate the A, B and D matrices in the unsymmetrically damaged laminate. The applicability of the effective stiffness in CLT to solve bending problems was validated analyzing bending of the damaged laminate in 4-point bending test which was also simulated by 3-D FEM.

  • 168.
    Quaresimin, Marino
    et al.
    Department of Management and Engineering, University of Padova.
    Carraro, P.A.
    Department of Management and Engineering, University of Padova.
    Mikkelsen, L.P.
    Department of Wind Energy, Technical University of Denmark, Risø Campus.
    Lucato, N.
    Department of Management and Engineering, University of Padova.
    Vivian, L.
    Department of Management and Engineering, University of Padova.
    Brøndsted, Povl
    Department of Wind Energy, Technical University of Denmark, Risø Campus.
    Sørensen, Bent F.
    Materials Department, Risø National Laboratory, Department of Wind Energy, Technical University of Denmark, Risø Campus, Danish Corrosion Centre.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Reprint of: Damage evolution under cyclic multiaxial stress state: a comparative analysis between glass/epoxy laminates and tubes2014In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 65, p. 2-10Article in journal (Refereed)
    Abstract [en]

    In this work an experimental investigation on damage initiation and evolution in laminates under cyclic loading is presented. The stacking sequence [0/θ2/0/-θ2]s has been adopted in order to investigate the influence of the local multiaxial stress state in the off-axis plies and the possible effect of different thickness between the thin (2-plies) and the thick (4-plies) layers. Results are presented in terms of S–N curves for the initiation of the first cracks, crack density evolution, stiffness degradation and Paris-like curves for the crack propagation phase. The values of the off-axis angle θ has been chosen in order to obtain local multiaxial stress states in the off-axis plies similar to those in previous studies for biaxially loaded tubes. Results concerning damage initiation and growth for these two specimen configurations are shown to be consistent for similar local multiaxial stress states

  • 169.
    Quaresimin, Marino
    et al.
    Department of Management and Engineering, University of Padova.
    Carraro, P.A.
    Department of Management and Engineering, University of Padova.
    Mikkelsen, L.P.
    Department of Wind Energy, Technical University of Denmark, Risø Campus.
    Lucato, N.
    Department of Management and Engineering, University of Padova.
    Vivian, L.
    Department of Management and Engineering, University of Padova.
    Brøndsted, Povl
    Department of Wind Energy, Technical University of Denmark, Risø Campus.
    Sørensen, Bent Fruergaard
    Department of Wind Energy, Technical University of Denmark, Risø Campus.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage evolution under cyclic multiaxial stress state: A comparative analysis between glass/epoxy laminates and tubes2014In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 61, p. 282-290Article in journal (Refereed)
    Abstract [en]

    In this work an experimental investigation on damage initiation and evolution in laminates under cyclic loading is presented. The stacking sequence [0/θ2/0/−θ2]s has been adopted in order to investigate the influence of the local multiaxial stress state in the off-axis plies and the possible effect of different thickness between the thin (2-plies) and the thick (4-plies) layers. Results are presented in terms of S–N curves for the initiation of the first cracks, crack density evolution, stiffness degradation and Paris-like curves for the crack propagation phase. The values of the off-axis angle θ has been chosen in order to obtain local multiaxial stress states in the off-axis plies similar to those in previous studies for biaxially loaded tubes. Results concerning damage initiation and growth for these two specimen configurations are shown to be consistent for similar local multiaxial stress states

  • 170.
    Rozite, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Analysis of the time-dependent behavior of bio-based composites made from highly non-linear constituents2012In: Proceedings of Mechanics of Composite Materials XVII International Conference, Riga, 2012Conference paper (Refereed)
    Abstract [en]

