GALCIT Colloquium

High strain and high strain-rate applications in aerospace, defense, and automotive industries have lead to interest in utilizing the ability of many polymers to withstand extensions to failure of several hundred percent, often without localization or necking and their strong rate dependence.  A broad range of characterization techniques will be presented for semi-crystalline polymers including elastic-plastic fracture, split Hopkinson pressure bar, plate impact and Taylor Impact. Temperature and strain-rate dependence will be reviewed in terms of classic time-temperature superposition and an empirical mapping function for superposition between temperature and strain-rate. The recent extension of the Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) technique to probe the dynamic tensile responses of polymers will be discussed, including probing incipient damage at very high strain-rate by linking in situ and post mortem experimental observations with high-fidelity simulation. Plate impact experiments have been performed on momentum-trapped shock assemblies with impact pressures above and below the phase II to phase III crystalline transition in polytetrafluoroethylene (PTFE) to probe subtle changes in the crystallinity, microstructure, and mechanical response. Building on a history of shock and Diamond Anvil Cell data it has long been know that PTFE exhibits a pressure induced phase transition at 0.7 GPa.  The transition has historically been assumed and reported to be dependent on the hydrostatic pressure.  However, studies employing neutron diffraction have suggested the phase transition is driven by only the principle stress applied to the compliant direction of crystalline domain.  A multiphase model was developed to include the stress deviator to drive potential martensitic transition and time to reflects the kinetic nature of the transitions demonstrating the need to account for both to capture subtle experimental observables.  Eric will also discuss the broader Dynamic Behavior of Materials program at Los Alamos National Laboratory.