What Level of Theretical Knowledge do FEA Analysists Need? 16

Let me give you a simple example of a true story I have run across. An "analyst" was doing a "simple" problem of solving the deflection of thin sheet fixed in all DOF on its edges with a pressure load applied. Seems very simple right ?

Why then were his results 5X, yes 500% off ? The answer is because he was lacking the theory and knowledge. More specificially, he did not understand the concept of stress stiffening in membranes.

So you see, even what appears to be a simple problem to many may require extensive knowledge. In my opinion, a Masters degree is a minimum. This is just my opinion, but think about the above story and try explaining why you got 500% error in your simple problem.

FeaGuru has written : 'a "simple" problem of solving the deflection of thin sheet fixed in all DOF on its edges with a pressure load applied. Seems very simple right ? '
I think the simplicity here depends on the magnitude of the load and hence the resulted deflection. It is well known that in plane stressed/tensioned elements (not only membranes, but also beams...or in general thin elements!) are stiffener, that is referred as stress stiffening. So a non linear FEA must be undertaken and the option stress stiffening should be also turned on, if the deflection is exactly very larger then expected. But if a "BSc"-Engineer has a long term experiences, then he must be aware about this situation, at least he has read the software manuals (if any), that explains it. Any comments?
I only find it good, if an analyst that does a non linear or dynamic FEA-problem should have a Masters degree. But a "BSc" one with equivalent experience is also capable for doing it.

cheers

FEAGURU, think about those programs in use out there which don't involve error estimates or that no one is checking!! Yikes!

My point is that how can someone seperate the straightforward problem from the one that isnt unless they have the background in such nature ? If your satisfied with results that are very accurate sometimes, but garbage other times great...do you know which are wich though ?

The reality is that it happens all the time, I see it all the time and am continually disgusted by it. I see it by people with Master's degrees and Phd's, but FAR more often from the ones with just a bachelors. There is no hard and fast rule, but I know which side of the bell curve I'd rather be on is all.

P.S. Im sure Ive made my share of mistakes as well, you can only hope to minimize them.

I understand your point well and only meant to emphasize it. For me there are too many general programs out there that cater to those with less than necessary understanding of FEA yet market the FEA as a tool to be used by anyone who merely picks up the user manual...sorry, I don't agree with that approach. Nor do I think that FEA is for everyone to work with.

Ok, Perhaps I was not specific enough in my original question. I was referring to FEA theory rather than engineering theory or knowledge in general. I admit that there is not always an easy distintion to be made.

FEAGuru's example is one where it is general engineering knowledge that was lacking rather then FEA theory inparticular. Presumably the analysist would have made the same mistake if using analytical solutions! Also I included the proviso that the analyist could read the manuals and was careful in checking the results.

I do think that properly qualified and trained engineers should be FEA analysists.


TERRY

I think, what Terry suggested, that there isn't a clear distinction between engineering and theoretical / mathematical backround. May be we should refer to the distiction made in that engineering = FEA and theory = FEM. That means in order to be an analyst, one should only know about engineering "face" of Finite Element, this is what normally described in many manuals of FEA-Tools concerning limitations of each elements and used solution methodes or the possibility of errors. But in order to be a FEM-Developer or at least PhD-Student in this discipline, then one should have the strong mathematical/theoretical background behind the FEM, such as Raleygh-Ritz or Galerkin methode, shape function, and so on.
From this reason I must say also, that properly qualified and trained engineers should be FEA analysists.

cheers

Well, zuardy, that is exactly what I mean - so why could I not say that in the first place!

TERRY

All Right, All Right..........Let's put the rubber on the road.......

I've got a BS and MS in CE (Structures Specialized in both)and have ~15 years doing FEA in the Aero and civil fields. The first couple of years I did a bang up job building models and pretty pictures but couldn't get answers that could be validated by a hand calculation. All the manual reading in the world didn't answer the questions I needed answered. So, I took a finite element class to learnabout matrix analysis and all that good stuff. The prof I had was a tenured prof who was a great theoretician but no applicator. I then took this knowledge to my peers who had gone the same route and then we all began studying what the heck we were doing until we learned that every problem is different and that the greatest advantage is having a great intuition of physical component behavior. After about the 7th year of this exercise I began to feel "confident" in my anlayses without having someone check my assumptions and procedure. Now, 15 years later, I beg to have my work checked to make darn sure I haven't overlooked some truly devastating number or input.

