Friction stress and normal stress in bulk metal-forming processes

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Abstract

When analysing pressure distributions in bulk metal-forming processes, it is conventional to adopt either Amonton's friction law, τ = μp, or the law of constant friction stress. Neither of these two laws however, is applicable universally. In this paper a general friction model including both laws is presented. Using a slip-line analysis of the plastic deformation of surface asperities, the influence on friction stress of normal pressure, friction factor and surface roughness (defined by asperity slope) is expressed analytically.

Examples of applications of the friction model when analysing bulk metal-forming processes are given: the tool/workpiece interface pressure and friction stress distributions are estimated for simple upsetting, forward bar extrusion and plate rolling. The calculated pressure distibutions show good accordance with those measured experimentally.

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