| 1. |
- Freire, Rodrigo T.S., et al.
(author)
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On Determination of the Linear Viscoelastic Compliance and Relaxation Functions for Polymers in One Tensile Test : [Об определении функций линейной вязкоупругой податливости и релаксации полимеров в одном испытании на растяжение]
- 2023
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In: Mechanics of composite materials. - : Springer Nature. - 0191-5665 .- 1573-8922. ; 58:6, s. 765-786
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Journal article (peer-reviewed)abstract
- Usually, the viscoelastic (VE) response of polymers for applications in composites is obtained in uniaxial strainor stress-controlled tests. However, analyzing multimaterial structures by the Finite Element Method (FEM) or by other numerical or analytical tools, a material model in terms of a complete set of compliance functions and/or relaxation functions is required. In this paper, a methodology and exact analytical expressions for calculating the whole set of VE functions is presented based on the relaxation modulus E(t)and Poisson’s ratio v (t) determined in strain-controlled tests. The method is based on Laplace transforms, where an exact inversion is possible if a linear VE model with functions in Prony series is used. Results of the analytical model are compared with the FEM simulation, where specific boundary conditions to determine each particular VE function are used. Finally, the applicability of the so-called quasi-elastic method is investigated, where the expressions of elasticity theory are used to calculate a given viscoelastic function at an instant of time tk using the instant values of E(tk) and v(tk). For isotropic materials, the three approaches render almost coinciding results.
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| 2. |
- Gonçalves Nunes, Stephanie, et al.
(author)
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SHIFT FACTOR DEPENDENCE ON PHYSICAL AGING AND TEMPERATURE FOR VISCOELASTIC RESPONSE OF POLYMERS
- 2022
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In: ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials. - Lausanne : Composite Construction Laboratory (CCLab), Ecole Polytechnique Federale de Lausanne (EPFL). - 9782970161400 ; , s. 431-438
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Conference paper (peer-reviewed)abstract
- As polymeric resins are used as matrix in reinforced composites, understanding of their viscoelastic-viscoplastic response is critical for long-term performance design. However, during service life, thermosets are not in a thermodynamic equilibrium state, resulting in physical aging, which affects failure and viscoelastic (VE) properties, becoming a concern for industries. In this paper, an alternative methodology for testing and parameter determination for aging polymer, at different temperatures (TA) and times (tA), is proposed. The experimental data analysis was performed using a Schapery's type thermo-aging-rheologically simple VE model with constant coefficients in Prony series and the effect of temperature and aging included by two shift factors (aT, aA). Results showed that the shift factor can be presented as the product of shifts aT and aA. Furthermore, for short tA the change rate of the aA with tA does not depend on TA, whereas for long tA at high TA the rate increases.
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| 3. |
- Gonçalves Nunes, Stephanie, et al.
(author)
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The effect of short carbon fibers on viscoelastic behavior of UHMWPE
- 2022
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In: ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials. - Lausanne : EPFL Lausanne, Composite Construction Laboratory. - 9782970161400 ; , s. 314-321
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Conference paper (other academic/artistic)abstract
- At service temperatures, ultra-high molecular weight polyethylene (UHMWPE) is a highly viscoelastic (VE) material due to its low glass transition temperature (≈-113 °C). Since the mechanical response changes over time, the ability to predict and improve its performance over lifetime is an engineering concern. Adding short carbon fibers (SCF) as reinforcement (10 wt%) is expected to improve the material instant and long-term properties. VE relaxation functions for UHMWPE and composite at different temperatures (25-100 °C) are obtained from experimental data used to find parameters in a Schapery's type linear VE model. Then, relaxation functions of the SCF (randomly distributed) composite are predicted using the quasi-elastic approach. The results show that fibers affect positively the VE properties of UHMWPE and that the temperature- and time-dependent matrix behavior affects the stress transfer to fibers However, due to uncertainty regarding the input parameters, limiting the applicability of the chosen quasi-elastic approach, the quantitative agreement is not perfect.
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| 4. |
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| 5. |
- Nunes, Stephanie Goncalves, et al.
(author)
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Does the viscoelastic behavior of fully cured epoxy depend on the thermal history during curing?
- 2022
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In: Journal of composite materials. - : Sage Publications. - 0021-9983 .- 1530-793X. ; 56:22, s. 3439-3453
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Journal article (peer-reviewed)abstract
- Residual strains and shape distortions in a polymer-based composite structure may depend on the cure schedule used for manufacture. Aiming to understand the cure history effects, the influence of the curing “path” (time tc and temperature Tc path during curing) on viscoelastic (VE) response of a fully cured (FC) (alpha = 0.992) epoxy was investigated. Five different “families” of the same epoxy were manufactured in constraint-free conditions using different sets of curing parameters. Then, tensile tests were performed at different temperatures (T = 30 to 110 °C), and the time-temperature superposition principle (TTSP) and Schapery’s type of linear viscoelastic (VE) model, accounting for physical aging of specimens tested at high temperature, were used. The results show that the VE properties of the studied epoxy are independent of the curing history provided that at the end all specimens are fully cured. Also, the physical aging rate at high temperatures of all “families” is the same and it can be described by a simple aging-temperature independent equation reported in Nunes et al1 It is expected that curing history of unconstrained and fully cured epoxy has an insignificant effect on final viscoelastic behavior, a knowledge which could assist in developing more time and cost-efficient cure cycles.
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| 6. |
- Nunes, Stephanie Goncalves, et al.
(author)
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On Temperature-Related Shift Factors and Master Curves in Viscoelastic Constitutive Models for Thermoset Polymers
- 2020
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In: Mechanics of composite materials. - : Springer. - 0191-5665 .- 1573-8922. ; 56:5, s. 573-590
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Journal article (peer-reviewed)abstract
- Reliable accelerated testing routines involving tests at enhanced temperatures are of paramount importance in developing viscoelastic models for polymers. The theoretical basis, the time-temperature superposition (TTS) principle, is used to construct master curves and temperature-dependent shift factor, which is the necessary information to simulate the material response in arbitrary temperature and strain regimes. The Dynamic Mechanical and Thermal Analysis (DMTA) TTS mode, being one of the most promising approaches in terms of time efficiency and maturity of the software, is compared in this paper with macrotests at enhanced temperatures in their ability to give reliable master curves. It is shown, comparing simulations with test data for a chosen epoxy polymer, that none of the three DMTA TTS mode-based attempts used (at different temperature steps during frequency scanning) was successful in predicting the epoxy behavior in tests. On the contrary, using one-hour macrotests at enhanced temperatures gives a viscoelastic model with a very good predicting accuracy. Simulations were performed using an incremental formulation of the previously published VisCoR model for linear viscoelastic materials.
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