Improved true stress-strain model of high-strength steels for fracture simulation in fire

Abstract

Fracture prediction of high-strength steel at elevated temperatures is particularly essential for the performance-based design of high-strength steel structures in fire. Accurate prediction of fracture behavior of high-strength steels in fire requires a precise true stress-strain model up to a large strain level (>1.0 mm/mm). This research investigates improved true stress-strain models of Q550, Q690, Q890, Q960, S690, and S960 high-strength steels at elevated temperatures up to 800 °C. The true stress-strain models are calibrated from test data and finite element analysis results. The calibrated true stress-strain model is improved in the following two aspects. First, to ensure the rationality of the true stress-strain models, they were examined by Considère’ Construction together with the physical law of ductile metals. Furthermore, by modifying the weight function, the true stress-strain models can accurately simulate the post-necking behavior of high-strength steels at high temperatures. Based on the research results, the true stress-strain behavior of three regions, including the elastic region, strain-hardening region, and post-necking region, need different forms of functions to express. They were linear function, combined exponential and linear functions, and weighted upper-bound and lower-bound functions, respectively. It was observed that the variation of the weighting factor in the weighting function may be related to the post-necking softening behavior of high-strength steels. For the investigated high-strength steels, at lower temperatures (< 550 °C), the weighting factor decreased with the increase of true strain, resulting in a significant softening of high-strength steels after the onset of necking.