    Large number of studies has been made about renewable bio-based (e.g. flax, hemp reinforced composites) composites during the last years. It has been showed that mechanical properties of bio-based composites compares well with glass fiber composites, especially if the properties are normalized with density. However, there are some drawbacks with respect to utilization of natural fiber properties to the extent that composites based on these reinforcements also can compete with synthetic materials, such as limited fiber length, sensitivity to the moisture, temperature etc. But probably one of the major drawbacks of natural fibers is variability of properties due to growth, processing conditions etc. Therefore, another type of reinforcement with high cellulose content is of interest – regenerated cellulose fibers (RCF). These fibers are manmade, but unlike materials with fossil origin, they are made out of the natural polymer directly. RCF is continuous and therefore it is easy to arrange them into fabrics with stable orientation and geometry. But RCF fiber and also bio-based resin performance is highly non-linear with presence of very significant viscoelastic and viscoplastic component.As basis material model developed by Schapery [1,2] was used in order to analyse time dependent behaviour of these materials. Model has been further modified to account for microdamage [3] In order to obtain all parameters needed for this model large number of experiments needs to be carried out. The numerous combinations of matrix and fibers are possible. Therefore it would be more convenient if material model is based on performance of constituents and test on composites should be performed only for verification of the model.The main objective of this investigation is to predict mechanical behaviour of these composites and their constituents by generalizing existing models to capture their time-dependent behaviour. In order to identify and quantify parameters needed for the modelling, extensive damage tolerance tests as well as creep experiments are carried out. REFERENCES1. Lou Y.C. and Schapery R.A., “Viscoelastic Characterization of a Nonlinear Fibre-reinforced Plastic,” Journal of Composite Materials, Vol. 5, pp. 208-234, 1971. 2. Schapery R.A., “Viscoelastic and Viscoplastic Constitutive Equations Based on Thermodynamics,” Mechanics of Time-Dependent Materials, Vol. 1, pp. 209-240, 1997. 3. Marklund E., Eitzenberg J. and Varna J. “Nonlinear viscoelastic viscoplastic material model including stiffness degradation for hemp/lignin composites,” Composite Science and Technology, Vol. 68, pp. 2156-2162, 2008.

  • 171.
    Rozite, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Characterization and analysis of time dependent behavior of bio-based composites made out of highly non-linear constituents2013In: 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] B. Antoun; H.J. Qi; R. Hall; G.P. Tandon; H. Lu; C. Lu, New York: Encyclopedia of Global Archaeology/Springer Verlag, 2013, Vol. 2, p. 109-115Conference paper (Refereed)
    Abstract [en]

    The objective of this investigation is to predict mechanical behavior of bio-based composites and their constituents by generalizing existing models to capture their time-dependent behavior. In order to identify and quantify parameters needed for the modeling, extensive damage tolerance tests as well as creep experiments are carried out.

  • 172.
    Rozite, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nyström, Birgitha
    Swerea SICOMP AB.
    Characterization and modeling of performance of polymer composites reinforced with highly non-linear cellulosic fibers2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: IOP Publishing Ltd , 2012, p. 12005-12014Conference paper (Refereed)
    Abstract [en]

    The behaviour of highly non-linear cellulosic fibers and their composite is characterized. Micro-mechanisms occurring in these materials are identified. Mechanical properties of regenerated cellulose fibers and composites are obtained using simple tensile test. Material visco-plastic and visco-elastic properties are analyzed using creep tests. Two bio-based resins are used in this study – Tribest and EpoBioX. The glass and flax fiber composites are used as reference materials to compare with Cordenka fiber laminates.

