基于弱磁理论的混凝土开裂区域锈蚀钢筋的疲劳特性研究

Owing to the poor tensile properties of concrete, transverse cracks of reinforced concrete structures are easily induced by static and cyclic loading. These compromising transmission paths for corrosive medium will result in non-uniformly corrosion of reinforcing steel bars, especially etch pits. At the bottom of hazardous etch pits, fatigue cracks will form and grow under dynamic loading until brittle fracture. As etch pit and fatigue crack of corroded steel bar growing, its weak magnetic effects, i.e. both the distributions of magnetic memory signals and the piezomagnetic-stress hysteresis loops are constantly changing. With which the fatigue behavior of steel bar in cracked concrete could be monitored and evaluated non-destructively. Based on the fatigue fracture theory, this project combines metal magnetic memory with piezomagnetism effect organically. It is proposed firstly to specify the corrosion characteristics of steel bars in transversely cracked concrete areas and then reconstruct the morphology of etch pit on the basis of distribution characteristics of magnetic field. Secondly, retrieve the dimensions of etch pit and fatigue crack based on coupled magnetic field and explore the influence mechanism of stress field at fatigue crack tip on piezomagnetic-stress hysteresis loop via magneto mechanical effect and energy principle, aiming at the characterizations of equivalent fatigue crack length and stress intensity factor amplitude with weak magnetic parameters. Through revealing the evolution of fatigue fracture of corroded longitudinal reinforcements in cracked concrete beam, a methodology for predicting its fatigue life would be proposed finally. The outcome of this project will provide theoretical basis for damage assessment and residual life prediction of in-service reinforced concrete structures suffering corrosive environment and repeated loads.

混凝土结构在荷载作用下产生和发展的横向裂缝为腐蚀介质提供了快速传输通道，引起开裂区域钢筋局部坑蚀。动态荷载作用下疲劳裂纹在蚀坑底部形成并扩展，最终导致钢筋脆性断裂。随锈蚀和疲劳损伤发展，钢筋的弱磁效应——磁记忆信号分布和压磁-应力滞回曲线均不断变化，可用来无损检测和评估混凝土开裂区域锈蚀钢筋的疲劳特性。本项目以断裂力学为基础，将金属磁记忆和压磁效应“点面结合”，拟首先探明混凝土开裂区域钢筋的锈蚀特性，基于磁场分布特征重构蚀坑形貌；然后基于耦合磁场分布特征反演蚀坑与疲劳裂纹的尺寸，通过力-磁理论和能量原理揭示裂纹尖端应力场对压磁效应的影响规律，构建坑蚀钢筋等效裂纹扩展长度和应力强度因子幅值的弱磁参数表征方法；最后掌握开裂混凝土梁锈蚀纵筋的疲劳特性，提出在锈蚀与疲劳共同作用下构件的疲劳寿命预测方法。研究成果有望为服役钢筋混凝土结构在腐蚀环境与反复荷载作用下的损伤评估和剩余寿命预测提供理论依据。

混凝土结构在荷载作用下产生和发展的横向裂缝为腐蚀介质提供了快速传输通道，引起开裂区域钢筋局部坑蚀。动态荷载作用下疲劳裂纹在蚀坑底部形成并扩展，最终导致钢筋脆性断裂。随锈蚀和疲劳损伤发展，钢筋的弱磁效应——磁记忆信号分布和压磁-应力滞回曲线均不断变化，可用来无损检测和评估混凝土开裂区域锈蚀钢筋的疲劳特性。本项目基于钢筋的弱磁效应研究了钢筋的裂纹扩展行为表征方法。主要内容包括：.1）研究了均匀锈蚀钢筋的锈蚀率、疲劳性能、塑性性能等与弱磁信号分布演变规律的相关性；对不同坑蚀大小钢筋进行了疲劳拉伸试验，基于连续损伤力学的弹塑性疲劳损伤模型进行坑蚀钢筋弹塑性疲劳损伤数值分析，研究了坑蚀对钢筋疲劳性能的影响，分析弱磁信号在疲劳作用下的变化规律。.2）研究了钢筋疲劳损伤机理与弱磁效应演化规律的内在机制并修正疲劳力磁耦合模型。根据钢筋残余塑性应变的拟合关系式模拟了各试件的模拟磁化强度，并通过疲劳损伤参数与试验磁感应强度相比较，验证了钢筋修正疲劳力磁模型的可靠性。.3）基于能量守恒定律研究裂纹扩展规律和弱磁效应的对应股演习，基于磁偶极子模型对缺陷表面的磁场分布进行了数值计算并分析了其和应力与缺陷尺寸间的关系；通过拉伸应力下钢材标准紧凑拉伸试件和坑蚀带肋钢筋的数值模拟以及坑蚀钢筋的弱磁场演化规律等试验研究对应力和弱磁信号间的联系进行验证。.4）进行钢筋标准试件预制半椭球蚀坑后的疲劳-弱磁试验，基于断裂力学的坑蚀钢筋疲劳裂纹扩展模型对裂纹扩展行为进行预测，分析了应力比和蚀坑参数对压磁信号和剩余磁场分布演化规律的影响，进行了基于压磁方法和断裂力学的坑蚀钢筋疲劳裂纹扩展评估的对比。.5）通过坑蚀钢筋混凝土梁疲劳-弱磁试验，分析坑蚀钢筋混凝土梁内疲劳过程中的弱磁信号变化规律，建立了混凝土梁坑蚀钢筋法向压磁滞回曲线变幅和裂尖应力强度因子间的定量关系，并给出了基于弱磁信号定量评估梁内坑蚀钢筋疲劳裂纹扩展过程的方法。