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Accurate determination of the forming limit strain of aluminum sheet metal is an important topic which has not been fully solved by industry. Also, the effects of draw beads (enhanced forming limit behaviors), normally reported on steel sheet metals, on aluminum sheet metal is not fully understood. This paper introduces an experimental study on draw bead effects on aluminum sheet metals by measuring the forming limit strain zero (FLD0) of the sheet metal. Two kinds of aluminum, AL 6016-T4 and AL 5754-0, are used. Virgin material, 40% draw bead material and 60% draw bead material conditions are tested for each kind of aluminum. Marciniak punch tests were performed to create a plane strain condition. A dual camera Digital Image Correlation (DIC) system was used to record and measure the deformation distribution history during the punch test. The on-set necking timing is determined directly from surface shape change. The FLD0 of each test situation is reported in this article.

Over the past twelve years the Auto/Steel Partnership (A/SP) Tube Hydroforming Materials and Lubricants Team has been conducting projects to aid the implementation of tubular hydroforming in automotive applications. The approach taken has been to initially gain a basic understanding of the hydroforming process and potential issues and to then extend learning to real world applications of increasing complexity. The experimental project investigations have encompassed various steel grades with a recent focus on advanced high strength steel (AHSS) and tailor welded tubes (TWTs) in free expansion and corner fill processes using several types of lubricants. Project plans are developed based on identified knowledge gaps, barriers to implementation and technology needs as follows: Effects of forming operations prior to hydroforming including tube making, pre-bending and pre-forming. Forming limits (e.g. forming limit diagrams (FLDs)) and other failure criterion.

A series of tests were performed to determine the influence of curvature on the forming limits of 1008 AK steel. Rectangular blanks cut from three thicknesses of the material from 0.69 mm to 1.04 mm were securely clamped at opposite edges and stretched over wedge shaped punches of different radii. A series of punches were used with radii that varied from 0.508 mm to 12.7 mm to produce bending effects that range from severe to mild. Measurements show that the neck forms on the convex surface when the strain on the concave side of the sheet reaches a value consistent with the forming limit in plane-strain for in-plane deformation.

The stress-based forming limit criterion has recently been shown to exhibit no significant dependence on strain path for a wide range of materials. This paper describes the effect of material models on the definition of the stress-based forming limit criterion. It is shown that although the shape of the forming limit curve in stress space is sensitive to the type of material model used, the degeneracy of the path dependent strain-based curves to a single curve in stress space shows little sensitivity to the material model. Consequently, the stress based forming limit criterion is shown to provide a more practical and robust measure of forming severity than is possible using the conventional forming limit diagram.