The strain-rate dependent mechanical properties of a sheet metal are indispensable for the reliability of the light-weight design and the crashworthiness of auto-body because the mechanical properties are remarkably changed by the level of the strain-rates. Since the level of the strain-rate in the crash analysis is between 10 /sec and 500 /sec, an appropriate experimental method has to be developed in order to acquire the material properties at the corresponding strain-rate
In this study, an experimental apparatus is developed with the servo-hydraulic press (HRS: Hot Rolling Simulator) that guarantees the fast crosshead speed. The apparatus is , capable of the tension test at intermediate strain rates ranged form 1 /sec to 300 /sec. A jig is devised to convert the compressive motion of the HRS into the tensile motion with sensors attached to measure the load and displacement of a specimen. Experiments are carried out between the strain-rate of 4 /sec and 40 /sec to investigate the capacity of the apparatus and the limit of strain rate that can be obtained with the apparatus. It is confirmed that well performed the apparatus from the result of experiments.
Design the specimen is important to perform the experiment because there is not standard tension specimen for the dynamic test. Numerical simulations are carried out in order to determine the dimensions and the shape of the specimen with an elasto-plastic explicit finite element method. In FE simulation and stress update, the plastic predictor - elastic corrector (PPEC) method is applied in order to integrate the stress tracking the Johnson-Cook constitutive equation over a time step. The scheme adopts secant iteration in the plastic predictor - elastic corrector method in order to satisfy the plane stress condition. Finite element simulations are carried out to investigate the effect of the variation of geometric parameters in specimen such as the gauge length, the width and the fillet radius. Optimum dimensions are determined for standard specimens at the various strain- rates considering the effect of parametric variation and the crosshead speed of a press.
