In this study, the effect of high strain rate on the stress triaxiality is examined. The commercial 6061-T6 aluminum alloy which has wide applications as structural materials in the transport machine and construction industries is selected for this study. The response of the material to varying degrees of stress triaxiality under static and dynamic loading conditions is measured for circumferentially round-notched tensile specimens. Tensile tests are performed at RT with an Instron and servo hydraulic testing machines at three kinds of strain rates (ε=7.2×10−4, 1.0×102 and 1.0×103 s−1). Fracture surface observations and finite element method simulations also reveal the effect of stress triaxiality on the mechanical response under dynamic loading condition. The plastic constraint factors of the round-notched specimen under dynamic loading condition are lower than those under static loading condition. Stress triaxiality is decreased by the reflection of the stress wave near the notch root of the round-notched specimen under dynamic loading condition. The decrease of the stress triaxiality under the dynamic loading condition affects the relaxation of the plastic constraint in the round-notched specimens. The fracture surface of the round-notched specimen with the high stress triaxiality level shows the tensile fracture type under static loading condition. When the stress triaxiality decreases with increasing strain rate in the round-notched specimens, the fracture surface changes to the shear type. On the other hand, the temperature rise during impact tests hardly affects the relaxation of the plastic constraint in the round-notched specimens under high strain rates.
