Experimental investigation of mixed mode I-II fatigue crack propagation in concrete using a digital image correlation method

The propagation of fatigue cracks leads to the degradation of the load-carrying capacity of con-crete structures and even fatigue failure. An in-depth investigation of the fatigue crack propa-gation process is essential to reveal the fracture mechanism and further assess the safety of concrete structures. This paper presents an experimental study of mixed mode I-II fatigue crack propagation in concrete. Fatigue tests are conducted on three-point bending (TPB) beams with an off-center initial crack with different fatigue load levels. The digital image correlation (DIC) method is employed to observe the complete fatigue crack propagation process, and the method is verified to be applicable. The results indicate that the mixed mode I-II fatigue crack propagation process can be divided into three stages, namely, the rapid propagation in the initial stage, a stable propagation stage, and the final fast propagation until unstable failure. The mixed mode I-II crack propagation path under static loading is a good approximation of that under fatigue loading. The results also show that the crack tip opening displacement (CTOD), crack tip sliding displacement (CTSD), mode I stress intensity factor (SIF), and mode II SIF corresponding to the unstable failure of the TPB beam are approximately constant and independent of the fatigue load level. In particular, the negligible CTSD and mode II SIF demonstrate that the mixed mode I-II fatigue failure of concrete is dominated by the mode I component.