高温后高强Q690钢材循环加载试验及本构模型研究

钢材经历火灾高温和冷却后其材料力学性能会发生不同程度退化，已有研究主要关注过火后钢材的残余静力性能。为研究经历火灾高温且不同方式冷却后高强钢的残余静力和滞回性能，对于历经500 ℃~1000 ℃高温后高强Q690在自然冷却和浸水冷却后的钢材进行了单调拉伸试验以及4种不同加载制度下的循环加载试验。结果表明，当过火极限温度不高于600 ℃时，在自然冷却和浸水冷却模式下，高温后Q690钢材单调拉伸和循环荷载下强度发展基本与常温未处理钢材类似。未过火高强Q690钢在循环荷载下表现出应变强化和循环软化效应。过火温度高于600 ℃且自然冷却下，过火后Q690钢随过火温度提高，其初始屈服强度下降。在浸水冷却下，过火冷却后高强钢初始屈服强度提升。且在循环荷载作用下，高温过火后Q690钢均表现出更为显著的应变强化效应，以及在等应变幅循环下均由未过火状态时的循环软化逐步转化为高温过火后的循环硬化效应。根据试验结果拟合了过火后Q690钢的Chaboche循环本构模型参数，可用于火灾后Q690钢结构的残余抗震性能评估。

CYCLIC LOADING TEST AND CONSITUTIVE MODEL OF POSTFIRE HIGH STRENGTH Q690 STEEL

The structural steels will experience different levels of material degradation after high temperature heating and cooling processes in fire hazards. Available studies are primarily about the residual static properties of postfire steels. An experimental study was performed to investigate the residual static and seismic performance of high strength Q690 steel after high temperature fire exposure and the following cooling process. Different peak temperatures varying from 500 ℃~1000 ℃ and two cooling methods, namely natural cooling and water-cooling, were considered. Monotonic tensile tests and cyclic loading tests (with 4 different loading protocols) were performed on postfire Q690 steel specimens. Results indicate that postfire specimens with the experienced peak temperature being lower than 600 ℃ exhibit similar mechanical properties with intact specimens without any fire exposure. The normal Q690 steel has strain-hardening and cyclic-softening behaviors under cyclic loads. After experiencing the high temperature fire-heating (temperature higher than 600 ℃) and the following natural-cooling process, the postfire Q690 steel displays a decreased initial yield strength as the increase of peak temperature. While postfire specimens after water-cooling from high temperature (higher than 600 ℃) have increased initial yield strengths. Under cyclic loads, the postfire Q690 steel displays more significant strain-hardening and cyclic-hardening behaviors instead of the cyclic-softening characters in non-fire-exposure specimens. Cyclic constitutive models of postfire Q690 steel were calibrated based on experimental data, which can be used for residual seismic performance evaluations of Q690 steel structures after fire.