Materials testing on deep-drawn sheets at high strain rates

The characteristic values for nonproportional elongation at maximum load an d for total elongation, as well as for the plastic strain ratio (r-value) a nd the strain-hardening exponent (n), serve to describe the deep-drawabilit y and crash behaviour of sheet materials. The objective of the following study was to measure the r-value of deep-dra wn sheets at high strain rates and to determine characteristic values to de scribe the crash and forming behaviour. The r-value depends to a greater or lesser extent on the strain rate, according to steel quality. The r-values normally determined under quasi-static test conditions no longer, therefor e, characterise the material behaviour at high strain rates well enough. A measurement method has been developed with which it is possible to measure the r- and n-values at high strain rates. The method has been applied to co ld-rolled batch-annealed steel grades. IF steel particularly, with very hig h r-values under quasi-static conditions, suffers a dramatic drop in this c haracteristic value. The sheet's resistance to thickness as a result. This heightens the reduction decreases significantly susceptibility to cracking in the deep-drawing process. At high strain rates, uniform elongation no lo nger correlates with the strain at maximum load. The n-value decreases with increasing strain rate for all the examined steels. The strain-hardening t hereby diminishes.