Loading rate effects on tensile properties and fracture toughness of steel

An overview of the effects of elevated ('dynamic') loading rates on the tensile and initiation toughness properties of steels and their effect on structural behaviour is given. Issues affecting engineering critical assessments are also considered. Dynamic loading rates affect both the material resistance and the structural response of engineering components and it is the combination of these two influences which determines the structural behaviour. Because of inherent complexities of testing at high loading rates, the development of suitable test techniques for measuring tensile and fracture toughness properties has been a topic of marked investment in the last 40 years or so, but increasing consensus has now been achieved leading to the development of test standards. Generally, the effect of increasing loading rate is to increase strength (positive strain rate dependence) and the strain rate sensitivity increases with temperature, but there are exceptions when dynamic strain ageing effects intervene. The effect of loading rate on the fracture toughness of ferritic steels is dependent on overall material behaviour: for temperatures below the brittle to ductile transition region, toughness decreases with increasing loading rate. At upper shelf temperatures, the ductile initiation toughness and tearing resistance generally increases with loading rate, hut exceptions exist which are outlined in the paper. In or near the transition region, increasing loading rates can cause a shift from fully ductile behaviour at static rates to brittle behaviour at high rates of loading. The brittle to ductile transition temperature of ferritic steels increases with increasing loading rates and methods have been proposed to predict this shift.