On Temperature-Related Shift Factors and Master Curves in Viscoelastic Constitutive Models for Thermoset Polymers
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ON TEMPERATURE-RELATED SHIFT FACTORS AND MASTER CURVES IN VISCOELASTIC CONSTITUTIVE MODELS FOR THERMOSET POLYMERS
S. G. Nunes,1 S. Saseendran,2 R. Joffe,3 S. C. Amico,1 P. Fernberg,3 and J. Varna3,4*
Keywords: polymers, viscoelasticity, stress relaxation test, dynamic mechanical and thermal analysis (DMTA), time-temperature superposition 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.
Materials Engineering Department, Federal University of Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Department of Polymer Materials and Composites, RISE Research Institutes of Sweden, Piteå, Sweden 3 Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden 4 Laboratory of Experimental Mechanics of Materials, Riga Technical University, Riga, Latvia * Corresponding author; e-mail: [email protected] 1 2
Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 56, No. 5, pp. 841-866, SeptemberOctober, 2020. Original article submitted August 24, 2020. 0191-5665/20/5605-0573 © 2020 Springer Science+Business Media, LLC
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Introduction In many applications where polymers or their composites are the part of a structure, residual stresses are building up during different stages of manufacturing and application of mechanical loads. Due to the viscoelastic-viscoplastic properties of polymers, the internal stresses relax in them with time, the polymer members of structures, after the removal of load, slowly return to their relaxed (equilibrium) state, leading to undesirable shape distortions of structural elements and redistribution of internal stresses. These processes take place on all length scales, starting with the fiber/matrix interaction in composites, throughout the interactions of anisotropic plies in composite laminates to the structural level where polymer composites interact with molds during processing and with other m
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