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Femap Joint Modeling Study using Rigid Elements

1 Objective Comparison of methods for the purpose of modeling joints using rigid elements in Femap and NX NASTRAN.

2 Introduction Joints are useful in transferring loads from one component to another component while allowing the

individual joint components to move relative to one another. In Finite Element Analysis, often times the

user is not interested in the output around the joint (eg: stresses, strains, displacement); however,

modeling the joint is necessary for transferring loads from one component to another.

The study encompasses two Finite Element Models, both comprising of the same Geometry, Materials,

Properties, Loads and Boundary Conditions. The difference between the two models is how the joint is

modeled using rigid elements.

Figure 1: Geometric Model to be Studied

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Model 1 (No CBUSH) incorporates a line element (rigid RBE2) connecting the independent nodes of two

spider elements (rigid RBE2) located at the centre of the lug bore.

Figure 2: Joint Model in Model 1 – No CBUSH

Model 2 (CBUSH) incorporates a line element (rigid RBE2) connecting two CBUSH elements which are

connected to the independent nodes of two spider elements (rigid RBE2) located at the centre of the lug

bore.

Figure 3: Joint Model in Model 2 – CBUSH

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3 Material and Property For the purposes of this study both lugs are assigned the material: 7050-T651 Al Plate .25-.5 which is one of the predefined materials available in the Femap material library. The element type used is “Solid Elements”. For the CBUSH elements the following properties were assigned:

Figure 4: CBUSH Properties

4 Mesh The geometry is meshed using Tet elements with midside nodes (Parabolic Tetra, 10-noded). The default

element size of 0.43229 was used which resulted in a total of 1025 Elements in Lug 1 and 1082 Elements

in Lug 2.

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Figure 5: Mesh (Left: Lug 1; Right: Lug 2)

5 Loads and Constraints A nodal force of 100 lbf in the positive Global Y direction (Cartesian Co-ordinate System) is applied at the end face of Lug 1. The end face of Lug 2 is constrained in all directions.

Figure 6: Loads and Constraints

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6 Joint

6.1 Model 1 – No CBUSH Spider Elements (Rigid RBE2) were modeled in the lug bore using the Custom Tools dialog box. The Spider Elements were constrained in the translation directions only (default) as shown in Figure 8.

Figure 7: Model 1: No CBUSH - Joint Formulation

Figure 8: DOF for Spider Elements

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A line element (Rigid RBE2) was used to connect the independent nodes of the two spider elements. All

DOF was constrained for the line elements as shown in Figure 9.

Figure 9: DOF for Line Element

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6.2 Model 2 – CBUSH Spider Elements (Rigid RBE2) were modeled in the lug bore using the Custom Tools dialog box. The Spider Elements were constrained in the translation directions only (default) as shown in Figure 11. Two new nodes were created on top of the existing nodes at the centre of the lug bore and were connected using CBUSH elements. The two CBUSH elements were connected by a line element (Rigid RBE2) with the DOF shown in Figure 12.

Figure 10: CBUSH - Joint Formulation

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Figure 11: DOF for Spider Elements

Figure 12: DOF for Line Element

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7 Analysis A static analysis using the NX Nastran solver was used to solve the problem. All settings were kept default except the NASTRAN Output Requests.

Figure 13: NASTRAN Output Requests

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8 Output The results comparing the Von Mises Stress, Min Principal Stress, Max Principal Stress and Total Translation are presented for the two analyzed models. A deformation scale equalling 10% of the max model is used to present the output illustrations.

Output Description Model 1 – No CBUSH Model 2 – CBUSH

Von Mises Stress 29527 29527

Min Principal Stress -34646 -34646

Max Principal Stress 42319 42319

Total Translation 0.0946 0.0946

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8.1 Model 1 – No CBUSH

Figure 14: Von Mises Stress

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Figure 15: Min Principal Stress

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Figure 16: Max Principal Stress

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Figure 17: Total Translation

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8.2 Model 2 – CBUSH

Figure 18: Von Mises Stress

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Figure 19: Min Principal Stress

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Figure 20: Max Principal Stress

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Figure 21: Total Translation

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9 Conclusion Comparing the output from both Model 1 – No CBUSH and Model 2 – CBUSH, it can be concluded that there is no difference in the results when using a CBUSH to connect the rigid elements in a joint as opposed to connecting the rigid elements directly without the use of a CBUSH element. Thus, rigid elements can be connected with other rigid elements to transfer loads between parts in a joint as long as the area of the interest in not near the joint.