Asperity interpenetration

The stress-mediated redistribution of mineral mass by pressure solution may be represented by the three serial processes of mineral dissolution beneath the contacting asperity, transport along an interfacial water-film to the fracture void, and then redistribution by precipitation within the fracture void, or efflux from the sample. The asperities interpenetrate as mass is removed from contacting asperities, in a similar manner to the mechanisms that drive grain interpenetration and cementation in sandstones [Yasuhara et al., 2003], as illustrated in Figure 10. 

Figure 10. Model for asperity interpenetration follows that for grain interpenetration in granular materials. [Yasuahra et al., 2(X)3].

The most general modern model used to describe frictional phenomena assumes that the friction between two unlubricated surfaces arises from two sources. The first and generally most important is that of adhesion between points of actual contact between the surfaces. We have seen on various occasions that real solid surfaces are almost never smooth. A very smooth surface will normally have asperities of between 5 and 10 nm so that the true area of contact between surfaces will be less that the apparent area (Fig. 18.1). At those areas of contact, the two surfaces will be bound by a certain adhesion force arising from the interaction between the materials at the molecular level—the same basic forces we have encountered before plus, in some cases, more physical interactions due to mixing, interpenetration, or locking. For the two surfaces to move tangentially, the points or areas of adhesion, welds, or junctions must be sheared or broken. If the real area of contact is A and the shear strength of the weld or bond is s, then the frictional force due to adhesion will be... 

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