  • 173.
    Rozite, 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.
    Pupurs, Andrejs
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nonlinear behavior of PLA and lignin based flax composites subjected to tensile loading2013In: Journal of Thermoplastic Composite Materials, ISSN 0892-7057, E-ISSN 1530-7980, Vol. 26, no 4, p. 476-496Article in journal (Refereed)
    Abstract [en]

    The effect of temperature (T = 22°C, 30°C and 35°C) and relative humidity (RH = 34% and 66%) on mechanical behavior of natural fiber reinforced bio-based matrix composites subjected to tensile loading was investigated. Three composites were studied (a) polylactic acid (PLA) composite with 10% weight fraction of flax fibers; (b) PLA composite containing 5% viscose fibers (filaments of regenerated cellulose); (c) lignin-based composite with 30% of flax fibers. Elastic modulus, the nonlinear tensile stress–strain curves and failure were analyzed showing that all materials are temperature sensitive. The nonlinearity was analyzed studying modulus degradation as well as development of viscoelastic and viscoplastic strains. The modulus reduction in PLA-based composites starts after reaching the stress maximum and is not significant, whereas the modulus reduction in lignin-based composites starts before the maximum and it can reach 50%. With increasing RH these effects are slightly larger. The time-dependent phenomena were analyzed in short-term creep and strain recovery tests demonstrating significantly higher viscoplastic strain in lignin composites. Both viscoelastic and viscoplastic strains are larger at higher RH.

  • 174.
    Saseendran, S.
    et al.
    Swerea SICOMP, Piteå.
    Wysocki, M.
    Swerea SICOMP, Mölndal.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of degree of cure and time on viscoelastic poisson's ratio2017In: ICCM21 Proceedings, ICCM, International Committee on Composite Materials , 2017Conference paper (Refereed)
    Abstract [en]

    The Poisson's ratio of a solid under deformation is classically defined as the negative of the ratio between the lateral or transverse strain and the axial strain. Ideally for an elastic material, the Poisson's ratio is assumed to be a constant. However, for viscoelastic materials like polymers and polymer matrix composites this is also likely influenced by various factors like time [1], temperature, degree of cure and also on the strain. In this work, the evolution of the viscoelastic Poisson's ratio of the commercial LY5052 epoxy resin is studied under uniaxial tension subject to constant deformation stress relaxation testing. Measurements of the Poisson ratios are performed using contact extensometers and strain gages. Samples at five different cure states are manufactured and investigated. The relaxation testing is performed by loading the samples to 0.5% longitudinal strain and monitoring the relaxation behavior over a period of 24 hours per cure state. Poisson's ratio is observed to evolve from 0.32 to 0.44 over time depending on the cure state. Moreover the data indicates that the individual Poisson's ratio curves can be shifted horizontally following time-cure superposition. The shift functions used for this horizontal shifting are similar to those identified for DMTA tests for storage modulus under identical conditions. Following horizontal shifting, master curves that show the evolution of Poisson's ratio over time can be created for a particular reference cure state. This similarity of the shift functions in both micro-scale DMTA testing and macro-scale relaxation testing is an indicator of the validity of the shift factors. The observation is used to further develop a viscoelastic model which identifies the total shift function as the product of the temperature and cure shift functions. 

  • 175.
    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.

  • 176.
    Saseendran, Sibin
    et al.
    Swerea SICOMP AB, Piteå, Sweden.
    Wysocki, Maciej
    Swerea SICOMP AB, Mölndal, Sweden. Swerea SICOMP AB, Piteå, Sweden.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Characterisation of Viscoelastic Material Properties During Curing Processes2016In: Challenges in Mechanics of Time Dependent Materials: Proceedings of the 2015 Annual Conference on Experimental and Applied Mechanics / [ed] Bonnie Antoun, Cham: Encyclopedia of Global Archaeology/Springer Verlag, 2016, Vol. 2, p. 45-54Conference paper (Refereed)
    Abstract [en]

    The present contribution is toward systematic characterisation of the thermo-viscoelastic properties of a curing epoxy resin system. Characterising the viscoelastic solid behaviour is performed using a dynamic mechanical analyser. The aim of this work is to investigate the dependence of the viscoelastic response on time, temperature and degree of cure and to derive a model that covers the dependency of the relaxation modulus on all three factors and also to investigate how various factors would influence each other in the overall evolution of the relaxation modulus. In particular, we investigate the linearity between the three factors above. To summarize, the results indicate that these three parameters indeed obey a linear relationship.

