Abstract

Three-point bending tests are performed on notched concrete beams under various cyclic loading regimes. Meanwhile, the digital image correlation (DIC) method is used to monitor the crack propagation. The fatigue fracture life decreases with the loading level increases for constant-amplitude cyclic loading. The residual crack mouth opening displacement (CMOD) accumulates and fracture stiffness degrades with the loading process. The size of fracture process zone (FPZ) can be determined based on the full displacement field monitored by DIC method. The length of FPZ increases linearly with the increasing of effective crack length before the crack tip opening displacement (CTOD) exceeds the critical value. When CTOD exceeds the critical value, a macroscopic crack formed in front of the initial crack tip. For cyclic envelope loading, the length of FPZ increases at beginning and then decreases with the increasing of effective crack length. For constant-amplitude loading, CTOD never reaches the critical value, and the macroscopic crack does not propagate forward. Finally, it is found that both the calculated effective crack lengths of concrete under constant-amplitude loading by two-parameter model (TPM) and DIC methods present three-stage feature, but the values calculated by TPM are smaller than those by DIC method.

摘要

三点弯曲试验是在各种循环加载状态下对开槽混凝土梁进行的。同时，采用数字图像相关（DIC）方法监测裂纹扩展。对于等幅循环载荷，疲劳断裂寿命随着载荷水平的增加而降低。残余裂口张开位移 (CMOD) 累积，断裂刚度随着加载过程而降低。断裂过程区（FPZ）的大小可以根据DIC方法监测到的全位移场来确定。在裂纹尖端张开位移（CTOD）超过临界值之前，FPZ的长度随着有效裂纹长度的增加而线性增加。当 CTOD 超过临界值时，在初始裂纹尖端前形成宏观裂纹。对于循环包络加载，FPZ的长度随着有效裂纹长度的增加先增加后减小。对于等幅载荷，CTOD 永远不会达到临界值，宏观裂纹不会向前扩展。最后发现，采用双参数模型（TPM）和DIC方法计算的等幅荷载下混凝土有效裂缝长度均呈现三阶段特征，但TPM计算值小于DIC方法。