How can I self-learn finite element analysis in a group that does minimal FEM work? 6

[EdwardNigma]

Hi all. As the subject states, I'm interested in learning FEA at work even if it's for simple applications. I currently work in a large stress engineering group where we check suppliers' FEM analyses for aircraft interiors but it comes in the form of a report. We perform simple hand calculations to validate the FEM analysis. However, there are a few people in my group who do model and analyze things from scratch using Catia and Patran/Nastran. Even for them, it's not a full-time job but they model different systems throughout the year. I don't work directly with them. On a day-to-day basis, I typically perform analysis using Excel templates for new designs, capture suppliers' analysis in a report for release, and also perform stress analysis on MRB issues (repairs mainly through reinforcing doublers).

Recently, I expressed interest to my manager about learning FEM, in particular FEA using Patran/Nastran. I listed it on my development plan and he paired me up with a lead stress engineer working in my group on a different aircraft model that does occasional FEM work. The plan as of now is for him to assign me some projects that were done in the past to get my feet wet in modeling and running analysis. Most are not too complex but it's modeling a system, of mostly composite structures, and applying certain loading conditions. However, although my mentor is helpful, he's there to help when I have questions but doesn't have time to hold my hand during this process.

A little about myself: when I first started my career in the aerospace industry, I started as a stress engineer doing extensive static and dynamic analysis using Patran/Nastran. I left job after 2 years to work for a large OEM but switched over to design engineering. Over time, I got the opportunity to work on stress engineering again but it was limited to hand calcs (static and fatigue analysis) of primary structures.

The question I have is, what would you recommend I do to self-learn FEA using Patran/Nastran? Are there books that you guys recommend? What advice would you give me considering my situation? What steps of progression would you recommend (ie tutorials, replicating things I analyze by hand, helping other groups). Are there certain guidelines to follow when deciding what modeling techniques to use?

I want to tell my mentor that I want to work on x, y, and z projects but I need to do some of that work myself and be proactive. I have access to the MSC Patran/Nastran course notes but can't take a one-week course with them because my manager doesn't think it's necessary due to the current statement of work. I found it easy to find information on modeling isotropic materials but not so much on composite materials. I am aware that it's good to go straight to Nastran input files and edit that on a text editor before looking at the results in Patran. I just haven't done it in a long time and would need to re-learn some of this again (debugging text files).

 

[SWComposites]

You need to create, run and post process models of actual parts and loading conditions. There is no better way to learn.
Doubt there is any useful books or tutorials beyond the very basics.
Just start learning from the assigned mentor. And learn how to ask good questions. And get all of the applicable manuals and refer to those as much as possible.
And remember, FEM is just a tool, it is not the end all method; thinking is still required; and many analyses don’t need a FEM.

 

[WKTaylor]

AGARD-LS-147 PRACTICAL APPLICATION OF FINITE ELEMENT ANALYSIS TO AIRCRAFT STRUCTURAL DESIGN

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[rb1957]

self study ... look at some models your company has made, talk to your lead about why it was done this particular way, how else could the model be made. When I started to learn FEA (back in uni, a long, long time ago) we did simple truss models by hand ... Bruhn examples would be good. Another thing I'd recommend would be patch tests ... a single element models to see how the element reacts to non-ideal things ... aspect ratio, skew, tension, bending, etc.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WKTaylor]

Hmmmm... interesting nom-de-guerre... EdwardNigma = ENigma???

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[EdwardNigma]

Thanks guys. I really appreciate the advice. From what I hear here I should stick to what my mentor points to as they'll definitely be real models with actual load cases which makes sense. I have taken FEA classes and have a book from college but it's emphasis is on 2D elements. It's highly theoretical, which is important but FEM is sort of like an art from what I've seen. ENigma is a reference to the Riddler from Batman WKTaylor.

 

[rb1957]

the trick is marrying the theoretical with the practical, applying our "limited" analysis approaches to the "messy" real world structures.

If you're working interiors, can you please not do something I see a lot. If you have a monument attached to the plane's floor at four points, please (please, please) don't constrain all four points. Definitely don't filly constrain (6 dof) the four points, and don't rigidly constrain the 4 points in 3 dof (X, Y, Z). This is a redundant support, and with 9g fwd crash case develops large, self-cancelling, in-plane load (Y loads if X is fwd/aft). Ideally, support the monument on finite stiffness constraints; I like to use three rods at each constraint point so I can quickly see X, Y, Z reactions. People will ask what stiffness, and I say there's little difference between 10^4 and 10^7 ... go with 10^4 if you like (Al, E = 10^7, 1" long, A = 0.001 in2). The validation of this approach would be the deflection of the monument node, a few 10s of thou of an inch.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

yep, agree completely with rb. getting appropriate boundary conditions for interiors components is critical, and something you need to discuss with your mentor in detail.

