Viscoelastic Properties of Semi-Crystalline Thermoplastic Polymers: Dynamic Analysis and Creep (original) (raw)
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Limits of linear viscoelastic behavior of polymers
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In the present study different approaches for determination of the limits of linear viscoelastic (LVE) behavior are considered on examples of some thermoplastic and thermosetting polymers. Stress or strain level, commonly considered as a limit of LVE behavior, are interrelated time-dependent functions strongly influenced by action of external factors. The concept of energy threshold has an advantage of combining into one physical function the effects of both stress and strain in initiating nonlinear behavior. The value of the stored deviatoric energy is considered as a limit of LVE behavior and is a material characteristic. The experimental data on tension at various constant strain rates and tensile creep at various stresses, temperatures, and moisture conditions are considered. It is proved for some polymers that LVE limits in stress-strain representation fall on a common curve that is an energy curve independent of time. Decrease of the test rate or growth of temperature or moisture content appears only in a shift down along the energy curve to the lower limit stresses and higher limit strains.
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Analysis of the thermo-mechanical behaviour of the polymers has been and still is the subject of many rheological studies both experimentally and theoretically. For small deformations, the modelling framework retained by rheologists is often of linear viscoelasticity which led us to the definition of complex moduli and to the rules of the renowned timetemperature superposition principle (TTSP). In this context, the effect of time (i.e., rate dependence) is almost unanimously associated with viscous effects. It has however been observed that the dissipative effects associated with viscous effects may be superimposed with thermo-elastic coupling effects, indicating a high sensitivity of polymeric materials to temperature variations (thermodilatability). Indeed, because of heat diffusion, it was also noticed that these strong thermo-mechanical couplings may induce a time dependence of the material behaviour. Using traditional experimental methods of viscoanalysis i.e., dynamic mechanical thermal analysis (DMTA) and via an experimental energy analysis of the behaviour using quantitative infrared techniques, the relative importance of thermoelastic heat sources compared to viscous dissipation was analysed with the increasing frequency of monochromatic cyclic tensile tests made at different ambient temperatures.
52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, 2011
Polymer and polymer composite materials offer a wide range of advantages such as high strength-to-weight ratio, impact resistance, high flexibility, recyclability, corrosion resistance, low cost and fast processing times, which make them very attractive materials . The vast applications of using such materials in both common and high-tech industries exhibit the importance of establishing and using accurate, reliable, simple, and practical numerical models to simulate and predict the behavior of these materials. There are numerous experimental observations indicating the nonlinear behavior of polymers and polymer composites in different loading conditions, especially at high temperatures or high stress levels . Due to the complex behavior of these materials especially the challenges in the modeling of damage nucleation and growth that depends on rate of loading, temperature, and the history of deformation, much less emphasis has been placed on predicting the damage evolution and fracture of polymer and polymer composites. Moreover, the combination of non-linear thermoviscoelasticity, thermo-viscoplasticity, and rate-and temperature-dependent damage (thermoviscodamage) effects seems unavoidable.
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The use of numerical simulations based on finite element analysis has become essential in designing new products, bridging basic material properties (obtained in various tests performed on material's specimens) and a product behaviour. Such simulations can account for a real geometry of designed components that can cause stress concentration as well as for in-service loading and/or environmental conditions. A challenging aspect for numerical simulations is anticipating the behaviour of advanced materials such as polymers and composites, demonstrating, i. a., anisotropy, heterogeneity and timedependent properties as well as non-trivial damage and fracture scenarios. The first step in achieving a valid product simulation is to perform simulations that can accurately reproduce the experimental results. The present work analyzes a possibility to simulate a response of a hyperelastic material (a semi-crystalline thermoplastic polymer) to monotonous uniaxial tensile loadings considering different strain-energy density functions as well as uniaxial cyclic loading using the commercial software ABAQUS/ CAE. The former case focuses on a stress-strain curve while the latter one deals with energy hysteresis, strain softening and strain hardening. The performed simulations produce good results for monotonous loading, but simulations of cyclic loading can only partially reproduce the material's behaviour.