Friction
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Friction Andrea. Ghiotti Department of Industrial Engineering, University of Padova, Padova, Italy
Synonyms Attrition
Definition Friction is defined as the resistance to relative motion exhibited by the contact surfaces of two bodies under the application of a normal load. The relative motion can be either sliding, rolling, or flowing over another body with which it is in contact. Friction is an energy-dissipating process, in which the kinetic energy of the moving body is transformed into heat, with consequent temperature variation of the contact surfaces.
Theory and Application Types of Friction The following main type of friction can be distinguished: – Static friction pertains two contacting bodies that do not present relative motion under the
influence of a resultant force. It is determined by the interlocking of the irregularities of two surfaces that prevent any relative motion up until some limit where motion occurs. – Kinetic friction is defined as the force that resists to motion when two surfaces are moving with a relative velocity; it is, therefore, equal and opposite to the force required to maintain the relative sliding of the surfaces. – Rolling friction is the force resisting the motion when a body (such as a ball, wheel, roll) rolls on a surface. At microscale, it is caused by the interference of small indentations formed as one surface rolls over another. The phenomena that affect rolling friction are mainly due to nonelastic effects that involve both hysteresis losses and plastic deformation of the contact surfaces. – Flowing friction determine an energy loss when a liquid flow in a cavity due to viscous effects generated by the surfaces that are in contact with the liquid. Friction has a significant impact in many metalforming processes, influencing the material flow in the dies cavities and consequently the deformation and the forces required in the processes. Furthermore, it may generate large amount of heat at the interface between dies and the component, with relevant impact on the tools wear phenomena and the microstructure of the formed components.
# CIRP 2016 The International Academy for Production Engineering et al. (eds.), CIRP Encyclopedia of Production Engineering, DOI 10.1007/978-3-642-35950-7_16825-1
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Friction
Adhesion Theory of Friction The theory was explained by Bowden and Tabor (Bowden and Tabor 1950) on the basis of the observation that two clean surfaces that are loaded against each other, in dry conditions, are in contact only for a fraction of their apparent area. Regardless of the surface roughness, at a microscopic level (see Fig. 1), the load is supported only by the contacting asperities, whose sum represent the real area of contact Ar. If the level of local pressure is kept lower than the material yield strength, the local deformation is elastic, while as the pressure increases and overcomes the material yield stress, the contact asperities deform plastically with an increase of the contact area until the real area of contact Ar is sufficient to support the load. Event
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