落锤冲击下钢筋对含缺口混凝土梁断裂机制的影响研究

As a kind of heterogeneous composite structure, the fracture mechanicsm of steel reinforced concrete is much complicated due to the influence of the mechanical properties of its component and the combinations. Interaction between the steel bars and concrete and its influence on the fracture mechanics behavior of the structure under impact loading become the current difficulties in the impact dynamics research. Addressing that the notched beams of reinforced concrete with low reinforcement ratio, this project aims to investigate the fracture behavior under different loading rates. Under low loading rates, the quasi-static three-point bending test is performed; In other cases, the drop-weight three-point bending test is used instead. The influence of loading rate on fracture energy and peak load are discussed, respectively. The relationship between the load and crack mouth opening displacement is established under different loading rates. The effect of the bond between reinforcement and concrete on the dynamic response and failure mode of reinforced concrete beam with different loading rates are investigated. The inertia effect under higher loading rates are also studied. Representing the reinforcement through beam element and introducing the equivalent dynamic bond-slip relationship in the finite element model, the pull-out behavior of reinforcement and the fracture process of concrete beam are simulated. The simulations will be compared with the experimental observations to improve the numerical analysis method. The research results of this project are significant for the further understand of the fracture behavior of reinforced concrete under low-velocity impact and could provide reference foundation for the related fracture constitutive model.

作为一种异质复合结构，钢筋混凝土受其各组成部分及结合面力学性能的影响，断裂机制非常复杂。冲击载荷下钢筋与混凝土基体间的相互作用及其对结构断裂力学行为的影响是当前冲击动力学研究的难点问题。本项目针对具有较低配筋率、含缺口的钢筋混凝土梁，分别采用准静态、落锤三点弯曲实验方法，研究其在六种加载速率下的断裂行为。探讨加载速率对断裂能和峰值载荷的影响。拟建立不同加载速率下的载荷-裂缝面张开位移关系。研究加载速率及钢筋混凝土的动态粘接特性对结构响应和破坏模式的影响，分析较高加载速率下惯性效应的影响。另外，用梁单元代表钢筋，并在有限元模型中引入等效的动态粘接滑移本构关系，模拟钢筋拔出及混凝土梁的断裂行为。将数值模拟结果与实验结果进行对比，修正数值分析方法。项目的成功实施，将为相关断裂本构模型的建立提供参考依据，对深入了解钢筋混凝土结构在低速冲击下的断裂行为具有重要意义。

动态加载下，混凝土中钢筋的阻裂性能一直是冲击动力学研究领域的难点之一。利用落锤试验机对含缺口的混凝土少筋梁进行三点弯曲试验，分析了不同加载速率下梁的冲击力、跨中挠度、混凝土起裂应变率和钢筋应变。试验结果表明：在一定加载速率范围内（0.885m/s-1.252m/s），混凝土预制裂缝尖端的裂纹起裂应变率、冲击力最大值、跨中挠度峰值与加载速率呈线性增长关系；当加载速率增至1.771m/s时，增长趋势减弱。冲击力卸载时，钢筋部分弹性变形恢复导致裂纹产生闭合，裂纹嘴张开位移（CMOD）逐渐减小至恒定值。对CMOD峰值前的部分曲线进行拟合后得到裂纹嘴张开位移率，结果表明裂纹嘴张开位移率随加载速率的提高而线性增大。建立了钢筋混凝土梁的全尺寸细观模型，可进行钢筋混凝土损伤演化与断裂过程的精确模拟，为研究该复合材料跨尺度失效问题提供了有效的工具。