Self‐oscillatory neck propagation in polymers

Abstract

The oscillatory neck propagation during cold drawing of PET was studied. The mechanism of selfoscillations is heat instability of neck propagation. Oscillations are observed at high velocities when the draw stress increases with an increase in cross-head speed. Neck propagation is described by three equations, which were solved numerically. The solution of these equations predicts appearance of oscillations at high elongation velocities in agreement with experimental observations. The necessary condition of appearance of oscillations in any polymer is existence of some interval of cross-head speeds V, where the draw stress decreases with an increase in V.

Stress-oscillation behaviour of semi-crystalline polymers: the case of poly(butylene succinate)

“…17 The SO mechanism is under debate although, it was proposed that the mechanical work was converted into thermal energy which generated local heat instability in the neck region of the polymer sample. 18 In the case of aPET, the deformation induced a local temperature rise in the neck section up to 90-140 1C, higher than its glass transition temperature (B75 1C), 19 which softened the polymer chain segments and resulted in a reduction of stress. 9 This non-isothermal process has been discussed in Barenblatt's model, 3 which is regarded as a dynamic system in a phase space of stress, drawing velocity and temperature at the neck.…”

Non-adiabatic heating of polymer films under drawing

“…On this reason, a fitting parameter, multiplying the temperature rise by a factor 2.5, was used (Toda 1994). Bazhenov corrected Toda's equation and obtained reasonable agreement with experimental data for high drawing rates without any fitting parameters (Bazhenov 2011). However, for low drawing rates, the equation used in (Bazhenov 2011) is not accurate.…”

“…(Hillmansen and Haward 2001;Bazhenov 2011). Note that the theory determines temperature rise ΔT, while in experiments, temperature T is measured.…”

“…For example, for PET and PA-6, the drawing ratio, λ, does not depend on temperature and drawing rate (Godovsky 1992). However, at high drawing rates (>100 mm/min), an increase in the drawing ratio, λ, was observed for PET and PP (Liu and Harrison 1988;Bazhenov 2011). An increase in λ with temperature was observed for PC (Chiang et al 2013) and HDPE (Serenko et al 2003).…”

The tensile force oscillation of polycarbonate at elevated temperatures

“…The tensile force oscillation phenomenon was observed as the Luder's band formation in metals after they were deformed plastically [8]. Later, this phenomenon was also observed in polymers such as poly(ethylene terephthalate) (PET) fibers [9–12], amorphous copolyester [13], and polyimide thin films [14] as well as the syndiotactic polypropylene and its nanocomposites with montmorillonite in concentrations of less than 0.5 weight percent [15]. Two characteristic instability phenomena usually occur in polymer processing and tensile testing.…”