## 3.21. Contact Friction

Contact friction (TB,FRIC) is a material property used with current-technology contact elements. It can be specified either through the coefficient of friction (MU) for isotropic or orthotropic friction models or as user defined friction properties.

### 3.21.1. Isotropic Friction

Isotropic friction is applicable to 2-D and 3-D contact and is available for all contact elements. Use the TB,FRIC command with TBOPT = ISO to define isotropic friction, and specify the coefficient of friction MU on the TBDATA command. This is the recommended method for defining isotropic friction.

To define a coefficient of friction that is dependent on temperature, time, normal pressure, sliding distance, or sliding relative velocity, use the TBFIELD command. Suitable combinations of up to two fields can be used to define dependency, for example, temperature and sliding distance as shown below:

```TB,FRIC,1,,,ISO     ! Activate isotropic friction model
TBFIELD,TEMP,100.0   ! Define first value of temperature
TBFIELD,SLDI,0.1     ! Define first value of sliding distance
TBDATA,1,MU          ! Define coefficient of friction
TBFIELD,SLDI,0.5     ! Define second value of sliding distance
TBDATA,1,MU          ! Define coefficient of friction
TBFIELD,TEMP,200.0   ! Define second value of temperature
TBFIELD,SLDI,0.2     ! Define first value of sliding distance
TBDATA,1,MU          ! Define coefficient of friction
TBFIELD,SLDI,0.7     ! Define second value of sliding distance
TBDATA,1,MU          ! Define coefficient of friction
```

See Understanding Field Variable Interpolation for more information on the interpolation scheme used for field-dependent material properties defined using TBFIELD.

To define a coefficient of friction that is dependent on temperature only, use the TBTEMP command as shown below:

```TB,FRIC,1,2,,ISO     ! Activate isotropic friction model
TBTEMP,100.0         ! Define first temperature
TBDATA,1,MU          ! Define coefficient of friction at temp 100.0
TBTEMP,200.0         ! Define second temperature
TBDATA,1,MU          ! Define coefficient of friction at temp 200.0
```

Alternatively, you can use MU on the MP command to specify the isotropic friction. Use the MPTEMP command to define MU as a function of temperature. See Linear Material Properties for details.

### 3.21.2. Orthotropic Friction

The orthotropic friction model uses two different coefficients of friction in two principal directions (see Frictional Model in the Mechanical APDL Theory Reference for details). It is applicable only to 3-D contact and is available for current-technology contact elements.

Issue the TB,FRIC command with TBOPT = ORTHO to define orthotropic friction, and specify the coefficients of friction, MU1 and MU2, on the TBDATA command.

To define a coefficient of friction that is dependent on temperature, time, normal pressure, sliding distance, or sliding relative velocity, use the TBFIELD command. Suitable combinations of up to two fields can be used to define dependency, for example, sliding relative velocity and normal pressure as shown below:

```TB,FRIC,1,,,ORTHO   ! Activate orthotropic friction model
TBFIELD,SLRV,10.0    ! Define first value of sliding relative velocity
TBFIELD,NPRE,200.0   ! Define first value of normal pressure
TBDATA,1,MU1,MU2     ! Define coefficients of friction
TBFIELD,NPRE,250.0   ! Define second value of normal pressure
TBDATA,1,MU1,MU2     ! Define coefficients of friction
TBFIELD,SLRV,20.0    ! Define second value of sliding relative velocity
TBFIELD,NPRE,150.0   ! Define first value of normal pressure
TBDATA,1,MU1,MU2     ! Define coefficients of friction
TBFIELD,NPRE,300.0   ! Define second value of normal pressure
TBDATA,1,MU1,MU2     ! Define coefficients of friction
```

See Understanding Field Variable Interpolation for more information on the interpolation scheme used for field-dependent material properties defined using TBFIELD.

To define a coefficient of friction that is dependent on temperature only, use the TBTEMP command as shown below:

```TB,FRIC,1,2,,ORTHO	  ! Activate orthotropic friction model
TBTEMP,100.0         ! Define first temperature
TBDATA,1,MU1,MU2     ! Define coefficients of friction at temp 100.0
TBTEMP,200.0         ! Define second temperature
TBDATA,1,MU1,MU2     ! Define coefficients of friction at temp 200.0
```

### 3.21.3. Redefining Friction Between Load Steps

If the friction behavior changes between initial loading and secondary loading (for example, during cyclic loading of seabed pipelines), you can reissue the TB,FRIC command between load steps to define new values for the coefficient of friction. This is true for both temperature-dependent friction (isotropic or orthotropic) defined via the TBTEMP command and field-dependent friction (isotropic or orthotropic) defined via the TBFIELD command. The following example shows the latter case:

```TB,FRIC,1,,,ORTHO	   !Activate orthotropic friction model
TBFIELD,SLDI,0.		!Define initial curve for coefficient of friction
TBDATA,1,0.0,0.0
TBFIELD,SLDI,0.25
TBDATA,1,0.0,1.25
TBFIELD,SLDI,0.5
TBDATA,1,0.0,1.0
TBFIELD,SLDI,20.
TBDATA,1,0.0,1.1

/SOLUTION
...
TIME,1
SOLVE

TB,FRIC,1,,,ORTHO	   !Activate orthotropic friction model
TBFIELD,SLDI,0.		!Define secondary curve for coefficient of friction
TBDATA,1,0.0,20.0
TBFIELD,SLDI,1.1
TBFIELD,SLDI,20.25
TBDATA,1,0.0,0.0
TBFIELD,SLDI,20.5
TBDATA,1,0.0,0.8
TBFIELD,SLDI,21
TBDATA,1,0.0,0.7
TBFIELD,SLDI,35
TBDATA,1,0.0,0.75

...
TIME,2
SOLVE
```

### 3.21.4. User-Defined Friction

You can define your own friction model with the user programmable friction subroutine USERFRIC. (See the Guide to ANSYS User Programmable Features for a detailed description of the USERFRIC subroutine.) User-defined friction is applicable to 2-D and 3-D contact and is available for current-technology contact elements.

The USERFRIC subroutine can only be used with penalty-based tangential contact (KEYOPT(2) = 0, 1, or 3 on the contact element).

To specify user-defined friction, use the TB,FRIC command with TBOPT = USER and specify the friction properties on the TBDATA command, as shown below. Also, use the USERFRIC subroutine to program the friction model.

```TB,FRIC,1,,2,USER     ! Activate user defined friction model
TBDATA,1,PROP1,PROP2  ! Define friction properties
```

Field variables specified with the TBFIELD command are not available for TB,FRIC,,,,USER.