Test method The effect of strain rate on the mechanical behavior of Teflon foam A. Tasdemirci * , A.K. Turan, M. Guden Dynamic Testing and Modeling Laboratory, Department of Mechanical Engineering, Izmir Institute of Technology, Gulbahce, Urla, Izmir 35430, Turkey article info Article history: Received 30 March 2012 Accepted 12 May 2012 Keywords: Split Hopkinson Pressure Bar Quartz crystal Strain rate sensitivity LS-DYNA abstract The quasi-static (1 Â 10 À3 ,1 Â 10 À2 and 1 Â 10 À1 s À1 ) and high strain rate (7200 and 9500 s À1 ) experimental and high strain rate numerical compression deformation of a Gore PolarchipÔ CP7003 heat insulating Teflon foam was investigated. High strain rate tests were conducted with the insertion of quartz crystal piezoelectric transducers at the end of the transmitter bar of a compression Split Hopkinson Pressure Bar (SHPB) set-up in order to measure the force at the back face of the specimen. A fully developed numerical model of the SHPB test on Teflon was also implemented using LS-DYNA. The simulation stresses showed close correlations with the experimentally measured stresses on the bars. The developed model successfully simulated the high strain rate loading. The damage initiation and progression of experimental high strain rate tests were further recorded using a high speed camera and found to be very similar to those of the simulation high strain rate tests. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Soft materials are distinguished by low mechanical impedance and low strength. In addition to many other applications, these materials are used where impact or shock loadings can occur. A well-known example for these applications is the rubber inter-layer between composite backing plate and ceramic front face of integrated composite armor [1,2]. The rubber inter-layer in the composite armor functions to distribute the incoming projectile momentum to a wider area of the continuous backing composite plate, diminishing the damage. In these applications, a thorough knowledge of the mechanical behavior of soft materials at increasingly high strain rates is certainly a prerequisite. In parallel with this, a dependable laboratory scale experimental set-up is also needed to mimic the actual impact loading conditions. The Split Hopkinson Pressure Bar (SHPB), originally developed by Kolsky in 1949 [3], is a widely used experimental method of testing a wide range of material groups, including composites [4], metals [5], ceramics [6] and viscous fluids [7], at strain rates higher than w100 s À1 . Nevertheless, testing soft materials using SHPB is known to be prob- lematic because of the impedance mismatch between the test and bar materials. In testing soft materials such as polymers, the transmitter bar signal is usually weak, leading to difficulties in distinguishing the experimental signals from noise. Various solutions to overcome this problem have been reported. An SHPB set-up made of viscoelastic bars was implemented in order to cover the impedance mismatch disadvantage of soft specimens; however, the nature of viscoelasticity of the bar materials resulted in intensified wave dispersion and attenuation effects [8,9]. The use of hollow SHPB transmitter bars was also investigated but it did not affect the amplitude of the noise [10]. The classical SHPB set-up was modified to test soft materials by measuring the sample forces by means of piezoelectric transducers [11–15]. With the insertion of quartz crystals at the impact ends of the incident and transmitter bars, the forces at the front and back of the specimen were measured directly and dynamic equilibrium was maintained throughout the experiment; hence, the validity of the tests was checked. * Corresponding author. Tel.: þ90 232 7506780; fax: þ90 232 7506701. E-mail address: alpertasdemirci@iyte.edu.tr (A. Tasdemirci). Contents lists available at SciVerse ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest 0142-9418/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2012.05.004 Polymer Testing 31 (2012) 723–727