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Recommended settings for Spotwelds


When using spotwelds, there are several non-default parameters that are recommended to improve the spotweld behavior and model robustness. Following are some of the important parameters:

1. CONTACT_CONTACT/OUTSEG
This parameter helps to get a list of all spotweld nodes and the parent segment information. This is helpful in earlier designs to ensure the spotweld node is tied to the desired parent element.

2. CONTROL_CONTACT/SPOTSTP
When a spotweld node is untied, LS-DYNA silently deletes the element and continues the calculations. It is recommended to set this parameter to 2 so LS-DYNA can delete the weld, print the node of the sptoweld and continue. This helps to tract the untied nodes and the deleted welds.

3. CONTROL_CONTACT/SPOTDEL
This option allows the deletion of spotweld elements when the parent element to which the nodes of the weld are attached to using a TIED interface. When the parent element is expected to erode during the calculations, this parameter is recommended to be set to 1 so the weld is deleted to avoid imposing constraints.

4. CONTROL_CONTACT/SPOTTHIN
When the spotweld parent elements are part of a penalty contact, the two parent element can interact due to contact and interfere with the weld forces. It is recommended that we set SPOTTIN=0.2-0.4, to eliminate the interference of the parent interaction with the weld forces.

5. Choosing the right TIED contact
When modeling spotwelds using beam elements, the recommended contact type is CONTACT_SPOTWELD_{TORSION} so translational and the rotational degrees of freedom of the spotweld node be coupled with the parent element. When using solid element, the recommended tied interface is CONTACT_TIED_NODES_TO_SURFACE since the solid nodes do not have rotational dofs.

6. Choosing the tolerance for tying the node to parent
Constraint based non-offset contacts require that the spotweld node be located on the parent element such that the projected distance between the node and the parent surface along the parent normal is zero. During initialization, this requires LS-DYNA to alter the spotweld geometry so as to move the node to lie on the parent surface. The tolerance to decide when the node is too far to project is based on the parent element shell thickness and its diagonal length. We can change this tolerance using a negative value of SST/MST which then is used to define a new tolerance that is ( abs( SST ) + abs ( MST ) ) * 0.5. It is recommended that the SST=MST=-0.1-0.3 so we limit LS-DYNA to move the nodes by a small number to avoid excessive geometry modifications during the initialization phase. Nodes farther than the tolerance will not be moved and tied and will be reported which can be monitored.

7. Using OFFSET based TIED contacts
If the spotweld node is desired to be offset by a certain distance from the parent and the resulting moment is important, it is recommended to use the option OFFSET in the TIED contact definition. TIED offset contacts uses a total displacement formulation by default and it is recommended to use an incremental displacement formulation by setting the TIEDID option in CONTACT/OptionalCard_D.

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