Frictional Mechanism of Ploughing

Abstract

lt seems that several failure and wearing phenomena observed in the contact and sliding of geologic materials, such as the ploughing os contact surfaces, cannot be explained only on basis of interlocking friction (Coulomb, 1781), adhesion (Terzaghi, 1925) or brittle failure mechanims (Byerlee, 1967). Reference is made to the suggestion presented by Jaeger (1971), according to which bisedes those mechanisms, another occurs, i.e. fracturing of comparatively flat surfaces, which is clearly shown when sliding takes place with materials of different hardness and the harder material scratches and ploughes out the softer material. On one hand, an analysis is made of what occurs in the friction of metals with different degress of hardness, a case in which the mechanisms of ploughing is well-known and whose frictional resistance is the sum of two terms: one being independent of ploughing and complying with Amontons's law, the other proportional to the cross-section of the track and not complying with that law (Bow­ den & Tabor, 1950). On the other hand, attention is given to studies by Nishimatsu (1972) on rock cutting by tunnel boring machines, in which cutting is carried out with tools that open successive contiguous tracks. Thus the shear strenght to be overcome by the machine was than deduced to be also the sum of a term independent of tracks that complies with Amontons's law, and other proportional to the depth of tracks, that is their cross-section. The analogy between the two mechanisms - the mechanisms of mettalic friction, which is microscopic, and the mechanism of rock cutting, of macroscopic characteristics - suggests that the friction between geologic materials sould also be determined by the same mechanism. Therefore, the coefficient of friction of those materials when ploughing occurs will also be the sum of a term complying with Amontons's law, thus independent of tracks, and other term proportional to the cross-section of tracks.