结构钢在受火降温段的力学性能及本构模型研究

Structural safety evaluation of steel building structures during the whole process of fire hazard is one of the hotspots of the research on fire resistance of structures. Research studies have shown that the steel structures will be further damaged in the cooling phase of a fire. However, the lack of research on mechanical properties of structural steel in the cooling phase of fire is the choke point of the current research. This project is aimed at conducting experimental studies on the mechanical properties of three kinds of steel materials (steels Q345, Q460 and Q690), which are commonly used in steel building structures, in the cooling phase of fire. The considered mechanical properties include the elastic modulus, proportional limit, yield strength, ultimate strength and ultimate strain. The effects of the maximum temperature that the material experienced, the lasting time of the maximum temperature and the cooling speed on the mechanical properties of steel will also be studied. Besides, microstructures of the unheated steel and cooled steel after heating will be explored with scanning electron microscopy to reveal the microcosmic mechanism of material deterioration. Besides, the calculation models of the reduction factors of the mechanical properties and mathematical expressions of the strain – stress curves, namely the constitutive models, of these steel materials in the cooling phase of fire will be built through data fitting. Finally, the obtained constitutive models of steel material will be adopted to analyze the performance of steel columns and steel beams in the whole process of fire, including heating and cooling. The study of this project will build a foundation for further studying the performance of steel structures in the cooling phase of fire and structural rehabilitation after fire.

建筑钢结构在火灾全过程的安全评估是目前结构抗火研究的热点之一，已有研究表明钢结构在火灾降温段会发生进一步的破坏。然而，缺乏对钢材在受火降温段力学性能的了解是目前研究的瓶颈。本项目将对建筑钢结构常用的三种钢材（Q355、Q460和Q690）在受火降温段的力学性能进行试验研究，揭示其弹性模量、比例极限、屈服强度、极限强度和极限应变等力学性能在受火降温段的退化规律；明晰钢材的最高升温、最高温度持续时间和降温速率等因素对其受火降温段力学性能的影响，并通过扫描电镜观测钢材常温下和受火冷却后的微观形貌，探索其力学性能退化的微观机理。然后，拟合得到钢材在受火降温段力学性能折减系数的计算公式和应力-应变曲线的表达式，即受火降温段的本构模型。最后，结合钢材受火降温段的本构模型建立有限元模型，分析梁、柱等钢构件的火灾全过程响应。本项目的研究成果将为建筑钢结构受火全过程的安全评估及灾后修复奠定基础。

钢材在火灾中的力学性能是钢结构抗火研究的基础。真实火灾包括升温阶段和降温阶段。目前，钢材在火灾升温段的性能已有较多研究，但其在火灾降温段的性能研究甚少，甚少有学者提出钢材在降温阶段的力学模型。然而，在实际火灾中，承受住火灾升温段的建筑结构可能在火灾降温段发生进一步的破坏，甚至倒塌。因此，进一步研究钢材在火灾降温阶段的力学性能，建立钢材在降温阶段的本构模型，对钢结构火灾全过程的抗火设计和火灾后性能评估与修复具有重要的科学意义。本项目对建筑结构常用的Q355、Q460和Q690钢材在火灾全过程中的力学性能进行了系统的试验研究。重要研究结果包括：1）对Q355、Q460和Q690钢材在常温和受火升温段的力学性能进行试验研究，并对比三种钢材在受火升温段的力学性能，提出三种钢材在受火升温段的力学性能折减系数的计算模型；2）对Q355、Q460和Q690钢材在受火降温段的力学性能进行试验研究，分析弹性模量、屈服强度、极限强度和极限应变等力学性能参数在受火降温段的变化规律；采用金相显微镜观察钢材试件的金相组织，分析钢材微观结构与其力学性能之间的联系；3）引入降温段影响系数，对比钢材在受火降温段和受火升温段力学性能的异同，并提出三种钢材在受火降温段的力学性能参数的计算方法；4）研究降温速率和最高受热温度下的恒温时间对钢材降温段力学性能的影响；5）提出适用于火灾全过程的Q355、Q460和Q690钢材的本构模型，初步建立了将提出的本构模型应用于钢构件火灾全过程的分析方法。