Dynamic tensile behaviour of high-strength steel bolt coupons after elevated temperature exposure

High-strength steel bolts are widely used in modern steel structures for connection, and the performance of these bolts determines the structural safety and resilience under various loading conditions. Some steel structures are vulnerable to sequential exposure to fire and impact, and may necessitate emergency use without maintenance, for example, the hard landing cases of helicopters on heliports during fire (with preliminary fire suppression) for fast rescue. The mechanical properties of high-strength bolts in these heliport structures may differ from their original state due to prior exposure to fire. Therefore, the dynamic behaviour of high-strength bolts after exposure to elevated temperatures is worthy of investigation, yet relevant research remains scarce. To address this, a comprehensive experimental study into the dynamic stress-strain response of G12.9 high-strength steel bolts at different strain rates (0.00025 s −1 –10 2 s −1 ) after exposure to elevated temperatures (20 °C–800 °C) has been conducted and is reported. The coupled effect of elevated temperature exposure, cooling conditions and strain rate on the mechanical properties of high-strength steel bolts is explored. The G12.9 high-strength steel bolts exhibited no significant difference in strain-rate sensitivity at exposure temperatures up to 400 °C, but showed a pronounced reduction in strength and an increased strain-rate sensitivity with an exposure temperature over 400 °C. Based on the test results, a dynamic constitutive model was then developed with the strain rate effect well considered, in order to facilitate relevant resilience assessments of the post-fire and impact performance of bolts and bolted connections.