Proper Element Choice 2

[DanStro]

Does anyone know of a site, book, or paper that gives a basic method of determining the proper element to use in common applications. For instance: some basic criteria such as when I can normally use linear elements and when I should consider going to higher order elements and when should I worry about hour glassing, volume locking,...

I have some books that throw in some information piecemeal but I haven't found a list of this sort of thing.

I know this is pretty general and that there are many applications that will be exceptions but I am hoping that I can find something that can help me get through the initial stages of an analysis.

Right now I am using Abaqus but I would like to keep this code agnostic so I can apply it to other codes if necessary.

Any help is appreciated.
Dan

 

[Andries]

DanStro,

"Google" the NAFEMS website; they have numerous publications (for purchase) on practical FE analysis. A good book is by Cook, Malkus and Plesha "Concepts and Applications of Finite Element Analysis".

Andries

 

[gurmeet2003]

Danstro,

The questions raised by you have general answers that may apply to all codes. The most general 3D elements are tetrahedron (tet) and brick elements. Linear tets (4 noded) are almost never used for structural analysis. On the other hand CFD generally does not use quadratic elements.

In my experience most common elements for structural analysis are quadratic tetrahedral (10 noded tets). These elements have nodes in the center of edges in addition to the nodes on vertices. In abaqus this would correspond to C3D10M. Today's auto-meshers can produce good quality tetrahedral meshes for complex shapes with minimum effort.

Degree of accuracy obtained by the mesh can be quantified by a mesh convergence study.

Some companies like GE tend to use brick elements exclusively. Both linear and quadratic brick elements are acceptable for structural analysis. However for complex shapes it is much more difficult to produce linear brick (8 noded) or quadratic brick (20 noded) elements. It takes much longer to produce a brick mesh. For complex shape lot of manual partitioning of the geometry is required.

Currently no auto-mesher exists that can produce brick mesh for a complex geometry.

In addition these elements can have different formulations, such as: reduced integration, full integration, hybrid etc.

Hour-glassing occurs only when reduced integration tet elements are used. Brick elements do not undergo hour-glassing.

Other type of elements are shell, beam, plate, plane stress, plane strain etc. Their use depends on type of loading and geometry.

ABAQUS offers a course on 'How to select elements'. You may consider attending it.

Gurmeet

 

[johnhors]

Gurmeet,

The Abaqus C3D10M (modified 10 node tet) element should only be used in contact analyses, it does not perform as well as the standard element C3D10 in non-contact analysis.

 

[DanStro]

Andries: I've ordered a book from NAFEMS, hopefully it will have these type of guidelines.

gurmeet2003: This is exactly what I am looking for. Did you come across these from experience or manuals? I saw the Abaqus course but I am afraid that it is more geared towards Abaqus options and not towards the method in general. But in these times it has been a hard sell to get approval for travel, missed time, and course costs. Maybe in the future though.This is exactly the kind of information I am looking for.

johnhors: I have been using the default Abaqus choice for now but I will definitely try those and compare the results. Thanks for the tip.

 

[gurmeet2003]

johnhors,

Could you explain the advantage of C3D10 elements over C3D10M elements in non-contact analysis?

Thanks,

Gurmeet

 

[gurmeet2003]

Danstro,

I have learnt this over a period of time. Some of the information is also available in Abaqus help files, website etc. I think that NAFEMS book that you ordered should also be useful. I have this book.

The book by Adams is good for practical tips on FEA.

Thanks,

Gurmeet

 

[jxc]

These are all good insights, please let me add mine. I have worked with a lot of thinwalled tubing and sheet metal products that do not lend themselves to using solid elements, but rather shell elements. A rule of thumb for when to use shell vs solid elements? Try 10:1 ratio of the surface area to the wall thickness. I use linear elements whenever possible because even in this day and age of 2.5 MHz processors a 150K DOF system still takes time. I have sacrificed some accuracy in the trade, but the results are usually within acceptable ranges.

As for references, NAFEMS is a great resource & also for practical analysis, "Bulilding Better Products with FEA", Abe Askenazi & Vince Adams is a successful resource.

 

[johnhors]

Gurmeet,

The C3D10M element as you know is a "modified" ten node tet element developed specifically for contact. When used in non-contact analyses it does not perform as well as a standard ten node tet element. Stress values can be significantly higher with the modified element compared to the standard elements of Abaqus, Nastran, Lusas, CalculiX and so on. Ever since Abaqus released this element, they have been working to improve it!