COMSOL 6.3 - Friction

By adding the subnode to a Contact or Interior Contact node, you can model static and dynamic friction in 3D, 2D, and 2D axisymmetry.

The selection in the node is the same as that of its parent or node. There can only be one node under a specific node.

In the case that the sliding velocity is known, you can replace the node by Slip Velocity, which provides a more efficient model, since there is no need to solve for the friction forces and slip direction. The and nodes are mutually exclusive.

If and Adhesion are present under the same node, the friction settings will be ignored at the locations where the adhesion criterion is fulfilled.

	This section is only present when Friction is used as a subnode to Contact. For Interior Contact, the coordinate system is selected in the parent node.

The friction forces are defined as boundary tractions with respect to the selected coordinate system. The selection is limited to boundary systems.

Make sure that the tangents of the selected boundary system are well defined on all destination boundaries.

Select a — , , or . The option is only relevant for time-dependent studies, since the value of the friction coefficient depends on the slip velocity.

For friction, enter a μ.

For friction, enter a μstat, a μdyn and a αdcf.

The effective friction coefficient is

where vslip is the slip velocity.

For both Coulomb type options, you can modify the rules for sliding by providing minimum and maximum shear tractions.

Enter a Tcohe to set a tangential traction that must be overcome before sliding can occur. The use of cohesion will give an offset to the friction force under sliding conditions.

Independently, you can enter a Tt,max. When the tangential traction exceeds this value, slip will occur, independent of the normal pressure. The default expression is Inf, indicating that no limit on the tangential traction is active.

The tangential friction force during sliding, Tt,crit, can be written as

Here, Tn is the contact pressure. If the tangential force is less than this value, there is no sliding.

When is set to , enter an expression to define Tt,crit. This expression can depend on any quantity. However, the implementation of the friction model is only correct if Tt,crit does not depend on the current friction force.

The settings in this section are used to control the penalty factor used by the friction model. The available settings depend on the chosen contact method in the parent or node. When the method is used in the parent node, the same penalty factor as in the parent contact node is used for friction. Hence, this section is not available.

When the or method is used in the parent or node, select the — (default); ; ; ; or .

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and provide predefined values for the penalty factor multiplier ft.

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For , enter a ft. The default value is 1, which corresponds to the option.

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For , the friction force penalty factor pt = pn/3. This option is not available for the method if the penalty factor control is used in the parent node.

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For , enter a pt. The default value differs depending on the type of parent node:
: (<phys>.<contact_tag>.E_char/<phys>.hmin_dst)/3.
: (<phys>.<contact_tag>.E_char/<phys>.<contact_tag>.hmin)/3.

When the or method is used in the parent node, select the — , , , , or .

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is available when is , and is then the default. When selected, the friction force penalty factor pt = pn/3

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is available when is in the parent or node and for the , for which is the default. It provides the same settings as described in the Contact Pressure Penalty Factor.

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is available when is in the parent or node and provides a predefined value for the penalty factor multiplier ft.

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For , enter a ft. The default value is 1. When is in the parent or node and for the , additional settings are available. These are the same as described for the Contact Pressure Penalty Factor section.

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For , enter a pt. The default value depends on the selected in the parent node.

If is or in the parent or node, enter values or expressions for the components of the initial force acting on the destination surface as Tt.

To determine whether friction effects are active when starting the solution or not, select the — or .

	The Previous contact state option is only available when Friction is used as a subnode to Contact.

For enter values or expressions for the xm, old. These serve as initial values to compute the tangential slip. The default value is (X, Y, Z) and indicates that the contacting boundaries are perfectly coincident in the initial state. The mapped source coordinates are defined as the location on the source boundary where it is hit by a certain point on the destination boundary.

	The entries in this section should be given as vector components in the material frame.

To display this section, click the button (

) and select in the dialog.

	When the parent node is Interior Contact, the only setting in this section is Compute frictional dissipation .

For numerical reasons, the gap value will not be exactly zero even when the boundaries are in contact. A certain small positive value of the gap must thus be used to determine that the boundaries touch each other, so that friction forces can be introduced. Select a — or . For , enter an absolute value for the Δfriction, which is the gap when the friction becomes active.

Select to compute and store to the energy dissipated by friction. This adds one extra dependent variable and an extra distributed ODE.