Bolt through multiple plates does not solve as expected in Nastran

Autodesk Support

Oct 8, 2023

Products and versions covered

Issue:

A stack of multiple plates is held together by a bolt. The bolt is created using the bolt connector in Nastran In-CAD or Inventor Nastran. The results of the analysis are one of the following:

  • The analysis completes, but the plates are not held together. The middle plate slips relative to the two outside plates.
  • The analysis does not complete. An E500x error occurs, such as E5000 (SINGULARITY DETECTED), E5001 (NON-POSITIVE DEFINITE DETECTED AT GRID id COMPONENT n), or E5004 (STIFFNESS MATRIX SINGULAR OR NON-POSITIVE DEFINITE).

Causes:

A bolt created by the bolted connector does not interact with the sides of the bolt hole. Any parts not connected to the bolt head or nut is free to slide.

bolt connector plate slide slip no contact
Figure 1: The figure on the left shows three plates held by a bolt. In reality, the sides of the hole can contact the bolt.
The figure on the right shows the simulation using a bolt connector. The bolt is simulated using beam elements. The head and nut (A and B) are attached to plates 1 and 3. The body of the bolt (C) does not contact any of the plates, so plate 2 is free to slide out from plates 1 and 3. A different setup is required to prevent plate 2 from moving freely.

Solution:

There are multiple methods to simulate the joint. Which one is used depends on what assumptions are acceptable.

  • Cap screw. Thread the bolt into plates 2 and 3. When creating the bolt connector, use a cap screw type bolt (and select the surfaces of the holes). This method prevents plate 2 from rotating around the bolt.
  • Prevent middle plate from sliding. Use a contact type between the middle plate and the outer plates that prevents the middle plates from sliding. For example, "Separation / No Sliding" would work. This method assumes that the friction is high enough to prevent sliding. The side of the hole does not need to contact the body of the bolt to prevent motion.
  • Model bolt with beam and rigid connectors. Instead of using the bolt connector, use beam elements to simulate the bolt. Rigid connectors are used to attach each hole to the beam. See Figure 2. The general steps are as follows:
    • In the Inventor modeling environment, create a sketch passing through the center of the hole. Draw line segments to represent the bolt. Position the ends of the line segments in the center of the hole in each plate. (In other words, the number of line segments is 1 less than the number of plates.)
    • In the Nastran environment, create an Idealization ("Prepare > Idealizations"). Set the Type to Line Elements. Define the Cross Section. Select the sketch lines for the Associated Geometry.
    • Create rigid connectors ("Prepare > Connectors > Rigid Body"). Select the face of the first hole for the "Dependent Entities". Select the end of the beam for the "Select Point". Click "Next" to define the next rigid connector and repeat the selections for the next hole.
    • Optionally, add end releases. The above steps will allow the plates to slide and bend the bolt appropriately, but it will not allow the plates to rotate about the axis of the bolt. That is, the bolt is "welded" to the center of the rigid connector and does not allow the rigid connector to rotate around the bolt (like a bearing). If rotation is important, end releases can be added to the beam elements at the appropriate connectors to free the axial rotation. First, mesh the model to create the beam elements. Then right-click the Model Tree's "Elements > End Releases > Rx" on either End A or End B, then select the appropirate beam elements to release the axial rotation. 
  • Model the bolt as a solid. Then define contact between the sides of the hole and the body of the bolt. (This approach creates the longest runtime since the number of elements is greater than the other approaches, and the separation contact between the bolt and holes requires multiple iterations during the analysis.)
Nastran bolt through multiple plates
Figure 2: Simulating a bolt with beam elements (A) and rigid connectors (B). The number of line segments in the beam is 1 less than the number of plates. The endpoints of each line segment should be center in the respective hole. The number of rigid connectors is the same as the number of holes.

Products:

Nastran; Inventor Nastran;

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