Page 31 - Tribology in Machine Design

18 Tribology in machine design

where A T is the real area of contact, r max denotes the ultimate shear strength
of a material and T S is the average interfacial shear strength.

2.6. Energy dissipation In a practical engineering situation all the friction mechanisms, discussed so
during friction far on an individual basis, interact with each other in a complicated way.
Figure 2.5 is an attempt to visualize all the possible steps of friction-induced
energy dissipations. In general, frictional work is dissipated at two different
locations within the contact zone. The first location is the interfacial region
characterized by high rates of energy dissipation and usually associated
with an adhesion model of friction. The other one involves the bulk of the
body and the larger volume of the material subjected to deformations.
Because of that, the rates of energy dissipation are much lower. Energy
dissipation during ploughing and asperity deformations takes place in this
second location. It should be pointed out, however, that the distinction of
two locations being completely independent of one another is artificial and
serves the purpose of simplification of a very complex problem. The various
processes depicted in Fig. 2.5 can be briefly characterized as follows:
(i) plastic deformations and micro-cutting;
(ii) viscoelastic deformations leading to fatigue cracking and tearing, and
subsequently to subsurface excessive heating and damage;
(iii) true sliding at the interface leading to excessive heating and thus
creating the conditions favourable for chemical degradation
(polymers);
(iv) interfacial shear creating transferred films;
(v) true sliding at the interface due to the propagation of Schallamach
waves (elastomers).

Figure 2.5

2.7. Friction under Complex motion conditions arise when, for instance, linear sliding is
complex motion combined with the rotation of the contact area about its centre (Fig. 2.6).
conditions Under such conditions, the frictional force in the direction of linear motion