Contents lists available at ScienceDirect Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech Experimental study on fracture and fatigue crack propagation processes in concrete based on DIC technology Dongyang Li a,b , Peiyan Huang a,c, ⁎ , Zhanbiao Chen a , Guowen Yao d , Xinyan Guo a , Xiaohong Zheng a , Yi Yang a a School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510640, China b Zhuhai Institute of Urban Planning & Design, Zhuhai 519000, China c State Key Laboratory of Subtropical Building Science, South China University of Technology, Guangzhou 510640, China d State Key Laboratory of Bridge and Tunnel Engineering in Mountainous Area, Chongqing Jiaotong University, Chongqing 400074, China ARTICLE INFO Keywords: Fatigue crack propagation Digital image correlation (DIC) method Fracture process zone (FPZ) Crack tip Concrete ABSTRACT A method to determine crack tip position and fracture process zone (FPZ) in concrete was pro- posed 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 (w cr ) 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 w cr 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 as- cending 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. 1. Introduction The accurate measurement of fracture process zone (FPZ), especially crack tip position is one of the difficulties to study fracture and fatigue crack propagation behavior of concrete. To accurately determine crack tip position and FPZ is of great significance to research fatigue and fracture performance of concrete. Using advanced experimental technique to measure FPZ, especially crack tip position is conducive to study the fatigue and fracture mechanism of concrete. Various experimental methods such as X-ray, Moire interferometry, scanning electron microscope, acoustic emission, laser speckle interferometry, etc. [1–7] were applied to research the fracture performance of concrete. The measurement based on the digital image correlation (DIC) technique is one kind of non-contacting methods that acquire images of an object, store images in digital form and perform image analysis to extract full-field shape, deformation or motion measurements [8]. DIC method has now been applied in concrete fracture test, taking advantages of non-contact, full-field mea- surement, simple devices and strong anti-interference [9–13]. Wu et al. [14] determined the crack tip of FPZ based on critical https://doi.org/10.1016/j.engfracmech.2020.107166 Received 13 April 2020; Received in revised form 15 June 2020; Accepted 16 June 2020 ⁎ Corresponding author at: School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510640, China. E-mail address: pyhuang@scut.edu.cn (P. Huang). Engineering Fracture Mechanics 235 (2020) 107166 Available online 23 June 2020 0013-7944/ © 2020 Elsevier Ltd. All rights reserved. T