When do you use FEM
When do you use FEM
(OP)
FEM analysis is quite expensive compared to hand analysis (for a simple problem). Just wondering how people in this forum decide when to use fem over hand analysis.
I have used fem on very large structure such as a 777 fuselage. However, this is a loads model not a stress model (i call it a stick model, no solids). Or perhaps to get a stress concentration on a very complex fitting (where i'll use a fine mesh model).
Anyway, just curious where most people draw their line.
I have used fem on very large structure such as a 777 fuselage. However, this is a loads model not a stress model (i call it a stick model, no solids). Or perhaps to get a stress concentration on a very complex fitting (where i'll use a fine mesh model).
Anyway, just curious where most people draw their line.





RE: When do you use FEM
Bottom line, every case is different. I do FEM for a living, so I hand calc to make sure I am still using the software properly, but people come to me for FEM's. If it can be hand calc'd, I usually tell them so and suggest they save their money.
RE: When do you use FEM
In the real world, eventually you can into normal modes, frequency response and transient analysis. Those things are done only by computer FEM analysis, since any hand calculation is on a VERY basic level.
RE: When do you use FEM
Ed.R.
RE: When do you use FEM
RE: When do you use FEM
Had a very simple design I'd done a few basic calcs on. My chief stress engineer did a full analysis, again using hand type calcs (he may have put them in mathcad, can't remember).
However because of the way the contract was we had to do a FEM analysis. Took weeks, cost thousands and if I recall correctly didn't tell us anything we didn't already know.
RE: When do you use FEM
As alluded to in the post above regarding contractual obligations, sometimes the client wants an FEA. Most of my clients are "internal" to the company. There are times when its simply good politics to run a quick hand calc to determine the answer, then make some pretty FEA "cartoons" to make the senior level managers who most likely do not have a stress analysis background happy. They just feel better when they see a pretty picture. One of my favorite quotes: "Red is bad, right?"
jt
RE: When do you use FEM
it depends... Please design a new ball valve for hydroelectric machinery and let me know if you can optimize this in less than 3 years using ONLY hand-calcs (or even computer-aided explicit-analytical maths)...
Regards
RE: When do you use FEM
I think, FEM can bring you there, where hand calcs can not.
Regards,
Alex
RE: When do you use FEM
OK, down off the high horse!
I wouldn't hand calc too many assemblies unless they are fairly simple, but I do trust some of the event simulation software out there today. As engineers, we have to make certain we apply the correct theory to whatever hand calc we may do...thick plates do not behave the way Roark's says plates behave because Roark's concentrates on thin plate theory making some assumptions about the shear deformation. Timoshenko makes some assumptions about shear in beams that standard beam theory does not...which is appropriate for your application? On...and...on...and...on, etc. etc.
Garland
Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
RE: When do you use FEM
Regards,
vangelis
RE: When do you use FEM
cheers,
Tobias
RE: When do you use FEM
just for the anecdote: when I ironically asked if anybody could calculate a ball valve for hydroelectric power-plant "from scratch" in less than 3 years using only hand-calcs, it was because in the company I work for, this has been done exactly in 1938.
When I had to re-calculate (cross-check) this in 2005 with the latest techniques available (i.e. FEM), I found that the deformation results determined by hand (using finite differences, I presume) corresponded to my FE results within a 2% error, globally... So, the results were "the same" but the time needed was 3 years in 1938 and two weeks in 2005... Should I repeat the same calculations right now, it would take me one week due to the improvements in hardware...
Regards
RE: When do you use FEM
Also, since your analysis is faster, have the problems become that much more complex, thereby negating speed advantage? I know that as computers speed up, what we analyzed has become more complicated mostly through our own choices to _not_ simplify structures, thereby negating any improvement in CPU speed or memory. In many cases (say airplanes), what was considered the analysis detail accepted say in the 1930s is no longer accepted by the manufacturer or the regulating authorities. I suppose the more complex analyses gives us better designs, but can't say for certain. There are an awful lot of airplanes designed 'back of the envelope' in the 50s that are still flying today.
Bravo to you for trying to repeat an analysis, though. I've never been able to find financial support for such endeavours. So many analysis methods are based on legacy analysis or experiments (for instance, Peterson's) that might be outdated, but no one seems to be willing to check the assumptions and try to improve the results.
RE: When do you use FEM
Prost, of course the cost for doing an analysis with the "same" precision as "yesterday" has dramatically dropped. But moreover, it's the "time-to-market" which has dropped even more. So, in my example, no-one would wait 1 year TODAY for a design that nobody would have claimed to have in shorter than 3 years YESTERDAY. That's why we can't afford TODAY to calculate "by hand" a very complex structure, although this may be possible. In fact, at least in my example, this HAS been possible indeed. And, probably, you were as safe in a new Boeing 747 as you are now in a new Boeing 777, but the improvements are elsewhere: aerodynamic efficiency, fuel savings, time-to-market, etc...
Regards
RE: When do you use FEM
Computers allow you to do many more analyses in shorter time; are the designs really improved? As the design margins go to zero, how's your numerical reliability? That is, how reliable do you think your computations are? I'm not talking about numerical convergence; I am talking about your squeezing of safety factors (or margins of safety) by assuming that because you can model increasingly complex structures, your numbers are better. The reliability of your numbers can be controlled by the quality of your inputs--say you are doing a plasticity analysis, how good do you think that plasticity model really is, and how much do you trust the model inputs such as yield stress?
Sometimes maybe we just need faster ways to do the same hand calcs, for instance in Excel or Matlab.
RE: When do you use FEM
Alternatively in a spaceframe chassis design, it is very easy to create models that will run through all the permutations of tube diameter for each member to pick the best. (Well OK it can't, the number of combinations is too large, but there are ways of getting round that).
I'm not saying a great analysis by hand could not have reached those designs, but it is nice to be able to take a step back from the grunt level work and spend more time on the two important steps that can't be automated: 1) how good are our data and assumptions and goals (2) how sensible is the result
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: When do you use FEM
If the geometry is complex; beyond a cube or a cylinder
FEM forces you to state the problem and think it through.
Many engineers that I have seen, who do not do FEM, do not do free body diagrams or many pages of hand written substructured sketches. Even a good engineer, without a free body diagram can miss something.