Post-fire mechanical behavior of ASTM A572 steels subjected to high stress triaxialities

Abstract

The goal of this paper is to understand the post-fire mechanical behavior of ASTM A572 Gr. 50 structural steels cooled from high temperatures using both air-cooling (slow cooling rate) and water-cooling (rapid cooling rate) methods under the influence of high stress triaxiality. In this study, various levels of stress triaxialities in uniaxial tension specimens are achieved by introducing notches in the test specimens. The uniaxial notched test specimens are subjected to temperatures ranging between 500 °C and 1000 °C at 100 °C intervals to simulate fire temperatures. The furnace-heated specimens are then either left outside the furnace for air-cooling or quenched in a water bath in the case of water-cooling method. Uniaxial tension tests are performed on both air and water cooled notched specimens to extract the post-fire mechanical properties. Non-linear finite element analyses are performed to obtain the triaxiality profiles along the critical cross sections of notched specimens by employing hardening curves obtained from post-fire uniaxial tension tests. Moreover, microstructures at selected temperatures are obtained to assess the microstructural changes that caused a significant change in post-fire mechanical properties. It is observed that ASTM A572 steels do no exhibit considerable change in mechanical properties when exposed to temperatures up to 600 °C, for both cooling methods. Beyond 600 °C, air-cooling resulted in a reduction in yield strength and ultimate tensile strength and increase in ductility of ASTM A572 steels. Water-cooling from temperatures beyond 600 °C increased the ultimate tensile strength and decreased the ductility of ASTM A572 steels due to the formation of the martensite phase. High stress triaxiality leads to the reduction in ductility and an increase in yield strength and ultimate tensile strength of ASTM A572 steels in both air-cooled and water-cooled conditions. The combined influence of high stress triaxiality and formation of martensite phase resulted in significant increase of up to 158% in yield strength and up to 172% increase in ultimate tensile strength of ASTM A572 steels that are water-cooled after exposure to temperatures beyond 800 °C. On the other hand, the presence of the high stress triaxiality and water-cooling resulted in up to 89% loss of ductility of ASTM A572 steels specimens that are exposed to temperatures beyond 800 °C.