Post-fire hysteretic and low-cycle fatigue behaviors of Q345 carbon steel

A series of material tests are carried out to study the cyclic loading behavior of Q345 carbon steel after fire. A total of 60 specimens are examined, which are heated to 500, 750 or 1000 degrees C, then cooled by air cooling, water spray cooling or water immersion cooling, followed by either incremental-amplitude cyclic loading or constantamplitude low-cycle/extremely low-cycle fatigue (LCF/ELCF) loading. The typical fracture failure mechanism of the specimens is analyzed by scanning electron microscope (SEM) and metallurgical microscopy (MM). The material parameters of Ramberg-Osgood model, Masson-Coffin model and a nonlinear combined hardening model are calibrated to facilitate the assessment of the residual seismic performance of the steel after fire. Among other important findings, the test results show that water immersion cooling with a rapid cooling speed following a medium or large fire (heated to 750 degrees C or 1000 degrees C) have a significant impact on the cyclic behavior of the Q345 steel. In these cases, the formation of martensite with high strength and poor ductility results in an increase in the cyclic stress, a decrease in the ductility and a sharp reduction of the fatigue life. These phenomena indicate that the seismic performance of steel structures may be significantly compromised after rapid cooling from a medium or large fire. Other heating-cooling processes have less pronounced effects on the hysteretic and low-cycle fatigue properties of the material.