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The circular grid system, in which a pattern of 0.05-0.25 in. diameter circles electrochemically marked on sheet specimens is used, permits a visual display of the magnitude and direction of the strain from point-to-point of a stamping. Strain values obtained from the grid are plotted relative to an empirical failure curve to indicate proximity of a stamping to failure. Analysis of the strain distribution allows one to reduce the die tryout period, assist in establishing material specifications, evaluate die modifications, and monitor die variables throughout production runs.

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.

Attempts at scientific understanding of sheet metal formability began approximately five decades ago. laboratory studies of the 1930's were aimed primarily at elimination of steel defects and characterization of steel properties; understanding of plastic deformation and its relationship to the forming process was just beginning. Lubricants, tool metal, die processing, and other associated decisions were based on trial and error experiences. Since then, the scope and volume of scientific literature addressing sheet metal forming has increased exponentially. While a significant increase in laboratory understanding has been achieved, introduction of this knowledge into the press shop has been more difficult. Current emphasis is directed at the systems approach to sheet metal forming and the attendant management of data.

The proliferation of new coated steels, primarily zinc-based, has highlighted the need for understanding the interactions among the components of the forming system - part design, die design, material, lubrication, and press. Specialized laboratory tests have recently emerged to duplicate and measure quantitatively the sum of these interactions as a function of different lubrication conditions. The coefficients of friction derived from the DBS (Draw Beam Simulator) and the DF (Dome Friction) tests are used in this paper to document a number of system interactions. The results from these simulative friction tests are confirmed by experiments conducted on a highly instrumented, laboratory controlled, test pan die. The conclusion is reached that press shop needs are best met by use of a single lubricant which is insensitive to other components of the forming system.

Mathematical modeling of sheet metal forming is now a reality for designing of automotive stampings. The input of one such mathematical model includes part geometry, forming mode, material properties, forming speed, and coefficients of friction. The output includes strain distributions and forming severity derived from the Forming Limit Diagram. The mathematical model identifies critical forming zones during the initial part design stage and allows what-if scenarios to be investigated before the final design is committed to soft or hard tooling. The designer can evaluate various design versus production options for reducing the severity of the stamping. This paper illustrates potential uses for this design tool with two design case histories - 1) a drawn corner of an inner door stamping and 2) a stretch zone for an outer door handle pocket.

The knowledge of the interrelationships among variables within the sheet metal forming system determines the success of the forming operation in terms of forming severity, stamping quality, and system control. The effects of certain system variables are qualitatively known only for a few simple stamping geometries. However, in the real world press shop, optimization of the forming system and subsequent specification of system variables require quantitative analyses capable of processing multi-variable interactions. Such analyses are beyond solution by normal press shop rules of thumb and require one of many finite element analysis (FEA) schemes. This paper presents several case histories illustrating how a PC or engineering work station can be used to analyze stampings at either the blueprint stage or the die tryout stage. A novel finite element program, based on the deformation theory of plasticity, is used to predict the strains within the stamping.