Full-range strain-hardening behavior of structural steels: Experimental identification and numerical simulation

The strain hardening behavior of structural steels is of significant importance for the elastoplastic response prediction and the collapse resistant evaluation of steel components. However, due to the necking occurrence in the uniaxial specimens, only the true stress-strain responses before necking are often obtained and applied for the plasticity identification of structural steels. This paper experimentally investigates the full-range strain-hardening behavior of four types of structural steels, including the ordinary structural steels Q235 and Q355, high strength steel Q460 and low-yield-point steel LYP225. The post-necking responses of these structural steels are determined with Bridgeman's correction theory and a proposed image-processing method. This method utilizes a series of morphological operations, processing the 2D grayscale images of smooth round bars under loading, to measure the instantaneous minimum radius and necking curvature. Based on the experimental results, a combination of the power and exponential laws is adopted to describe the full-range hardening behavior of structural steels. Numerical simulations of the experiments validate the Bridgeman and the MLR correction theories. A hybrid numerical-optimization method, employing a standard PSO algorithm, is further presented for the inverse identification of the full-range strain hardening behavior, and the applications to the Q460 and LYP225 steels validate this method.