What's my point??? It takes 5 to 10 years of very intense grinding study and application to collect all the tools, skills, intuition, and experience to be an FE analyst.

Want a good test of my hypothesis???......take a job as an independant contract finite element analyst, if you can make it through your customer's expectations and produce a marketable product to their liking AND have them ask you to do more.......you have what it takes. Try doing that by reading a few manuals..........

ISEAG
Well said - it is true that the best theoretician in the world may not make the best analyst. I agree you do need to be a competent engineer to be a good analyst and this is much more important than knowing lots of FEA theory. However the theory can be helpful but it is more useful than a necessity.

TERRY

"...every problem is different and the greatest advantage is having a great intuition for physical component behavior."

Set that in bold type and put it at the top of the page! The rest you can get with time and a Mastercard, but if this priceless capacity is not in your genes to build upon, you need to find that out as quickly as possible and then find something else to do. If you have a mind that can take a complex structure, intuitively break it into a set of FBDs, then predict the resultant behaviors, you will probably do fine with FEA. If you never learn what the results are going to be before you get them, then FEA had better be someone else's tool.

Well stated Darrell!

I strongly agree with Darrell et al about physical intuition.

Finite element models are often so complicated that you can't hold all the information about the model in your head at one time. A small inconsistency in one of the 100,000 elements can cause the whole thing to produce funny results (or worse, produce funny results intermittently). My experience is that the majority of modeling errors come from stupid mistakes in the construction of the model, and not lack of understanding of theory.

Whether the bug in your model is that you assigned a concrete element steel material properties, or something to do with convergence, physical intuition is invariably what alerts you to it. Maybe you could argue that further academic study enhances your understanding of of the physical world, but most of the theory we are talking about is just high calibre math. Math and physical intuition are totally different.

I agree with the last few comments. But this applies to any design aid, not just FEM software.

You do not necessarily need to know the theory well enough to write the program in the first place. You DO need to understand the theory well enough to allow you to model the item and interpret the results.

If a designer does not know what results to expect from a computer program (within 5-10%), then he should not be using it.

I had a case about 10 years ago where a soils engineering company did an analysis on a raft slab for project we were doing and supplied us with the results on which to base our design. The person doing the analysis was a well respected Phd in the field. We looked at the results and they looked logical. We then asked for some changes to the shape (architect again) so he reanalysed and gave us a new set of results. A junior engineer in out office then designed the reinforcement for the raft. He complained to me that he could not make the design work in one area, it was having ductility problems and he simply could not add enough reinforcement to make it work. I looked at the 3D moment plot at the point where he had the problem and asked how he could develop a moment at that point, let alone 6000KNm/m. It was perpendicular to a free edge. We questioned the Phd and he fixed the bug in his program for rectangular elements (in the first run he had used square elements and have changed to rectangular to get a quicker solution) and a week later gave us a new output which showed a moment at the same point of 0. He had been using that software for several years and never noticed a problem.

The Phd should have picked the problem. The moment plot "LOOKED WRONG". That is the understanding of FEM that a design engineer needs. An understanding of structures and how they act and what the results should look like. With FEM, there are too many numbers to look at every one and check its vaule and logic. But the designer needs to know what the pattern of results should look like, what the order of the numbers should be.

Computer software is not meant to replace engineering knowlege, understanding and ability, it is meant to do more indepth calculations to allow an engineer to make better decisions about a design situtaion. Too many "engineers" use design software because it obviates the need for them to learn how to design properly and understand what they are doing. They are wrong. They are not engineers, they are technicians.

Many software developers push the ability of their software to do the job of an engineer. Noone should buy or use their software. Software is a tool to be used by knowlegeable people who understand the field. A good example of this is the number of designers who have absolutely no understanding of prestressed concrete design who are putting out designs using Black Box software. They do not understand what they are doing or what the software is doing and the limitations of the design methodology they are using. They are dangerous and so is the software.

