Influence of stress concentration and cooling methods on post-fire mechanical behavior of ASTM A36 steels

This study aims to investigate the influence of stress triaxiality and cooling methods on post-fire mechanical behavior of ASTM A36 steels. To this end, ASTM A36 notched steel specimens are designed to generate a range of stress triaxialities. These specimens are subjected to target temperatures of 500 °C, 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C, and then cooled down to room temperature using air-cooling and water-cooling methods. These specimens are then uniaxially tested to determine their post-fire mechanical properties. Non-linear finite element analysis is conducted using post-fire mechanical properties to obtain stress triaxiality distribution in notched test specimens subjected to different target temperatures and cooling methods. Finally, a Scanning Electron Microscope (SEM) study is conducted on fractured surfaces of representative un-notched and notched test specimens to investigate the influence of high stress triaxiality and cooling methods on fracture initiation and propagation mechanisms. The post-fire mechanical properties of ASTM A36 steels are found to remain almost unaffected when cooled from 600 °C, irrespective of cooling method. ASTM A36 steels experienced up to 14% degradation in ultimate tensile strength and up to 22% increase in fracture strain when air-cooled from temperatures beyond 700 °C. Post-fire ultimate tensile strength is observed to increase by up to 146% whereas fracture strain is observed to decrease by up to 76% when ASTM A36 specimens are water-cooled from high temperatures. High stress triaxiality resulted in up to 37% increase in ultimate tensile strength and up to 74% reduction in ductility of air-cooled specimens. Presence of high stress triaxiality and water-cooling from temperatures beyond 700 °C is observed to significantly increase the ultimate tensile strength (up to 252%) and substantially reduced the ductility (up to 98%) of ASTM A36 steels.