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.
For more information on how to specify friction in a contact analysis, see Choosing a Friction Model in the Contact Technology Guide.
Isotropic friction is applicable to 2D and 3D 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 Variables 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.
Note that if the coefficient of friction is defined as a function of temperature, the program always uses the contact surface temperature as the primary variable (not the average temperature from the contact and target surfaces).
The orthotropic friction model uses two different coefficients of friction in two principal directions (see Frictional Model in the Theory Reference for details). It is applicable only to 3D contact and is available for current-technology contact elements.
Issue the TB,FRIC command with
TBOPT = ORTHO, EORTHO, or FORTHO 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 Variables for more information about the interpolation scheme used for field-dependent material properties defined using TBFIELD.
For TBOPT = ORTHO, the friction
coordinate system is always attached to the contact element and rotates with the contact
element.
For TBOPT = FORTHO, the friction
coordinate system is essentially fixed in space and does not rotate as the contact
element rotates.
The option TBOPT = EORTHO is similar
to TBOPT = ORTHO, except when the frictional coefficients are
defined as a function of sliding distance or sliding velocity. The difference occurs in
the way the coefficients are interpolated. For TBOPT = ORTHO,
the friction coefficient in each direction is a function of sliding distance or velocity
in that direction only. For TBOPT = EORTHO, the friction
coefficient in each direction depends upon the magnitude of total sliding or total
velocity, thus causing sliding in one direction to affect the friction coefficients in
both directions.
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
Note that if the coefficient of friction is defined as a function of temperature, the program always uses the contact surface temperature as the primary variable (not the average temperature from the contact and target surfaces).
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 !* LOAD STEP 1 ... 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 !* LOAD STEP 2 ... TIME,2 SOLVE
As an alternative to the program-supplied friction models, you
can define your own friction model with the user-programmable friction
subroutine, USERFRIC. The frictional stresses
can be defined as a function of variables such as slip increments,
sliding rate, temperature, and other arguments passed into the subroutine.
You can specify a number of properties or constants associated with
your friction model, and you can introduce extra solution-dependent
state variables that can be updated and used within the subroutine.
User-defined friction is applicable to 2D and 3D contact elements.
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; NPTS = 2 TBDATA,1,PROP1,PROP2 ! Define friction properties
Field variables specified with the TBFIELD command are not available for TB,FRIC,,,,USER.
For detailed information on using the USERFRIC subroutine, see Writing Your Own Friction Law (USERFRIC) in the Contact Technology Guide.