Strain energy density factor theory hypothesizes that the crack will propagate in the direction of minimum distortion energy density

while the stress intensity factors which being used in the modified theory are calculated in tension stress field. However

rock mass is usually being in compression in the nature. So the modified strain energy density theory with the stress intensity factors which has been calculated in compression is incorporated

the friction between crack faces also being took into consideration additionally; and a modified strain energy density theory which aims at analyzing the rock crack propagation in compression is proposed. The influence of crack angle

confining pressure and crack surface friction on crack propagation angle are analyzed

and the analyzing results have been compared with experiment results and numerical simulation results of other researchers and a good agreement has been obtained. The analysis shows that the critical load for cracking decreases first and then increases with the increase of crack angle under uniaxial compression. And one crack angle corresponds to more than one crack propagation angle

which means that the crack can propagate in different directions. Finally

the crack propagation angle is related to the confining pressure under triaxial compression. The results can provide reference for numerical simulation of rock crack propagation in compression.