In studies that been carried out at LNEC On the frictional resistante of particles of granular materials [1] analyses were made of the main mechanisms referred to by several authors to explain the classic Amon- tons' s friction laws, and recentlythe stick-slip of geologic materials, such as the interlocking mechanism attributed to Coulomb [2], the adhesion mechanism proposed by Terzaghi [3] and the brittle failure mechanism presented by Byerlee [4].
Nevertheless it seems that some phenomena of failure and wearing observed in the contact and sliding of geologic materials, such as the Ploughing of contact surfaces, cannot be explained only on basis of those mechanisms. Jaeger [5] suggests that in addition to the mechanisms of failure of saliences pro- Posed by Byerlee there should occur a further mechanism of failure on rather flat surfaces parallel to the direction of sliding. In fact this becomes clear enough when sliding occurs with materials of different hardness, and the harder one scratches or ploughes out the softer material [5, p, 117]. Bearing i n mind what happens with metals of different hardness, in which the mechanism by which the harder metal Ploughes out the softer one is perfectly identified [2], this suggestion seems reasonable and worth to be analysed. On the other hand the recent development of studies on rack cutting, mainly related to tunnel-boring machines [6], has shown-same analogy between rock cutting by means of tracks opened by the tools of the boring machine and the ploughing mechanism used in the theorization of metallic friction. This-analogy provides a new outlook regarding the problem of the friction of geological materials, which may be of interest.
These are thus the guidelines for and the motivation behind the present work.
It is a well-established concept that the frictional resistance of metal surfaces can be considered as the sum of two terms. One represents adhesion, that is, the shearing of the metallic junctions established at the true points of contact between the surfaces, the area of these contacts being small as compared to the apparent area of the surfaces, or large as compared to the mollecular dimensions. The other term expresses the ploughing process.
The mechanism by which these two terms participate in the process of metal friction is briefly as follows [2, p.90]. Let us consider sliding between surfaces of two metals of different hardness. A given asperity of the harder metal sinks into the surface of the softer one until the contact area is sufficient to withstand load N applied to the asperity. If A is the projection of the area of contact thus generated and q is the yield stress of the sfter metal, we shall obtain.
The force T required to make the asperity slide in a direction parallel to the surface will be the sum of two terms as already said: one, Ts, is the force required to over-some the shearing strength s resulting from the metallic junctions
