The MTL Torsional Split-Hopkinson Bar

Abstract : A Stored Torque Split-Torsional Hopkinson bar (THBar) was developed (based on the principle of the one located at Brown University) and constructed at the U.S. Army Material Technology Laboratory (MTL). This report provides a detailed description of the THBar and its shear, axial and bending characteristics, and use of it to generate high strain rate shear stress-strain data of annealed OFHC copper (99.99% copper). With the present clamp of the THBar, shear stress-strain data can be generated for strain rates from 200 to 1300 s-1. At the highest strain rate, a specimen can be deformed in shear to a maximum strain of 85% with the shear pulse of 655 micro seconds. Three tests have been conducted for copper at strain rates of 400, 800, and 1200 s-1. The shear stress-strain curves that have been generated at these three strain rates show a strain rate sensitivity for strains less than 35%; stress increases with strain rate for a constant strain. At higher strain rates and higher strains (greater than 35% strains), shear stress saturates to a constant value independent of the strain rate. High strain rate, shear stress-strain data correlate favorably with the trend found in the quasi-static rate data from a servohydraulic test machine. (jes)