# *FRICTION

 Specify a friction model. This option is used to introduce friction properties into a mechanical surface interaction model governing the interaction of contact surfaces, a contact pair, or connector elements. It must be used in conjunction with the SURFACE INTERACTION option, the CONNECTOR FRICTION option, or in an Abaqus/Standard analysis with the CHANGE FRICTION, the GAP, the INTERFACE, or the ITS options.
 Related Topics *CHANGE FRICTION *CONNECTOR FRICTION *GAP *INTERFACE *ITS *SURFACE INTERACTION In Other Guides Connector behavior About mechanical contact properties Frictional behavior FRIC FRIC_COEF VFRIC VFRIC_COEF VFRICTION

ProductsAbaqus/StandardAbaqus/ExplicitAbaqus/CAE

TypeModel or history data in Abaqus/Standard; History data in Abaqus/Explicit

LevelPartPart instanceAssemblyModel in Abaqus/Standard; Step in Abaqus/Explicit

Abaqus/CAEInteraction module

## Optional, mutually exclusive parameters

ELASTIC SLIP

This parameter applies only to Abaqus/Standard analyses.

In a steady-state transport analysis set this parameter equal to the absolute magnitude of the allowable elastic slip velocity ($γi˙$) to be used in the stiffness method for sticking friction. In all other analysis procedures set this parameter equal to the absolute magnitude of the allowable elastic slip ($γi$) to be used in the stiffness method for sticking friction. If this parameter is omitted, the elastic slip (or elastic slip velocity) is defined by the SLIP TOLERANCE value.

LAGRANGE

This parameter applies only to Abaqus/Standard analyses and cannot be used when friction is defined for connector elements.

Include this parameter to choose the Lagrange multiplier formulation for friction.

ROUGH

This parameter cannot be used when friction is defined for connector elements.

Include this parameter to specify completely rough (no slipping) friction.

SLIP TOLERANCE

This parameter applies only to Abaqus/Standard analyses.

Set this parameter equal to the value of $Ff$ (defined as the ratio of allowable maximum elastic slip velocity to angular velocity times the diameter of the spinning body in a steady-state transport analysis or as the ratio of allowable maximum elastic slip to characteristic contact surface face dimension in all other analysis procedures). The default is SLIP TOLERANCE=.005.

When friction is defined for connector elements, $Ff$ is defined (when possible) as the ratio of allowable maximum elastic slip to a characteristic element dimension in the model. In this case the default is SLIP TOLERANCE=.0001.

USER

This parameter cannot be used when friction is defined for connector elements.

In an Abaqus/Standard analysis, set USER=FRIC (default) if the friction model is to be defined in user subroutine FRIC. Set USER=COEFFICIENT if the friction coefficient is to be defined in user subroutine FRIC_COEF.

In an Abaqus/Explicit analysis, set USER=FRIC (default) if the friction model is to be defined in user subroutine VFRIC. Set USER=FRICTION if the friction model is to be defined in user subroutine VFRICTION. VFRIC is applicable to contact pairs, whereas VFRICTION is applicable to general contact. Set USER=COEFFICIENT if the friction coefficient is to be defined in user subroutine VFRIC_COEF. VFRIC_COEF can be used only with general contact.

## Optional parameters

ANISOTROPIC BEHAVIOR

Include this parameter if anisotropic friction is being defined. This parameter cannot be used when friction is defined for connector elements.

Set ANISOTROPIC BEHAVIOR=LEGACY to specify anisotropic friction with directional preference associated with a contact orientation. This parameter value applies only to Abaqus/Standard analyses.

Set ANISOTROPIC BEHAVIOR=SURFACE PROPERTY to specify anisotropic friction with directional preference as a surface property. This parameter value applies only to Abaqus/Explicit analyses.

