Model Element

Class Name

Contact

Description

Contact defines a 3-D contact force between geometries on two rigid bodies.  Each body is characterized by a set of one or more geometries.  Whenever any geometry on the first body penetrates any geometry on the second body, a contact normal and frictional force are generated.  The normal force tends to repulse motion along the common normal at the contact point.  The frictional force tends to oppose relative sliding velocity at the contact point.  The contact force vanishes when the geometries no longer intersect.

Attribute Summary

Usage

Contact supports four normal force models. You must select one of these and specify its parameters.

Contact also supports some optional parameters that are independent of the choice of the normal force. You may specify any combination of these.

Finally Contact supports two friction models.

Specification of friction at the contact patch is optional. If you want to model friction at the contact patch, you may specify the parameters for one of these models. If you use neither, friction forces are NOT applied at the contact patch.

# Type = “IMPACT”

Contact (type=”IMPACT”, igeom=list, jgeom=list, stiffness=Double, exponent=Double, damping=Double, dmax=Double, optional_attributes)

# Type = “Volume”

Contact (type=”VOLUME”, igeom=list, jgeom=list, i_elastic_modulus=Double, i_layer_depth=Double, j_elastic_modulus=Double, j_layer_depth=Double, exponent=Double, damping=Double, optional_attributes)

# Type = “POISSON”

Contact (type=”POISSON”, igeom=list, jgeom=list, penalty=Double, normal_trans_vel=Double, restitution_coefficient=Double, optional_attributes)

# Type = “USER”

Contact (type=”USER”, igeom=list, jgeom=list, normal_routine=String or Python function pointer, normal_function=userString, optional_attributes)

Normal force type = “IMPACT”

Attributes

Description

type

String “IMPACT”

Specifies that the IMPACT, normal force model is to be used for calculating the contact normal force.

A contact force is computed as a function of the penetration between the intersecting geometries defined for the contact.  In other words, as the geometries intersect, a repulsive force is generated which is based on the type of contact selected. See Comments 2-3 for more information on the IMPACT model.

This attribute is optional. MotionSolve will automatically set type = ”IMPACT” when type is not specified.

igeom

List of Graphics objects.

This is a list of the Graphics objects on the first body to be considered for contact.

Note     All the GRAPHICS entities must belong to the same body.

The attribute igeom is mandatory.

jgeom

List of Graphics objects.

This is a list of the Graphics objects on the second body to be considered for contact.

Note     All the GRAPHICS entities must belong to the same body.

The attribute jgeom is mandatory.

stiffness

Double

Specifies the stiffness parameter of the contact; this is the same parameter as found for the IMPACT function.

A large value for stiffness permits only a small penetration between the two contacting geometries; a small value permits a larger penetration.

Consider using a stiffness value that is as small as possible, but still captures accurate deformation, to improve solver performance. See Comment 6 for more information on how to select a value for stiffness.

The attribute stiffness is mandatory for type=”IMPACT”.

stiffness ≥ 0

damping

Double

Specifies the maximum damping coefficient that is to be used for generating a damping force. See Comment 3 for more details.

A value around 0.1% of the stiffness is a good value to use. A large value for damping will make the contact “sticky”. Conversely, a small value for damping will tend to cause the contact to be “intermittent”.

Also note that the damping force adds to the spring force during penetration, but subtracts from the spring force during separation.

The attribute damping is mandatory for type=”IMPACT”

damping ≥ 0

exponent

Double

The exponent of the force deformation characteristic of the contact interface. For a stiffening spring characteristic, it must be greater than 1.0.

The attribute exponent is mandatory for type=”IMPACT”.

exponent ≥ 1.0

dmax

Double

Specifies the penetration at which full damping is applied.  It must be a positive number.

Note     dmax should be somewhat smaller than the maximum penetration that is expected. If dmax is too small, the damping force is turned on quite rapidly. Conversely, if dmax is too large, the full damping may never be turned on.

The attribute dmax is mandatory for type=”IMPACT”.

dmax > 0.0

Normal force type = “VOLUME”

type

String “VOLUME”

Specifies that the VOLUME normal force model is to be used for calculating the contact normal force.

