Digital imaging of crack evolution in granite containing unparallel flaws under fatigue loading

Abstract Failure in jointed rock masses presents a major challenge in tunneling, mining, and rock slope engineering, typically occurring along surfaces formed by interacting cracks at pre-existing flaws. Understanding how cracks initiate, propagate from these flaws, and eventually coalesce can contribute to more reliable and safer rock structure design. This study investigates the influence of the unconfined compressive monotonic and cyclic loading conditions and varied unparallel flaw geometries on the crack propagation process and mechanical properties of granite. Five distinct flaw geometries were analyzed, each containing two pre-existing flaws. The upper flaw (Flaw number ①) had a fixed inclination angle of 45°, while the lower flaw (Flaw number ②) was oriented at 0°, 45°, 90°, 135°, and 180°, both relative to the horizontal axis. Based on the orientation of Flaw number ②, the specimens were labeled S0, S45, S90, S135, and S180. These configurations ranged from non-overlapping geometries (S0, S45, and S90) to overlapping ones (S135 and S180). The analysis was conducted under monotonic and then cyclic loading conditions with stress amplitude set at 75%, 80%, and 85% of the peak uniaxial compressive strength (₂) obtained from monotonic tests for each geometry. Crack development was captured using Digital Image Correlation (DIC) via GOM Correlate software, enabling high-resolution monitoring of surface strain and displacement fields at the pixel level. The results reveal that flaw geometry significantly influences the crack coalescence path and final failure pattern. A transition from non-overlapping to overlapping flaw configurations shifted the crack coalescence from indirect to direct trajectories. Cyclic loading, in some cases, causes changes in crack propagation paths at lower amplitudes, as well as the formation of new cracks, referred to as cyclic cracks, unique to this type of loading, leading to distinct overall failure patterns compared to other loading types. While the loading type (monotonic vs. cyclic) had minimal effect on the mechanism of crack initiation, it notably altered the sequence of crack growth and propagation mechanism. Mechanical properties also varied with both geometry and loading conditions. Under monotonic loading, overlapping geometries exhibited higher strength and a greater crack initiation stress ratio (CI/CP, defined as the ratio of crack initiation stress to the peak compressive strength of the specimen) compared to non-overlapping ones. Specifically, specimens S90 and S45 demonstrated the highest values of ₂ (161 MPa) and CI/CP at 51%, respectively. In contrast, S180 showed the lowest values, approximately 124 MPa in strength and 17% in CI/CP. Finally, flaw geometry was found to govern both fatigue life and its sensitivity to loading amplitude. As the loading amplitude increased from 75 to 80% and 85%, fatigue life decreased across S0, S45, S90, and S135. Notably, S180 exhibited non-linear behavior, with the fatigue life dropping from 425 to 257 cycles at 75–80%, then increasing slightly to 310 cycles at 85%.