Friction
By adding the Friction subnode to a Contact node, you can model static and dynamic friction.
The selection in the Friction node is the same as that of its parent Contact node. There can only be one Friction node under a specific Contact node.
In the case that the sliding velocity is known, you can replace the Friction 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 Friction and Slip Velocity nodes are mutually exclusive.
If Friction and Adhesion are present under the same Contact node, the friction settings will be ignored at the locations where the adhesion criterion is fulfilled.
Coordinate System Selection
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.
Friction Parameters
Select a Friction modelCoulomb, Exponential dynamic Coulomb, or User defined. The Exponential dynamic Coulomb option is only relevant for time dependent studies, since the value of the friction coefficient depends on the slip velocity.
For Coulomb friction, enter a Friction coefficient μ.
For Exponential dynamic Coulomb friction, enter a Static friction coefficient μstat, a Dynamic friction coefficient μdyn and a Friction decay coefficient 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 Cohesion sliding resistance 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 Maximum tangential traction 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 Friction model is set to User defined, 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.
Penalty Factor
The settings in this section are used to control the penalty factor used by the friction model. The settings available depend on the chosen Formulation in the parent Contact node.
Settings for the Penalty and the Penalty, Dynamic Methods
When Formulation is Penalty or Penalty, dynamic in the parent Contact node, select the Penalty factor control Automatic (default); Automatic, soft; Manual Tuning; From Parent; or User defined.
Automatic and Automatic, soft provide predefined values for the penalty factor multiplier ft.
For Manual Tuning, enter a Penalty factor multiplier ft. The default value is 1 which corresponds to the Automatic option.
For From Parent, the friction force penalty factor pt = pn/3.
For User defined, enter a Friction force penalty factor pt. The default value is (<phys>.<contact_tag>.E_char/<phys>.hmin_dst)/3.
Settings for the Augmented Lagrangian Methods
When Formulation is Augmented Lagrangian or Augmented Lagrangian, dynamic in the parent Contact node, select the Penalty factor control From Parent, Preset, Automatic, Manual Tuning, or User defined.
From Parent is available when Formulation is Augmented Lagrangian, and is then the default. When selected, the friction force penalty factor pt = pn/3
Preset is available when Solution method is Segregated in the parent Contact node and for the Augmented Lagrangian, dynamic, for which is the default. It provides the same settings as described in the Contact pressure Penalty Factor.
Automatic is available when Solution method is Fully coupled in the parent Contact node and provides a predefined value for the penalty factor multiplier ft.
For Manual Tuning, enter a Penalty factor multiplier ft. The default value is 1. When Solution method is Segregated in the parent Contact node and for the Augmented Lagrangian, dynamic, additional settings are available. These are the same as described for the Contact pressure penalty factor section.
For User defined, enter a Friction force penalty factor pt. The default value depends on the selected Solution method in the parent Contact node.
Initial Value
If Formulation is Augmented Lagrangian or Augmented Lagrangian, dynamic in the parent Contact node, enter values or expressions for the components of the initial force acting on the destination surface as Friction force Tt.
To determine whether friction effects are active when starting the solution or not, select the Previous contact stateNot in contact or In contact.
For In contact enter values or expressions for the Previous mapped source coordinates 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.
Advanced
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.
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 Friction detectionAutomatic or Manual. For Manual, enter an absolute value for the Friction detection tolerance Δfriction, which is the gap when the friction becomes active.
Select Compute frictional dissipation to compute and store to the energy dissipated by friction. This adds one extra dependent variable and an extra distributed ODE.
You can also request that the total accumulated slip distance it stored by selecting the Store accumulated slip check box.
Including Friction, and The Friction Node in the Structural Mechanics Modeling chapter.
Tangential Contact with Friction in the Structural Mechanics Theory chapter.
Location in User Interface
Context Menus
Ribbon
Physics tab with Contact selected in the Model Builder tree: