A novel intermediate strain rate testing device: The serpentine transmitted bar

• A device for testing at intermediate strain rates (5/s to 500/s) is presented.
• The results from this testing are free of ringing typical in servo hydraulic machines.
• The device can reproduce the results of a long Hopkinson bar in a more compact form.
• Testing shows good correlation with low and high strain rate data.
• The device can be readily used for crash and metal forming characterization experiments.

A material's stress–strain behavior at intermediate strain rates (between 5 s−1 to 500 s−1) is essential for characterization of important events such as a car crash or a metal forming process. In addition, a material's stress–strain behavior can be strongly strain rate dependent, such that calibrating and validating the constitutive model at the actual strain rate of interest are important if finite element analyses are used for components that experience these strain rates. Testing of materials below 5 s−1 is easily accomplished with conventional electro-mechanical or servo-hydraulic load frames. Rates above 500 s−1 are typically performed with the split Kolsky/Hopkinson pressure bar (SHPB) and other devices depending upon the strain rate. However, the intermediate strain rate regime is a demanding test regime in which researchers have extended the use of specially instrumented servo-hydraulic load frames or very long Hopkinson bars. We describe a novel design of a serpentine Hopkinson transmitted bar that allows for accurate and robust load acquisition at intermediate strain rates in a compact form. Our new design produces repeatable stress–strain results without stress oscillations typical of a specially instrumented servo-hydraulic load frame and produces data for a longer loading time than a conventional Kolsky/Hopkinson bar of the same length. We demonstrate the intermediate bar's stress–strain response on a 6061-T6 Al alloy in which low rate and high rate data from the literature bounded the intermediate bar's response.