Plastics fatigue wear

Abrasive wear is caused by sharp asperities cutting the plastic fatigue wear is caused by particles of plastic being detached as a result of dynamic stressing on a localised scale adhesive wear is the transfer of plastic to another surface as a result of adhesive forces between the two surfaces. There can also be corrosive wear due to the direct chemical attack on the surface and the term erosive wear is sometimes used for the action of particles in a liquid stream. 

Figure 6.2. Schematic diagram of probable mechanism of plastic fatigue wear (from Briscoe and Evans, 1987) (a) Formation of plastically deformable grooves in series (b) Deformation of the grooves pushed in one direction (c) Sway back to the opposite direction (d) Deterioration of ridges after repeated fluttering (e) Detachment of ridges in the form of band-shaped debris.

The mechanisms by which wear of a plastic occurs when it is in moving contact with another material are complex but the principal factors involved are cutting, fatigue and friction. It is possible to categorise wear mechanisms in various ways and commonly distinction is made between abrasive wear, fatigue wear and adhesive wear. 

Fatigue Wear (Hailing, 1975) Hailing model Wf, = K 4 Fn Wf3 = wear rate j] = line distribution of asperities 7 = constant defining particle size ej = strain to failure in one loading cycle H = hardness of the softer material K = wear coefficient Incorporates the concept of fatigue failure as well as simple plastic deformation failure. 

Wear, scoring, material flow, pitting, fracture, creep, and fatigue cause plastic and metal gears to fail. Continuous lubrication can increase the allowable bending stress by a factor of at least 1.5. However there are plastics (acetals, nylons, fluoropolymers, and others) that operate efficiently with no lubrication. There are plastics with wear resistance and durability of plastic gears makes them exceptionally useful. 

The preliminary impact test results are shown in Figure 4a. The plots show the evolution of impact-induced damage with time (i.e. plastic deformation, fatigue wear and fracture) throughout the test. The height of the surface is recorded with sub-nm precision when the probe is on the surface. 

A probable mechanism of erosion for plastics is illustrated in Fig. 6.2 [Briscoe and Evans, 1987]. Initially, a series of plastically deformed grooves can be formed by the abrasion of particle flows. The subsequent directional or random impacts of particles may push the deformed grooves from side to side. The fatigue limits of the plastics would eventually make the ridges between the grooves detach to form ribbonlike debris. Brittle cracks also occur when the wear tracks interact. 

The abrasive wear of plastics occurs as a result of strong adhesive interaction, fatigue, macroshearing, abrasive action, thermal and thermooxidative interaction, corrosion, cavitation, etc. Fillers are involved in these processes because mineral... 

Under boundary lubrication in conditions of elastic contact the wear rate is defined by the corrosion processes on freshly formed surfaces in the actual contact spots. In conditions of plastic contact the wear rate is composed of two constituents, that is, the low-cycle fatigue in the actual contact spots and solution on freshly formed surfaces. 

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