Talk Title: Modelling of fretting wear: Limits and perspectives of the friction energy concept
Fretting is a surface degradation process invariably observed when two bodies in contact experience small oscillatory movements. Wear and fatigue cracking mechanisms may interact, depending on the loading conditions. When larger sliding amplitudes are imposed, wear processes induced by debris formation and ejections prevail. Considered as a plague for modern industry, fretting is encountered in all quasi-static loadings subjected to vibrations and thus concern many industries (aeronautics, train, nucleation plant, electrical connectors, etc). Prediction of fretting wear rate is therefore a key issue. However it is very difficult due to the presence of the debris layer maintained within the interface and by the fact that only a small part of the fretted interface is exposed to the ambient air inducing a complex and differential oxidation process. One strategy to predict the wear volume extension consists in applying the friction energy wear approach where the wear volume is expressed as a function of the accumulated friction work dissipated in the interface multiplied by a so called energy wear factor. This approach appears reasonably relevant as long as a single wear process is operating in the contact. However, many investigations demonstrate that depending on the loading conditions, contact configurations and contact size, various wear process (i.e. abrasive wear versus adhesive wear) may be activated inducing significant fluctuations of the energy wear rate. To formalize the evolution of the energy wear rate, it is necessary to better expressed the evolution of each wear possess and their relative distribution within the interface. Various strategies, including the so called “p.v” approach are considered to better formalize the transition from adhesive wear to abrasive wear. This approach is discussed taking into account the necessity to consider the so called “contact oxygenation concept” (i.e. distribution of the partial pressure of dioxygen available in the interface) which, by monitoring either metal oxidation or adhesive metal transfers, controls the distribution of wear phenomena in the fretting scar. Alternatively, a crucial aspect in tribology engineering concerns the prediction of the wear depth extension. A review of local approaches including Archard and friction energy density concepts is discussed. Comparisons with experiments underline the necessity to consider the presence of the debris layer (i.e. third body) and its evolution during the surface wear extension. The typical example of high temperature debris glaze layer structures, whose quasi pure plastic response allows a complete accommodation of the friction energy and a wear rate converging to zero is investigated. Finally the progress toward multi-physical wear simulation taking into account mechanical and rheological properties of debris layer but also tribochemical aspects is discussed.
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