On the Use of a Torsional Split Hopkinson Bar to Study Rate Effects in 1100-0 Aluminum

TLDR

Accurate measurement of flow stress at high strain rates is challenging, and this paper reviews those difficulties and discusses methods to overcome them. The authors developed an explosive‑generated split Hopkinson bar for torsion testing, incorporating a mechanical pulse smoother to reduce large amplitude fluctuations, and described its operation, calibration, and potential inaccuracies. The study concludes that a split Hopkinson bar adapted for torsion testing with a short, square loading pulse yields reliable data, and demonstrates that 1100‑0 aluminum exhibits distinct flow stress behavior at ~800 s⁻¹ compared to 10⁻⁴ s⁻¹ and to compressive test results from other researchers.

Abstract

The difficulties involved in the accurate measurement of the flow stress of materials at high rates of deformation are reviewed, and methods of overcoming these difficulties are discussed. It is concluded that the best experimental method is that in which a split Hopkinson bar is adapted for torsion testing, and the loading pulse is approximately square in shape and has a relatively short duration. A description is given of apparatus which was developed to achieve this type of loading, the input wave being generated explosively. This wave was found to contain large amplitude fluctuations, and to eliminate these a mechanical “pulse smoother” was used. The operation of this device is described, and the test procedure, method of calibration, and possible sources of inaccuracy are discussed. Results are presented for 1100-0 aluminum alloy deformed at strain rates of the order of 800 sec−1 and are compared with those obtained at a nominal strain rate of 10−4 sec−1; the results are also compared with those obtained in compressive tests by other workers.