DEPENDENCIES

Set this parameter equal to the number of field variable dependencies included in the definition of the friction coefficient in addition to slip rate, contact pressure, and temperature. If this parameter is omitted, it is assumed that the friction coefficients have no dependencies or depend only on slip rate, contact pressure, and temperature. See Material data definition for more information.

DEPVAR

This parameter is valid only if the USER parameter is included.

Set DEPVAR equal to the number of state-dependent variables required for user subroutine FRIC in an Abaqus/Standard analysis or for user subroutines VFRIC and VFRICTION in an Abaqus/Explicit analysis. The default is DEPVAR=0.

EXPONENTIAL DECAY

Include this parameter to specify separate static and kinetic friction coefficients with a smooth transition zone defined by an exponential curve.

The ANISOTROPIC BEHAVIOR and TAUMAX parameters cannot be used with this parameter.

NOMINAL

This parameter applies only to Abaqus/Explicit analyses and is meaningful only when ANISOTROPIC BEHAVIOR=SURFACE PROPERTY.

Set NOMINAL=AVERAGE (default) to specify an average friction coefficient on the data lines.

Set NOMINAL=MINIMUM to specify a minimum friction coefficient on the data lines.

Set NOMINAL=MAXIMUM to specify a maximum friction coefficient on the data lines.

PROPERTIES

This parameter is valid only if the USER parameter is included.

Set this parameter equal to the number of property values needed as data to define the friction model in user subroutine FRIC and FRIC_COEF in an Abaqus/Standard analysis or in user subroutines VFRIC, VFRIC_COEF, and VFRICTION in an Abaqus/Explicit analysis. The default is PROPERTIES=0.

SHEAR TRACTION SLOPE

This parameter applies only to Abaqus/Explicit analyses.

Set this parameter equal to the slope of the curve that defines the shear traction as a function of the elastic slip between the two surfaces. If this parameter is omitted or frictional forces are not present, shear softening will not be activated. This parameter cannot be used in conjunction with user subroutines VFRIC, VFRIC_COEF, and VFRICTION.

TAUMAX

Set this parameter equal to the equivalent shear stress limit; that is, the maximum achievable value of the equivalent shear stress. The value given must be greater than zero.

TEST DATA

This parameter is valid only if the EXPONENTIAL DECAY parameter is used.

Include this parameter if the exponential decay coefficient, $dc$, is to be computed by Abaqus. If this parameter is omitted, the decay coefficient must be given directly on the data line.

WEIGHT

This parameter is valid only if ANISOTROPIC BEHAVIOR=SURFACE PROPERTY is used.

Set WEIGHT=BALANCED to specify a balanced weighting method for combining surface-based directional preferences for the anisotropic friction model.

Set WEIGHT=PROPORTIONAL to specify an epsilon-proportional weighting method for combining surface-based directional preferences for the anisotropic friction model.

Set WEIGHT=DOMINANT to specify the maximum-epsilon-dominant weighting method for combining surface-based directional preferences for the anisotropic friction model.

## Data lines to define the coefficient of friction if the USER, ROUGH, EXPONENTIAL DECAY, and ANISOTROPIC BEHAVIOR parameters are omitted

First line
1. Friction coefficient, $μ$.

2. Slip rate, $γ˙e⁢q$. If this value is omitted, the friction coefficient is assumed to be independent of the slip rate.

3. Contact pressure, p. If this value is omitted, the friction coefficient is assumed to be independent of the contact pressure.

4. Average temperature at the contact point, $θ¯$, between the two contact surfaces. If this value is omitted, the friction coefficient is assumed to be independent of the surface temperature.

5. Average value of the first field variable, $f¯1$.

6. Average value of the second field variable, $f¯2$.

7. Etc., up to four field variables.

Subsequent lines (only needed if the DEPENDENCIES parameter has a value greater than four)
1. Average value of the fifth field variable, $f¯5$.

2. Etc., up to eight field variables per line.

Repeat this set of data lines as often as necessary to define the friction coefficient as a function of contact pressure, slip rate, average surface temperature, and other predefined field variables.

