Materials Science and Engineering A 464 (2007) 202–209 Instrumented anvil-on-rod tests for constitutive model validation and determination of strain-rate sensitivity of ultrafine-grained copper M. Martin a , A. Mishra b , M.A. Meyers b , N.N. Thadhani a,∗ a School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States b Department of Mechanical and Aerospace Engineering, University of California, San Diego, CA 92093, United States Received 8 December 2006; received in revised form 28 January 2007; accepted 31 January 2007 Abstract Anvil-on-rod impact tests were performed on as-received (cold-rolled) OFHC copper rods and copper processed by 2- or 8-passes of equal channel angular pressing (ECAP). The average grain size ranged from ∼30 m for the as-received sample to ∼440 nm for the 8-pass sample. The dynamic deformation states of the samples were captured by high-speed digital photography and velocity interferometry was used to record the sample back (free) surface velocity. Computer simulations utilizing AUTODYN-2D hydrocode with the Johnson–Cook constitutive model were used to generate free surface velocity traces and transient deformation profiles for comparison with the experimental data. The comparison of experimental results with AUTODYN simulations provided a means for extracting the strain-rate sensitivity of copper as a function of grain size. Strain-rate sensitivity was found to increase as grain size decreased. © 2007 Elsevier B.V. All rights reserved. Keywords: Strain rate sensitivity; Grain size effects; Dynamic deformation; Ultrafine-grained (UFG) copper 1. Introduction Nanocrystalline and ultrafine-grained (UFG) metals have unique mechanical properties (e.g., strength, hardness, and fatigue resistance) that render them good candidates for vari- ous structural applications [1–6]. Recent results indicate that strain-rate sensitivity in UFG metals is enhanced in compari- son with conventional polycrystalline metals having micro-scale grains [7–12]. The strain-rate sensitivity of UFG copper has been studied by Gray et al. [13] by performing quasistatic com- pression tests and split Hopkinson pressure bar experiments on ECAP-processed specimens. This study revealed that the strain- rate sensitivity of UFG Cu is significantly higher than that of typical annealed, polycrystalline Cu, and its yield strength is above that extrapolated from the Hall-Petch relation. The work described in this paper is an extension of what has been previ- ously done to determine the strain-rate sensitivity enhancement in UFG, ECAP-processed Cu at strain rates on the order of 10 3 to 10 5 s -1 using dynamic reverse Taylor [14] anvil-on-rod impact tests. ∗ Corresponding author. Tel.: +1 404 894 2651. E-mail address: naresh.thadhani@mse.gatech.edu (N.N. Thadhani). The rod-on-rigid-anvil impact experiment developed by Tay- lor [14] in 1948 has become a standard method for investigating the high strain rate (∼10 3 to 10 5 s -1 ) response of materials. In Taylor’s impact experiment, a cylindrical specimen is acceler- ated to impact a rigid anvil and deformation propagates through the cylinder as a wave. After impact, the specimen is recovered and the changes in its dimensions are used to infer its dynamic flow strength [14,15]. This test has become a common tool for investigating the constitutive response of materials by attempt- ing to reproduce the final deformed shape of the specimen with a constitutive model [15–19]. However, simply matching the final shape of the specimen does not necessarily provide a robust val- idation of the constitutive model since the deformation path is not considered [20]. Constitutive models based on empirical relationships (i.e., Johnson–Cook [16]) as well as physically based relationships (i.e., Zerilli–Armstrong [17]) have been commonly used in the past for comparison with experimental results. It is not the intent of this paper to choose one model or type of model over another, but simply to explain the validation method which was used in this study. In recent years, the Taylor impact test has been performed in its reverse configuration, with the rigid anvil impacting a stationary rod-shaped sample, allowing for simultaneous 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.147