Characterisation of velocity dependent flow-curves and yield-surfaces

An innovative approach for material characterisation in terms of strain rate dependant flow curves for high speed forming and cutting is proposed. Two driving concepts for accelerating the hammer in high velocity testing are presented: a pneumatic one for plastic strain rates of up to 1000/s and an electromagnetically driven one for strain rates of up to 10000/s. As directly measuring the time dependent local distribution of stresses and strains in the specimen is impossible at high-velocity testing, auxiliary test parameters are recorded and used as input data for an inverse numerical simulation that provides the desired material characteristics as a result. Specifically, the hammer displacement is determined by optical measurement principles and the forces affecting the specimen are determined via strain gauges at the specimen holder. Velocity and strain rate courses are measured for impact and acceleration modes. In the impact modes the hammer hits the specimen after being accelerated. In the acceleration mode hammer and specimen are contact at the beginning of the process and thus both are accelerated together. While the impact mode leads to higher strain rates, a nearly constant amplitude, and oscillations by stress waves, the acceleration mode leads to lower strainrates, an evolving amplitude, and reduces oscillations.