Strain rate effect of G8.8 high-strength steel bolts after elevated temperatures

Bolts, as essential components of connections, have been widely employed in steel structures. During the service life of steel structures, they are inevitably subjected to extreme conditions such as fire loads. Repaired steel structures that have experienced fire are inevitably susceptible to subsequent impacts and other dynamic loads during their service life. In this context, the dynamic mechanical behaviour of bolts after exposure to elevated temperatures constitutes a key issue. This work presents the static and dynamic mechanical properties of G8.8 high-strength steel bolts at different strain rates after exposure to various elevated temperatures. The influence of strain rate(0.00025 s⁻¹, 0.1 s⁻¹, 10 s⁻¹, 10² s⁻¹), temperature (20 °C, 200 °C, 400 °C, 600 °C, 800 °C), and cooling conditions (cooling in air and cooling in water) on the mechanical properties of high-strength bolts is investigated. It was found that, under air cooling condition, the yield strength and ultimate strength were reduced by 559.90 MPa and 446.37 MPa, respectively, as the temperature increased from 200 °C to 800 °C. However, under water cooling condition at 800 °C, a partial recovery of the ultimate strength was observed, with an increase of 85.30 MPa. Meanwhile, under water cooling conditions at 800 °C, the strain rate effect is the most prominent due to the phase transformation of the metallic material – at a strain rate of 10² s⁻¹, the strength increases by 33 %.Finally, based on the Cowper-Symonds model and the temperature damage factor, a dynamic constitutive model is developed to characterize the stress-strain relationship and dynamic mechanical properties of G8.8 high - strength steel bolts under varying strain rates, heating temperatures, and cooling conditions.