How long should this take? 2

[trainguy]

Hi all.

Here's an open ended question, but I'd like to get a feel from the industry (you) about estimated level of effort.

Starting from a 3D step file of mostly thick plates (so solid meshing is an option), how many hours of FE analyst time would you allocate to the following:

Import geometry into FEA package
Mesh the geometry with 3D (tet) elements, using automated meshing. (assume approx 50 000 elements)
Apply straightforward boundary conditions.
Appply a single set of loads (say 4 loads over 4 small rectangular areas)
Perform a linear static analysis.
Prepare a few stress / deflection plots to be able to comment on the structural performance for this single load case, without doing any redesign.

The structure would consist of the first 10 feet of a locomotive underframe, which is essentially 2 heavy I beams side by side with structure between them which receives compressive load. The load case is not fatigue related.

How different would your estimate be for a mapped (non free) mesh of mostly Hex elements?

I realize the answers could vary wildly, but it would really help to know how you perceive this.

Thanks in advance.

tg

 

[corus]

Personally I've found from experience that it's easier to draw something from scratch rather than rely on a CAD drawing, which was drawn for a different purpose. But why people think you just have to point and click and the job's done I have no idea. Spend a little time on the geometry and use hex whenever possible and you get better results, and prettier pictures. For something that looks like a ladder I'd have though it was just lazy if someone used tets on it, and would naturally think everything else they did was just as slip-shod.

To get back to the original question. I'd have said maybe a few days but with any estimate it's always wise to double the original time you think it can be done in. If you say you can finish it by Tuesday and do it ny Monday you're thought of being good. Do the job on Tuesday, like you said you would, and no one thinks anything of it.

corus

 

[GregLocock]

corus- you are ignoring one very useful use of tets - in optimisation work. I recently optimised a ladder type chassis in HM, and to pick up points from various posts above, I drew the design volume from scratch, so it was a very pure shape, with cutouts for the engine, rack, mufflers and fuel tank. Then I hit automesh and it went away and did its thing, and then optimised it. This gave me a good picture of the load paths and some idea of the relative size of the members.

And then, just like cbrn, I turned it back into a beam model, which I then optimised again in my own program, using the standard beam sections available. This gave me the practical structure which is now being detailed, and I hope, built.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[corus]

Greg,
In that particular case you're doing a comparative study, and as such using comparable meshing techniques of tet elements. That may be ok when you just wish to see which design is 'better' than the other. I don't know if it's just Abaqus though, but I find that the results from quadrilateral tet meshing doesn't give me the smooth contour results I'd have confidence in as you get with hex meshing, unless you go berserk with the number of elements. Results that appear to be mesh dependent should always be avoided. In the final analysis I'd still avoid tets unless it was obvious that the geometry was so complex it would just take too much time to partition something down in order to hex mesh it.

corus