  • 177.
    Saseendran, Sibin
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP.
    Wysocki, Maciej
    Swerea SICOMP.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Cure-state dependent viscoelastic Poisson’s ratio of LY5052 epoxy resin2017In: Advanced Manufacturing: Polymer & Composites Science, ISSN 2055-0340, Vol. 3, no 3, p. 92-100Article in journal (Refereed)
    Abstract [en]

    It is shown, using thermodynamically consistent linear viscoelastic material model that accounts for properties dependence on test temperature and cure state parameters, that for rheologically simple materials the cure and temperature related reduced times and shift factors are the same for all viscoelastic compliances, relaxation modulus, and Poisson’s ratio as well as for the storage and loss modulus. A necessary condition for that is that the cure and temperature parameters are affecting the reduced time only. This means that the Poisson’s ratio of polymeric materials, which for simplicity is often assumed constant, in fact exhibits a small dependence on time which is affected by temperature and state of cure. In this work, the evolution of the viscoelastic Poisson’s ratio of the commercial LY5052 epoxy resin is studied in relaxation test subjecting the specimen to constant axial strain. Specimens at several cure states are studied and Poisson’s ratio, defined as the lateral and axial strain ratio, is shown to evolve from 0.32 to 0.44 over time. Moreover, the data confirm that the cure state-dependent reduced time controlling the Poisson’s ratio development leads to the same shift functions as those identified in DMTA tests for storage modulus. The latter measurements also confirmed that the total shift can be considered as a sum of two shifts in the frequency domain, which means that function for reduced time calculation can be written as a product of two functions: one dependent on the test temperature and another one dependent on the cure state.

  • 178.
    Saseendran, Sibin
    et al.
    Department of Mechanics and Processes, Swerea SICOMP.
    Wysocki, Maciej
    Department of Structural Analysis and Modeling, Swerea SICOMP.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Evolution of viscoelastic behavior of a curing LY5052 epoxy resin in the glassy state2016In: Advanced Manufacturing: Polymer & Composites Science, ISSN 2055-0340, Vol. 2, no 2, p. 74-82Article in journal (Refereed)
    Abstract [en]

    The aim of this work is to develop a methodology to analyze the influence of the curing history on the viscoelastic storage modulus. Two different experimental approaches are presented exposing the material to various cure temperature and cure time sequences. The evolving viscoelastic properties are characterized using standard Dynamic Mechanical and Thermal Analysis (DMTA) equipment. Therefore, the present study is limited to infinitesimally small strains and linear viscoelasticity only. The methodology is demonstrated using the LY5052 epoxy resin system for its storage modulus E′ in the frequency domain. Results indicate that evolution of thermo-viscoelastic properties could be indeed assumed independent of the cure history for the investigated LY5052. We observe that the shift factor in the reduced time expression for the viscoelastic model examined in this paper is a product of two shift functions, namely the temperature and cure shift functions.

  • 179.
    Saseendran, Sibin
    et al.
    Swerea SICOMP, Piteå .
    Wysocki, Maciej
    Swerea SICOMP, Mölndal .
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Evolution of viscoelastic behaviour of a curing LY5052 epoxy resin in the rubbery state2017In: Advanced Composite Materials, ISSN 0924-3046, E-ISSN 1568-5519, Vol. 26, no 6, p. 553-567Article in journal (Refereed)
    Abstract [en]

    In this work, we investigate the relationship between the rubbery modulus and the degree of cure for partially to fully cured LY5052 epoxy resin. In particular, this paper experimentally tests an existing model formulated for shear modulus by redefining for in the tensile storage modulus. Experiments to characterize viscoelastic behaviour were performed in a dynamic mechanical and thermal analysis (DMTA) instrument in the frequency domain. Master curves are then created from DMTA using general time–temperature–cure superposition. The master curves are then normalized using the model so that the master curve does not depend on the properties in the rubbery region. This results in a unique master curve that describes the viscoelastic behaviour of the LY5052 epoxy resin for the given conditions. Once the relationship between the rubbery modulus and the degree of cure has been established, the amount of experimental characterization can be reduced. This could lead to the development of simplified experimental methodologies and simplified models to characterize the viscoelasticity of low molecular weight resins like the LY5052 epoxy resin system.

