Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel Part 1 - Mechanical behaviour

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0.15, 0.3, and 0.5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10(2) to 5 x 10(3) s(-1) at room temperature, with true stains varying from 0.1 to 0.3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (a-ay). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0.5 prestrain, 0.3 strain, and strain rate of 4.8 x 10(3) s(-1). Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.