Yield Strength Model and Thoughts on an Ignition Criterion for a Reactive PTFE-Aluminum Composite; Conference paper

摘要

We studied a pressed and sintered reactive composite of 74 wt% polytetrafluoroethylene (PTFE, or Teflon) and 26 wt% aluminum powder. A model, which we call 'JCP', was developed to relate the yield strength of this material to the equivalent plastic strain, total strain rate, and temperature. The model was fit to Instron compression data at 0.1/s strain rate and split Hopkinson pressure bar (SHPB) compression data at approximately 2900/s strain rate. The SHPB database included initial temperatures of 297K and 325K. The model predicted a high susceptibility to shear localization due to the material's thermal softening and strain hardening characteristics. The hypothesis of shear localization as a precursor to ignition led us to consider equivalent plastic strain as a basis for an ignition initiation criterion. The JCP model was installed into the EPIC finite element wavecode and used to simulate Taylor impact tests involving impact speeds of 104 and 222 m/s. The computed boundary shapes versus time were compared with digitized images from a framing camera. Agreement was good at 104 m/s throughout the 36-microsec time range of observation. Agreement was good at 222 m/s until 16 microsec after impact. Thereafter the photographs displayed a greater degree of mushrooming than did the EPIC simulation. We speculated that internal ignition, fracture of various modes including petalling, and inaccuracies associated with interpolations and extrapolations of our JCP fit all may accounted for some of the mushrooming in excess of that predicted using JCP.