Experimental Investigation of the Coupled Influence of Rate of Loading and Contact Time on the Frictional Behavior of Quartz Grain Interfaces under Varying Normal Load

In this study, the coupled influence of contact time and rate of loading on the interface behavior of quartz grains was examined experimentally. The rate of loading is considered not just a tangential (or sliding) rate but a combination of both normal and tangential loading rates, which leads to the target normal load and tangential displacement for a given contact time. Two classes of tests were conducted; in the first class, steady-state sliding was first allowed to be reached by shearing the grains at a constant normal load before the application of the simultaneous normal load and tangential displacement variation, whereas in the second class, the normal load and tangential displacement were varied simultaneously from the start of the test. The experimental results from monotonic and cyclic micromechanical tests highlight the significant influence of the coupled load rate and contact time on the interparticle friction, tangential stiffness, and energy loss of the contacted interfaces. The results are compared with standard micromechanical experiments that were conducted under a constant normal load during the shearing of the grain contacts. Indirect qualitative analysis was performed to substantiate the increase in contact area with increasing contact time, which is not expected in constant normal load sliding.