 

[EdwardNigma]

Thanks RB1957. I know what you're saying. Recently, I took a composites course and our instructor emphasized that for modeling composite plates subjected to out of plane pressures. I'll have to get with my mentor to fully understand this.

Just a little more about myself and my current situation. I used Patran/Nastran straight out of college and I performed linear/non-linear static and dynamic analysis of propulsion systems but that was only for about 2 years. I worked in a development team that did this supplying products for major OEMs. I later went on to work at a large OEM thinking I'd get better experience and worked designing tools for aircraft using Catia, so I switched over to design engineering. I used Nastran to analyze simple lifting equipment I designed to compare with my hand calculations used in stress reports. The FEM part was more of individual projects I did on my own. The analysis was very limited and only done in parts that took large lifting loads or supporting equipment. Eventually, I moved onto performing fleet support stress analysis of one-off fuselage repairs where the only FEA I modeled were joints using 1D models with rivets using an in-house FEA code and the main job in that role was to interrogate the certification FEM model, isolate the area I will analyze, extract loads from elements, and perform static and fatigue analysis through hand calcs.

Now, I work on aircraft interiors for new configurations or designs. The work is basically reviewing suppliers' analyses and Catia models, finalizing SCNs, and also performing stress analysis of MRB issues in the factory (hand calcs). I'm really just looking to become a generalist stress analyst where FEM is a tool in my toolbox. I've lost the limited FEA capabilities I had but would like to do more FEM in the future whether it's in my current group or being proactive and helping a different group in my organization.

 

[SWComposites]

some possibly useful info:

https://gama.aero/documents/gama-pu...structural-substantiation-method-version-1-0/

https://www.icas.org/ICAS_ARCHIVE/ICAS2014/data/papers/2014_0460_paper.pdf

I thought there was an FAA doc on FEM substantiation/validation for interior monuments, but can't seem to find it.

Also, if you want to do more real stress work and FE analysis, get out of the interiors group and into an airframe stress group.

 

[GregLocock]

...and make friends with the modal analysis and vibration labs.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Sparweb]

On the subject of Validation, this is fairly detailed, from the FAA:

 

[WKTaylor]

SW... another document for FEA validation... I wonder how this track s with the FAA document?

ASTM F3601 Standard Practice for Structural Finite Element Model Verification and Validation

NOTE1. Richard Abbott, Abbott Aerospace Canada, has real heartburn with regulatory over-site of aviation... especially by the 'non-technical idiots' at the FAA... and especially about FEA/FEM validation... in this editorial... Your thots???

How much oversight is too much?

https://mailchi.mp/6260244e947f/abbott-aerospace-newsletter-may-2024?e=2a6566061a

So the counter-question to 'How much oversight is too much?'... is... 'How much oversight is too little?'

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[rb1957]

I don't think it's matter of too much oversight, but too much oversight from people who don't know what they're looking at. Oversight is good, and we go wrong when we don't have enough of it (ie the right oversight at the right time).

If you go to work for an OEM, and work in the technical world you quickly become more knowledgeable (on some aspects) that someone who works in the certification world. And of course someone who has lived with a design for some years will know much more about it than some one parachuted in for a few hours (days?).

It works best when both the technical side and the certification side work together, each acknowledging the other's contribution, each appreciating that the other wants to get the job done ... as quickly and as safely as possible. If the technical side "hides" stuff from certification, if certification dig their heels in on "trivial" matters, then it falls apart.

I hate it when we spend months reviewing and tweeking a report with no change to the design and with very little change in the analysis, just more words.

On a recent project I had a good encounter with a certification authority where we started going down a rabbit hole but I quickly showed the approach was extremely conservative, and certification knew enough to see that too, and the problems went away. Later, another encounter not so successful, we spent several iterations of a report including things that were (should have been) obvious but the reviewer made a comment so we had to reply, which created a counter comment, etc ...

sigh

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[EdwardNigma]

Hey Sparweb, just noticed that the fuselage stress analysis I did was on slides 26-28 of the FAA handout you provided. Just basic FEA for joints but analyzing more through hand calcs by performing net area checks, joint analysis, making sure fasteners were like 4D-6D in spacing, performing fatigue analysis on the critical fatigue detail, sizing the repair doubler and tripler and writing a disposition to the customer. This brought back some memories.

