Very High Strain Rate Range

International audienceThe classical Split Hopkinson Pressure Bar (SHPB) system is considered to be able to perform tests at strain rates ranging from 102 to 104s−1. However, some modifications can be carried out to extend the reachable strain rate within the specimen. The mean strain rate defined within the specimen:$$\dot{\varepsilon}_s =\frac{V_\text{out}-V_\text{in}}{l_s}$$where $V_\text{out}$ and $V_\text{in}$ stand for the velocity of output and input cross-sections of the specimen respectively, shows that the achievable strain rate varies inversely proportionally to the length of the specimen $l_s$, while the achievable stress is confined by the elastic limit of the bars, especially by the incident bar sustaining the entire impacting energy. From this viewpoint, extending the strain rate in the test can be either achieved by scaling down the size of the specimen and consequently that of the entire device, or by dispensing with the limit on the stress of the incident bar by removing it. Two modified Hopkinson devices are widely adopted to test the material at the strain rates beyond 104s−1, referred to as the miniaturized Hopkinson bar and Direct-Impact (DI) devices