Linear FEA in stress design!
There is nothing wrong with using Linear FEA! The trouble starts when you use it, unaware of the problems associated with this approach.9 March 2021
It’s funny how these things go. It never occurred to me to write about prerequisites for studying FEA. And when Sesha asked about this in a recent email, I was really taken aback! It’s so easy to treat FEA as a “complete skill”, but like almost everything in life, it’s interdependent on other skills you should possess. Let’s ponder today what you should learn/understand before learning FEA:
First of all, you can use Finite Element Analysis in a lot of fields. I will be referencing the structural design field here since this is what I do. And as you will use FEA to design stuff… actual engineering design skills will be mandatory. But what are “engineering design skills”? Let’s dive deeper!
To begin, I feel I should point out that I will not mention things you definitely don’t need to know (while they are being taught everywhere anyway!). Yea… I’m looking at you: complex mathematics! I have already shared my thoughts on this here, so you can take a look if you are interested!
Today, let’s focus on what skills you will really need to start learning FEA!
Let’s start like this:
Imagine that you are an inventor (it’s uncanny that spell checker tries to correct this into “investor”…). After a few years of work, you’ve created a super-cool-surgery-robot-5000TM. You know, a machine that is extremely accurate and is capable of internal surgery without cutting patient in half or whatever.
Naturally, you know the ins and outs of the machine, and you are the most knowledgeable about how it works, how accurate it is, and how to operate it and even fix it.
An investor calls you to ask for a presentation, promising a fat cheque. Without much thought, you pack the stuff and drive to his office…
… there you find a few guys in suits and an unconscious man laying on the floor. “So dear inventor, show us how it works, and operate on this dude lying on the floor here!” says the investor.
“But Mate! I’m an inventor, not a surgeon!”
… yea. FEA is like a super-cool-surgery-robot-5000TM
We live in a very complex world. And while I would love to know a lot, it’s impossible to know it all. Heck, it’s impossible to know 0.1% of it all! This is why it would be super hard to be an inventor and a surgeon at the same time. Both professions require a lifetime of learning to be good at either of them!
However, this is no excuse at all! In fact, you should know what you are good at, and what you can accomplish. And not just “try hard” to do stuff you have no idea about! So perhaps our inventor might stand a chance to do a super simple surgery (like a simple tooth removal) after sufficient training. But such “tooth removal training” would definitely be required! You can’t just “wing it” regardless of how much you know about super-cool-surgery-robot-5000TM!
On the other hand, I wouldn’t trust even the best surgeon if they were operating on me while using the super-cool-surgery-robot-5000TM for the first time in their life! It’s quite obvious that to use such a machine you need to be “trained” in using it. Not necessarily how it works precisely, but you definitely need to know how to use it properly!
So in order to successfully operate with the super-cool-surgery-robot-5000TM you need two things:
As I wrote before, FEA is like the super-cool-surgery-robot-5000TM. It’s a wonderful and super complex tool. But in order to use it in design, you also (or maybe even dominantly!) need to be a “surgeon with proper medical knowledge”. And in our terms that would be “design engineering skills”. Let’s see what those are, what is useful, and what you should not waste your time on!
It’s actually funny, but it’s hard to pinpoint precisely what engineering design skills are! To me, it would mostly be about understanding how things work, and what to expect. Plus knowing some basic procedures of designing stuff.
But the above is far more of an “end result” rather than a “direction”. And it’s useless advice when it comes to where to start learning. Heck, how can you even learn to “understand how things work”?! So let’s try to boil it down to some key elements.
I know you cringed! No worries, “strength of materials” has the same effect on me. I well remember the professor who with his deep and sleepy voice went through one differential equation after another. It’s amazing how long a 1.5h lecture can be, eh?
But this is not the strength of the materials I have in mind! While universities did their best to punch us with the complex mathematics in this region (making the subject universally hated I would add!), this is not what is needed. What you need is to UNDERSTAND THE BASICS of the strength of materials. And as far as I know, you don’t need to know a single differential equation to get there!
Instead, you should understand several key concepts that will guide your decisions. And while I would love to make a complete list, I’m afraid that some other things that haven’t crossed my mind should be added (and if you happen to notice that something is missing – let me know in the comments below!). So, while I do not claim this is a complete list in any sense (to be honest, I’m not too sure if it is even possible to make a complete list!), below are a few concepts that I find really important.
Please note that I wrote “understanding” as a requirement.
I don’t care if you even know the equations that govern a given field, as long as you know how stuff will behave, and what the consequences of any given phenomenon are! Knowing all the equations doesn’t necessarily give you that understanding!
