New developments in material testing at very high strain rates

The determination of material properties under high-speed loading has been a challenge for many years. Structural vibrations, also called system ringing, in conventional testing machines deteriorate the quality of force measurement, which makes a precise determination of stress-strain curves and corresponding mechanical properties impossible. In this work, a new specimen geometry with its basic mechanical principle and the corresponding measurement technique are presented and discussed. The new method allows the determination of true stress-strain curves at high strain rates free from oscillation. Due to the additionally minor plastic deformation area in the new Generation III specimen, a quasi-movable bearing condition for the specimen fixation was created. Forces, based on the natural frequency of the specimen, deform the cross-section and create a displacement, which keeps the kinetic energy of the measurement area high. In this way, the elastic ringing effect has been reduced significantly. Any kind of filtering, smoothing or similar manipulations of the result are no longer needed. This new method has been validated on three steels types and one type of aluminium alloy with different strain hardening behavior through measurements and numerical analysis. The Generation III specimen can also be used for the quasi-static test and cover the strain rate range from 4.4·10-4 - 103 /s accordingly.