Determination of Forming Limits by Digital Image Processing Methods

A deep-drawing grade AKDQ steel sheet was tested to determine the Forming Limit Diagrams (FLDs) using two different forming methods to identify points where the incipient necking started. With the conventional out-of-plane spherical forming test, incipient necking points were determined from the formed dome samples at areas diametrically opposite to the necked and fractured locations. With the Marciniak in-plane forming test, incipient necking points were determined from real-time video images obtained during the experiment. These images were analyzed with strain-measurement software to determine the strain distributions in the samples. Various data plotting schemes were explored to produce meaningful results for the flat-bottom in-plane tests. These methods included “best fit” line analysis and strain vs. time plots. The final outcome is reasonably consistent with published FLD data.