On the influence of tangential modulus in work-hardening part of stress-strain curve on ductility of stainless-steel tubular columns

Regarding the dependence of ductility response in structural components on their ability to keep stability after yielding of the material, in this paper, the influence of change in tangential modulus of work-hardening part of stress-strain response, was observed on the load carrying capacity and plastic buckling response of stainless-steel tubular columns. To keep cost-effectivity in the research, the objective of the study was followed by FE modeling, which was verified by simulation of plastic buckling in an experimental specimen with D/t=60, made of duplex stainless-steel. For all components of the models, S4R elements were used and both material and geometrical nonlinearity were included in the models. To conduct deformation of the columns according to the experimental observations, an initial imperfection equal to t/100 to combination of first three mode shapes of the columns was imported. The material stress-strain response after yield point was determined for the model by a multilinear curve according to the tensile stress-strain curve, obtained experimentally. The main parameters for comparison of the FE model and experimental observation were force-displacement curves. The FE study was extended by modeling of stainless-steel columns with various D/t ratios in range of D/t=30-120. Two main parameters comprised of energy absorption capacity and deformation of the column related to yield of the section and collapse threshold (e/g) were compared to the columns with various D/t ratios.According to force-deformation curves, by decrease of D/t ratio, the energy absorption capacity increased considerably for the columns, for example the energy absorption capacity and e/g ratio increased by 12% and 12%, respectively for comparison of the columns with D/t=60 and D/t=30, however, e/g ratio for the columns of D/t=120 and 100 were less than two, categorized as a force-controlled column. Two models with D/t=30, 60 were selected to follow the objective of the study. The work-hardening response of the material was approximated by two linear segments, the first by a tangential modulus equal to E=7.9 GPa and the second was by the tangential modulus equal to E=2.4 GPa. The influence of change in tangential modulus through a range between 0-200% was observed on the structural parameters related to ductility, comprised of energy absorption capacity up to collapse threshold and deformation of column at the collapse threshold. The results showed different reaction of the columns with different D/t ratios, increase of tangential modulus at the first work-hardening part was more significant than increase of the influence by the later part of the work-hardening response for the column of D/t=60, however an inverse effect was observed for the column of D/t=30, i.e. the influence of tangential modulus at the later part was more significant for this column. By doubling the tangential modulus of earlier part of the work-hardening response, the energy absorption capacity and e/g ratio for the column of D/t=60 increased by 26% and 46%, respectively. By doubling the tangential modulus of later part of work-hardening response, the energy absorption capacity and e/g ratio for the column of D/t=30 increased by 111% and 71%, respectively. The results showed significant parts of work-hardening response of duplex stainless-steel, to be exploited for development of ductility in the tubular columns.