Abstract: To investigate the crack propagation rule and failure mechanism of concrete under cyclic loading, digital image correlation (DIC) technology was employed to analyze the evolution process of the load-crack mouth opening displacement (P-CMOD) curve. The variation characteristics of the crack mouth opening displacement (CMOD), crack length (a) and the residual strain in the fracture process zone, over the number of cycles and the unloading position, were quantitatively analyzed. The research results indicate that under constant-amplitude cyclic loading, the evolution of CMOD exhibits a three-stage characteristic: rapid accumulation, linear growth, and instability leading to failure. The crack length (a) is highly correlated with the changes of CMOD. At a stress level of 0.4 Fmax, a exhibits linear slow increase and then stabilizes. With increasing stress levels, a undergoes rapid extension in the early stage, followed by a deceleration in the growth rate, where residual stress promotes stable crack propagation. Under envelope cyclic loading, the growth rate of a converges while the CMOD continues to extend, indicating a gradual increase in the width of the fracture process zone. The accumulation rate of residual strain increases significantly with the load level. At 0.6 Fmax, the strain growth rate is 2.6 times that at 0.4 Fmax, and the high unloading residual stress exacerbates strain concentration. Based on the segmented mechanical response and dynamic damage evolution mechanism, a P-CMOD curve model that could predict the crack growth rate, failure critical point and response under multiple loading conditions is established, which provides a basis for the fatigue damage assessment and life prediction of concrete structures.

Key words: port engineering, residual strain, fracture process zone, three-point bending test, digital image correlation technique, cyclic loading

摘要： 为探究循环荷载下混凝土裂缝的扩展规律与失效机理，采用数字图像相关(DIC)技术分析了荷载-裂纹张开位移曲线(P-CMOD)的演变过程，量化分析了裂缝口张开位移(CMOD)、裂缝长度a及断裂过程区残余应变随循环次数与卸载位置的变化特征。研究结果表明：在等幅循环荷载下，CMOD的演化呈现出快速累积、线性增长、失稳破坏等3个阶段特征。a与CMOD变化高度相关：在0.4Fmax下，a呈线性缓慢增长后趋于稳定，随应力水平提高，a前期快速扩展，后期增速减缓，残余应力促进裂纹稳定扩展；在包络循环荷载下，a增速收敛而CMOD持续扩展，断裂过程区宽度增加，残余应变累积速率随荷载水平显著提高；在0.6Fmax下，其应变增速为0.4Fmax的2.6倍，高卸载残余应力加剧应变集中。基于分段力学响应与动态损伤演化机制，建立了可预测裂纹扩展速率、失效临界点及多荷载工况响应的P-CMOD曲线模型，为混凝土结构疲劳损伤评估与寿命预测提供基础。

关键词: 港口工程, 残余应变, 断裂过程区, 三点弯曲实验, 数字图像相关技术, 循环荷载