EXTENSIONAL DEFORMATION, STRESS-RELAXATION AND NECKING FAILURE OF VISCOELASTIC FILAMENTS

We investigate the transient viscoelastic behavior of weakly strain-ha rdening fluids in filament stretching devices during uniaxial elongati on and following the cessation of stretching. The numerical results ar e compared with experimental observations on a concentrated shear-thin ning polystyrene solution which is well characterized by a multi-mode Giesekus model. The finite element computations incorporate the effect s of viscoelasticity, surface tension, and fluid inertia and the time- dependent moving-boundary problem is solved using the code POLYFLOW. A detailed comparison of multi-mode computations with single-mode solut ion is presented in order to examine the differences in the predicted viscoelastic behavior and the role of the fluid relaxation spectrum. T he evolution in the transient Trouton ratio at different deformation r ates is compared with experimental measurements and with the theoretic al predictions of ideal homogeneous uniaxial elongation. Simulations o f the filament stretching device using the multi-mode viscoelastic mod el demonstrate a significant improvement in the agreement between the predicted and observed extensional viscosity at short times. The compu ted Trouton ratio is also in good agreement with theoretical expectati ons for ideal homogeneous uniaxial extension, despite the strongly non homogeneous viscoelastic necking of the fluid column observed during e longation in the filament stretching device. Following the cessation o f elongation, numerical simulations predict an interesting and complex evolution in the kinematics of the fluid filament. Initially the tens ile stresses in the column relax in the non-linear form predicted theo retically, indicating that filament stretching devices can be used to monitor transient extensional stress relaxation, provided that the evo lution of the tensile force at the end-plate and the filament radius a t the mid-plane are carefully measured. However, at longer times after cessation of stretching, the local extension rate at the axial mid-pl ane begins to increase rapidly, leading to a 'necking failure' that is greatly accelerated compared to that expected in a corresponding Newt onian filament. The calculations show that this unstable necking is no t driven solely by the surface tension but also by the viscoelasticity of the fluid, and is coupled with significant elastic recoil of the m aterial near the end-plates. The rate of necking in the column is a se nsitive function of the extensional viscosity predicted by the constit utive model, in particular the magnitude and the rate of strain-harden ing that occurs during uniaxial elongation. This phenomenon can also b e simply and accurately described by an appropriate set of coupled one -dimensional thin filament equations that use the finite element compu tations to provide a suitable initial condition for the axial distribu tion of the polymeric stresses in the filament. (C) 1998 Elsevier Scie nce B.V. All rights reserved.