Mixed Mode Ⅰ-Ⅱ Fracture Process Zone Of Concrete And Numerical Simulation Of Crack Propagation Process

The linear elastic fracture mechanics is no longer suitable for concrete material because of the existence of fracture process zone (FPZ). The research on the fracture process zone is one of the most important basic research topics to reveal the mechanism of concrete fracture all the time. Considering that I-II mixed mode fracture is one of the most familiar fracture patterns, the study on the characteristics of the mixed mode FPZ get the international academic attention and is becoming more and more remarkable. Up to now, few researches have been down on mixed mode FPZ because of its complexity. In this paper, experimental method is used to measure the size of mixed mode I-II fracture process zone of concrete. Then the numerical calculation of mixed mode I-II crack growth process considering the influence of FPZ is also carried out. The main contents of this paper are as follows:(1) Digital image correlation method (DICM) is used to measure the size of fracture process zone, the crack opening displacement and shearing displacement of four point shearing beams of different sizes including 700mm×150mm×40mm,580mm×120mmx40mm, 460mm×90mm×40mm and 340mmx60mmx40mm under various loading levels. Results indicate that in the mixed mode I-II fracture crack, opening displacement within FPZ decreases linearly along the direction of crack propagation, while shearing displacement within FPZ decreases rapidly in the first half and the attenuation slows down in the second half. The ratio of shearing displacement and opening displacement on precast crack tip decreases with the growth of FPZ length. The FPZ length which corresponds to the peak load increases with the growth of specimen height, while the ratio of the FPZ length in times of peak load and length of initial ligament is always 0.365.(2)Maximum tangential stress criterion and maximum circumferential strain criterion are most frequently used when doing concrete fracture analyses. Based on these two criteria, new crack propagation criteria are proposed for mix mode Ⅰ-Ⅱ fracture. Finite element analysis is carried out to calculate the crack extension path and load vs. crack mouth shearing displacement (P-CMSD) curves for four shearing concrete beams of different height including 60mm,90mm,120mm and 150mm. The results indicate that the difference between P-CMSD curves calculated by two criteria is quite small and both of them show a good agreement with the experimental results. The crack extension path simulated by the maximum tangential stress criterion is more close to the experimental results. The crack propagation criterion based on the maximum tangential stress criterion is more suitable to analyze the mix mode Ⅰ-Ⅱ fracture of concrete.