Comparison Of The Quasi-Static Method And The Dynamic Method For Simulating Fracture Processes In Concrete

Concrete is heterogeneous and usually described as a three-phase material, where matrix, aggregate and interface are distinguished. To take this heterogeneity into consideration, the Generalized Beam (GB) lattice model is adopted. The GB lattice model is much more computationally efficient than the beam lattice model. Numerical procedures of both quasi-static method and dynamic method are developed to simulate fracture processes in uniaxial tensile tests conducted on a concrete panel. Cases of different loading rates are compared with the quasi-static case. It is found that the inertia effect due to load increasing becomes less important and can be ignored with the loading rate decreasing, but the inertia effect due to unstable crack propagation remains considerable no matter how low the loading rate is. Therefore, an unrealistic result will be obtained if a fracture process including unstable cracking is simulated by the quasi-static procedure

Lattice type of fracture model for concrete

Concrete is usually described as a three-phase material, where matrix, aggregate and interface zones are distinguished. The beam lattice model has been applied widely by many investigators to simulate fracture processes in concrete. Due to the extremely large computational effort, however, the beam lattice model faces practical difficulties. In our investigation, a new lattice called generalized beam (GB) lattice is developed to reduce computational effort. Numerical experiments conducted on a panel subjected to uniaxial tension show that the GB lattice model can reproduce the load-displacement curves and crack patterns in agreement to what are observed in tests. Moreover, the effects of the particle overlay on the fracture process are discussed in detail. (C) 2007 Elsevier Ltd. All rights reserved

非均质材料准脆性损伤断裂的广义梁链网模型

陶瓷、岩石和混凝土等非均质材料的准脆性损伤断裂,既不同于玻璃的理想脆性断裂,又有别于金属材料的韧性断裂。本研究建立了该类材料的微观断裂力学模型——广义梁链网模型~([1,2]),根据应力重分配的思想实现渐进失效的模拟。重点考虑了三个方面的问题:1.微结构预应力的影响~([3,4])。微结构预应力是指混凝土风干过程中基体与骨料间的非协调变形导致的初始非零应力场。研究表明,微结构预应力可显著提升材料的延展性,并且造成卸载后不可恢复的变形。2.惯性效应~([5,6])。在新裂纹生成的瞬间,其表面邻域内物质点突然获得非零的加速度,将产生应力波。研究表明,该动态效应将导致加载曲线的震荡特性。我们发展了一种动静结合的算法,既能够捕捉局部断裂导致的惯性效应,又能很大程度上避免计算量的激增(与纯动态算法相比)。3.纸等纤维编织材料的撕裂~([7,8])。考虑了纤维-纤维结合键与纤维的断裂两种微观失效机制,分析了纤维长度、结合键强度等因素对断裂性能的影响

Algorithm for simulating fracture processes in concrete by lattice modeling

To simulate fracture behaviors in concrete more realistically, a theoretical analysis on the potential question in the quasi-static method is presented, then a novel algorithm is proposed which takes into account the inertia effect due to unstable crack propagation and meanwhile requests much lower computational efforts than purely dynamic method. The inertia effect due to load increasing becomes less important and can be ignored with the loading rate decreasing, but the inertia effect due to unstable crack propagation remains considerable no matter how low the loading rate is. Therefore, results may become questionable if a fracture process including unstable cracking is simulated by the quasi-static procedure excluding completely inertia effects. However, it requires much higher computational effort to simulate experiments with not very high loading rates by the dynamic method. In this investigation which can be taken as a natural continuation, the potential question of quasi-static method is analyzed based on the dynamic equations of motion. One solution to this question is the new algorithm mentioned above. Numerical examples are provided by the generalized beam (GB) lattice model to show both fracture processes under different loading rates and capability of the new algorithm