On the other hand, knowing some of the basic equations will really come in handy later on… you will see!
I’m pretty sure that this is not the end of the list, but I hope I covered some useful things and got you thinking. I’m the first to admit, that I didn’t understand ALL of that when I first started. It came in time. However, you really should have a strong base… otherwise, there will be nothing you will be able to build understanding on!
Just when you thought this couldn’t get any worse, eh? Well, maybe it’s not worse, I definitely favored static design over the strength of materials in University! But it wasn’t a terribly practical subject, to be honest. I remember that my professor (and awesome guy BTW!) stood in front of us at the beginning of the lecture and said: “I know that I’m a theoretician, and to all of you, future designers, it may seem stupid and unneeded! But these theoretical studies of mine, allowed my wife and I to travel all over the world to various scientific conferences, and I really enjoyed that!”. I was really taken aback by that, but now, years later I’m not even surprised!
Static calculations are more or less like the strength of materials. You don’t need the fancy statically indeterminate solution schemes to go along with your design. Your computer will be more than happy to solve those for you! However, you need to know your way around the basics… so you can more or less control what the computer is doing!
Funny enough, there isn’t much I can list here since static calculations are a super focused thing!
… and that is it! I had like 3 or 4 semesters of static design at university! They taught me at least several different methods of solving statically indeterminate systems (including some graphical methods) etc. I don’t think I ever used them in any meaningful way, but perhaps learning that made me more “aware” of how the structures behave (I don’t think that is the case, but I’m open to the possibility!).
Perhaps some imagination is useful here. You know, the type that allows you to imagine, how to divide your system into the set of cantilevers and beams you can easily calculate by hand if needed. But I think it just “comes” as a natural result of having the skills I listed above!
This is the last topic I want to touch on in the article. While the strength of materials and static calculations could be seen as “theoretical” due to how they are taught, structural design skills are clearly practical in nature. This is the last step that you will need to be successful at FEA I think!
I’ve heard many times, people saying: “you should not start to solve an FEA problem if you would not be able to solve it by hand”. In some simple cases, that would mean that you need to have practical skills in the strength of materials and static (that I already described). But in the real world, people associate this with design skills way more often.
While I’m not sure if I would be so absolute in my view, knowing how to design stuff is definitely useful. And funny enough, it is more important at the beginning! Mostly, because if you know how to use FEA in one “field” you can try to extrapolate your skills to other fields. But starting really does require design skills… simply to avoid problems! Not to mention that design skills in one field, can be more or less translated to design skills in another field. After all, while rules and codes can be different, the underlying physics is the same everywhere!
So let’s take a look at what should be useful:
The only problem with design skills is, that it’s not simple to pick them up! You could of course read some books (or even codes/standards, if you think you are that tough!). But this will just give you shallow knowledge in most cases (it was definitely like that in my case). It takes a dedicated teacher or a few years of experience (or both!) to *understand* how things work! And as I wrote, it’s not only about you memorizing the needed equations! In fact, what you need is an understanding of how things work.
If you want to check what I mean by this, I did a post some time ago about hand calculations of a given case, based on the code procedure. You can read about it here. Please note how I explain stuff and “what is what, and why” in the equations. This is the understanding that I’m talking about! There are also things “I just knew”, like the fact that we were expecting problems with buckling, etc. This also comes as “part” of the knowledge on how to design things!
This all seems like a LOT right? Heck, maybe this even is a lot! But, please… don’t get discouraged!
The truth is, that the above skills are needed if you want to be a good design engineer anyway. In some sense, they are even FEA independent. So even if you feel that you don’t want to learn FEA after reading this… if you still want to have a career as a design engineer… you still need to know all this!
But there is also a glimmer of light. You don’t have to know everything at the start! I know I didn’t! Some things just take time to sink in, I observe that the more I learn, the more I understand things I used to learn in the past. It all somehow connects in your head, but time is definitely a factor as well!
So, don’t write yourself off too easily! Just always be curious and question everything (within reason of course!). It’s not the lack of knowledge that is the biggest problem. What will damage you the most is not learning anymore!
Even if you don’t know some of these things, you can still start doing FEA, and even have fun with it! Just remember, that you have gaps in your knowledge, and be careful… while working hard to fill those gaps with good practical knowledge! If you constantly learn – there is nothing you cannot accomplish!
I really hope that I showed you something useful here. If you don’t agree, or you would add or remove something from the list above, please voice your opinion in the comments below. I always read them, reply to them, and I’m happy to discuss this all in a very open manner. In short, I really care about engineering as art, propagating practical FEA knowledge, and simply helping other engineers!
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10 Lessons I’ve Learned in 10 Years!