Common FEM Element Types

Element Types
Common FEM Element Types

The basic building blocks of any structure model can be broken down into the following common FEM element types:

  1. Tube: These elements are typically used for structure overhead or side tie rod attachments
  2. Bar/Beam: These elements are typically used to model any type of screw or bolted joints with appropriate releases (you will learn more about degree of freedom releases in the intro course)
  3. Plate: Any metallic plate, cover plates, splice plates, or other structural plate members
  4. Laminate: This element is essentially based on plate theory, but customized to be used for composite structure modeling
  5. Rigid: RBE2 (hard rigid), and RBE3 (soft rigid or interpolation rigid element) is used for fittings, joints etc. where you know the stiffness of the joint is very high between the fasteners or load/moment transfer interface
  6. Mass/Weight: Used to model concentrated mass/weight such as inserts or equipment installed on the panels using brackets or screws
  7. Spring/CBUSH/GAP: These elements are used to define stiffness based connections such as panel pins, or brackets, directional contact elements, or aircraft structure interface stiffness elements

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So there you have it, no matter how big or complex the structure installed in the aircraft is, the above element types can do it all and give you the data you need to certify any structure by analysis or test. If you want to learn more about what FEM is, check this post: Finite Element Analysis

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FEM is very interesting. As with any FEM modeling practice, the key is to make sure the results make sense. You can read more about this in the post Finite Element Analysis. Ideally, you want to take some college level courses on the finite element method to get deeper into the method. Or, just read about the freely available material on the web. However in the real job world, all you need to know is the basics and the practical application side of these common FEM element types. The reason is that the companies that make the FEM programs must go through rigorous bench marking to the standards set by NAFEMS and become certified to be acceptable as the FEM solvers for the FAA. NASTRAN is pretty much the standard in the space and aerospace industries, but there are also other programs such as ANSYS, ABAQUS etc. that are used widely. All of these programs have extensive element libraries, some of them pretty advanced. And the common FEM element types listed above are certainly a fundamental part of any program's element library.

I hope you found this post useful. Make sure you comment and let me know your feedback.

 

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Surya Batchu
Surya Batchu

Surya Batchu is the founder of Stress Ebook LLC. A senior stress engineer specializing in aerospace stress analysis and finite element analysis, Surya has close to a decade and a half of real world industry experience. He shares his expertise with you on this blog and the website via paid courses, so you can benefit from it and get ahead in your own career.