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In most forming analyses, the stresses normal to the sheet surface are usually neglected, and the “plane stress problem” is the one being solved. Most FEA codes use shell elements in which the “third” stress component is not considered, or more properly, iterated until it goes to zero. This simplification yields accurate results for most cases. However, when high curvature bending under tension is present, the effect of the normal pressure on yielding is significant, and should be considered for more accurate results. This paper presents various analytical and numerical solutions that highlight and quantify such an effect.

Metal forming is complex; hence conducting computer simulations with reasonable turn around time requires a number of assumptions and simplifications of the physical reality. Since a number of commercial software codes are now available for metal forming simulation, it is very important that the underlying assumptions are well understood. It is not a simple case of one code being “better” than another; rather it is to understand the results and to apply them effectively. The goal of this investigation was to compare the simulation results between a full bending formulation (FBF) code and a modified membrane formulation (MMF) code, with particular emphasis on how friction should be “treated”. The strategy followed was to simulate the stamping of a simple U-channel FEA first in the FBF code and then to adjust the MMF results to be comparable by changing the friction coefficient. (The full bending code has been previously quantitatively correlated to many soft and hard tool stampings.)