Experimental study on fracture and fatigue crack propagation processes in concrete based on DIC technology

A method to determine crack tip position and fracture process zone (FPZ) in concrete was proposed based on Bazant 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 ascending crack propagation functions based on the increment of crack Delta a and stress intensity factor Delta K, respectively. The da/dN-Delta a curves for the descending segment showed obvious scatter. Paris' law was applied to describe fatigue crack propagation rate well for the ascending segment.