混凝土结构裂缝扩展的双G准则

断裂力学基本上有两种分析裂缝稳定性的方法 :应力强度因子法和能量法。在混凝土断裂性能的描述上两者应该是等效的。针对目前大多数混凝土断裂模型以应力强度因子型的断裂韧度作为判定准则 ,本文尝试从能量的角度出发 ,结合线弹性断裂力学和虚拟裂缝区上的黏聚力分布 ,以能量释放率G作为断裂性能判定参数 ,建立了混凝土结构裂缝扩展的双G准则。与双K断裂参数相对应 ,双G准则引入了两个重要的裂缝扩展判定参量 :起裂断裂韧度GiniIc 和失稳断裂韧度GunIc 。其中 ,起裂断裂韧度GiniIc 对应于起裂荷载Pini和初始裂缝长度a0 ;失稳断裂韧度GunIc 对应于极值荷载Pmax和临界有效裂缝长度ac。根据线性渐进叠加假定 ,可以把 (Pini,a0 )和(Pmax,ac)代入线弹性断裂力学相对应的公式直接求得GiniIc 和GunIc 。考虑到起裂荷载Pini的不易确定性 ,本文引入黏聚力分布引起的能量损耗GcIc,通过三者GiniIc 、GunIc 和GcIc的关系 ,给出了起裂断裂韧度GiniIc 和失稳断裂韧度GunIc的适用计算公式 ,并通过三点弯曲梁实验得到了验证。

A DUAL-G CRITERION FOR CRACK PROPAGATION IN CONCRETE STRUCTURES

At present there are two main approaches describing the fracture behavior of concrete structures: one uring stress intensity factor(SIF) as the main fracture parameter, and the other being energy based. Theoretically, these two methods should be consistent with fracture mechanics. Most existing fracture models for concrete belong to the former category,i.e., these models express fracture toughness in terms of SIF. In this paper, an attempt is made to establish a fracture criterion from the standpoint of energy balance. Paralleling to the dual-K fracture model(SIF-oriented),a dual-G energy criterion is proposed to characterize the fracture behavior of the crack propagation in concrete, where,G is in terms of the energy release rate in the conventional fracture mechanics. Two important fracture parameters are included: the initial fracture toughness ,G ini Ic,corresponding to the initial load, P min,and the initial crack length,a 0,and the unstable fracture toughness G un Ic referring to the critical unstable state with the load reaching its maximum value ,P max,and the crack growing to its critical value,a c. Based on the assumption of linear asymptotic superposition, both of these two fracture parameters can be determined by using linear elastic fracture mechanics(LEFM). Because of the difficulty in defining the initial load,P min,the energy consumption,G c Ic, contributed by the cohesive force in the fictitious crack zone is taken into account in determining the initial fracture toughness,G ini Ic. Combining LEFM and G c Ic, practical expressions for calculating those two fracture toughness, parameters G ini IcandG un Ic, are obtained and verified by using three-point bending beam experiments. Although only the concrete is studied here, the proposed fracture criterion can be easily extended to other quasi-brittle materials.