A contact force is computed as a function of the penetration between the intersecting geometries defined for the contact.  In other words, as the geometries intersect, a repulsive force is generated which is based on the type of contact selected. See Comments 2,5 for more information on the VOLUME model.

igeom

List of Graphics objects.

This is a list of the Graphics objects on the first body to be considered for contact.

Note     All the GRAPHICS entities must belong to the same body.

The attribute igeom is mandatory.

jgeom

List of Graphics objects.

This is a list of the Graphics objects on the second body to be considered for contact.

Note     All the GRAPHICS entities must belong to the same body.

The attribute jgeom is mandatory.

i_elastic_modulus

Double

Specifies the elastic modulus (sometimes also called the P-wave modulus) for the geometries belonging to Body I for the Volume force model.  The value of this elastic modulus can be derived from the Bulk and Shear Modulus of a material. See Comment 5 for an explanation on how this parameter is used to calculate the contact stiffness.

The attribute i_elastic_modulus is mandatory when type=”VOLUME”

i_elastic_modulus > 0

i_layer_depth

Double

Specifies the size of the layer depth for the geometries belonging to body I. See Comment 5 for an explanation on how this parameter is used to calculate the contact stiffness.

The attribute i_layer_depth is mandatory when type=”VOLUME”.

i_layer_depth > 0

j_elastic_modulus

Double

Specifies the elastic modulus (sometimes also called the P-wave modulus) for the geometries belonging to Body J for the Volume force model.  The value of this elastic modulus can be derived from the Bulk and Shear Modulus of a material. See Comment 5 for an explanation on how this parameter is used to calculate the contact stiffness.

The attribute j_elastic_modulus is mandatory when type=”VOLUME”.

j_elastic_modulus > 0

j_layer_depth

Double

Specifies the size of the layer depth for the geometries belonging to body I. See Comment 5 for an explanation on how this parameter is used to calculate the contact stiffness.

The attribute j_layer_depth is mandatory when type=”VOLUME”.

j_layer_depth > 0

EXPONENT

Double

The exponent of the force deformation characteristic of the contact interface. For a stiffening spring characteristic, it must be greater than 1.0.

The attribute exponent is mandatory for type=”VOLUME”.

exponent ≥ 1.0

damping

Double

Specifies the coefficient of damping used to calculate the damping force for the VOLUME force model.

The attribute damping is mandatory for type=”VOLUME”.

damping > 0

Normal force type = “POISSON”

type

String “POISSON”

Specifies that the POISSON, normal force model is to be used for calculating the contact normal force.

A contact force is computed as a function of the penetration between the intersecting geometries defined for the contact.  In other words, as the geometries intersect, a repulsive force is generated which is based on the type of contact selected. See Comments 2,4 for more information on the POISSON model.

igeom

List of Graphics objects.

This is a list of the Graphics objects on the first body to be considered for contact.

Note     All the GRAPHICS entities must belong to the same body.

The attribute igeom is mandatory.

jgeom

List of Graphics objects.

This is a list of the Graphics objects on the second body to be considered for contact.

Note     All the GRAPHICS entities must belong to the same body.

The attribute jgeom is mandatory.

penalty

Double

Specifies the stiffness parameter that is used to calculate the spring force.

A large value for penalty permits only a small penetration between the two contacting geometries; a small value permits a larger penetration. Consider using a penalty value that is as small as possible, but still captures realistic deformation, to improve solver performance.

See Comments 2,4 for more information on the POISSON model.

The attribute penalty is mandatory when type=”POISSON”.

penalty > 0

restitution_coef

Double

Defines the coefficient of restitution (COR) between the geometries in contact. This is used in the computation of the contact force. See Comment 4 for more information on the use of restitution_coef in the POISSON model.

A value of zero specifies perfectly plastic contact meaning that the two bodies coalesce after contact.

A value of one specifies perfectly elastic contact.  No energy is lost in the collision and the relative velocity of separation equals the relative velocity of approach.

COR = (relative speed after collision)/(relative speed before collision)

The attribute restitution_coef is mandatory when type=”POISSON”.

0 ≤ restitution_coef ≤ 1

normal_trans_vel

Double

Defines the velocity limit between the two bodies at which full damping is applied in the contact force.

