Friction on a granite to granite interface

Summary

Direct shear tests were conducted on a granite to granite interface for the purpose of tracing the evolution of frictional resistance as the initially smooth and polished surface wears during continuing shearing displacement. At the moment when sliding on the freshly manufactured interface starts (first slip), the friction angle is very low, between 15° and 20°, but then it increases with displacement rapidly without reaching a peak in the first test (maximum displacement less than 25 mm). Upon repeated shearing of the same surface (without re-finishing), this process of “displacement-strengthening” continues until a total accumulated displacement of about one half of one meter is reached. At this point, the angle of friction would typically be between 42° and 44°.

Once the residual surface was established, the effect of time-related parameters, the duration of stationary contact under constant normal and shear load and variable displacement rate, were investigated. The frictional resistance is shown to increase with decreasing displacement rate and to increase with the duration of stationary contact.

At a normal load greater than about one half of one MPa, shear displacement on a residual surface proceeds in an unstable, stick-slip manner. The change from stable to unstable sliding, that can be effected by either decreasing the sliding velocity or increasing the normal load, is not instantaneous, but occurs over a finite amount of displacement.