The Mechanical and Optical Response of Polychlorotrifluoroethylene to One-Dimensional Shock Loading
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THE use of polymers in high loading rate situations has a number of sources of interest. The most significant comes from the energetic materials community, where materials such as polyurethane rubbers (estane),[1] KelF 800 (a copolymer of chlorotrifluoroethylene and vinylidene difluoride),[2] and hydroxyterminated polybutadiene (HTPB)[3] find application as the binder phases of plastic-bonded explosives (PBXs). Polychloroprene has been used as a structural material in its own right for the protection of buildings from earthquakes,[4] while epoxy and phenolic resins are used as the matrix materials in fiber composites.[5,6] Finally, some polymers such as polyvinylidene difluoride (PVDF)[7] can be conditioned such that they become piezoelectric in nature, and as such have found application as stress gages. Fluorinated polymers find application in aggressive environments where conditions of pressure, temperature, and strain rate can be extreme. They are also noted for their resistance to chemical attack. The most common of this class of material is polytetrafluoroethylene (PTFE or the trade name Teflon), and to date has been studied most extensively under shock loading conditions. This material is a semi-crystalline polymer, with the chains JEREMY C.F. MILLETT, Senior Scientist, and MICHAEL R. LOWE, Scientist, are with the AWE, Aldermaston, Reading, RG7 4PR, U.K. Contact e-mail: [email protected] GARETH APPLEBY-THOMAS, Lecturer, and ANDREW ROBERTS, Senior Technician, are with the Centre for Defence Engineering, Cranfield University, Defence Academy of the United Kingdom, Shrivenham, Swindon SN6 8LA, U.K. Manuscript submitted April 22, 2015. Article published online July 24, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
adopting a number of crystalline domains around ambient pressures and temperatures. Of most relevance to shock loading experiments is the ambient temperature phase II to III transition at a pressure of 0.5 to 0.65 GPa. Phase II consists of a helical rotation of 13 CF2 units, in a hexagonal array, giving way to a planar zig–zag conformation with the chains lying in either an orthorhombic or monoclinic lattice. Such behavior has also been noted under shock loading conditions, for example Champion[8] observed a change in the shock velocity (US)–particle velocity (up) corresponding to a pressure of ca. 0.5 GPa. Bourne and Gray[9] made similar observations, drawing much the same conclusions. Nagao et al.[10] used a laserdriven flyer plate to shock load PTFE to 1.0 GPa, using Raman spectroscopy to probe the response. Results showed that there was a stretching of the carbon–carbon bonds along with a twisting of the CF2 groups, again suggesting the phase II to III transition. Rae et al.[11] used Taylor impact experiments to study the high strain-rate response of PTFE. They showed that (at room temperature) PTFE underwent an abrupt change in response, showing a ductile response at an impact velocity of 133 m s1, and a brittle one at 134 m s1. Brown et al.[12] performed one-dimensional shock recovery expe
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