Phase transition modeling of polytetrafluoroethylene during Taylor impact

Abstract

The complex pressure and temperature dependent phase behavior of the semicrystalline polymer polytetrafluoroethylene (PTFE) has been investigated experimentally. One manifestation of this behavior has been observed as an anomalous abrupt ductile-to-brittle transition in the failure mode of PTFE rods in Taylor cylinder impact tests when impact velocity exceeds a narrow critical threshold. Earlier, hydrocode calculations and Hugoniot estimates have indicated that this critical velocity corresponds to the pressure in PTFE associated with the transition from a crystalline phase of helical structure to the high pressure crystalline phase (phase III) of a planar form. The present work represents PTFE as a material in a simplified phase structure with the transition between the modeled phases regulated by a kinetic description. The constitutive modeling describes the evolution of mechanical characteristics corresponding to the change of mechanical properties due to either an increase of crystallinity or the phase transition of a crystalline low-pressure component into phase III. The modeling results demonstrate that a change in the kinetics of the transition mechanism in PTFE when traversing the critical impact velocity can be used to explain the failure of the polymer in the Taylor cylinder impact tests.

On the Shock Response of Polymers to Extreme Loading

“…At the present time most polymer constitutive models in use in hydrocodes are top level, semi-empirical fits to behaviour that have severe limitations in their applicability since they borrow concepts based on viscoplasticity and are fit to data in which the stress state is rarely steady (e.g. [47,52,88,90,91]). Future developments in theory and experiment must advance in tandem if models are to become more accurate and predictive.…”

“…Any length scale has three phases of behaviour; localisation, flow and interaction and at least in the densification regime, polymer stress state stability is frequently not achieved in short impulse, low strain, laboratory tests [16]. Thus integrated loading techniques developed for metals (such as Taylor impact) reveal the transit to stress state stability over milliseconds for polymers where microseconds were sufficient in metals [16,43,88,89]. At the present time most polymer constitutive models in use in hydrocodes are top level, semi-empirical fits to behaviour that have severe limitations in their applicability since they borrow concepts based on viscoplasticity and are fit to data in which the stress state is rarely steady (e.g.…”

High Strain Rate Mechanics of Polymers: A Review

“…The complex loading in compression of the Taylor test, where stress, strain rate, and final strain vary within the specimen makes it an ideal test for validating constitutive models, where the complex loading is thought to provide a more robust test of the model [36]. In polymers, the test has been used to understand the behavior of the materials under complex loading for modeling efforts [108][109][110][111][112][113] and to elucidate phase transitions in the polymeric materials [12,114].…”

On compression and damage evolution in PTFE and PEEK

“…Polytetrafluoroethylene (PTFE, trade name Teflon), is a widely employed fluoropolymer found in [6]. Under laboratory conditions at room temperature, a pressure-induced phase transition has been reported in PTFE at 0.50-0.65 GPa (Phase II-III) [7]. The phase transition results in a 13% local volume decrease within the crystalline domains and a considerable reduction in compressibility.…”

“…There is also more radial crack bifurcation seen with increasing impact speed. In the shock region (loaded in 1D in a conical geometry), there is a central segment where failure appears restricted that corresponds to the region identified previously through experiment and simulation as going through the phase transformation at 0.7 GPa or 12% strain perpendicular to polymer chains [6,7,11,12]. Fracture in the PEEK samples was seen to be different in nature to that of the PTFE samples; being fully contained within the cylinder and not extending to the impact face or the cylinder circumference in the targets shown.…”