Keep in mind that loads are transferred from one element to the adjacent element only if the nodes are connected. Typically, the elements within one part are connected directly through the nodes since the mesh is continuous.
How different parts in the model are connected is determined by the Contact heading in the browser (tree view). When two surfaces of different parts touch, they will be connected together based on the default contact type, unless a specific contact pair is created and defined as a different type of contact. See the page Types of Contact for an explanation of the available contact types and the mesh requirements.
Generate Contact Pairs
Specific contact pairs need to be created and assigned to the appropriate contact type when the default contact type is not appropriate for the pair.
Automatic Contact Pairs
When working with a CAD model, you can choose whether the interface automatically creates the list of all contact pairs in the model, or whether you want to create the contact pairs manually. This choice is set from the 
Options CAD Import tab; Global CAD Import Options button, then set the Automatically generate contact pairs option accordingly.
- Automatically generate contact pairs activated. When a CAD model is opened in the interface, the surfaces that are physically in contact (zero gap) are listed in the Contact branch of the browser. The contact type for each pair is Default so that they follow the setting of Contact (Default:). To set certain contact parameters to be used between specific surfaces, select the contact pair in the browser, right-click, and choose the new type of contact.
- Automatically generate contact pairs not activated. When a CAD model is opened in the interface, the time required to detect all of the surfaces that are physically in contact is not expended. You can manually choose parts or surfaces to be in contact as described next.
Add Additional Contact Pairs
Regardless of whether the contact pairs are generated automatically or not, you can create additional contact pairs. This may be needed because you want contact between two parts (instead of between the individual surfaces on the parts), or the two surfaces are not initially in contact, or because the original entry was deleted. Also, the contact pairs in a hand-built model must be specified manually.
There are two methods to generate the additional contact pairs:
- Create pairs one at a time. This method is appropriate for CAD and hand-built models. First select two surfaces, two parts, or one surface and one part in the browser. Once they are selected, right-click the heading for one of the surfaces or parts and select the Contact command. Select the appropriate type of contact for this pair. A new heading will be created under the Contact heading in the browser for this pair. Alternatively, the parts and surfaces can be selected in the display area, then right-click and choose Contact. Normally, which surface is selected first and which is selected second does not matter, other than controlling the order of the listing in the browser. For surface-to-surface contact in a nonlinear analysis, the order may be important when defining the Contact type as Point to Surface. The first part or surface selected becomes the primary surface, and the second selection becomes the secondary surface. (See the page Setting Up and Performing the Analysis: Nonlinear: Loads and Constraints: Surface-to-Surface Contact: Surface-to-Surface Contact Options for details on the Contact Type.)
- Create multiple contact pairs at a time. This method is appropriate only for CAD models. First select two or more parts in the browser or in the display area. Then, right-click and select ContactCreate Contacts Between Parts. Any pair of surfaces that are in contact between the selected parts will be listed in the Contact branch of the browser. The type of contact for each pair can then be changed individually by right-clicking on the entry in the browser.
Contact between different surfaces of the same part is not permitted except in nonlinear stress surface-to-surface contact. (Use the Contact tree to generate a contact entry within a single part, but the processors will not handle it.)
Sort Contact Pairs
When contact pairs are added to the model, they are listed at the bottom of the contact branch of the browser in the order that they are created. The contact pairs will be sorted whenever the model is opened again. Given the three different types of contact entries --- Part A Surface B with Part C Surface D (A/B with C/D in browser), Part A with Part C Surface D (A with C/D), and Part A with Part C (A with C) --- the sorting of the contact list will be by A, then C, then B, and then D.
Color of Contact Pairs
Contact pairs that are created automatically (or prior to version 23) use black text in the browser. Contact pairs that you create explicitly use blue text in the browser.
Appearance Visual Style Edges or View Appearance Visual Style Mesh. If model colors make it difficult to see highlighting, use View Appearance Color By Part or View Appearance Color By Surface to change the display. In addition, the highlight color can be set from the Setup tab under the Options dialog. Consider surface contact between three different parts where the materials of these parts will create different static coefficients of friction. You must create two contact pairs and assign a static coefficient of friction for each pair. You would not be able to set the default contact type to surface and set up two different friction coefficients.
As another example, consider a model has 50 surfaces in two different parts that interact with each other. Assume that various contact pairs have different types of contact. Then, selecting entries and changing the contact type for each pair may be the best approach. However, if only two pairs of surfaces are in contact and the other 48 pairs of surfaces are considered to be bonded, then you should set the default for the entire model to Bonded. The other two surfaces would then be selected and set to Surface contact. The idea is to minimize the number of contact pairs that must be defined. Specify the default contact type that is most prevalent within the model.
Order of Contact Affects the Model
If the exact same contact pair is created (that is, duplicate contact pair), only the first entry in the browser is used. Subsequent entries are ignored. When both part to part and part/surface are defined at the same face, the part/surface entry controls the contact type for that pair, and the other surfaces that are in contact follow the part setting. See Figure 1 for examples.
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| (a) The same part/surfaces are defined in contact. Free contact is the result because it is listed first | (b) The same part/surfaces are defined in contact. surface contact is the result because it is listed first. |
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| (c) The contact between part 1 surface 6 and part 2 surface 5 (1/6 with 2/5) is a smaller more detailed region, so that face will have surface contact. Any other contact between parts 1 and 2 (1 with 2) will have free contact. |
(d) Same contact pair, different order. Since the two entries are not exactly the same, the outcome is the same as (c): part 1 surface 6 has surface contact with part 2 surface 5, and any other faces in contact between parts 1 and 2 are free. |