The influence of the dynamic loading rate on tensile failure properties of metallic materials

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Abstract

The influence of the dynamic loading rate on the tensile failure properties of a series of tungsten alloys and an austenitic stainless steel were investigated by evaluating the flexural strength with unnotched and notched Charpy specimens. These data were generated with a newly developed Hopkinson pressure bar technique. The technique consists in impacting with a striker, at velocities ranging from 25 to 160 m/s, a round bar specimen placed against two incident Hopkinson pressure bars. Through the recording of the striker velocity before and after impact, failure energy is deduced. At impact velocities greater than 30 m/s, the results reveal a surprising increase of the Charpy energy with the increase of the impact velocity for both types of metallic materials. The results have been interpreted through numerical simulations of the Charpy test, the dependence of the material flow stress with the strain rate, and observations of the failure mechanisms. It was found that at impact velocities greater than 30 m/s, tangential strain rates exceed 3000s⁻¹ at the failure initiation site of the Charpy specimen. These strain rates are within the strain rate regime where strengthening occurred due to the viscous behaviour of the dislocations. Data generated with moderate stress concentration using notched round bar Charpy specimens indicate that the strengthening occurring at high strain rates continues to pilot the tensile failure processes.