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 µd and µs 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 µs to µd, 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 | 0 for contact when COD<0 and stress<0, 1 for contact whenever COD<0, <l0 for contact when COD<0 and stress<-displacementOnly (in pressure units) | none | 0 |
| Dc | Imperfect interface parameter for detection of contact in the normal direction. If Dc is specified, the contact law using imperfect interface methods to find separation, otherwise it uses standard contact methods. | pressure/length units | none |
The displacementOnly 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. For on thing, it is not clear how to calculate friction sliding force when interface is in tension. The code treats tensile stress as a frictionless interface, but will enforce sticking in the normal direction.
Displacement between two surfaces transitions from 0 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. It use adds another parameter with little guidance for selecting that parameter.