Influence of the strain-rate on the mechanical properties of mild carbon steel at elevated temperatures

This paper presents an experimental study on the influence of the temperature and especially the strain rate on the stress–strain relationship of mild carbon steel with regard to fire conditions. The study shows that the level of strain rate has a marked effect on the material behaviour at elevated temperatures. An accurate prediction of the elevated temperature material stress–strain response is a key factor for an appropriate assessment of the structural fire resistance in engineering practise. The material characteristics of mild carbon steel at elevated temperatures differ from those at ambient temperature. Under fire conditions, the strength and stiffness reduce with increasing temperature, the clearly defined yield point and the pronounced yield plateau – characteristically for mild carbon steel at ambient temperature – vanish and the stress–strain curve becomes distinctly nonlinear. Many material test series performed in recent years did not specifically focus on the influence of strain or heating rates on mechanical properties at elevated temperatures. Therefore an experimental study focusing on the influence of the strain rate was performed. On the basis of the presented test results, a comparative study has also been performed. It has been found that the reduction factors for material properties given in current European and American fire design rules fit quite well for high strain rates (short fire duration times) but do slightly overestimate the material properties for low levels of strain rate (long fire duration times). Finally the test results are compared to existing stress–strain models. The study revealed that a description of the stress–strain relationship by means of an adapted exponential (similar to the Ramberg–Osgood model) formulation is able to model the stress–strain relationship for fire design.