Analysis the evolution of friction coefficient of sandstone joint during sliding process

Interior of rock mass,such as sandstone,has joint in general.The sandstone penetrates along joints to form a slip surface when disturbance by external loading.The sandstone slides along the joint surface,the asperities on the surface of joints are constantly worn leading to the constant changes of the roughness of the joint surface,which affects the sliding stability of sandstone.The reciprocating sliding friction test is performed on the split sandstone joints using the self made sliding friction test device.The changes of wear area,wear mass,friction coefficient,and surface roughness are measured through a high resolution camera,electronic analytical balance,dynamic strain gauge,and laser confocal scanning microscope during the sliding process.The results indicate that the sliding process can be divided into three stages,such as the initial sliding stage,the running in stage,and the stable sliding stage.In the initial sliding stage,the initial value of roughness of the sandstone joint surface is relatively large,the wear area is small,and the wear mass and friction coefficient are large.In the running in stage,the asperities on the surface of the wear area are constantly sheared off and ground flat,the decreased trend of surface roughness slows down,the new wear area constantly decreases.The proportion of newly added wear area in total wear area is still higher than 10%,and the wear mass and friction coefficient appear to have a jump phenomenon.In the stable sliding stage,the surface of the wear zone is matched with the diamond lapping,and the roughness of the wear zone tends to be a stable value.The proportion of newly added wear area in total wear area is less than 10%.The wear mass and friction coefficient finally tend to be stable and fluctuate at the surrounding of a stable value.The surface profile of sandstone joints is measured through the laser confocal scanning microscope,and the root mean square of the profile is chosen to evaluate the joint roughness coefficient.It demonstrates that the joint roughness coefficient is mainly affected by the wear area,while the effect of local roughness is small for the joint roughness coefficient.The prediction model of the friction coefficient of a sandstone joint based on the variable quantity of the wear area is established,and the change of joint roughness coefficient during the sliding process can be reflected by the model.The estimated value is in good agreement with the test value when the correction coefficient a=2.5 and the minimum error is 2.89%.