The influence of production routes on the behaviour of stainless steel structural members

Abstract

Current structural design codes for stainless steel employ material strengths, or 0.2% proof stress values that are significantly lower than the 0.2% proof stress of material taken from stainless steel structural cross sections. This discrepancy is attributed to the ability of stainless steel to significantly cold work during plastic deformation, which occurs in sheet rolling and cross section forming processes. The resulting under-estimation of the material strength in stainless steel cross sections leads to overly conservative structural designs. As the comparative expense of stainless steel demands efficient design, this study proposes models to predict the strength enhancements offered by different cross section production routes to increase the efficiency of stainless steel structural design. This research project includes a substantial experimental program that has produced 0.2% proof stress distributions from over 450 tensile coupon tests for a total of 19 cross sections formed via three standard production routes: press braking, cold rolling and hot rolling. To obtain 0.2% proof stress variations to a higher resolution, Vickers hardness values have been obtained and correlated with the 0.2% proof stress values. Significant strength increases in the flat regions of cold rolled box sections have been found and related to the strain history of the sheet material used in production and the strain caused during section forming. Existing models to predict further strength enhancements in the comer regions have been modified and the extension of the region of cold work associated with comer forming has been quantified, defining the material strength distributions for both press braked and cold rolled sections. In addition, geometric profiles of 31 complete section lengths have been measured and over 800 residual strain readings performed. Since production routes also influence the geometric imperfections and residual stress distributions, they must be quantified to accurately predict structural behaviour. Simple models have been proposed to predict global and local imperfections and membrane and bending residual stresses in the three types of sections. Based on available test data, the proposed 0.2% proof stress distribution for press braked sections offers, on average, cross section resistances 1.4 times those predicted using the minimum 0.2% proof stress. The 0.2% proof stress distribution proposed for cold rolled sections provides cross section compression resistances, column buckling resistances and inplane bending resistances, on average, 2.1, 1.5 and 1.9 times respectively the values obtained using the minimum specified material strength. This research therefore identifies large increases in efficiency for stainless steel structural design which, combined with low maintenance requirements greatly increases the competitiveness of specifying stainless steel structures.