Ductile to brittle failure transition of HSLA-100 Steel at high strain rates and subzero temperatures

Mechanical properties of HSLA-100 steel at room and subzero temperatures of −40 °C, −80 °C and −196 °C were investigated in this work. Dry ice and liquid nitrogen were used to reach the low temperatures. Also, the material response to different strain rates was studied. Quasi-static tensile test rig and tensile Hopkinson split bar were employed to carry out the experiments. Furthermore, fracture behavior and the transient temperature of this steel was investigated using the curve of stress triaxiality factor (P/Y) against true fracture strain (εf) using Bridgman equation. In addition, fracture mechanisms for this steel at different testing conditions were studied. Finally, SEM observations were performed to back the results found in this research. The results indicated that the material behavior at room temperature to −40 °C was quite ductile regardless of the strain rate level. Fracture mechanism transition from ductile to brittle occurred at −80 °C. For −196 °C, the material behavior was completely brittle for high strain rate and entirely ductile for lower strain rates. It was also found that the yield stress of material progressively increased with the decrease of temperature and the increase of strain rate. The maximum increase was 160% and occurred for −196 °C at the strain rate of 10,800 s−1.