Behaviour of Materials at High Strain Rates and Cryogenic Temperature

Abstract

An experimental technique has been developed for investigating behaviour of materials at high strain rates and cryogenic temperature using a split Hopkinson bar system and a liquid helium cryostat. Yield stress and fracture toughness for a mild steel (JIS-S25C) and an aluminum alloy (Al 7075-T651) were measured at strain rates (loading rates) up to 3×103 s−1 (3×106MPa√m/s) and temperatures down to 4.6K. For the mild steel, the dynamic and static yield stresses increased with decreasing temperature and the dynamic yield stress was fairly larger than the static one. The variation of the dynamic yield stress due to temperature was not seen below 77K. The fracture toughness was dropped with decreasing temperature and increasing loading rate. The brittle-to-ductile transition was shifted to higher temperature range by the increase of loading rate. For the aluminum alloy, the yield stress scarcely depended on the temperature. The influences of temperature and loading rate on the fracture toughness were not clearly seen for Al 7075-T651.