Contacts may be defined automatically or created manually. Contact pairs can be automatically detected and generated in Mechanical when you first attach geometry. If a required contact is not detected and defined automatically, you will need to create the contact conditions manually.

Figure 2.27: Automatic and Manual Contact Creation

 

2.3.1.1. Flexible-Rigid Contact Definition

When creating contact between a rigid body and a flexible body, select the flexible body as the Contact body and the rigid body as the Target body. If you do not create a Contact Properties object, the parameters are set to default values.

Figure 2.28: Contact Between Rigid and Flexible Bodies

Figure 2.29: Equivalence between Motion in Workbench and Standalone Motion

	Motion Standalone	Motion in Workbench
Base (Contact)	Rigid (Faceset)	Flexible Behavior
Action (Target)	FE (Patchset)	Rigid Behavior

• Contact in Workbench corresponds to the Base geometry in the Standalone Preprocessor.

• Target in Workbench corresponds to the Action geometry in the Standalone Preprocessor.

• A Patchset creates a group of triangular or rectangular patches on an FE body to define contact.

• A Faceset can represent the contact surface on a rigid body.

 

2.3.1.3. Defining Beam Contact

You may select an edge on a beam body for a Contact Region in a Motion analysis. This beam body must have a circular cross section. In this case, based on the R dimension of the beam cross section properties, the solver uses a virtual shape of hexahedrons surrounding the middle beam line to represent the real contact surface (see figure below).

Figure 2.30: Shape of beam contact surface

Limitation

Partial selection of edges for contact is not supported. You may therefore need to split the beam body to use for contact.

 

2.3.1.4. Defining Edge Contacts

To use Edge Contact for a Contact Region in a Motion analysis, the contact (base) must always be an edge. The target body can be an edge or face.

Figure 2.31: Edge Contact Between Bodies

The following face and edge combinations are supported for Edge Contact:

Table 2.5: Support for Edge Contact

	Contact - Edge (Flexible)	Contact - Edge (Rigid)
Target - Face (Flexible)	Supported	Not Supported
Target - Face (Rigid)	Supported	Not Supported
Target - Edge (Flexible)	Supported	Not Supported
Target - Edge (Rigid)	Not Supported	Not Supported

Contact Point Detection Algorithm

• Edge to Face

  • Consists of a Vertex-to-Patch search using an FE Node belonging to the Edge as the Vertex, and an Element face belonging to the Face as the Patch (if the element is of high order, the mid-node is also used as a vertex).

  • Contact point example:

    Figure 2.32: If a contact point is found and is on a patch on the face

    

    
    

    Figure 2.33: If the contact point cannot be found

    

    
    

• Edge to Edge

  • Consists of a Vertex-to-Patch search using an FE Node belonging to the Contact Edge as the Vertex, and an Element face including the Target Edge as the Patch (if the contact edge element is of high order, the mid-node is also used as a vertex).

  • Contact point example:

    Figure 2.34: If a contact point is found

    

    
    

    Figure 2.35: If the contact point cannot be found

    

    
    

There are 4 types of Contact:

Contact Regions of type Bonded or No Separation are supported only for Nodal flexible bodies .

The contact type for the solver is determined by the Body Flexibility and Surface Type selected in the contact properties for Frictional/Frictionless contacts.

In Workbench, Bonded and No Separation contact are represented internally as Tie Constraints and are only supported between FE Nodal bodies.

The detection method executes two searches to find contact pair candidates. The first is a global search and the other is a local search. If the bounding boxes in the global search overlap, those boxes are divided to sub-domain boxes to be inspected for overlapping contact patches. The solver then uses a local search to find overlapping patches (see below).

Figure 2.36: Contact Detection

If the relative displacement d_ba is perpendicular to the tangent direction at a point on the base surface, the point can be considered a contact point, as shown below.

Figure 2.37: Contact Point Detection

The contact penetration p can be calculated from the relative displacement and normal direction, as shown below.

Figure 2.38: Contact Penetration Detection

Frictional or Frictionless Contact

Motion uses a penetration method to determine friction at the contact point.

Figure 2.39: Contact Friction detection

Where:

Bonded or No Separation Contact

The nodes on the action surface are tied to each corresponding point on a base surface. Tie contact constrains full DOFs of each node relative to the other surface. See Tie Contact in the Motion Preprocessor User Guide for more information.

Figure 2.40: Contact Bonding Detection

Insert a Contact Properties object to specify additional contact characteristic values. This can be applied to a set of Contact Region objects whose type is Frictional or Frictionless.

