Direct shear tests were conducted on sandstone specimens under different constant normal stresses to study the coalescence of cracks between non-persistent flaws and the shear sliding characteristics of the shear-formed fault. Digital image correlation and acoustic emission (AE) techniques were used to monitor the evolution of shear bands at the rock bridge area and microcracking behaviors. The experimental results revealed that the shear stresses corresponding to the peak and sub-peak in the stress-displacement curve are significantly affected by the normal stress. Strain localization bands emerged at both the tip of joints and the rock bridge, and their extension and interaction near the peak stress caused a surge in the AE hit rate and a significant decrease in the AE b value. Short and curvilinear strain bands were detected at low normal stress, while high normal stress generally led to more microcracking events and longer coplanar cracks at the rock bridge area. Furthermore, an increase in normal stress resulted in a higher AE count rate and more energetic AE events during friction sliding along the shear-formed fault. It was observed that the elastic energy released during the crack coalescence at the pre-peak stage was much greater than that released during friction sliding at the post-peak stage. More than 75% of AE events were located in the low-frequency band (0–100 kHz), and this proportion continued to rise with increasing normal stress. Moreover, more AE events of low AF value and high RA value were observed in specimens subjected to high normal stress, indicating that greater normal stress led to more microcracks of shear nature.

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Keywords: Shear band evolution, Acoustic emission (AE), Crack coalescence, Normal stress, Shear sliding