Cold Formed Steel - Ductility Criteria

Ductility Criteria

Ductility is defined as ‘‘an extent to which a material can sustain plastic deformation without rupture.’’ It is not only required in the forming process but is also needed for plastic redistribution of stress in members and connections, where stress concentration would occur. The ductility criteria and performance of low-ductility steels for cold-formed members and connections have been studied by Dhalla, Winter, and Errera at Cornell University. It was found that the ductility measurement in a standard tension test includes local ductility and uniform ductility. Local ductility is designated as the localized elongation at the eventual fracture zone. Uniform ductility is the ability of a tension coupon to undergo sizeable plastic deformations along its entire length prior to necking. This study also revealed that for the different ductility steels investigated, the elongation in 2-in. (50.8-mm) gage length did not correlate satisfactorily with either the local or the uniform ductility of the material. In order to be able to redistribute the stresses in the plastic range to avoid premature brittle fracture and to achieve full net-section strength in a tension member with stress concentrations, it is suggested that:

• The minimum local elongation in a - 1–2 in. (12.7-mm) gauge length of a standard tension coupon including the neck be at least 20%.
• The minimum uniform elongation in a 3-in. (76.2-mm) gauge length minus the elongation in a 1-in. (25.4-mm) gage length containing neck and fracture be at least 3%.
• The tensile-strength-to-yield-point ratio Fu /Fy be at least 1.05.