 

[WKTaylor]

RB... I've worked with USAF since 1983.

Others in our group work with the *** on 'similar-to' Acft. Our folks 'row-to-hoe' is made much tougher by excessive oversight by a staff of XXX advisors/POCs with little/no in-depth technical expertise in the aircraft [in-general]... and little/no practical stress/durability/fatigue, systems troubleshooting, in-service corrosion, practical service engineering experience... etc. Sighhhh. I saw this phenomena when I worked overseas... it was always funny-odd to work with XXX and XXXX folks... as an experienced USAF engineer. Ahhhhhhh the stories I could tell.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[rb1957]

Yeah, but the military understand that situation. Junior officers are expected to listen to their sergeants, and the good ones do (listen and learn). The problem would be colonels and above who think they know something, who expect to be listened to (and obeyed) and often don't know.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Sparweb]

Wil,
I stopped receiving newsletters from Mr. Abbott a few years ago. He must have some strange friends because his political views (I won't discuss, go look for yourself if you care) have become extreme. Yet why he still cares after living in the Cayman Islands for a decade or more is also hard to fathom.

So why has the FAA decided that a formal validation of finite element models is required now?
Straw-man question set-up, right at the start.

Now, any Finite element model requires validation, no matter how simple or determinate.
Make sure we think he's a reasonable person before he gets rolling.

Some years ago the certification agencies started to get nervous about the integrity of finite element models. The reason for this is not clear.
Has he had his head in the sand for decades? FAA and EASA have been on guard about garbage-in-garbage-out for my entire career. I was taught about model validation in school (in the 90's), had a boss (DER) who basically never accepted a FEM solution for 10 years, and have been handed some lame FEA results by junior analysis many times. No wonder the regulators are edgy about this - everyone has been ever since FEM models were invented. Why should the FAA ignore their concerns?

“The reason that the regulator wants greater oversight over FE modeling is because most people at the FAA do not really understand FE modeling. Getting a detailed plan agreed and approved for exactly how you are going to show validation of your FE models is very difficult and can take a very long time because most people at the FAA do not really understand FE modeling.”

That probably is true, but it doesn't justify distrust of the FAA. Their job is not to be a group of analysts, and nobody comes to the FAA/EASA/TCCA to build and solve FEMs for them. As RB1957 rightly pointed out, they have a different goal, and it can be accomplished without each of them knowing how many degrees of freedom will over-constrain a node in a joint.

This is a bit of a diatribe. I guess I've written a few screeds like this myself in my time, hopefully nowhere that my colleagues can find and bring back to haunt me.

Maybe Abbott has had a bad experience that went out of control. I have too, but I've also had good discussions on these matters, and successfully passed through the experience without headaches. It really helps to come prepared!

 

[Sparweb]

Enigma,
I think a lot of aero structures engineers have done that sort of analysis (referring to those figures on pages 26+27). Skin penetrations, repairs, antennas and a common modification. I'm a little surprised to see FEA done for a 6" blade, but the OEM's do it because they have all the wheels in their analysis machine spinning already. A guy like me in a little office looks for simpler ways. Hand calcs are pretty easy for those. A few of Roark's formulas go a long way.

Wil,
Were you and your colleagues experiencing something of a power-trip dynamic? The so-and-so gets a buzz out of bossing the USAF guys around? Another colleague of mine (since passed away) had an experience like that with South Korean officials, and had some stories to tell.

 

[SWComposites]

Sigh,
Given the FEM abuse and rubbish I’ve seen over the years (and I’ve probably been guilty of it at times), the FAA is right to be concerned. Particularly now that almost any fool can push a couple of “auto model” buttons in a CAD program and pop out lots of FEM pretty pictures. And then say “see, no red spots, all margins are positive”.

Where people get into deep trouble with FEM “validation” is:
- crappy test setups and locations of strain gages and LVDTs
- making claims that FEA can accurately predict detailed stress concentrations and then trying to correlate those
- attempting to correlate results of a complex post-buckled structure with a couple of strain gages.
- and so on
The FAA is not going to help you dig yourself out of a deep hole that you have foolishly created on your own.