  • 180.
    Saseendran, Sibin
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP.
    Wysocki, Maciej
    Swerea SICOMP.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Viscoelastic Behavior of LY5052 Epoxy Resin in Rubberystate During Curling2016In: ECCM17: Proceeding of the 17th European Conference on Composite Materials, European Conference on Composite Materials , 2016, Vol. 2, p. 190-197Conference paper (Refereed)
    Abstract [en]

    The aim of the presented work is to investigate the relationship between the rubbery modulus and thedegree of cure for partially to fully cured LY5052 epoxy resin. In particular, this work experimentallytests an existing model defined in shear modulus by redefining into the elastic tensile modulus.Experiments were performed in a Dynamic Mechanical and Thermal Analysis (DMTA) machine inthe frequency domain. After the model is tested, super-master curves generated using timetemperature-cure superposition are normalized using the model so that the rubbery modulus madeindependent on the state of cure, which further simplifies the super-master curves. This results in aunique master curve that describes the viscoelastic behavior of the LY5052 epoxy resin for the givenconditions. This consequently could help formulate simplified models to predict viscoelastic behaviourand also develop better experimental methodologies to characterize them

  • 181.
    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

  • 182.
    Sjögren, A.
    et al.
    Structures Department, The Aeronautical Research Institute of Sweden.
    Krasnikovs, Andrejs
    Department of Strength of Materials, Riga Technical University.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Experimental determination of elastic properties of impact damage in carbon fibre/epoxy laminates2001In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 32, no 9, p. 1237-1242Article in journal (Refereed)
    Abstract [en]

    This paper describes an investigation of in-plane elastic properties of impact damaged regions in composite laminates. Quasi-isotropic carbon fibre/epoxy laminates were impacted and the impact damage examined by ultrasonic C-scanning, optical microscopy and thermal deplying. After impact damage observations, specimens were cut from the laminates and tested in tension and compression. The elastic modulus of the impact damage was, in both tension and compression, mainly controlled by the amount of fibre breakage. Interestingly, layers with broken fibres could sustain some load in compression, which led to higher elastic modulus in compression than in tension. The effect of delaminations on the elastic modulus was quite small in both tension and compression. The through-the-thickness variation of in-plane stiffness was studied by successively removing plies. The variation in stiffness was negligible, probably as a result of the very uniform distribution of delaminations and fibre breakage through the thickness of the laminates.

  • 183.
    Sparnins, Edgars
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Andersons, Janis
    Institute of Polymer Mechanics, University of Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Applicability range of the one-parameter ply plasticity model for prediction of the nonlinear response of laminates2005In: Advanced Composites Letters, ISSN 0963-6935, Vol. 14, no 1, p. 23-28Article in journal (Refereed)
    Abstract [en]

    A one-parameter lamina plasticity model is applied to predict the nonlinear deformation of an E-glass / epoxy cross-ply composite laminate under quasi-static uniaxial tensile loading at different angles to the material orthotropy axes. It is shown that the laminate theory yields accurate results within the plastic strain range covered in unidirectional continuous-fibre reinforced composite tests that are used to determine the plasticity model parameters

  • 184.
    Sparnins, Edgars
    et al.
    Institute of Polymer Mechanics, University of Latvia.
    Pupurs, Andrejs
    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.
    Nättinen, Kalle
    VTT Technical Research Centre of Finland, Espoo.
    Lampinen, Johanna
    VTT Technical Research Centre of Finland, Espoo.
    The moisture and temperature effect on mechanical performance of flax/starch composites in quasi-static tension2011In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 32, no 12, p. 2051-2061Article in journal (Refereed)
    Abstract [en]

