Factors influencing the FLD of automotive sheet metal

This 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° increments between 0° and 90°; (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, e1P versus e2P, or the major in-plane and thickness strains, e1P versus e3P. 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° orientation is beneficial to the limiting formability when e2P is negative. The thickness strain e3P becomes sensibly constant within the negative minor strain (e2P) 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.