Stainless Steel Cold-formed Square Steel Residual Stress And Its Impact On The Ultimate Load Analysis

The accuracy of structure analyses is highly dependent on mechanical properties of material and initial imperfections of the member. Significant residual stresses and equivalent plastic strains are induced in cold-formed members, and their magnitude and distribution are highly dependent on the cold work operations in their manufacturing process. The cold work may play an important role in determining structural behaviour of cold-formed members. Now analytical solutions for residual stresses in press-braked, cold-formed channel sections and circular hollow sections have been carried out. However, analytical solutions for residual stresses and equivalent plastic strains in cold-formed thin-walled square hollow sections are not available. Hence, an analytical solution for predicting residual stresses and equivalent plastic strains in cold-formed stainless steel square hollow sections is carried out in the current study, and the analytical solution is then applied to study the compressive behaviour of cold-formed columns with square hollow sections under axial compression. The main contents and conclusions of the thesis include:1. Based on the plane strain elastic-plastic pure bending assumption, a general analytical solution for residual stresses and equivalent plastic strains in cold-formed stainless steel square hollow sections has been developed. The material anisotropy and non-linear strain hardening behaviour of stainless steel are all carefully taken into account. Residual stresses and equivalent plastic strains due to coiling, uncoiling and flattening, cold forming and reforming stages of the manufacturing process can be predicted by the analytical solution.2. To get the final residual stresses and equivalent plastic strains in cold-formed stainless steel square hollow sections, the finite element models for simulating the spring-back and the whole reforming stages are carried out. The distribution of residual stresses across the whole section as well as through plate thickness can be predicted. The accuracy of the analytical solution and the finite element models is verified by comparing their results with test results.3. Based on the analytical solution, an advanced 3-D shell element model is carried out to study the ultimate bearing capacity of cold-formed columns under compression. The material nonlinearity, initial geometrical imperfections, residual stresses as well as equivalent plastic strains, due to the manufacturing process, are all carefully taken into account. Various column lengths with different boundary conditions of cold-formed thin-walled square hollow sections have been carried out. The axial strength of each column predicted by the finite element model is compared with that predicted by the mainstream standards.4. To achieve a complete understanding of the cold work effect on structural behaviour of cold-formed members, a parameter study has been carried out to study the effect of residual stresses, equivalent plastic strain and their co-action, respectively, on the ultimate bearing capacity of cold-formed columns. Various column lengths and different cross-sectional dimensions of stub columns and pinned-end long columns have been studied. In general, residual stresses have negative effect on ultimate bearing capacity and stiffness, while equivalent plastic strains have positive effect. The enhancement in ultimate bearing capacity, due to cold work, of cold-formed square hollow sections increases as the column length increase, while decreases as the cross-sectional dimension increases.