Determination of Forming Limits in Automotive Stampings

Forming limits are determined by analyzing strain distributions obtained from 0.2–0.05 in. grids imprinted on blanks by photographic or electrochemical marking techniques. Failure generally occurs in a region of stretch forming where all strains are positive (tensile) in the plane of the sheet. Experimental results derived from laboratory specimens and production automotive stampings indicate a predictable critical strain level beyond which failure is anticipated. The most effective method of increasing the stretching limits is the development of a more nearly uniform strain distribution. Factors controlling this distribution are material strain hardening characteristics, die design, stamping geometry, and lubrication.