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LectureNotes_FRICTION - MATERIALS CIVL2110 FRICTION...

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MATERIALS CIVL2110 FRICTION – fundamentals and its relevance to soil and rock Introduction Friction is important in many engineering applications particularly in mechanical engineering where friction between moving parts, and the wear that this causes, are undesirable. The branch of engineering science that deals with frictional problems is known as Tribology. In Civil Engineering friction is important in many areas, such as the resistance of fluids to flow in pipes and rivers, the interaction between structures and the ground, and generally in the behaviour of soils and rocks. In these lectures the basics of friction between solid bodies will be introduced and this will be used to explore the nature of friction between soil particles and rock masses. Laws of Friction The physical laws governing friction between solid bodies in contact have been understood for many centuries. These are generally expressed as: The friction force is proportional to the normal load φ μ tan N F = = In materials science it is normal to discuss friction in terms of the coefficient of friction, μ . In soil and rock mechanics the friction angle, φ is used. The friction force is independent of the apparent area of contact The friction force is independent of velocity These laws are based on empirical observation. For constant environmental conditions the first two laws are obeyed by most materials (polymers are a notable exception). The third law breaks down at very high velocities. It is also found that the force required to initiate sliding is greater than when sliding occurs. This leads to distinction between the static coefficient of friction ( μ s ) and the dynamic coefficient of sliding ( μ d ). To explain these observations it is necessary to consider the interactions occurring at the points of contact.
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Physical Origins of Friction Our current knowledge of the physics of friction is based on the work of Bowden and Tabor, who proposed what is known as the adhesion model of friction in the 1960’s. Research suggests that the real contact area between two bodies in contact is very much smaller than the superficial area. As most surfaces are rough on an atomic scale, contact can only occur occasionally where the asperities allow it. The source of the frictional resistance is provided by the welding together of molecules at the true points of contact. By assuming that the true contact area A c is proportional to the normal force N, Bowden and Tabor showed that the conventional frictional law, F = μ N can be derived. They argued that the initial contact area is infinitesimal, and that there is a limiting normal stress σ c at any asperity. As the normal load is applied asperities will be flattened until the load is spread over an area A c such that: N A c c = σ The shear force required to break the contact is simply given by the shear strength of the bond τ c times the contact area F A c c = τ so that F N c c = τ σ
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