To investigate the mechanical properties and failure characteristics of bedding coal under different stress paths

mechanical tests were conducted using the MTS816 rock mechanics testing system. Coal samples with bedding angles of 0°

30°

45°

60°

and 90° were subjected to tests under three stress paths: conventional triaxial loading

increasing axial pressure unloading confining pressure

and constant axial pressure unloading confining pressure. The study analyzed the influence of stress paths and bedding angles on the strength

deformation

and failure characteristics of the coal samples. The results indicate the following: Under the path of conventional triaxial loading

the stress-strain curves of the coal samples show a nearly linear relationship before reaching the peak point

followed by a rapid drop in stress after reaching the peak. Under the path of increasing axial pressure unloading confining pressure

the axial strain increment ratio exhibits a linear increasing trend as the unloading ratio increases. The circumferential strain increment ratio and volumetric strain increment ratio display a three-stage change characteristic

involving low-speed growth

rapid growth

and stable growth. Under the path of constant axial pressure unloading confining pressure

the axial strain increment ratio

circumferential strain increment ratio

and volumetric strain increment ratio exhibit a two-stage change characteristic

involving low-speed growth and rapid growth. The peak strength and axial peak strain of the coal samples exhibit a V-shaped trend with increasing bedding angle under different stress paths. They initially decrease and then increase

reaching the maximum value at 0° and the minimum value at 60°. As the unloading ratio increases

the deformation modulus of the coal sample initially stabilizes and then rapidly deteriorates. The Poisson’s ratio first slowly increases and then exponentially increases. The critical unloading ratio initially increases and then decreases with increasing bedding angle. When the bedding angle is the same

the critical unloading ratio under the path of increasing axial pressure unloading confining pressure is lower than that under the path of constant axial pressure unloading confining pressure. Under the path of conventional triaxial loading

the coal sample undergoes brittle shear failure. However

under the paths of increasing axial pressure unloading confining pressure and constant axial pressure unloading confining pressure

the coal sample exhibits a mixed tensile-shear failure mode. When the bedding angle is 60°

the angle of the main shear fracture surface of the coal sample is almost consistent with the bedding angle.