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Viscoelastic Constitutive Equation (viscoelastic + constitutive_equation)
Selected AbstractsSimulation of dry-spinning process of polyimide fibersJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2009Gang Deng Abstract As one type of high-performance fibers, the polyimide fibers can be prepared from the precursor polyamic acid via dry-spinning technology. Unlike the dry-spinning process of cellulose acetate fiber or polyurethane fiber, thermal cyclization reaction of the precursor in spinline with high temperature results in the relative complex in the dry-spinning process. However, the spinning process is considered as a steady state due to a slight degree of the imidization reaction from polyamic acid to polyimide, and therefore a one-dimensional model based on White-Metzer viscoelastic constitutive equation is adopted to simulate the formation of the fibers. The changes of solvent mass fraction, temperature, axial velocity, tensile stress, imidization degree, and glass transition temperature of the filament along the spinline were predicted. The effects of spinning parameters on glass transition temperature and imidization degree were thus discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source] Theoretical and experimental studies of anisotropic shrinkage in injection moldings of semicrystalline polymersPOLYMER ENGINEERING & SCIENCE, Issue 6 2006Keehae Kwon A novel approach to predict anisotropic shrinkage of semicrystalline polymers in injection moldings was proposed using flow-induced crystallization, frozen-in molecular orientation, elastic recovery, and PVT equation of state. The anisotropic thermal expansion and compressibility affected by the frozen-in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in-plane anisotropic shrinkages. The frozen-in orientation function was calculated from amorphous and crystalline contributions. The amorphous contribution was based on the frozen-in and intrinsic amorphous birefringence, whereas the crystalline contribution was based on the crystalline orientation function, which was determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen-in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with temperature- and crystallinity-dependent viscosity and relaxation time. Occurrence of the flow-induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman-Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs on polypropylene of various molecular weights were carried out by varying the packing time, flow rate, melt temperature, and mold temperature. The anisotropic shrinkage of the moldings was measured. Comparison of the experimental and simulated results indicated a good predictive capability of the proposed approach. POLYM. ENG. SCI., 46:712,728, 2006. © 2006 Society of Plastics Engineers [source] Effective Dynamic Material Properties of Foam-like MicrostructuresPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005S. Alvermann The effective material parameters of a microstructured material can be found using homogenization procedures based on calculations of a Representative Volume Element (RVE) of the material. In our approach the RVE is calculated in frequency domain and inertia is taken into account, leading to a frequency dependent behavior of the RVE. With the frequency response of the RVE, effective dynamic properties of the material are calculated using an optimization procedure. Due to the frequency dependent material behavior on the microscale a viscoelastic constitutive equation is applied on the macroscale. An example calculation is presented for an auxetic 2-D foam-like microstructure which is modelled as a plane frame structure. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Simulations of instability in fiber spinning of polymersPOLYMER ENGINEERING & SCIENCE, Issue 7 2010Atanas Gagov This work determines the critical regimes beyond which the melt fiber spinning for noncrystallizable polymeric liquids becomes unstable. The critical draw ratio of the process is established using linear stability analysis for both isothermal and nonisothermal fiber spinning regimes. In addition, nonlinear isothermal analysis describes the complete range of the stable and unstable conditions for fiber spinning. Unlike previous studies, this research uses quite realistic viscoelastic constitutive equations extensively tested for five polymer liquids, which provides a good comparison of our calculations with available experimental data. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers [source] | ||||||||||