Linear stability analysis has been applied to a coextrusion fiber spinning flow that consists of a Newtonian fluid as a core layer and a Phan‐Thien/Tanner (PTT) fluid as a skin layer. These two chosen fluids show entirely different rheological behaviors. The stability of this coextrusion system was affected by the choice of three characteristic parameters: the skin layer fraction (f), the extensional parameter (ϵ), and the shear thinning parameter (ξ) in the Phan‐Thien/Tanner model. The linear stability results indicate that the viscoelastic skin layer (PTT fluid) has a stabilizing effect that delays the onset of draw resonance. Under fixed compositions (f is fixed), the stability envelopes changed from upturned curves to flattened ones as extensional force dominated the system. The neutral stable curves closed to the horizontal line at a critical draw ratio of around 20, showing similar behavior to a Newtonian fluid where the system has a very high Deborah number or it is dominated by shear thinning effects.

The steady flow of isothermal bicomponent coextrusion fiber spinning has been investigated. A model has been chosen in which a Newtonian fluid and a Phan‐Thien/Tanner (PTT) fluid were considered to be the core and the skin layer, respectively. This model was adopted to study the effect on spinline velocity of an interaction between two fluids with quite different extensional rheology. The effects of the hoop stress, gravity, inertia, and surface and interfacial tensions were disregarded. Only viscous and viscoelastic forces were considered. A uniaxial extension was also assumed as the radius variation in the axial direction is small. The Newtonian fluid has been considered prone to fluctuate during melt processing while the viscoelastic skin layer has a stabilizing effect. The velocity profile was affected by the choice of two characteristic parameters: extensional and shear‐thinning parameters, as well as the viscoelastic skin flow rate ratio. Both shear‐thinning and extensional parameters play important roles in melt flow. The results show that as the draw ratio increases, the system is dominated by the extensional parameters, whereas slow drawing is dominated by shearing, in spite of the thin viscoelastic skin.

An impression test using a flat indenter with three different diameters 0.5, 1, and 2 mm is proposed to study the plastic flow of a polypropylene copolymer. By comparing the flow characteristics of the impression test with the uniaxial tension and compression tests, it is found that a yield drop occurs in the compression, and geometrical necking appears in tension, but no load drop and only strain hardening is found in the impression tests. Furthermore, the plastic behaviors of the impression before the yield are also similar to those of the compression test because there is a correlation of the stress‐strain curve between impression and compression by only adjusting the scaling factors. The plastic zone size underneath the indenter is only slightly larger than the indenter diameter and, as the load is applied, the mean pressure across the face of the indenter is smaller than that of the metals due to the large intermolecular distance in the polypropylene copolymer. The impression test is also used to measure the strength, elastic modules, strain‐hardening exponent, and strain‐rate sensitivity in a manner of shallow penetration and miniature deformation, which indicates that the impression test can be employed to study the mechanical properties of the thin‐film polypropylene copolymer. © 1996 John Wiley & Sons, Inc.