Effects of positions of material imperfection at right edge of rock specimen under plane strain compression on initiation and evolution of shear bands as well as stress-strain curves of rock specimen were modeled numerically by FLAC. Two material imperfections were introduced in the form of null element at left and right edges of specimen

respectively. The material imperfection at left edge was fixed; however

the positions of material imperfection at right edge were different for different schemes. The adopted failure criterion was a composite Mohr-Coulomb criterion with tension cut-off and post-peak constitutive relation of rock was linear strain-softening. Only one shear band bisecting specimen through two imperfections is formed and only full development of shear band below right material imperfection is permitted due to end restraint at top of specimen when right imperfection is close to the top. When right imperfection is not close to the top

shear bands are initiated in the vicinity of material imperfections and propagate along their inherent directions so that patterns of shear bands are very complex. Stress-strain curve beyond the peak strength exhibits steeper behavior when the number of shear bands intersecting specimen is lower. Otherwise

strain-softening branch of stress-strain curve undergoes ductile behavior. In the process of shear band propagation

when the tip of shear band head-on approaches two triangular regions affected by strong restraint at top or base of specimen

direction of shear band has to be changed. If the tip is very closer to the symmetrical line in vertical direction of specimen

direction of shear band will be slightly changed (the phenomenon is called shear band refraction). However

if the tip is not very closer to the symmetrical line

the direction will be changed greatly (the phenomenon is called shear band reflection).