基于宏观断裂力学的混凝土细观裂缝扩展过程

The heterogeneity of concrete is caused by the random distribution of coarse aggregate, resulting in the variation of crack propagation trajectory and macroscopic fracture properties, which is more significant for the fully-graded aggregate concrete. In this study, macroscopic fracture method is employed to analyse crack propagation process on mesoscopic level. A mixed mode crack propagation criterion expressed by stress intensity factor will be proposed, which can determine the crack propagation in different mesoscopic medium(motar,coarse aggregates and their interfaces) when a complex stress filed acts on the crack tip. This research focuses on the three potential crack propagation modes, i.e. crack crosses through the coarse aggregate , crack propagates from interface to mortar or to coarse aggregate, and their effects on the mechanism of crack evolution and macroscopic fracture properties. Moreover, the softening constitutive relationship of interface and evolution of fracture process zone are studied based on the experimental observation. Further, the relationship of macroscopic fracture energy and microstructure are discussed, and a mesoscopic discrete crack method based on fictitious crack model is developed to simulate the crack initiation, stable propagation and unstable fracture. The method proposed in current research explores the crack evolution mechanism, which cannot be solved using macroscopic mechanics, and bypasses these mechanical parameters hardly measured, which is essential in mesoscopic analysis. It is expected that the proposed method can provide a new pathway to assess the safety of fully-graded aggregate concrete structures.

粗骨料的随机分布造成混凝土材料的不均质性，影响裂缝扩展规律，进而改变混凝土宏观断裂特性，这在全级配大骨料混凝土中体现得尤为明显。本项目拟采用宏观断裂力学从细观层次研究裂缝扩展过程，提出以应力强度因子为参量的复合型裂缝扩展准则，以判别混凝土不同介质（砂浆、骨料及界面）在复杂应力场下裂缝扩展模式，重点研究裂缝尖端遇到骨料时产生的三种扩展路径，即裂缝穿过骨料、沿界面扩展至砂浆及沿界面扩展至骨料，以及不同扩展路径对裂缝演化、宏观断裂性能的影响。通过试验研究界面软化本构关系、细观断裂过程区形成机理，在此基础上探讨混凝土细观结构与宏观断裂能之间的关系，发展基于细观层次的虚拟裂缝粘聚力的分离裂缝模型，结合理论分析实现对裂缝起裂、稳定扩展及失稳破坏的全过程模拟。本项目提出的方法克服了宏观力学无法解释裂缝扩展规律及细观力学难以获取界面力学参数的困难，为评价全级配大骨料混凝土结构安全性提供了一个新的思路。

开展了砂浆-岩石界面抗拉强度、断裂特性试验研究，分析了界面粗糙度、砂浆强度、岩石种类对界面参数的影响，并设计预埋骨料试件进行三点弯曲试验，得到了裂缝尖端遇到骨料后不同的裂缝扩展路径。进行了岩石的断裂试验，验证基于材料起裂韧度的裂缝扩展准则在岩石材料中的适用性，并提出了以裂缝口张开位移为控制变量的计算方法，避免了粘聚力的迭代运算，提高了计算精度与计算效率。设置垂直双材料界面裂缝试件，探究了裂缝尖端在界面时的扩展准则，后将界面单独作为一相材料，采用应力外推法得到界面的断裂韧度，研究界面断裂韧度的尺寸效应。基于本课题组提出的基于材料起裂韧度的裂缝扩展准则，采用数值方法得到裂缝扩展全过程的p-cmod曲线，通过试验验证了裂缝扩展准则的在多相材料中的准确性。最后，将裂缝扩展准则用于随机模型以探究混凝土非均匀性对裂缝扩展过程的影响，通过统计分析得到极限荷载、断裂过程区长度等要素的概率分布函数，为工程检测和工程可靠性分析提供依据。本项目的课题研究对评估细观层次的混凝土裂缝扩展规律和机理具有重要的理论意义和工程应用价值。