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Interface friction between sheet metal and tooling in sheet metal forming is examined in different forming modes using laboratory simulative tests. Stretchability is studied by the limiting dome height test; drawability is investigated by a four inch Swift cup draw test and the coefficient of friction is measured by the draw bead simulator under bending and unbending deformation. The responses of the interface friction in six different coated and uncoated steel sheets are studied using seven different lubricants. It is found that the interface friction between sheet metal and tooling is very sensitive to the forming mode and the type of coating. For the same lubricant and coated material, two different forming modes may produce very different results in interface friction. However, overall good and bad lubricants for all forming modes can be determined for a given coated material using these three tests.

Strain rate sensitivity is an important material property in the formability of sheet metal. In this study, strain rate sensitivity is evaluated for several different grades of steel. Strain rate sensitivity varies from 0.01 to 0.022 for the steels tested. It was found that formable steels such as IF and AKDQ steels have both high n-value (strain hardening) and m-value (strain rate sensitivity). Positive strain rate sensitivity results in a significant increase in the yield strength and tensile strength at higher strain rates. The n-value decreases with strain rate for all of the steels. The total elongation decreases slightly with strain rate for the lower strength steels but is constant or even increases slightly with strain rate for high strength steels. For a typical AKDQ steel, the increase in yield strength can be as high as 43% for an increase in strain rate from 0.002 /s to 2.0 /s.

In previous investigations, it has been shown that the dent resistance of an auto body panel depends upon the yield strength of the material. However, it is known that the yield strength of steel increases with prestrain due to strain hardening. Panel design and material selection based on the material properties obtained from unstrained sheet steels may lead to inaccurate prediction of the dent resistance of the formed panel. In this study, the effect of prestrain on the static dent resistance of auto body panels was investigated. Using existing empirical relationships between dent resistance and panel properties, it was found that the static dent resistance of an auto panel depends not only on the part geometry and material properties but also on the strain level in the panel. The improvement in dent resistance resulting from a material change from an AKDQ steel to a bake hardenable steel or a high strength steel was determined at different strain levels.

The effects of sheet thickness, yield strength, strain aging and prestrain on the dynamic dent resistance of sheet steel are investigated using an instrumented drop-weight test. It is found that the dynamic dent resistance is less dependent on the sheet thickness and the yield strength of the material than the static dent resistance. The dent resistance of automotive steels under dynamic loading conditions increases with prestrain in a manner similar to static denting. The relative ranking of different strength steels in the performance of dynamic denting is provided at various strain levels. An empirical relation of dynamic denting force and energy with the sheet thickness and the yield strength of the material is derived for a flat panel. Data confirm that dynamic dent resistance is improved using high strength steels including bake hardenable steels and rephosphorized steels.