Factors influencing the FLD of automotive sheet metal

• Published in: Journal of materials processing technology 2001-12, Vol.118 (1), p.1-8
• Main Author: Rees, D.W.A
• Format: Article
• Language: English
• Subjects: Forming limits; Orientation; r- and n-values; Strain history; Thickness
• ISSN: 0924-0136
• DOI: 10.1016/S0924-0136(01)01030-5

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, 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° 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.