Factors influencing the FLD of automotive sheet metal

Ms paper examines the formability of a zinc clad automotive CR sheet steel. This material's extended ductility allows a diffuse instability condition to define the limit of formability at the onset of necking under in-plane biaxial stressing. It is shown how this theory can admit material anisotropy, sheet orientation, thickness, strain history and changes to its r- and n-values under any ratio of applied principal stresses. Shown here are the influences upon limiting formability of: (i) orientation between the principal stress axes and the sheet's rolling direction in 15 degrees increments between 0 degrees and 90 degrees; (ii) n-values between 0.1 and 0.3; (iii) r-values between 1.1 and 1.8; (iv) equivalent prestrains from -15 to +15%; (v) sheet thickness between 0.25 and 1.5 mm. The ratio between the in-plane principal stresses is allowed to vary between 0 and +/-1 for constructing two forming limit diagrams. These forming limit diagrams (FLDs) plot either the limiting in-plane principal engineering strains, e(1)(P) versus e(2)(P), or the major in-plane and thickness strains, e(1)(P) versus e(3)(P). Each diagram shows the influence of (i)-(v) so allowing the conditions for optimum formability to be established. In general, high n, low r and negative prestrains will improve formability. The influence of sheet orientation can be beneficial, but will depend on the strain path. For example, a 45 degrees orientation is beneficial to the limiting formability when e(2)(P) is negative. The thickness strain e(3)(P) becomes sensibly constant within the negative minor strain (e(2)(P)) region. Thus the alternative FLD is proposed to provide the dependence of the parameters (i)-(iv) upon a thickness limit. Ultrasonics provides a means to monitor thickness of pressed panels to this limit. (C) 2001 Elsevier Science B.V. All rights reserved.