Micromechanics of rock friction and wear processes: A theoretical and experimental study.

摘要

The research described in the thesis is an attempt to understand the physics of rock friction at the micromechanical scale. Frictional sliding is always associated with damage and erosion of the surfaces which is known as wear. The micro-processes of wear during frictional sliding on the surfaces of Westerly granite have been investigated experimentally and theoretically. Wear experiments are conducted on the surfaces with different roughnesses, under normal stress ranging from 1 to 10 MPa and over displacements up to 2 meters. The wear is modeled with two kinds of micromechanisms, shearing-off and riding-over wear. There is a critical overlap distance that determines which of two processes will occur. A quantitative study on micromechanics of base friction with a combined experimental and theoretical approach has been carried out. The base friction, the first order frictional resistance, is the sum of a number of distinct frictional interactions which progressively develop in the early stages of slip. (1) Initial friction; (2) Interlocking; (3) Surface evolution due to wear; and (4) Friction due to the work of wear. The stability of rock friction, which determines whether faulting is seismic or aseismic, depends on second order frictional phenomena. There are a number of second order effects on friction. We have begun studying two of them, the effect of variable normal stress, and the effect of slip velocity. For the first time we observed time dependent closure between surfaces during static loading and unloading. The results from both creep and relaxation experiments on Westerly granite surfaces show that this time-dependent deformation of contacting asperities is significant. This may be the mechanism for time- and velocity-dependent friction. The scaling of friction parameters is a crucial step for us to study a geological faulting based on our laboratory results. In Chapter Four the model interprets that the critical slip distance {dollar}Dsb{lcub}C{rcub}{dollar} is the distance at which the majority of contacts have changed from partially sliding to fully sliding. The synthetic spectrum method has been used to generate fractal surfaces on different scales. {dollar}Dsb{lcub}C{rcub}{dollar} has been calculated for these synthetic fractal surfaces with the contact model. It is found that {dollar}Dsb{lcub}C{rcub}{dollar} is mainly controlled by the uncorrelated part of surface topography. The relationship between {dollar}Dsb{lcub}C{rcub}{dollar} and the critical correlation length of the surface is a power law. If {dollar}Dsb{lcub}C{rcub}{dollar} for nature faults is in the range, 1 mm to 1 cm, then the calculations show they have correlation lengths of 10s to 100s of meters.