Chapter 36 A Novel Torsional Kolsky Bar for Testing Materials at Constant-Shear-Strain Rates Jason R. York, John T. Foster, Erik E. Nishida, and Bo Song Abstract Kolsky bars, also known as split-Hopkinson bars, have been widely used in the dynamic characterization of engineering materials for over 50 years. Kolsky bars can be made to test materials in compression, tension, or torsion, and until recently, have been generally employed in the testing of high-impedance ductile materials to collect rate-dependent stress-strain data during large-strain inelastic flow. The advancement of “pulse-shaping” techniques in the last decade has allowed Kolsky bars to be utilized for testing low-impedance and brittle materials as well. Pulse-shaping is a processes of tailoring the dynamic loading during a test, with consideration given to the material being tested, in order ensure that the sample achieves a state of dynamic stress equilibrium and constant strain-rate if desired. The most common design of torsional Kolsky bars currently in widespread use offer no way to incorporate pulse-shaping. This limits their use mostly to high-impedance, large-strain applications. A novel torsional Kolsky bar design is presented in this work, which allows for straightforward pulse-shaping, similar to the method employed in compression testing, that can be used to test brittle and low-impedance materials as well as to design experiments that ensure the sample is undergoing a constant-shear-strain-rate deformation. Details of the design as well as some preliminary data demonstrating the pulse shaping capabilities collected during tests are presented. Keywords Kolsky bar • Split-Hopkinson bar • Torsion • Constant-strain-rate • Shear strain 36.1 Introduction A typical Kolsky bar apparatus [1, 2] is shown in Fig. 36.1, the experimental technique associated with this apparatus uses the elastic response of incident and transmission bars interfaced with a test specimen to characterize its material properties at varying strain-rates. To initiate the experiment a stress wave is applied to one end of the incident bar typically by striking the bar with a projectile fired from a gas gun. As the stress wave interacts with and propagates through the specimen, due to mechanical impedance differences between the sample and bars, a portion of the wave is reflected back through the incident bar, and the remaining pulse is transferred to the transmission bar. From the measured incident, reflected, and transmitted strain histories γI, γR, and γT, the material response can be derived [2]. Material characterization in shear loading can be conducted by modifying the method such that the incident and transmission waves are torsional stress/strain waves. This is typically carried out via astored energytechnique, whereby the incident bar is, at once, clamped near the center and a torque is hydraulically applied to the end of the bar, imposing a shear strain in one-half of the bar. The clamp is designed such that it J.R. York • J.T. Foster (*) Mechanical Engineering Department, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA e-mail: john.foster@utsa.edu E.E. Nishida Terminal Ballistics Technology Department, Sandia National Laboratories, Albuquerque, NM 87185, USA B. Song Experimental Solid Mechanics Department, Sandia National Laboratories, Albuquerque, NM 87185, USA B. Song et al. (eds.), Dynamic Behavior of Materials, Volume 1: Proceedings of the 2013 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-00771-7_36, #The Society for Experimental Mechanics, Inc. 2014 301
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