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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.

Drawbeads in production stamping dies often have insufficient penetration of the male bead into the female cavity. With insufficient penetration, the actual bending radii of the sheet metal are larger than the geometrical radii of the drawbead. The actual bending radii in the sheet directly affect the force that restrains sheet movement. To predict the restraining stress due to a drawbead, it is necessary to know the actual bending radii in the sheet as it passes though the drawbead. Data from a previous study are used to develop empirical regression equations for predicting measured radii of the sheet that is bent around the radii in a drawbead. A physical model for the evolution of the sheet radii as the drawbead closes is proposed. This model is consistent with the empirical equations and the mechanics of the sheet bending process.

In a study of the Bauschinger effect, data were collected from three sources in the published literature. Quantitative stress-strain data were taken from these papers, and the results re-analyzed. The resulting database has 44 lots of sheet steels, including drawing quality, interstitial free, bake hardening, HSLA (and related grades), dual phase, TRIP, recovery annealed, and martensitic grades. In analyzing the data, it is found that use of the 0.05% yield strength on reversal instead of the conventional 0.2% yield strength provides more generality in explaining the results. In this analysis, the Bauschinger effect is characterized by a term (BE), which is the difference between the steel strength just prior to reversal and the 0.05% yield strength on reversal normalized by the strength just prior to reversal. An initial prestrain of 2% is needed to establish a dislocation morphology that can be generalized across many of the steel grades.

A three-dimensional surface profilometer has been used to examine the surface morphology in various regions of stamped automotive steel parts. This analysis provides insight into the nature of the surface morphology development during stamping. For this work, four stamped zinc-coated sheet steel sections were considered. Each of the sections was examined with the 3-D surface profilometer in a variety of regions suspected to show different surface morphology due to differences in forming. Upon analysis of the surface height frequency histograms, different modes of deformation on the part surface were identified and categorized. Three modes of surface deformation were observed - “simple pressing”, “pressing with small scale sliding and bending” and “pressing with gross sliding, stretching and bending”. Each mode had a distinctive set of characteristics in the frequency height histograms. These modes were observed in both galvannealed steel sheet and hot-dipped galvanized steel sheet.