Structural design of cold worked austenitic stainless steel

When stainless steel is cold worked, it undergoes substantial strain hardening, leading to significant strength enhancement. This strength enhancement has not generally been utilised in practice due to a lack of knowledge of the structural behaviour of this material. This research project carried out extensive experimental and numerical studies on material specimens, structural members and connections in order to generate valuable data on the key aspects of the structural design of cold worked stainless steel. From the results of these investigations, design rules were developed which facilitate the economic structural design of members made from cold worked stainless steel. A summary of each Work Package is given below. Work Package 1: Material properties The objective of this work package was to determine basic material properties for cold worked stainless steel. The process of strain hardening, whether by stretching or cold working, results in increased proof and tensile strength and reduced ductility. A programme of uniaxial (tension and compression) and biaxial tests was carried out. Based on the test results, a constitutive model was developed to predict the anisotropic and non-symmetric behaviour of cold worked stainless steel. Compared to other constitutive models, the model developed in this study was significantly better, both qualitatively and quantitatively, in predicting the response to subsequent loadings. Cold worked stainless steels are strongly anisotropic (i.e. exhibit different strengths parallel and transverse to the rolling direction) and asymmetric (i.e. exhibit different strengths in compression and tension). The compression strength in the transverse direction is generally the highest strength. The compression strength in the rolling direction is usually the lowest strength. The minimum specified material strength values given in stainless steel European standards are tensile strengths, transverse to the rolling direction. These values can be used in design when the orientation of the member is such that it is loaded transverse to the direction of rolling. For loading in the rolling direction, lower strength values need to be determined, unless the manufacturer guarantees the higher values in compression. The 0.2% proof strength in compression in the rolling direction can be taken as 85% of the 0.2% proof strength in tension in the rolling direction. Work Package 2: Structural hollow sections The objective of this work package was to study the local buckling behaviour and overall member response of rectangular hollow sections made from cold worked stainless steel grades 1.4318 and 1.4571 (C700 and C850). A programme of bending, internal support and flexural buckling tests was carried out on rectangular hollow sections made from cold worked stainless steel. The tests were modelled numerically and good agreement was achieved between the measured and predicted behaviour. Even better agreement was achieved when face specific material properties, rather than weighted average properties, were used. The test results were compared with the resistances predicted by the design rules in prEN 1993-1-4 for flexural buckling, bending and web crippling behaviour of RHS members. The measured/predicted load ratios for the cold worked specimens were very similar to the measured/predicted load ratios for annealed specimens.