Study On Dynamic Frictional Properties Of Joint Plane Under Impact Load

Dynamic frictional properties of joint planes in rock mass were significant in rock dynamics,civil engineering,etc.Lots of works focused on the quasi-static frictional properties,but few works was on the investigation of dynamic frictional properties of rock mass due to the limitation of loading method,localized deformation and slipping on surfaces.To measure the local properties of the joint planes during the process of dynamic slipping,two sets of Hopkinson bundle bars,i.e.single-bar incidence and three-bar transmission,and three-bar incidence and three-bar transmission,were designed and fabricated to study the combined dynamic loading of cement,aluminum,and granite,respectively.The digital speckle distribution histogram was used to analysis the qualities of natural and artificial speckles,and the dynamic deformation process was analyzed by digital imaging correlation method(DIC)to verify the reliability of experimental system.The propagation properties of stress wave during joint surfaces slipping were also discussed.The local failure strength of cement under single-bar incidence and three-bar transmission was analyzed,and the decreased degree of groove point in transmission stress profiles was used to determine the local damage states.The mechanism of stress drop during joint surfaces slipping was further investigated for the cement under three-bar incidence and three-bar transmission,and the relationship of transmission stress to incident stress was used to determine the starting point of slipping.Experimental results show that the combined process can be divided into four stages:the loading stage,the initial slipping stage,the stress drop stage and the unloading stage.In order to simplify the problem,the aluminum alloy specimens with the same material component as the bars were tested under different impact velocities and pulse durations.It was revealed that the sample with small oblique angle,i.e.20 degree,did not slip,which resulted from the facts that the transmission stress was approximately equal to the incident stress.But the slipping happened when the oblique angle was large enough,e.g.30 degree.Moreover,with the impact velocity increasing,rates of the ratios of vertical/horizontal stress and the frictional strength decreases more quickly.The frictional strength and the start slipping time of the interface varied with the initial friction coefficient,for example,the start slipping time gradually brought forward with the decreasing of friction coefficient.Finally,series experiments were conducted to investigate the dynamic slipping properties of granite joint surface under different angles.The dynamic slipping process was observed by high-speed camera,and corresponding whole strain fields on specimens were analyzed by DIC method.It was revealed that the start slipping time gradually brought forward and the whole slipping time gradually increased with the increasing of the oblique degree of joint surface,which eventually caused the increasing of slipping displacement.It was also demonstrated that the granite joint surface did not slip under a shorter loading duration,but slip under a longer loading duration.Then,the concept of displacement gradient was introduced to illuminate the relationship between the oblique angle and the deformation uniformity of joint surface,and a novel method was proposed to calculate the whole slipping displacement.