The phenomenon of stiffness deterioration（progressive damage） beyond the peak stress has been observed in cyclic loading tests of coals

rocks

concretes

soils and sands.In this paper

different cases of degraded stiffness in a system of shear-band（or fault） and its outside elastic rock were taken into account to derive the post-peak stress-deformation curve of the system in the direct shear condition.When stiffness deterioration in the shear-band is considered

the total shear deformation of the shear-band does not change but the elastic deformation increases while the plastic deformation decreases.Stiffness deterioration in the elastic body leads to the nonlinear post-peak stress-deformation curve of the system

which is different from the previous result with stiffness deterioration neglected.In the process of strain-softening

the structural response of the system changes from ClassⅠto ClassⅡbehavior

which is attributable to the stiffness deterioration in the elastic body.Larger the elastic body

higher the post-peak brittleness

shorter the internal length

softer the initial elastic modulus

more strong the shear strength

less the residual shear stress

and stiffer the residual shear modulus will all lead to steeper the post-peak structural response of the system

even result in the classⅡbehavior

and be liable to make the system losing the stability （rock bursts

bumps

mining induced earthquakes or mine tremors）.