2D vs 3D FEA...
2D vs 3D FEA...
(OP)
Hi everyone..
I just got back from a meeting and there's something discussed in the meeting that I can't resolve. It should be simple but...
Anyway, a colleague was presenting the difference between running a simple cantilever beam deflection using a 2D planar (simplified) and using 3D model. Now, we all know that simplifying a 3D model that has some form of symmetry in 2D can reduce the run time, cost, etc, but when the colleague presented the output of the two different simulations he gave the following unit translations that I do not understand.
For 2D:
Length - mm
RF - N/mm
Stress - N/mm2
Pressure - MPa
For 3D:
Length - mm
RF - N
Stress - N/mm2
Pressure - MPa
What had me confused was the difference of reaction force unit between 2D and 3D. Why shouldn't it be the same for both 2D and 3D? If stress is F/A, and F is really N/mm2 in 2D... then won't it translate to stress being (N/mm)/mm2 for 3D? (My understanding is that the units would all be the same)
I raised my hand to ask and a senior explained that its because the 2D planar model has no thickness and the beam in the model actually has a thickness of 10mm. I still can't reconcile this with the fact that force unit translates differently but stress unit stays the same... I'm just a junior engineer and I'm confused... can anyone explain?
thanks in advance,
jo
I just got back from a meeting and there's something discussed in the meeting that I can't resolve. It should be simple but...
Anyway, a colleague was presenting the difference between running a simple cantilever beam deflection using a 2D planar (simplified) and using 3D model. Now, we all know that simplifying a 3D model that has some form of symmetry in 2D can reduce the run time, cost, etc, but when the colleague presented the output of the two different simulations he gave the following unit translations that I do not understand.
For 2D:
Length - mm
RF - N/mm
Stress - N/mm2
Pressure - MPa
For 3D:
Length - mm
RF - N
Stress - N/mm2
Pressure - MPa
What had me confused was the difference of reaction force unit between 2D and 3D. Why shouldn't it be the same for both 2D and 3D? If stress is F/A, and F is really N/mm2 in 2D... then won't it translate to stress being (N/mm)/mm2 for 3D? (My understanding is that the units would all be the same)
I raised my hand to ask and a senior explained that its because the 2D planar model has no thickness and the beam in the model actually has a thickness of 10mm. I still can't reconcile this with the fact that force unit translates differently but stress unit stays the same... I'm just a junior engineer and I'm confused... can anyone explain?
thanks in advance,
jo





RE: 2D vs 3D FEA...
With a 2D plane strain model, softwares generally assume a unit depth in an infinitely thick plate. Because of this, loads and reaction data are "per unit thickness of 'infinite' plate". As for the units, you still input a force, but it is also per unit thickness and your results generally report standard force units, but because your model assumes a unit thickness, the results must be interpreted as "per unit thickness".
Stress results are already appropriate because they are area-based (per mm^2).
Not sure this is very clear, but hope it helps.
RE: 2D vs 3D FEA...
example, NASTRAN forece output is force (N) for rods, but running load (N/mm) for shells. there are good reasons for this, but that is another story.
bottom line, from what you presented, the 3D model is outputing directly reaction forces (in N), but the 2D model is providing an intermediate answer (N/mm), and you need to know what length dimension to apply to get the true reaction force (in N).
you are very confused ... stress/pressure is N/mm2 ... always. Force is always N. 2D elements do have thickness (it's on the propert card), it's not expressly modelled like it is in 3D elements, but it is there.
RE: 2D vs 3D FEA...
corus
RE: 2D vs 3D FEA...
rb1957 is right..I AM confused. I thought stress is always F/A = N/mm2 too (or MPa etc), and load is always N. Which is why it stumped me yesterday evening. But the way you guys explain it make sense to me now. We use mainly use Abaqus over here, and yesterday we were discussing plane strain models. My area has always been axisymmetric stuff. My colleague was probably talking about what happens while the software 'does its thing'.
Any good books you can point me to so I can understand FEA better? I'm currently working through Schaum's Outlines FEA. I was never exposed to FEA back in uni and had to learn on the job. But I really like what I do now; just want to understand the theory better.. though when I ask questions I usually get cut off at meetings by my impatient boss.
thanks a lot!
jo
RE: 2D vs 3D FEA...
i agree with your boss, that meetings aren't the time to learn stuff (i'm assuming your questions are about the very basic methodology ones rather than appropriate to the solution). get some time with the gurus (or maybe the lesser-gurus who can talk and think at the same time !!), maybe take a course at a local college or something.
keep grounded ... as long as you remember stress is stress and load is load you won't go too wrong !
"axi-symmetric" = solid rocket fuel ??
RE: 2D vs 3D FEA...
Did you by chance misspell this person's name?
RE: 2D vs 3D FEA...
actually since i've just been on amazon it's john robertson "understanding finite element stress analysis"
memory ain't what it was ...
RE: 2D vs 3D FEA...
http://ww
RE: 2D vs 3D FEA...
RE: 2D vs 3D FEA...
RE: 2D vs 3D FEA...
RE: 2D vs 3D FEA...
An excellent primer is Logan's book as well. It is not as in-depth and old-school as the aforementioned but is a great resource to get you started. Some of the older books are a little snobby and not very forgiving when it comes to a beginner but their value is simply unquestionable.
There is also a great text written by Carlos A. Fellipa that you can get for free at the University of Colorado website. It is written in a course format (pdf of the text, including slides used in the classroom) that will take you from basic linear FEM all the way through to non-linear analysis. Not small though, we're talking about 1000 or so pages here.
Glad to hear you'd like to understand the fundamentals. Too many people just blindly apply the method to produce pretty contour plots and don't have the faintest idea what they have done; let alone lucidly interpret the results. FEM is never really wrong but we often ask it the wrong question. The onus is on you, the analyst, not the code.
A detailed look at these references and your days of confusion will be behind you. Good luck.
RE: 2D vs 3D FEA...
RE: 2D vs 3D FEA...
site:.edu intitle:"index.of" (pdf) title.of.document
style google search to find what you are looking for. Not sure since I haven't checked it out myself yet but I'm sure it will turn up something.
In general, you can search .edu sites for fantastic reference material suited for learning at an appropriate pace, complete with examples and often some working code that can be used to develop and understand simple examples. I know for a fact that MIT posts all of their lecture material for free. This is just the kind of resource I'd have loved about ten years ago.
My opinion is that one will never truly understand FE unless they have sat down and sweated through the development of at least a simple 1-D code themselves; it minimizes the whole "black box" mentality commonly associated to powerful software tools...but that's just what I think....
RE: 2D vs 3D FEA...
Linear FEM:
http:/
Advanced FEM:
http:/
Non-Linear FEM:
http:/
While this is not a replacement for the references mentioned before (I think I'm required to say that) and certainly won't get one from zero to hero (only time and patience can do that), it is definitely worthwhile checking it out, whether you are a beginner (mizzjoey) or simply interested (rb1957). I found it to be a good read, presented in a manner not reeking of pretentiousness or solipsism.
Regards.
RE: 2D vs 3D FEA...
rb1957, axisymmetric as in shaft seals and the like. My background is materials engineering, so I always try to understand a problem in terms of forces acting in the body as opposed to a computational approach.
thanks everyone! great help!
RE: 2D vs 3D FEA...
Other software wants you to input Force/Length^2 as the traction units. Usually FE software is consistent in this regard, not requiring Force/Length and spitting out Force/Length^2. "RTFM" is still good advice here.