基于统一相场理论的混凝土结构高温随机损伤破坏分析

Concrete exposed to elevated temperatures exhibits strong nonlinearity and notable variability in mechanical behaviors. Reasonable representation of these properties is the basis for refined damage and failure analysis as well as catastrophe behavior prediction of concrete structures under fire attack. Proceeding from the recently developed unified phase-field damage theory by the applicant’s group, the phase-field damage model for concrete at elevated temperatures is proposed in this project. The effect of temperature variation on the mechanical properties combined with the damage and failure behaviors of concrete is taken properly into consideration in the model. The damage evolution law of concrete at high temperatures is physically established in accordance with energy release and competition mechanism. The modeling approach towards multi-dimensional multi-variable random fields with respect to the mechanical and thermal parameters of concrete is investigated incorporating the synthesis techniques for multi-scale random fields developed earlier by the applicant. Systematic combination of the developed random fields and the phase-field damage model gives the stochastic damage and failure analysis method for concrete structures exposed to high temperatures. The corresponding numerical algorithm is established with the staggered iterative solution scheme. Random damage and failure tests on typical structural components under fires are implemented roundly to validate the developed model and the analysis method. Nonlinear and random behaviors of concrete structures subjected to damage and failure can be fully represented with the proposed analysis method. Meanwhile, the problems regarding strain localization, stress locking, and mesh dependence in numerical simulations can be resolved. Research achievements of the present proposal will provide scientific basis for performance-based fire resistance analysis, design, and assessment of concrete structures.

混凝土高温力学行为具有强烈的非线性和显著的随机性，对其合理反映是开展混凝土结构高温损伤破坏精细化分析和灾变行为预测的基础。本项目从申请人所在团队新近提出的统一相场损伤理论出发，合理考虑温度对混凝土力学性质和损伤破坏行为的影响，建立混凝土高温相场损伤模型，并基于能量释放与竞争机制，物理地确立混凝土高温损伤演化法则。结合申请人前期发展的多尺度随机场建模技术，研究混凝土力学和热工性能参数的多维多变量随机场建模方法，并与高温相场损伤模型有机结合，建立高温作用下混凝土结构的随机损伤破坏分析方法，发展其交错求解数值算法。系统开展典型结构构件的受火损伤破坏随机试验，对建议模型和分析方法的有效性进行验证。项目提出的分析方法可以全面描述混凝土结构高温损伤破坏行为的非线性和随机性，同时解决数值模拟中的应变局部化、应力闭锁和网格相关性等难题。项目研究成果将为基于性能的混凝土结构抗火分析、设计与评估提供科学基础。

热-力耦合作用下混凝土的损伤破坏行为极易对结构的安全性和耐久性造成重大影响，其准确模拟是混凝土结构高温损伤破坏精细化分析和灾变行为预测的基础。固体结构损伤破坏的统一相场理论采用能量等效的几何规则化相场模型来模拟裂缝扩展，可以统一地描述脆性、准脆性材料结构的各种复杂开裂行为，而无需进行裂缝路径追踪及假定起裂和扩展准则，解决了数值模拟中的应变局部化和应力闭锁等关键难题，且模拟结果不存在网格敏感性和相场尺度参数敏感性等问题。在统一相场理论的基础上，通过合理考虑混凝土的热膨胀和热蠕变效应，高温作用对混凝土的热工、力学性能退化的影响，以及裂缝相场对温度场的开裂热阻效应等，推导了热-力耦合作用下混凝土的损伤演化控制方程，建立了满足热力学原理的混凝土高温相场损伤模型。通过发展交错求解和全耦合格式的多场有限元数值实现算法，系统建立了混凝土结构的高温损伤破坏分析方法。若干数值算例表明，高温相场损伤模型能够合理再现热-力耦合试验观测的混凝土多裂缝起裂、扩展、分叉和汇聚等复杂的损伤破坏行为，能够量化温度作用对混凝土结构开裂行为和力学性能的影响。建议模型和方法可为实现复杂混凝土结构高温灾变全过程力学行为模拟提供关键工具。项目设计开展了混凝土高温应力-应变全曲线随机试验，首次完整获得了高温后混凝土关键力学性能参数及其统计特征，同时得到了不同温度、强度下混凝土损伤本构关系大样本及其均值和方差曲线等，为建立混凝土高温随机相场损伤模型并进行模型参数标定奠定了基础。钢筋混凝土无腹筋梁的多工况损伤破坏对比试验，深刻揭示了常温下设计受弯破坏的梁，高温作用后其破坏模式存在不确定性，即常温下按照“强剪弱弯”设计的梁在高温作用下或可能发生剪切破坏。进一步探究了火灾高温对无腹筋梁弯剪破坏模式竞争的影响机制和影响规律。项目成果可为混凝土结构的多场耦合损伤分析和长期服役性能评估提供重要基础。