I think the last discussions "converge" to the conclution that advanced FEM-Software can not supersede the FEA-Engineer's knowhow. To emphasize this, let's refer to the well known error posibilities in using FEM-Software :
1. Idealization and/or modeling error (physic)
2. Discretization error (nature)
3. numerical error (mathematic)
The FEM-Software actually can only capture the 3rd. error, since this is completely done in the FEM-solver. But due to the influences of the 1st. and 2nd. error (if any), the complexity of the error rises and therefore the FEM-Software can only give a "User Message" to the engineer (if it can!), so the engineer should undertake himself the selective changes to the FEM-Model. E.g.
- a not properly constrained DOF (1st.error) would call User-Message : singularity for this DOF or
- too fine meshed dynamic structure would create a numerical error type of User-Message, due to the complexity of algorithm used in dynamic solver.
And yet many examples.
In general the 1st. error is under the responsibility of the engineer and fortunately (^-^) can not be fully superseded until now by the FEM-Software (Preprocessor), since it is a blind tool, so it can't "see" the real problem and can't make the decision concerning the element type and the soulution type. This is a huge challenge of FEM-Developer to create an algorithm that enable the preprocessor to analyze the shape of given structure and then mesh it automatically with the suited elements. At least they can implemented an expert system in their FEM-Software to support the engineer during preprocessing.
The most recent developed element free Galerkin methode or sometimes called meshfree or meshless could make this kind of engineer job obsolete, but fortunately it can't solve thin or slender sturcure problems well, so it remains the engineers job region, yeah ;-).
I hope this will make clear where the FEA-engineers stand and where the FEM-software stand. We all do not want to loose the job in the near future at least ;-)

cheers

My two-cents on this. Nowhere did anybody mention what level of sophistication we are talking about. Great comments, but I would counter Terry's original posting by stating that at a certain level, the FEA theory IS the engineering theory.
When one starts dealing with the very high-level problems, often the behavior of the structure cannot be understood by the most knowledgeable "classical mechanist", because the frontier is solely in the FEA algorithms employed.
A good code will state these distinctions within their theory manual, but not at a level which can be easily discerned by anybody short of an advanced applied mechanician.
I agree that for more simple problems, a good engineer can run good FEA without knowing the nuances of FEA solvers. However, as soon as somebody starts stumbling into singularities, zero pivots, negative eigenvalues, wavespeed problems, and convergence difficulties (to name a few), one will have a hard time interpreting what is breaking down without the theoretical basis of FEA.
Thanks for the interesting posting!
Brad

Thanks all,
In my original post I was trying to play devils advocate, but the discussions have helped me to make some sense of the original question. I am now certain that:

1) A good analyst is first and formost a good engineer, this should include good physical intuition for the problem. In particular I think I would sum-up the requirement in that "if an engineer cannot make back-of-the-envolope calculations of the problem they should not be using FEA/FEM to solve the problem".

2) Depending on the complexity of the analysis, a detailed knowledge of FEA theory is not required to be a competent analysist.

3) Any good engineer that is using FEA will familirise himself with the appropriate theory. See point 1).

So perhaps this has been a bit of an accedemic discussion. When hiring your next engineering analysist look for someone with a passion for engineering and an inqusitive mind - perhaps the rest will follow!

Brad, thought we had lost you - welcome back.


TERRY

Terry,
Great summary.

I'm still alive and kicking. However, the presence of a 13-month old running me ragged (and warming outside weather and "honey-dos" that come with it) are starting to limit my surfing time.


Brad

Tubyguru,

If you get hired buy a small, large or any company straight from school with a BS in engineering, you should not even be designing simply members without an experienced designer reviewing your work. You are an apprentice designer and need several years of supervision before you are a real engineer.

How can you tell if you have not made a modelling mistake if you have no experience as to what the answers should look like?

Do you know how the results of the FE analysis should be applied in design?

In structural engineering, many designers using FE analysis for slab systems still think they will get reduced design moments from an FE analysis compared to a 2D frame analysis. The only thing the FE analysis does is give a transverse distributiion of the moments. The total moment in a panel is the same for both types of analysis. They are simply reading the numbers wrongly and do not realize it and consequently underdesign.

A designer must understand the basics of FE and have the experience to interpret and apply the results, or have direction, advice and supervision from an experienced designer. That is how yoou become an experienced designer. Not by trying things unsupervised until you eventually produce a building that does not fall down when it is built. Clients hate this approach for some reason. You learn from an experienced supervisor who fixes your mistakes before you build them.