Figure 2.41: Inserting Contact Properties Objects

Surface Type

If Surface Type is set to Surface, and the contact is defined between a rigid and a flexible body (as the Target and Contact bodies, respectively), the base and action are flipped internally during analysis. With this option, Stiffness Type can be set to Program Controlled, Factor, Constant or d-F Spline. See available options in Figure 2.42: Available Contact Properties: Surface Type = Surface.

Figure 2.42: Available Contact Properties: Surface Type = Surface

If Surface Type is set to Multi-Point, Stiffness Type has additional options for Considering Contact Area and d-P Spline. See available options in Figure 2.43: Available Contact Properties: Surface Type = Multi-Point.

Figure 2.43: Available Contact Properties: Surface Type = Multi-Point

Maximum Penetration and Buffer Range

Usually, Max. Penetration is set first and Buffer Range Factor does not need to be changed. The purpose of the buffer range is to detect contact candidates before the contact has begun. If the model has contact between fast-moving bodies, a larger buffer range is required to detect this. To increase the buffer range, set Buffer Range to Factor, and Buffer Range Factor to a larger value.

The Max. Penetration setting determines buffer range as follows:

Figure 2.44: Determining Maximum Penetration

Offset

Offset is used to offset the Contact or Target surface. When one body is enveloped by another body, use a small offset to help establish contact and improve numerical stability in the Motion solver.

Figure 2.45: Determining Offset

Stiffness

You can choose a contact Stiffness Type and Coefficient Factor from the Advanced properties of a Contact Properties object.

Damping

Set Damping Type to Factor, Constant, Ratio or v-F Spline.

Selecting Ratio means that the Damping Coefficient is used as a scale factor for the contact Stiffness Coefficient currently being applied.

When v-F Spline (Velocity-Force Spline) is selected, contact damping force can be defined using a predefined spline.

Usually, a maximum damping coefficient value in the range of 1E-3 ∼ 1E-6 times the stiffness coefficient value is used.

Bounce Off Damping Factor

The Bounce Off Damping Factor is intended to prevent excessive adhesion when the contact surfaces move outside the penetration range. It only affects the solution when an object is moving away from the contact surface (penetration velocity is negative) and has no effect when it is approaching or stationary. The limit of the adhesive force is determined using a factor proportional to the spring force component of the contact normal force (see Figure 2.46: Normal force calculated using bounce off damping factor).

This factor can be specified as a value between 0 and 1 as follows:

This can be expressed as shown in the formula and graph below.

Where f_n is the normal force, R_b is the bounce off damping factor, and f_s is the spring force component of the contact normal force.

Figure 2.46: Normal force calculated using bounce off damping factor

Enable this option by setting Use Bounce Off Damping Factor to Use and then specify a numerical Bounce Off Damping Factor to give different damping values depending on the state of contact. Values in the range 0 to 1 are supported, as described above.

Max. Step Size

This property sets the modification factor for the step size used during contact detection.

Contact between two bodies is imminent when their buffer ranges start to overlap. The solver therefore monitors this condition and when overlap is detected, it reduces the step size by the specified factor (usually smaller than 1). This avoids the risk of the bodies passing through each other without making contact, due to a large change in displacement between two time steps.

If you do not specify a factor, it will default to a value of 1, meaning that there will be no modification to the step size when buffers ranges start to overlap.

Use Increment Time

If this option is set to Use (default = None), the Increment Time parameter is used and the applied contact force F_cnt is calculated as follows:

This is useful if you need to gradually increase the contact force, such as in a shrink-fit simulation.

Point Check

This option is only exposed when Surface Type is set to Multi-Point. You can use this to set one of the following options for finding contact points:

Factor for Internal Point

This parameter is exposed when the Use Factor for Internal Point option is set to Use and it generates additional internal contact points on a patch. This is useful for models where it is otherwise impossible to increase the number of contact points, such as with a Patchset on an FE body. This is a good option when you get a poor contact solution due to a rough mesh. The number of additional internal contact points can be determined as shown in the table below.

Figure 2.47: Original and Internal Contact Points

Sliding Contact Option

In sliding contact, contact force is only generated at nodes within the maximum penetration range, as shown in the figure below. If the penetration is greater than zero, a repulsive force is generated. The Sliding Contact Option reduces the repulsive force by adding opposing forces on the nodes located in the penetration area. This can improve the stability of time integration under higher contact forces and is useful in reducing chattering effects in sliding contact between two bodies.

Figure 2.48: Sliding Contact

Use Limit Increment in NR

If the Newton-Raphson (NR) increment is larger than the maximum penetration, the Motion solver cannot detect contact. However, if the Use Limit increment in NR option is selected, the solver can limit the solution to the specified value. Since using this option decreases the integration step size, the solving time may be longer.

Figure 2.49: Limit Increment Option