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Material toughening due to crack surface friction under mixed mode loading is studied. A mechanistic model is developed for a crack with an intergranular fracture surface under mixed Mode I/Mode III loading. The model takes into account several microstructural and material parameters such as grain shape and friction coefficient between grain boundaries. By employing a self-consistent, fracture mechanics approach, the governing integral equation with a singular kernel is obtained. A special numerical technique is used to solve the integral equation. The frictional stress distribution along the crack face is obtained for different grain shapes and frictional coefficients. The results show that, because of crack surface friction, the material toughens significantly. The toughening ratio increases rapidly with the relative magnitude of the Mode III component of the applied load. Moreover, two parameters, the oblique grain boundary angle and the friction coefficient, are identified as the controlling parameters for the toughening behavior. Using an energy-based crack tip fracture criterion, fracture loci in KI - KIII space are constructed to serve as a failure criterion for mixed mode fracture. The model predictions are also compared with experimental measurements. Good agreement is obtained. Potential applications of the model to surface crack growth under rolling contact loading in the presence of lubricating fluids are discussed.
A finite element study was performed on driving forces of short cracks at inclusions in bearing steel exposed to rolling contact load. The inclusions were assumed to be elastic and the steel matrix was elasto-plastic with material parameters from cyclic loading tests. The surface load on the raceway was simulated with a moving Hertzian pressure distribution. An inclusion was situated in the steel below the raceway. Short cracks were allowed to grow from the inclusion between subsequent passages of the contact load. The inclusion had a size of 20?m in diameter. The cracks were oriented 45° to the rolling contact plane. Five inclusion configurations were considered, namely a pore, a manganese sulfide inclusion, a through-cracked alumina inclusion, an alumina inclusion which was uncracked but which could debond from the matrix and finally a titanium nitride inclusion. The driving forces of the cracks were evaluated in terms of energy release rates. The magnitudes of these rates were significantly influenced by inclusion type, crack configuration, crack length, Hertzian loading level and metal plasticity. A ranking list was made of the different inclusions with respect to expected detrimental effect on contact fatigue life.
The 31st Leeds-Lyon Symposium on Tribology was held at Trinity and All Saints College in Leeds under the title "Life Cycle Tribology" from Tuesday 7th September until Friday 10th September 2004. Over the three days of presentations that followed, life cycle tribology was explored across a range of areas including automotive tribology, bearings, bio-degradability and sustainability, bio-tribology, coatings, condition monitoring, contact mechanics, debris effects, elastohydrodynamic lubrication, lubricants, machine systems, nanotribology, rolling contact fatigue, transmissions, tribochemistry and wear and failure. Invited talks in these fields were presented by leading international researchers and practitioners, namely C.J. Hooke, J.A. Williams, R.J.K. Wood, G. Isaac, S.C. Tung, D. Price, I. Sherrington, M. Hadfield, K. Kato, R.I. Taylor, H.P. Evans, R.S. Dwyer-Joyce and H. Rahnejat.
Improving our understanding of friction, lubrication, and fatigue, Modeling and Analytical Methods in Tribology presents a fresh approach to tribology that links advances in applied mathematics with fundamental problems in tribology related to contact elasticity, fracture mechanics, and fluid film lubrication. The authors incorporate the classical tenets of tribology while providing new mathematical solutions that address various shortcomings in existing theories. From contact interactions to contact fatigue life, the book connects traditionally separate areas of tribology research to create a coherent modeling methodology that encompasses asymptotic and numerical techniques. The authors often demonstrate the efficacy of the models by comparing predictions to experimental data. In most cases, they derive equations from first principles. They also rigorously prove problem formulations and derive certain solution properties. Solutions to problems are presented using simple analytical formulas, graphs, and tables. In addition, the end-of-chapter exercises highlight points important for comprehending the material and mastering the appropriate skills. Unlocking the secrets that govern the physics of lubricated and dry contacts, this book helps tribologists on their quest to reduce friction, minimize wear, and extend the operating life of mechanical equipment. It provides a real-world industrial perspective so that readers can attain a practical understanding of the material.