Self‐oscillation mechanism of necking on extension of polymers

Abstract

A new phenomenon in necking of some polymers, including poly(ethylene terephthalate) (PETP) was detected. It was found that extension of PETP films under certain conditions results in periodic stress oscillations and a periodic change in appearance of the sample. The conditions at which self‐oscillations appear have been determined, and the principal regularities of this regime of deformation are described. The following factors are critical for the appearance of self‐oscillation: speed of straining and compliance of the sample. The self‐oscillation of stress and formation of the periodic transverse bands is attributed to heat dissipation during necking corresponding to local temperature jumps and periodic strong variation of elasticity modulus due to poor heat conductivity of the polymer. Changing the external conditions of heat transfer influences the possibility and development of the effect. The phenomenon is common for various crystallizing polymers, being dependent on physical properties of the polymer and conditions of deformation.

Modelling tensile force oscillation during the tensile test of PET specimens

“…Model parameters, such as E 0 , E 1 , η 0 , η 1 , ε c and ε f 0 can be calculated from Equation (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). If σ 0 is known E 0 can be determined from Equation (8) and solving the Equation (18) τ 0 can be obtained: (18) and from Equation (6) η 0 can be calculated.…”

“…The reasons of stress oscillation have not been clarified in the literature and are hotly debated [5]. Among possible explanations one can find local heating caused by orientational elongation [6][7][8][9][10]; oscillation of local deformation rate during necking within a critical stress range [11]; orientation crystallization induced by adiabatic heat formation [12].…”

Neck propagation as a shock wave

“…At this juncture the authors speculate that the role of the BCD part may be clarified by taking inertia into consideration in a transient analysis of initial neck formation and of the propagation of the neck. It was also speculated that the dip BCD may be causin,the'random or cyclic vibrations known [19,20,23] to a~cpmpany neck propagation in solid filament extension. These problems are discussed in section 5 below.…”

“…These waves behind the neck influenced both by the shape of t? (X) and the presence or absense of plasticity hysteresis are probably physically real, possibly accounting for the vibrations known to accompany the neck propagation [19,20,23]. The 1/X(2) curves shown in Fig.…”

Study of the stress oscillation phenomenon in syndiotactic polypropylene/montmorillonite nanocomposites

“…However the exact mechanism behind this phenomenon remains elusive. Adrianova et al [8] has suggested that the formation of the alternating opaque/transparent zones is related to differential heat diffusion. Temperature at the necking region tends to fluctuate, because of the polymer poor heat conductivity and this causes a reduction in the local elastic modulus of the material, triggering thus stress oscillation.…”