Philip Rae

Pressure-induced phase change in poly(tetrafluoroethylene) at modest impact velocities

Although poly(tetrafluoroethylene) (PTFE) is an unusually ductile polymer, it undergoes an abrupt ductile-brittle transition at modest impact velocities. No previous explanation for this behavior has been found after an extensive literature search. In this paper, we examine the role of a pressure-induced phase transition in PTFE in the dynamic failure of Taylor cylinder samples. There is a known phase transition in PTFE with a marked decrease in volume and compressibility that occurs at 0.5–0.65GPa at 21°C, with the transition pressure inversely related to temperature. Varying the temperature of the samples in the experiment revealed that the phase transition is probably involved in sample failure because the ductile/brittle transition velocity increased for decreasing temperature, despite the material fracture toughness decreasing. Additionally, Taylor tests were carried out on samples of poly(chlorotrifluoroethylene) PCTFE to investigate the behavior of a similar material to PTFE but without a pressure-...

Soft recovery of polytetrafluoroethylene shocked through the crystalline phase II-III transition

Polymers are increasingly being utilized as monolithic materials and composite matrices for structural applications historically reserved for metals. High strain-rate applications in aerospace, defense, and the automotive industries have lead to interest in the shock response of polytetrafluoroethylene (PTFE) and the ensuing changes in polymer structure due to shock prestraining. We present an experimental study of crystalline structure evolution due to pressure-induced phase transitions in a semicrystalline polymer using soft-recovery, shock loading techniques coupled with mechanical and chemical postshock analyses. Gas-launched, plate impact experiments have been performed on pedigreed PTFE 7C, mounted in momentum trapped, shock assemblies, with impact pressures above and below the phase II to phase III crystalline transition. Below the phase transition only subtle changes were observed in the crystallinity, microstructure, and mechanical response of PTFE. Shock loading of PTFE 7C above the phase II-III...

Phase transition modeling of polytetrafluoroethylene during Taylor impact

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.

The Taylor Impact Response of PTFE (Teflon)

Whilst Polytetrafluoroethylene (PTFE) is an unusually ductile polymer, it undergoes an abrupt ductile‐brittle transition at modest impact velocities. No previous explanation for this behaviour seems to have been presented. In this paper we examine the role of a pressure‐induced phase transition in PTFE in the failure of Taylor cylinder samples. Whilst a phase transition occurs at approximately 0.65 GPa at 21°C, the transition pressure is inversely related to temperature. Varying the temperature of the fired Taylor cylinders shows that the phase transition is likely to be involved because the critical velocity increased for decreasing temperature, despite the material fracture toughness decreasing.

Dynamic-tensile-extrusion of polyurea

Polyurea was investigated under Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) loading where spherical projectiles were propelled at 440 to 509 ms-1 through a conical extrusion die with an area reduction of 87%. Momentum of the leading edge imposes a rapid tensile deformation on the extruded jet of material. Polyurea is an elastomer with outstanding high-rate tensile performance of interest in the shock regime. Previous Dyn-Ten-Ext work on semi-crystalline fluoropolymers (PTFE, PCTFE) elucidated irregular deformation and profuse stochastic-based damage and failure mechanisms, but with limited insight into damage inception or progression in those polymers. The polyurea behaved very differently; the polymer first extruded a jet of apparently intact material, which then broke down via void coalescence, followed by fibrillation and tearing of the material. Most of the material in the jet elastically retracted back into the die, and only a few unique fragments were formed. The surface texture of all failed surfaces was found to be tortuous and covered with drawn hair-like filaments, implying a considerable amount of energy was absorbed during damage progression.

Mechanical Characterization and Preliminary Modeling of PEEK

Poly-ether-ether-ketone (PEEK) is a high-performance semi-crystalline polymer with mechanical and thermal stability characteristics that are superior to most tough polymers. The mechanical characteristics of this polymer are modeled over a broad range of mechanical loading conditions using a thermodynamically consistent modeling process. This preliminary model, which ignores the thermal response and the possible recrystallization of this material during loading, shows an outstanding ability to capture the multidimensional nonlinear response of PEEK up to 60 % compression, with loading rates from 0.0001 to 3000 1/s at room temperature. The model includes the measured anisotropy in the wave response that develops with plastic flow, captures the evolution of the measured equilibrium stress, and correctly matches the evolution of the tangent modulus at equilibrium. This broad range of rates and experimental conditions are achieved by using a two-element nonlinear thermodynamically-consistent model.

Free-field microwave interferometry for detonation front tracking and run-to-detonation measurements

A quadrature interferometer used in a free-field measurement mode has, with the aid of a high directivity horn antenna, been successfully used to measure the detonation front of PBX-9501 within a dielectric can. Using the known length of explosive, a relative dielectric permittivity of 3.84 has been calculated for the 34 GHz frequency used. Using this value, the displacement vs. time of the detonation front can be found and hence the velocity of detonation may be calculated. This technique shows good promise as a method of measuring the run-to-detonation distance in explosives using a totally non-contacting technique.

Damage & Fracture of High-Explosive Mock Subject to Cyclic Loading

We use four-point bend specimen with a single shallow edge notch to study the fracture process in Mock 900-21, a PBX 9501 high explosive simulant mock. Subject to monotonic loading we determine quantitatively the threshold load for macroscopic crack initiation from the notch tip. The four-point bend specimen is then subject to cyclic loading in such a way that during the first cycle, the applied force approaches but does not exceed the threshold load determined from the monotonic loading test and in the subsequent cycles, the overall maximum deformation is maintained to be equal to that of the first cycle. It is expected and is also confirmed that no macroscopic damage and cracking occur during the first cycle. However, we observe that sizable macroscopic crack is generated and enlarged during the subsequent cycles, even though the applied force never exceeds the threshold load. Details of the process of damage formation, accumulation, and crack extension are presented and the mechanical mechanism responsible for such failure process is postulated and discussed.

THE YOUNG’S MODULUS OF 1018 STEEL AND 6061‐T6 ALUMINIUM MEASURED FROM QUASI‐STATIC TO ELASTIC PRECURSOR STRAIN‐RATES

The assumption that Youngs modulus is strain-rate invariant is tested for 6061-T6 aluminium alloy and 1018 steel over 10 decades of strain-rate. For the same billets of material, 3 quasi-static strain-rates are investigated with foil strain gauges at room temperature. The ultrasonic sound speeds are measured and used to calculate the moduli at approximately 10{sup 4} s{sup -1}. Finally, ID plate impact is used to generate an elastic pre-cursor in the alloys at a strain-rate of approximately 10{sup 6} s{sup -1} from which the longitudinal sound speed may be obtained. It is found that indeed the Youngs modulus is strain-rate independent within the experimental accuracy.

NON‐RANDOM CRACK OPENING IN PARTIALLY CONFINED, THERMALLY DAMAGED PBX 9501 AND OBSERVATIONSON ITS EFFECTS ON COMBUSTION

In this work, we present evidence for how strong radial confinement can result in aligned macro‐scale crack opening. We damage cylinders in a tight‐fitting quartz sleeve, open on both ends, and observe the occurrence of aligned cracks opening normal to the longitudinal axis. This geometry and confinement is common in experimental arrangements such as strand burners and DDT tubes. Further, we observe, with high‐speed photography, how this non‐random crack opening affects combustion, and propose mechanisms, garnered from time‐lapse photography and elastic stress analysis, for how it occurs.