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6 minutes read
5 October 2017

5 steps of FEA results verification

6 minutes read

Whatever we design life will verify it at the end… but I would rather know if my design has a fighting chance before any major collapse occurs!

This is why FEA results verification is a very important thing.

When you are solving a known problem (i.e. as a benchmark) this is easy to do. You just compare the outcome of your calculations with the correct known results… and that is all (or you have some tweaking to do, which is more likely!).

But usually, you won’t know the correct answer right? This is when things become a bit tricky. And if you are anything like me you may be in trouble! You see, I have a tendency to find “weird” justifications to unexpected outcomes I get from the analysis. I’m so good at it that I even managed to convince myself once or twice that something was ok… until further analysis showed that it was not!

I think a lot of people simply want to “succeed” in what they do. This means that if something unexpected happens, we tend to search for a justification of the fact. This is a great mental exercise for sure, but I’ve learned long ago to actually verify the outcomes I receive. Below is a small process I try to follow in doing so!

1. Check the shape of deformations

I think that shape of the deformation is more important than its value! Checking how the shape of the deformation looks gives you a chance to think about whether the model works as intended! This is not a code-check or a demand your Customer will have. This step is only for you, as it allows you to see if everything went well.

Note that you may need to increase the scale of deformations dramatically to see the model behavior. Again: this is not about the value of the deformation, but about the shape of it!

FEA results verification: Checking of deformations

2. Check deformation values

Checking deformation values may be a bit tricky. This is because of the software usually “auto-scales” deformations based on the model size. This means that what you see on a screen is not the “actual” deformed shape, but rather a shape with a scale factor selected by the software automatically (to “enhance” user experience).

I admit this is usually a nice feature, but it can be greatly misleading. Be sure to set the deformation scale to 1.0 and vied the deformed model there as well.

Bear in mind that too big deformations aren’t the only issue here. If you know that the model should deform, but it didn’t something is wrong just as well!

3. Check reaction forces

There are a few things here that you should be interested in:

  • Firstly, check if all simplifications in supports you have made still hold

If you modeled a flower pot standing on the ground, you may have omitted contact definition and model vertical support on the entire bottom surface of the book. This is great… unless you get a tensile force in that support! Such load could not appear there (unless the pot is glued to the ground of course).

With this in mind, if you would get a tensile reaction force in that support you know that the assumption you have made (to ignore contact) doesn’t hold!

This is a very common mistake, that can lead to big mistakes in boundary conditions – more on this in one of the lessons of my free FEA essentials course.

  • Check the force – reaction force balance

Unless you do serious dynamic analysis active forces should be in equilibrium with reaction forces. Since you usually know how much load you have applied (a good thing to know!) just check the total reaction force in any given direction, and see if the value of total load in that direction is a match. It should be! If not, something went wrong…

  • Check the “soft springs” (aka the “air hooks”)

From time to time, you will analyze problems that are “a bit” unstable. You will most likely stabilize it with a very soft spring (sometimes called an “air hook”). This is completely fake support but it helps with the numerical calculation of the problem.

When you finish the analysis it is a very good idea to check reaction forces in those soft springs… Since there is no real support there, the reaction force should be minimal!

If that is not the case… you need to rethink the softness of the spring or a place where you have added it.

If you have changed the placement of soft springs several times, and you still get huge forces in air hooks, maybe your model is so unstable that it simply won’t work?

  • Consider boundary conditions you have chosen

If you have doubts about what boundary conditions to use, use both sets and compare outcomes. This way you will see if the assumption you are uncertain about will play any significant role in design.

Also, thanks to the fact that you can take a look at a deformed model in both cases, you may be able to decide which solution is more realistic.

4. Take a look at stresses

I usually do this while checking if stresses are acceptable. But the goal here is not to assess if the part you have calculated is good or needs strengthening. We are checking if the outcomes we get make any sense at all!

By default, your software will most likely show you the average stress. This may not be the perfect approach. Just find the place where averaging is being used, and turn it off. You will be surprised how much the outcomes may change!

FEA results verification: Stress Averaging

If the differences in stresses are big for adjacent elements, you should consider refining mesh in that region!

Usually, there are several options on how outcomes are displayed in your software. Learn about them and use them wisely!

5. Do the hand calculations!

This always gets me at least a few weird looks but I feel that verification of FEA outcomes with simplified calculations is a freaking superpower!

This post is getting long, and hand calculations really deserve a post in FEA results verification. You may be sure I will get back to this on the blog!

Want to learn about FEA?

