Coulomb Friction Law

Description

This frictional contact law implements simple Coulomb friction law where

      [math]\displaystyle{ S_{slide} = \mu_d N \quad{\rm if}\quad S_{stick}\gt \mu_s N }[/math]

and µ_d and µ_s are the dynamic and static coefficients of friction. In other words, the sliding will begin when it overcomes the static frictional force, but thereafter will slide with the dynamic coefficient of friction. Note that in dynamic simulations, a static coefficient of friction often has little effect. One solution is to implement a smooth transition from µ_s to µ_d, which can be done by switching to the Adhesive Friction Law.

If the friction coefficient is entered as a negative number, then surfaces stick whenever they are in contact but are allowed to move freely when separated. If the static friction coefficient is less than the dynamic friction coefficient, then it is ignored.

Properties

The properties for this law are:

Property	Description	Units	Default
coeff	The dynamic coefficient of friction	none	0
coeffStatic	The static coefficient of friction. If this optional static coefficient of friction is changed to a positive number, it must be greater than the dynamic coefficient or friction.	none	-1
displacementOnly	Set to: 0 to detect contact when COD<0 and stress<0; 1 to detect contact whenever COD<0 regardless of stress; <0 to detect contact when COD<0 and stress<-displacementOnly (in pressure units)	none	0
Dc	<0 to find separation assuming perfect interfacial contact, ≥0 to use imperfect interface methods to find separation.	pressure/length units	none

Comments on Parameters

1. The displacementOnly Parameter: this parameter should almost always be zero. In simulations that are known to always be in contact, displacementOnly=1 sometimes helps the simulation results. The displacementOnly<0 option to require some tensile stress before loss of contact rarely helps. For one thing, it is not clear how to calculate frictional sliding force when interface is in tension. The code treats tensile stress as a frictionless interface, but will enforce sticking in the normal direction.
2. The Dc Parameter: Displacement between two surfaces transitions from zero stiffness when separated to infinite stiffness when in contact. The Dc parameter changes the stiffness when in contact to be treated like a Linear Imperfect Interface and uses interface methods to re-evaluate interfacial separation. This parameter can sometimes help, but also can have a very large effect. Its use adds another parameter with little guidance for selecting its' value.