一种基于粘性力场存放机制的损伤模型

针对混凝土、陶瓷和岩石等非均匀脆性介质,建立了基于粘性力场演化的损伤断裂模型。考虑了局部损伤导致的两种应力重分配机制:(1)损伤单元内力的瞬时释放。假设该过程发生在无限短时间内,所以对于周围介质相当于载荷突变,粘性效应导致"变形滞后"。我们引入"粘性力场"的概念来定量的反映内力的瞬时释放所引起的粘性特征。(2)粘性力场的逐渐释放。在内力释放过程中储存的"粘性力场",随着时间推移而消减,伴随着周围介质变形场的持续演化。前者对应于"瞬态弹性"响应,后者对应于"粘性"响应。该算法易于实现到有限元和无网格等多种计算程序中,本研究基于链网模型而进行。对单轴拉压、巴西圆盘压缩和锚杆拔出实验进行了模拟,结果表明,该模型有效克服了传统链网模型软化曲线"过脆"的缺陷。能够反映混凝土等材料的应变率效应,随着加载速率的增加,加载曲线表现出更强的延展性,断裂模式更为弥散。该模型还可重现循环荷载下的"迟滞回线"。本研究对于预测复杂材料损伤断裂乃至地震和山体滑坡等,具有一定的理论意义和应用价值

BEAM LATTICE MODELING FOR THE FRACTURE OF PARTICLE COMPOSITES

通过发展一种三段式单元，改进了颗粒复合材料的梁网格断裂模型。根据单元内各段交接处的平衡条件推导了单元刚度矩阵。与已有的模型相比，改进的模型能够更加真实的反映颗粒和基体之间的界面区域，并且能够容纳小于网格尺寸的颗粒。计算结果表明利用改进的模型能够得到合理的力-位移关系和断裂模式

两种弹性损伤模型的基本方程与色散关系讨论

分析了传统连续介质损伤理论的控制方程在损伤过程中的变化特点,方程类型的改变导致错误预测波不能在损伤区域传播.二阶隐式应变梯度损伤理论是对传统理论的一种改进,严格证明了其控制方程在损伤过程中类型不变,这意味着损伤区域能传播波,同时有助于克服病态的网格相关性.色散分析结果表明:传统连续介质损伤理论不能反映色散现象,隐式梯度模型可以并对波长有上限截断作用

A simple method to simulate shrinkage-induced cracking in cement-based composites by lattice-type modeling

A new numerical procedure is proposed to investigate cracking behaviors induced by mismatch between the matrix phase and aggregates due to matrix shrinkage in cement-based composites. This kind of failure processes is simplified in this investigation as a purely spontaneous mechanical problem, therefore, one main difficulty during simulating the phenomenon lies that no explicit external load serves as the drive to propel development of this physical process. As a result, it is different from classical mechanical problems and seems hard to be solved by using directly the classical finite element method (FEM), a typical kind of "load -> medium -> response" procedures. As a solution, the actual mismatch deformation field is decomposed into two virtual fields, both of which can be obtained by the classical FEM. Then the actual response is obtained by adding together the two virtual displacement fields based on the principle of superposition. Then, critical elements are detected successively by the event-by-event technique. The micro-structure of composites is implemented by employing the generalized beam (GB) lattice model. Numerical examples are given to show the effectiveness of the method, and detailed discussions are conducted on influences of material properties

Numerical investigation of crack growth in concrete subjected to compression by the generalized beam lattice model

The beam lattice-type models, such as the Euler-Bernoulli (or Timoshenko) beam lattice and the generalized beam (GB) lattice, have been proved very effective in simulating failure processes in concrete and rock due to its simplicity and easy implementation. However, these existing lattice models only take into account tensile failures, so it may be not applicable to simulation of failure behaviors under compressive states. The main aim in this paper is to incorporate Mohr-Coulomb failure criterion, which is widely used in many kinds of materials, into the GB lattice procedure. The improved GB lattice procedure has the capability of modeling both element failures and contact/separation of cracked elements. The numerical examples show its effectiveness in simulating compressive failures. Furthermore, the influences of lateral confinement, friction angle, stiffness of loading platen, inclusion of aggregates on failure processes are respectively analyzed in detail

取向构元组集模型的弹性损伤理论

"对泛函势"是多种材料势能的很好近似.在计算材料势能过程中,分别由对势和嵌入势抽象出取向构元和体积构元,这两种构元的能量和就是材料的势能.材料损伤的本质是原子键合力的破坏,由于取向构元是原子间键的抽象,反映在构元层次上,损伤是构元刚度的减小.组集所有构元的响应函数,得到了材料的弹脆性损伤本构关系,这种基于构元组集的材料本构被称为"取向构元组集"材料模型.理论分析和数值实验表明,本模型具有概念清晰,物理直观,以及表示各向异性自然等特点