Definitely check out my FREE FEA essentials course. You can get it by subscribing below.

Author: Łukasz Skotny Ph.D.

I have over 10 years of practical FEA experience (I'm running my own Engineering Consultancy), and I've been an academic teacher for a decade. Here, I gladly share my engineering knowledge through courses, and on the blog!

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Comments (18)

Shubham - 2021-07-26 19:45:34

That was a really nice blog. As someone who got attracted by the colourful post - processing abilities of FEM softwares, I used to make several blunders during my studies. I still make some mistakes though but your blogs have really helped in thoroughly criticising my results before accepting it as it is.
Keep up the good work!

Reply
Łukasz Skotny Ph.D. - 2021-07-27 12:16:23

Thank you Shubham! I'm so glad that you like the blog, and find it helpful!

All the best!
Ł

Reply
Daniel Gaspar - 2020-07-10 11:47:27

Great review, maybe I would suggest also to perform a quick modal analysis to check that there isn't something "flying" or not correctly merged, because sometimes if it happens far from the clearly deformed parts of the model you don't realize it (and then if you make another analysis later you go crazy haha).

I am very interested also to know how you make the hand calculation checks and which are the best practices cause is something where I need to improve a lot, so I look forward for that post :)

Reply
Łukasz Skotny Ph.D. - 2020-07-10 18:14:52

Hey Daniel!

Nice idea - that is a pretty cool thing. I admit that I do a quick linear buckling for that, but it's essentially the same thing :) Indeed - sometimes, stupid things strike from the shadows in the least preferable time :)

Ach, that is a tricky part! I was thinking about this post for a while... but to be honest, there aren't many "general rules" that I follow. For whatever reason (I guess 10 years fo teaching by-hand-calculations at Uni) I simply know how to estimate stuff by hand. I'm doing some things for my nonlinear FEA course, but I will think about a post for sure!

All the best!
Ł

Reply
Lucjan Nastalek - 2020-05-29 13:20:06

Hello Lukasz. Have you covered a topic of a V&V?

Reply
Łukasz Skotny Ph.D. - 2020-05-30 18:40:09

No not really... I think that a lot of folks understand this term in very different ways... and somehow it always threw me off, as I felt that whatever I will write on it, it won't be "good enough"...

What would you write in such a post Lucjan if I may ask... I mean, what topics would you describe :)

All the best!
Ł

Reply
Kashif Salman - 2020-04-21 00:50:33

This really reflects your great understanding of the FEM solution. Thanks for your kind sharing.
Stay Blessed

Reply
Łukasz Skotny Ph.D. - 2020-04-22 05:59:56

Thank you, Kashif!

I'm really glad that you enjoy my content :)

All the best!
Ł

Reply
Manuel - 2018-04-01 21:22:48

I shocked with your honesty when you wrote "... find “weird” justifications to unexpected outcomes I get from the analysis. I’m so good at it that I even managed...". I'm usually involved on civil calculations easier than subjects that you explain and sometimes I find myself on the same point and it scares me! Lucky me that I also revise twice.
Great blog! I can't stop reading! Thanks for sharing your knowledge!

Reply
Łukasz Skotny Ph.D. - 2018-04-02 17:18:36

Hey Manuel!

Designing stuff is pretty responsible! At some point, I realized I need a second person that is not easy to convince by my "random theories justifying weird outcomes" to check my designs... and all is good :)

all the best, and thanks for writing!
Ł

Reply
Alexandros Theodoridis - 2018-01-20 16:30:05

Very good! Go on with such intresting articles!

Reply
Łukasz Skotny Ph.D. - 2018-01-20 19:00:50

Hey Alex!

Thank you for the kind words :)

All the best!
Ł

Reply
Kumaresh - 2017-10-11 07:09:39

Got some useful stuffs. Thanks for the nice share and your efforts.

Reply
Łukasz Skotny Ph.D. - 2017-10-11 07:39:26

Hi, Kumaresh!

I'm very happy that you like it!

All the best
Ł

Reply
Sathish kumar SM - 2017-10-10 22:46:41

Very nice!

Reply
Łukasz Skotny Ph.D. - 2017-10-11 07:00:48

Hey, Sathish!

I'm really glad that you like this!

All the best
Ł

Reply
Puneekumar - 2017-10-10 08:54:14

It's one of best information u shared sir
Thanking you sir

Reply
Łukasz Skotny Ph.D. - 2017-10-10 08:56:03

Hey Mate!

I'm very glad that you like it!

All the best
Ł

Reply

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