    The effect of temperature and moisture on mechanical behavior of flax fiber/starch based composites was investigated experimentally. Elastic modulus, the nonlinear tensile loading curves, and failure strain were analyzed. Neat matrix and composites with 20 and 40% weight content of fibers were tested. It was found, performing tests with different amplitudes, that microdamage development with stress is rather limited and the related elastic modulus reduction in this type of compositesis not significant. It was shown that the composite elastic modulus and failure stress are linearly related to the maximum tensile stress in resin. The sensitivity of the maximum stress of the resin with respect to temperature and moisture is the source of composites sensitivity to these parameters. Constant interface stress shear lag model for stress transfer assuming matrix yielding at the fiber/matrix interface has been successfully used to explain the tensile test data. It indicates that the sensitivity of the used composite with respect to the matrix properties change could be significantly reduced by increasing the average fiber length from 0.9 mm to 1.5 mm.

  • 185.
    Ståhlberg, Daniel
    et al.
    Materials Technology, Scania CV AB.
    Nordin, Lars-Olof
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Johansson, Mats
    Royal Institute of Technology, Fibre & Polymer Technology, Stockholm.
    Mechanical response of thermoset polymers under high compressive loads, 12005In: Macromolecular materials and engineering (Print), ISSN 1438-7492, E-ISSN 1439-2054, Vol. 290, no 11, p. 1063-1072Article in journal (Refereed)
    Abstract [en]

    The present study describes the mechanical response of thermoset polymers under high compressive loads. A well-defined free radically cured vinyl ester resin has been used and studied in six different geometries in order to determine the dependence of apparent mechanical properties on the particular size and shape of a sample. The mechanical response in compression has also been compared to the response in tensile tests. Variation of the film thickness, boundary conditions and loading conditions reveal that there is a significant effect on the mechanical performance (apparent properties) of the polymer. When the thickness-to-width ratio of the sample is reduced in a compression test, the friction between the sample and the compression plates proves to be of great importance. The yield stress increases dramatically when the thickness of the sample is reduced, whereas it decreases when the friction between sample and the compression plate is reduced. The creep decreases when the thickness of the material is reduced and it decreases even more due to reaction of the material surrounding the compressed part of the sample. The described test conditions and observed phenomena will be subject to simulation in Part 2 of this study.

  • 186.
    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.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Time-dependent nonlinear behavior of recycled polypropylene in high tensile stress loading2011In: Journal of Thermoplastic Composite Materials, ISSN 0892-7057, E-ISSN 1530-7980, Vol. 24, no 5, p. 625-652Article in journal (Refereed)
    Abstract [en]

    Inelastic mechanical behavior in tension of a recycled polypropylene (rPP and a rPP with addition of 10% of maleic anhydride grafted polypropylene (rPP + MAPP) was characterized and compared. The time-dependent response was decomposed into nonlinear viscoelastic and viscoplastic parts and each of them quantified. It was found that the elastic properties did not degrade during loading. The addition of MAPP did not change the mechanical properties of the rPP. A nonlinear material model was developed and the involved parameters (stress-dependent functions) were identified. The model was then validated in a stress controlled test at a constant stress rate.

  • 187.
    Talreja, Ramesh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Joffe, Roberts
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    High temperature aging effects on damage and mechanical response of carbon/BMI composites1999Conference paper (Refereed)
  • 188.
    Talreja, Ramesh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, JanisLuleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modeling Damage, Fatigue and Failure of Composite Materials2016Collection (editor) (Refereed)
  • 189.
    Talreja, Ramesh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Joffe, Roberts
    Effect of aging on damage and stiffness changes in IM7/K3B laminates1998In: Mechanical behavior of advanced materials, American Society of Mechanical Engineers , 1998Conference paper (Refereed)
  • 190.
    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.

