Necking in thermoplastic polymers is often viewed as a failure-related instability. In conventional tensile tests, once a neck forms, most additional deformation occurs through neck propagation rather than straining of the necked material, which obscures the intrinsic post-yield response. Here we show that stable neck propagation under an in-plane plane-strain constraint in a widely used commercial polymer film, poly(ethylene terephthalate) (PET), acts as a mechanical transformation that can create a new highly oriented material state. Under certain loading conditions, this state exhibits exceptional specific strength and strain rate insensitivity. PET films were stretched under conditions that promote steady neck propagation. Specimens were then excised from the necked zone and reloaded along directions parallel and transverse to the draw to decouple neck formation from post-neck deformation. The necked film exhibits strong anisotropy: along the draw direction, the elastic modulus increases from 1.67 GPa to 4.55 GPa, and the yield stress more than doubles, while transverse specimens display reduced yield stress and secondary necking. Although the parent material exhibits pronounced rate dependence in yielding and neck initiation, the draw-direction response of the necked state is nearly rate-independent over strain rates from 10 − 5 to 10 − 3 / s , and remains rate-insensitive under cyclic loading. These findings recast necking from an instability into a route for creating rate-insensitive, high-specific-strength polymer states and provide a compact experimental benchmark for constitutive models coupling localization, anisotropy evolution, and rate sensitivity.
Sun et al. (Wed,) studied this question.