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  • 1.
    Pupure, Liva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Saseendran, Sibin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Swerea SICOMP, Piteå, Sweden.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Basso, Margherita
    Department of Chemistry, Materials and Chemical Engineering, Politecnico di Milano, Italy. The Research Hub by Electrolux Professional, Pordenone, Italy.
    Effect of degree of cure on viscoplastic shear strain development in layers of [45/−45]s glass fibre/ epoxy resin composites2018In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 24, p. 3277-3288Article in journal (Refereed)
    Abstract [en]

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

  • 2.
    Saseendran, Sibin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of Degree of Cure on Viscoelastic Behavior of Polymers2016Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Reinforced polymer composites consist of continuous fibers embedded in a polymer matrix. The matrix is usually a thermoplastic or thermosetting resin. When thermosetting matrices are cured during the manufacture of composite parts, residual stresses develop within the part during the manufacture due primarily the thermally and chemically induced volumetric strains imposed on them. This can lead to shape distortions and sometimes weakening of the structure itself.Curing is the manufacturing process in which the thermoset resin is transformed from a liquid to a solid material. The molecular mechanisms involved in this process are quite complex and not well understood. In the macro-level, in addition to volumetric strains, heat is also generated since most thermoset polymerization reactions are exothermic. The mechanical properties of the thermoset also undergo dramatic changes. The material changes from its initial liquid state to a rubbery gel and finally to its vitrified glassy state.In modern day composite manufacturing, to accommodate for the shape distortions caused due to residual stress formation, the mold geometry is compensated. To do this, accurate predictions of the distortion behavior is required via computer simulations. This in turn requires simple mathematical models that can replicate the complex processes that take place during manufacture. One such process that requires attention is the curing of the thermoset. While models exist that assume elastic behavior during cure, they are not accurate throughout the entire cure process. Models based on viscoelastic material during cure offer better prospects in this perspective. However, currently models that are based on full viscoelasticity are either not well defined or are computationally tasking. Viscoelastic materials can be classified further in to thermorheologically simple and complex materials depending on their molecular weights. In layman’s terms, thermorheologically simple materials re those that obey the principles of time-temperature superposition (TTS). TTS requires that all response times (i.e., all relaxation or retardation time), depend equally on temperature. This is expressed by the temperature shift function. Master curves can be then generated extending the time scale beyond the range that could normally be covered in a single experiment. However to fully understand the development of viscoelasticity during cure it is also necessary that the effects of the degree of cure of the thermoset on these times be included in the model definition. This requires defining a cure shift function along with the temperature shift function. In the presented work, an attempt is made to develop a simplified methodology to characterize the viscoelastic material properties during curing. In the first paper, two different methods are investigated in a DMTA instrument to determine the effects of curing on the glassy state of the resin system LY5052/HY5052. A cure shift function was identified in the process. Based on observations it was concluded that the total shift function could be possibly defined as a product of the temperature and cure shift functions. Unique super-master curves were generated as a result. However, these curves showed a dependency of the rubbery modulus on the degree of cure. Hence, in the second paper, the effect of the degree of cure on the rubbery modulus was investigated. Following this a model was reformulated from an existing one and this was used to further simplify the super-master curves.

  • 3.
    Saseendran, Sibin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics. Swerea SICOMP.
    Effect of Degree of Cure on Viscoelastic Behavior of Polymers and their Composites2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Reinforced polymer composites consist of continuous fibers embedded in a polymer matrix. The matrix is usually a thermoplastic or thermosetting resin. When thermosetting matrices are cured during the manufacture of composite parts, residual stresses develop within the part during the manufacture due primarily the thermally and chemically induced volumetric strains imposed on them. This can lead to shape distortions and sometimes weakening of the structure itself. Curing is the manufacturing process in which the thermoset resin is transformed from a liquid to a solid material. The molecular mechanisms involved in this process are quite complex and not well understood. In the macro-level, in addition to volumetric strains, heat is also generated since most thermoset polymerization reactions are exothermic. The mechanical properties of the thermoset also undergo dramatic changes. The material changes from an initial liquid state to a rubbery gel and finally to a vitrified glassy state. In modern day composite manufacturing, to accommodate for the shape distortions caused due to residual stress formation, the mold geometry is compensated. To do this, accurate predictions of the distortion behavior is preferred via computer simulations. This in turn requires simple mathematical models that can replicate the complex processes that take place during manufacture. One such process that requires attention is the curing of the thermoset. While models exist that assume elastic behavior during cure, they are not accurate throughout the entire cure process. Models based on viscoelastic material during cure offer better prospects in this perspective. However, currently models that are based on full viscoelasticity are either not well defined or are computationally tasking. Viscoelastic materials can be classified further in to thermorheologically simple and complex materials depending on their molecular weights. In simpler terms, thermorheologically simple materials are those that obey the principles of time-temperature superposition (TTS). TTS requires that all response times (i.e., all relaxation or retardation time), depend equally on temperature. This is expressed using the temperature shift function. Master curves can be then generated extending the time scale beyond the range that could normally be covered in a single experiment. However to fully understand the development of viscoelasticity during cure it is also necessary that the effects of the degree of cure of the thermoset on these times be included in the model definition. This requires defining a cure shift function along with the temperature shift function. In the presented work, an attempt is made to develop a simplified methodology to characterize the viscoelastic material properties during curing. Two different methods are investigated in a DMTA instrument to determine the effects of curing on the glassy state of the resin system LY5052/HY5052. A cure shift function was identified in the process. Based on observations it was concluded that the total shift function could be possibly defined as a product of the temperature and cure shift functions. Unique super-master curves were generated as a result. However, these curves showed a dependency of the rubbery modulus on the degree of cure. Hence, in the second paper, the effect of the degree of cure on the rubbery modulus was investigated. Subsequently a model was reformulated from an existing one and this was used to further simplify the super-master curves. Following dynamic testing, it was necessary that macroscopic testing is performed to corroborate the results. The macroscopic experiments utilized for this purpose was stress relaxation tests to determine the viscoelastic Poisson’s ratio of neat resin. The Poisson’s ratio in particular is an important property to study, since it’s interaction with the fiber during curing is critical in the study of residual stresses. The focus of the study is to determine if there is a dependency of the Poisson’s ratio on degree of cure and whether master curves can be generated by horizontal shifting of data. Literature pertaining to the dependency of the Poisson’s ratio on degree of cure is scarce. If appropriate horizontal shifting can be performed, it can be easily compared to the results from dynamic testing to check if the shift factors are truly universal. Also presented is a brief study of the effect of degree of cure and time on the development of viscoplastic strains during curing. This is done by performing creep tests on composite specimens with varying degrees of cure. The experimental results were then used to validate the well-known Zapas-Crissman model for viscoplastic strain evolution with time and investigate how it is influenced by the cure state.

  • 4.
    Saseendran, Sibin
    et al.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Wysocki, Maciej
    Swerea SICOMP AB, Mölndal, Swerea SICOMP AB, Box 271, 941 26, Piteå.
    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.

  • 5.
    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.

  • 6.
    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.

  • 7.
    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.

  • 8.
    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

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