Theory of wear - Measurement, contacts characterization and wear rate

Relative movement between two solid bodies generates two inseparable phenomena: friction, i.e. mechanical resistance to this relative movement (forces, torque) and wear, i.e. loss of material from the antagonistic bodies. Friction has negative consequences (energy dissipated), but also positive ones: it ensures the stability of our footing on the ground, the braking of vehicles, the entrainment of sheet metal in the rolling mill gap, the heating of parts and their welding in the friction or friction stir welding process. On the other hand, wear and tear on rubbing parts and manufacturing tools (plastic forming, mechanical cutting, crimping, riveting, clinching, friction stir welding, molding, extrusion and polymer injection processes) has only negative aspects, and must be minimized. In fact, wear and tear brings manufacturing lines to a standstill, as well as means of transport, heating and energy production... and the need to replace worn components - in short, it has very costly repercussions on human activities. According to a study carried out by CETIM, wear and tear represents a cost of around 4% of France's GNP.

It should be noted, however, that abrasive machining processes (grinding, ultrasonic, lapping, polishing) or certain physico-chemical machining processes (chemical, electroerosion) aim to remove material from a material at the highest possible speed, in order to obtain, at the lowest possible cost, a part with the desired geometry and optimum surface properties (roughness, residual stresses, microstructure). The field of wear modelling is thus in many ways similar to that of machining process modelling, although the practical objectives are quite different.

It should be stressed, however, that contact involves two antagonists, and that it is often desirable to fix wear phenomena, a priori unavoidable, on one of the two parts, the easier and less costly to change (sacrificial wear part, in the same way as a sacrificial anode can be used to counter corrosion). For example, a judicious choice of material for the brake pads of a motor vehicle concentrates wear on the pads, which are easy to change, to maintain brake performance and protect the brake discs, which are integral with the vehicle and more massive. Similarly, in a bearing, it's preferable to concentrate wear on the bush, which is easy to change... This requires knowledge of simple rules for designing the contact. Wear must also be taken into account in the design of machines and manufacturing operations, whose specifications generally include minimum device life. For example, the cost price of a manufactured part can be highly dependent on the service life of the tools used (hot forming of metals by extrusion and forging, machining). This is why it is so important to have models that can predict...