## Data lines to define the coefficient of friction if the ANISOTROPIC BEHAVIOR=LEGACY parameter is used and the USER, ROUGH, and EXPONENTIAL DECAY parameters are omitted

First line
1. Friction coefficient in the first slip direction, $μ1$.

2. Friction coefficient in the second slip direction, $μ2$.

3. Slip rate, $γ˙e⁢q$. If this value is omitted, the friction coefficient is assumed to be independent of the slip rate.

4. Contact pressure, p. If this value is omitted, the friction coefficient is assumed to be independent of the contact pressure.

5. Average temperature at the contact point, $θ¯$, between the two contact surfaces. If this value is omitted, the friction coefficient is assumed to be independent of the temperature.

6. Average value of the first field variable, $f¯1$.

7. Average value of the second field variable, $f¯2$.

8. Etc., up to three field variables.

Subsequent lines (only needed if the DEPENDENCIES parameter has a value greater than three)
1. Average value of the fourth field variable, $f¯4$.

2. Etc., up to eight field variables per line.

Repeat this set of data lines as often as necessary to define the friction coefficient as a function of contact pressure, slip rate, average surface temperature, and other predefined field variables.

## Data lines to define the coefficient of friction if the ANISOTROPIC BEHAVIOR=SURFACE PROPERTY parameter is used and the USER, ROUGH, and EXPONENTIAL DECAY parameters are omitted

First line
1. Average ($μnom$), minimum ($μmin⁡$), or maximum ($μmax⁡$) friction coefficient, depending on the parameter used at the keyword level.

2. Slip rate, $γ˙e⁢q$. If this value is omitted, the average friction coefficient is assumed to be independent of the slip rate.

3. Contact pressure, p. If this value is omitted, the average friction coefficient is assumed to be independent of the contact pressure.

4. Average temperature at the contact point, $θ¯$, between the two contact surfaces. If this value is omitted, the average friction coefficient is assumed to be independent of the temperature.

5. Average value of the first field variable, $f¯1$.

6. Average value of the second field variable, $f¯2$.

7. Etc., up to three field variables.

Subsequent lines (only needed if the DEPENDENCIES parameter has a value greater than three)
1. Average value of the fourth field variable, $f¯4$.

2. Etc., up to eight field variables per line.

Repeat this set of data lines as often as necessary to define the average friction coefficient as a function of contact pressure, slip rate, average surface temperature, and other predefined field variables.

## Data line to define the static and kinetic friction coefficients if the EXPONENTIAL DECAY parameter is used and the decay coefficient is specified directly

First (and only) line
1. Static friction coefficient, $μs$.

2. Kinetic friction coefficient, $μk$.

3. Decay coefficient, $dc$. The default value is zero.

## Data lines if the EXPONENTIAL DECAY and TEST DATA parameters are used

First line
1. Friction coefficient for the first data point, $μ1$. This value corresponds to the static friction coefficient.

Second line
1. Friction coefficient for the second data point, $μ2$. This value corresponds to the dynamic friction coefficient measured at the reference slip rate, $γ˙2$.

2. Slip rate of the second data point, $γ˙2$. This value corresponds to the reference slip rate used to measure the dynamic friction coefficient.

Third line (optional)
1. Kinetic friction coefficient, $μ∞$. This value corresponds to the asymptotic value of the friction coefficient at infinite slip rate, $γ˙∞$. If this data line is omitted, Abaqus/Standard automatically calculates $μ∞$ such that $(μ2-μ∞)/(μ1-μ∞)=0.05$.

## Data lines to define the user subroutine properties if the PROPERTIES parameter is used

First line
1. Enter the values of the friction properties, eight per line.

Repeat this data line as often as necessary to completely define all of the properties needed by user subroutines FRIC, FRIC_COEF, VFRIC, VFRIC_COEF, and VFRICTION as indicated by the value of PROPERTIES.