Modified Johnson-Cook constitutive model for butt-welded Q345 structural steel after high temperature cooling

This study examines the mechanical behavior of butt-welded Q345 structural steel (Q345-BW) joints after exposure to elevated temperatures and subsequent cooling by different methods. Uniaxial tensile tests were performed on specimens subjected to temperatures ranging from 200 °C to 900 °C and cooled by air, water, or fire-fighting foam. The results indicate that the mechanical properties, including yield and ultimate strengths, remain largely stable below 600 °C, whereas significant degradation occurs at higher temperatures. With increasing exposure temperature, the fracture location shifts from the heat-affected zone to the necking region, and the cooling method has a pronounced influence on both fracture morphology and mechanical performance. A modified Johnson–Cook constitutive model is developed, capable of accurately predicting the stress–strain behavior of welded joints after fire exposure. These findings contribute to a better understanding of the fire-resistant design of steel structures.