Abstract A method to determine crack tip position and fracture process zone (FPZ) in concrete was proposed based on Bažant Crack Band Model (CBM) and digital image correlation (DIC) technique, which was described as: firstly, the critical crack opening displacement of concrete (wcr) was determined based on CBM and the basic mechanical properties; secondly, DIC method was used to obtain the displacement field on the surface of specimens; thirdly, the position where the crack opening displacement equaled to wcr was defined as the crack tip position. In the fatigue crack propagation tests, the whole fatigue crack was FPZ at the beginning of fatigue loading until the non-cohesive crack grew and the length of FPZ decreased. The length of FPZ tended to be stable when the number of loading cycles was close to half of the fatigue life. Fatigue crack propagation curves (a-N curves) were obtained and fitted by logistic function to reduce the influence of the data scattering. Fatigue crack propagation rate da/dN was separated into descending and ascending crack propagation functions based on the increment of crack Δa and stress intensity factor ΔK, respectively. The da/dN-Δa curves for the descending segment showed obvious scatter. Paris’ law was applied to describe fatigue crack propagation rate well for the ascending segment.

中文翻译：

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基于DIC技术的混凝土断裂与疲劳裂纹扩展过程试验研究

摘要 基于Bažant裂纹带模型（CBM）和数字图像相关（DIC）技术，提出了一种确定混凝土裂纹尖端位置和断裂过程区（FPZ）的方法。混凝土（wcr）是根据煤层气和基本力学性能确定的；其次，采用DIC方法获得试样表面的位移场；第三，裂纹张开位移等于wcr的位置定义为裂纹尖端位置。在疲劳裂纹扩展试验中，疲劳加载开始时整个疲劳裂纹为FPZ，直至非内聚裂纹扩展，FPZ长度减小。当加载循环次数接近疲劳寿命的一半时，FPZ的长度趋于稳定。获得疲劳裂纹扩展曲线（aN 曲线）并通过逻辑函数拟合以减少数据散射的影响。疲劳裂纹扩展速率da/dN分别基于裂纹增量Δa和应力强度因子ΔK分为下降裂纹扩展函数和上升裂纹扩展函数。下降段的 da/dN-Δa 曲线表现出明显的分散性。应用巴黎定律很好地描述了上升段的疲劳裂纹扩展速率。下降段的 da/dN-Δa 曲线表现出明显的分散性。应用巴黎定律很好地描述了上升段的疲劳裂纹扩展速率。下降段的 da/dN-Δa 曲线表现出明显的分散性。应用巴黎定律很好地描述了上升段的疲劳裂纹扩展速率。