  • 191.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Caractérisation de l’endommagement des matériaux composites pour l’aéronautique à partir des mesures plein champ des déplacements et modélisation2012Conference paper (Other (popular science, discussion, etc.))
  • 192.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: COST Action FP0802 Experimental and computational methods in wood micromechanics2009Conference paper (Other (popular science, discussion, etc.))
  • 193.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Damage Accumulation and Stiffness Degradation in Composite Laminate2011Conference paper (Other (popular science, discussion, etc.))
  • 194.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Degradation of thermo-elastic properties of non-uniformly composite laminates2012Conference paper (Other (popular science, discussion, etc.))
  • 195.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: International Conference on Composites Testing and Model Identification, COMPTEST 2008: the key to matrix dominated composites failure2008Conference paper (Other (popular science, discussion, etc.))
  • 196.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Mechanics of Composite Materials, XVI International Conference2010Conference paper (Other (popular science, discussion, etc.))
  • 197.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Mechanics of Composite Materials, XVI International Conference2010Conference paper (Other (popular science, discussion, etc.))
  • 198.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Analysis of transverse composite cross-ply laminates1992Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Advanced polymer composites are commonly based on thin prepreg plies stacked at different angles to form a laminate. When the laminate is loaded, intralaminar transverse cracking in off-axis plies is one of the first damage modes. In the presented thesis, cross-ply laminates are used to study the transverse cracking phenomenon. Micromechanical modeling was performed in order to predict thermoelastic properties of the laminates as a function of crack density. The stress state between two existing cracks was calculated from two models developed to different degree of accuracy, both based on the principle of minimum complementary energy. As compared with the established model by Hashin, the so called 2-dim 0-model included a non-uniform x-axis stress distribution across the 0-layer thickness, the 2-dim 0/90-model also has a non-uniform x-axis stress distribution across the 90-layer thickness. The predictions of the 2-dim 0/90 model show the best agreement with test data for brittle matrix composites. If the 90-layer is thin as compared with the 0-layer, the 2-dim 0-model shows better agreement as compared with Hashin's model. Models for prediction of the first transverse cracking strain and crack density versus applied load were also developed. Fracture criteria based on linear elastic fracture mechanics were used. A so-called through-the-thickness flaw model showed good agreement with test data in laminates with thin 90-layers. For laminates with thick 90-layers, the failure strains were underestimated. A model with flaw growth in two directions (thickness and width) was therefore developed. The agreement between first transverse failure strain predictions and experimental data was very good for all 90-layer thicknesses. Finally, transverse cracking in [90n/0m]s and [0m/90n]s laminates was compared and the differences in local delamination behaviour were explained based on an analysis of the stress state.

  • 199.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Characterization of viscoelasticity, viscoplasticity and damage in composites2011In: Creep and fatigue in polymer matrix composites, Cambridge: Woodhead Publishing Materials , 2011, p. 514-542Chapter in book (Refereed)
  • 200.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Crack Separation Based Models for Microcracking2017In: Reference Module in Materials Science and Materials Engineering / [ed] Saleem Hashmi, Elsevier, 2017, p. --Chapter in book (Refereed)
    Abstract [en]

    Intralaminar cracking in layers of multidirectional laminates lead to reduced ability of these layers to carry load which is the reason for damaged laminate thermo-elastic properties reduction. The average stress reduction in the damaged layer is uniquely linked with displacements of the crack faces and, therefore, the crack opening displacements (CODs) and the crack face sliding displacements (CSDs) are alternative descriptors in damaged laminate stiffness predictions. In this chapter, exact closed form relationships are established linking thermo-elastic constants of the damaged laminate with crack density in layers and local parameters of the crack (average normalized COD and CSD). These robust local parameters depend on surrounding and damaged layer stiffness and thickness ratio-relationship, which is investigated numerically and described by simple fitting functions. It is also shown that the energy release rate (ERR) for propagation of the intralaminar crack in steady-state conditions has simple expression through the COD and CSD, leading to simple predictive tool for thin ply laminates where the cracking is propagation controlled.

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