Study on ductile–brittle transition law and micro-fracture evolution mechanism of rock under dry–wet cycle

The deterioration and damage law of rock under dry–wet cycles is the basis for studying the stability of reservoir bank slopes. Existing research mainly focuses on the damage law of rock mechanical properties under the action of dry–wet cycle. The mechanical behavior of rock after dry–wet cycles and the fracture evolution mechanism in the destruction process still need to be further studied. In this paper, uniaxial compression and Brazilian splitting tests were carried out on sandstone under 2, 4, 8, and 16 dry–wet cycles, and different brittleness indices were used to characterize the changes in the brittleness of the samples under dry–wet cycles. The results show that with the increase in the times of dry–wet cycles, the brittleness of the rock decreases, the macroscopic failure mode of the sample gradually changes from splitting failure to shear failure. The variations of acoustic emission b value and energy rate during uniaxial compression are analyzed; the results show that with the increase in the times of dry–wet cycles, the dominance of large fractures decreases at first and then increase. The distribution density of ratio of risetime to amplitude and average frequency values at different loading stages is analyzed, and the results show that with the increase in the times of dry–wet cycles, the proportion of shear fractures in each stage increases, especially in the instability stage and post-peak stage, which gradually changes from tension fracture dominance (shear fracture accounts for less than 50%) to shear fracture dominance (up to 60%).