Model fatigue wear

In the model most commonly used for fatigue wear of polymers, the wear rate Is assumed proportional to the volume of a wear particle divided by the number of stress cycles N necessary to produce the wear particle. The number of cycles N Is given by... 

The most comprehensive fatigue wear model (2) proposed In the literature was used to predict wear rates to within 30 percent or less of the experimentally measured values (J). The exponent t was determined from notched cylindrical specimens In reverse bending. The number of contacts and the areas were determined from surface profiles of the polymer and the counterface after steady-state wear was attained. The wear data was obtained from a polymer pin sliding on a rotating cylinder. 

For the polylmides, at the 5 N load, the wear rates were negatively correlated with the cycles to initiate wear. This result is consistent with the fatigue wear model which states that the wear rate is Inversely proportional to the number of cycles to fatigue failure. Comparing the cycles to initiate at the 5 and 10 N load, it is noted that cycles to initiate significantly Increase for the... 

The wear rate-load data for the modified epoxies have load exponents less than one. Several fatigue models for wear predict that the exponent should be greater than one ( ). Explanation for... 

The positive correlation of wear rates with elastic moduli E of the polylmides is consistent with the fatigue model for wear ( ). The reason for this correlation is that the surfaces stresses which are calculated using the Hertzian contact equations are proportional to Higher stresses lead to high wear rates because less stress cycles are required to cause a fatigue failure and produce a wear particle. 

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. 

The four wear mechanisms described in this section all lead to Archard s law, but the physical interpretation of the wear coefficient differs. In the adhesive wear model the wear coefficient expresses the probability that an adhesive junction leads to formation of a wear particle. In the abrasive wear model the wear coefficient depends only on the geometry of the abrasive. The wear coefficient in delamination wear characterizes the critical number of cycles leading to fatigue fracture of microscopic subsurface cracks. Finally, the wear coefficient in oxidative wear includes the growth constant and the critical thickness of a surface oxide film. 

Quahtative simulation is a specific KBS model of physical processes that are not understood well enough to develop a physics-based numeric model. Corrosion, folding, mechanical wear, equipment failure, and fatigue are not easily modeled, but decisions about them can be based on qualitative reasoning. See Refs. 178 and 292. 

For the slloxane modified epoxies and the polylmides, the wear rates correlated positively with the elastic moduli, which is also in agreement with the fatigue model. However, the wear rate-load relationship predicted by fatigue models was not corroborated by the data. Likewise, the effect of load on the cycles to initiate for the polylmides was not consistent with the fatigue model. 

Observation of worn surfaces and wear debris suggest three types of wear mechanisms—adhesion, when PE that adhered to the metal surface is torn off abrasion when bone cement particles get into the bearing and cut grooves in the soft PE and fatigue, where surface features are deformed back and forward until they fall off. Baudriller et al. showed a mechanism for the detachment of a PE flake (Fig. 15.17), but they could only start the FEA modelling of microscopic wear process. 

A thorough understanding of the mechanics of UHMWPE is important for efforts to improve the performance of orthopedic components. Elastic properties, resistance to plastic deformation, stress and strain at failure, fatigue behavior, and wear resistance of UHMWPE are believed to play roles in the life expectancy of an UHMWPE bearing. There exists a fundamental relationship between a material s intrinsic mechanical properties, akin to state variables, and how a structure made of the material will respond under mechanical stimuli. This material-specific fundamental relationship is referred to as a constitutive model. A validated constitutive model is a required input to a finite element (FE)-based simulahon of a structure made of the material in question. 

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