Tensile Fracture Behaviour of Thin G550 Sheet Steels (No. R773)

Abstract

Cold formed structural members are fabricated from sheet steels which must meet the material requirements prescribed in applicable national design standards. The Australian / New Zealand standard for cold-formed steel structures (AS/NZS 4600) allows for the use of thin (t < 0.9 mm), high strength (fy = 550 MPa) sheet steels in all structural sections. However, due to the low ductility exhibited by sheet steels which are cold reduced to thickness, the engineer must use a yield stress and ultimate strength reduced to 75% of the minimum specified values. The American Iron and Steel Institute (AISI) Specification further limits the use of thin, high strength steels to roofing, siding and floor decking panels. Sheet steels are required to have a minimum elongation capability to ensure that members and connections can undergo small displacements without a loss in structural performance, and to reduce the harmful effects of stress concentrations. The ductility criterion specified in the Australian / New Zealand and North American design standards is based on an investigation of sheet steels by Dhalla and Winter, which did not include the thin high strength G550 sheet steels available today. A previous research report entitled Ductility of G550 Sheet Steels in Tension – Elongation Measurements and Perforated Tests (No. R735) details the basic material behaviour of G550 sheet steels. The authors concluded that the ability of G550 sheet steels to undergo deformation is dependent on the direction of load within the material, where transverse specimens exhibit the least amount of overall, local and uniform elongation. Furthermore, the G550 sheet steels that were tested for this project do not meet the Dhalla and Winter material requirements regardless of direction, except for the uniform elongation of longitudinal coupon specimens. This document reports on the failure mechanisms, as well as the fracture properties of G550 sheet steels tested in tension. Descriptions of failure surfaces, which were observed through a scanning electron microscope are provided. The failure behaviour of perforated and solid coupon specimens is documented, the fracture resistance of G550 sheet steels is measured for a range of temperatures and a numerical study of the effect of cracks on structural performance in the elastic load range is completed using the FRANC2D finite element computer program.