Sheet Thinning during Plane-Strain Bending

Knowledge of the net thinning strain that occurs in a sheet as it is bent over a single radius is an important component in understanding sheet metal formability. The present study extends the initial work of Swift on thinning during plane-strain bending to sheet steels with power law stress-strain behavior and with the inclusion of friction. The experimental data come from studies on the enhanced forming limit curve on DQSK steel and analysis of the curl behavior of 590R and DP600 steels. Results for single radius bending from these studies are used in the present investigation. It has been found that the amount of net thinning strain depends on back tension, initial plane-strain yield strength, and the maximum true bending strain calculated for the neutral plane at the mid-thickness of the sheet. It has also been found that for a constant initial plane-strain yield strength, as back tension increases, the amount of the thinning strain associated with unbending (i.e. straightening) progressively increases compared to amount of the thinning strain associated with bending.