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The two regimes of dry friction are 'static friction' ("stiction") between non-moving surfaces, and kinetic friction (sometimes called sliding friction or dynamic friction) between moving surfaces. Coulomb friction, named after Charles-Augustin de Coulomb , is an approximate model used to calculate the force of dry friction.
φ' = the effective stress friction angle, or the 'angle of internal friction' after Coulomb friction. The coefficient of friction is equal to tan(φ'). Different values of friction angle can be defined, including the peak friction angle, φ' p, the critical state friction angle, φ' cv, or residual friction angle, φ' r.
It does assume Coulomb's friction law, which more or less requires (scrupulously) clean surfaces. This theory is for massive bodies such as the railway wheel-rail contact. With respect to road-tire interaction, an important contribution concerns the so-called magic tire formula by Hans Pacejka. [7] In the 1970s, many numerical models were devised.
where is a parameter, is the value of when the plastic strain is zero (also called the initial cohesion yield stress), is the angle made by the yield surface in the Rendulic plane at high values of (this angle is also called the dilation angle), and (,) is an appropriate function that is also smooth in the deviatoric stress plane.
Since the Painlevé paradoxes are based on a mechanical model of Coulomb friction, where the calculated friction force can have multiple values when the contact point has no tangential velocity, this is a simplified model of contact. It does, nevertheless, encapsulate the main dynamical effects of friction, such as sticking and slipping zones.
For a given experiment and at normal stresses (σ n) below about 2000 bars (200 MPa) the shear stress increases approximately linearly with the normal stress (τ = 0.85 σ n, where τ and σ n is in units of MPa) and is highly dependent on rock type and the character (roughness) of the surfaces, see Mohr-Coulomb friction law.
The critical state friction angle is not nearly as variable as the peak friction angle and hence it can be relied upon with confidence. [3] [4] [11] Not recognizing the significance of dilatancy, Coulomb proposed that the shear strength of soil may be expressed as a combination of adhesion and friction components: [11]
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law [1] of physics that calculates the amount of force between two electrically charged particles at rest. This electric force is conventionally called the electrostatic force or Coulomb force . [ 2 ]