Automated Surface Contact Generation (ASCG) is included in the V9.1 release of NEi Nastran. This feature improves the ability to create models that simulate true surface-to-surface contact. It is ideally suited for models in which the contact areas are initially touching and will experience little sliding.

Due to the complexities in the geometry of a lens mount, only some parts were easily hex meshed. Thus, a hybrid model was built with linear hex elements and parabolic tet elements. The part was setup with a body load of 10g’s in the lateral direction with the base fully fixed. A linear static analysis was performed. ASCG was added by means of one NeiNastran Case Control command to achieve the results shown above.

Engineers involved in computer modeling and analysis of structures containing discontinuous parts often find that getting realistic results can be difficult, time consuming, or simply not possible using traditional Finite Element Analysis (FEA) tools. ASCG automatically finds and sets up the appropriate contact between different bodies within a specified proximity. This feature can cut run times from hours to minutes.

It works in three phases. First, you define parameters such as which elements should be considered for each contact pair, what the near tolerance is for objects to be considered in contact with each other, and the type of contact to be generated (i.e., general, welded, bi-directional sliding, or rough). Then, the first phase consists of identifying the external surfaces of these elements. For shells it would be the entire element. For solids it would be any exterior surface. The second phase looks for grid points positioned on or near and above (within the user defined tolerance) each surface. The third phase eliminates invalid contact scenarios by considering surface normals. It is important that surfaces that are to be welded are not offset and that shell normals are oriented properly.

For a lug assembly, a 1000 lb force was applied in the negative Y direction. Due to the loading, there will be separation between the stud and the lug, requiring a nonlinear analysis. One approach is to define all the areas of contact as Rough Contact, which ensures no sliding between the surfaces. With the load and the assumption of little sliding between the lug-stud and nut-lug interfaces, the ASCG shouldprovide accurate results. As can be seen above, the program shows a visible separation between lug and stud as would be expected in a real world application.

ASCG can be used on legacy models. With it, you can easily add surface contact to an existing Nastran model input file or replace gap contact with the more robust surface contact. The process is completely automated and only requires a single command. It works at the FEA level with an existing Nastran model allowing true legacy model support.

ASGC also opens the way to achieve higher levels of real world fidelity for virtual testing. In particular, a wide range of assembly scenarios can be addressed, such as sliding, welded, and free movement between surfaces and modeling difficult features like dissimilar shell meshes from extracted mid plane models, shell face to shell edge with or without gaps, mid surface meshes, and offset welds. ASCG has been credited with productivity gains and better quality models in diverse applications ranging from large ship models to production fixtures.

Noran Engineering, Inc.
www.NEiNastran.com

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