The attribute normal_trans_vel is optional when type=” POISSON”. When not specified, MotionSolve initializes it to 0.01.

normal_trans_vel > 0

Normal force type = “USER”

TYPE

String “USER”

Specifies that the USER, normal force model is to be used for calculating the contact normal force.

A contact force is computed as a function of the penetration between the intersecting geometries defined for the contact.  In other words, as the geometries intersect, a repulsive force is generated which is based on the type of contact selected.

IGEOM

List of Graphics objects.

This is a list of the Graphics objects on the first body to be considered for contact.

Note     All the GRAPHICS entities must belong to the same body.

The attribute igeom is mandatory.

JGEOM

List of Graphics objects.

This is a list of the Graphics objects on the second body to be considered for contact.

Note     All the GRAPHICS entities must belong to the same body.

The attribute jgeom is mandatory.

normal_function

String

Specifies the list of parameters that are passed from the data file to the user defined subroutine.

The normal_function attribute must be specified when type=”USER”.

normal_routine

String or a pointer to a Python function.

Specifies the path of the shared library containing the contact force calculation subroutine and the function within that library that is to be executed.  The path and function name are to be separated by “∷”. MotionSolve uses this information to load the user subroutine in the library at run time.

The normal_routine attribute is optional when type=”USER”.

When normal_function is specified, but normal_routine is not, normal_routine defaults to “CNFSUB”.

Other optional Parameters that can be used with all normal force models

id

Integer

Specifies the element identification number.  This number must be unique among all the Contact objects in the model.

This attribute is optional. MotionSolve will automatically create an ID when one is not specified.

Range of values: id > 0.

label

String

Specifies the name of the Contact object.

This attribute is optional. When not specified, MotionSolve will create a label for you.

master_surface

String. Choose one from: “I”, “J”, “IANDJ”, “AUTO”

Specifies which surface should be used as the master surface while calculating the penetration depth, point of contact, contact normal etc. You can choose from:

The attribute master_surface is optional.

When not specified, master_surface = “AUTO”. This is also the recommended option.

active

Bool

Select one from True or False.

The attribute active is optional. When not specified, active defaults to True.

Optional friction force specification – These can be used with all normal force models

There are two options: Using the built-in friction model or using your own custom friction model. You can only select one of the two.

Using the built-in friction model

coulomb_friction

String. Choose one from: "ON", "OFF", "DYNAMICONLY”, “USERCFF”

Specifies the friction force model that will be used to compute the contact friction force.

See Comment 7 for more details.

mu_static

Double

Defines the coefficient of static friction when the friction is in the static regime.

Used only when coulomb_friction is not “USERCFF”.

MotionSolve uses a step function to transition between the static and dynamic friction regimes. See Comment 7 for more details.

mu_static ≥ 0

mu_static ≥ mu_dynamic

mu_dynamic

Double

Defines the coefficient of dynamic friction when the friction is in the dynamic regime.

Used only when coulomb_friction is not “USERCFF”.

MotionSolve uses a step function to transition between the static and dynamic friction regimes. See Comment 7 for more details.

mu_dynamic ≥ 0

stiction_transition_velocity

Double

Defines the slip velocity at which the static coefficient of friction, MU_STATIC, is applied. See Comment 7 for more details.

Used only when coulomb_friction is not “USERCFF”.

stiction_transition_velocity > 0

friction_transition_velocity

Double

Defines the slip velocity at which the dynamic coefficient of friction, MU_DYNAMIC, is applied. See Comment 7 for more details.

Used only when coulomb_friction is not “USERCFF”.

friction_transition_velocity ≥ stiction_transition_velocity > 0

Using your own custom friction model

friction_function

String

The list of parameters that are passed from the data file to the user defined subroutine.  The user defined subroutine CFFSUB is used.

The attribute must be specified when coulomb_friction= “USERCFF”.

friction_routine
 

String or a pointer to a Python function.

Specifies the path and name of the shared library containing the friction force calculation subroutine.  MotionSolve uses this information to load the user subroutine in the library at run time.

The friction_routine attribute is optional. When not specified

Comments

Example