Hand Stress Analysis
Hand Stress Analysis
(OP)
Not to disparage anybody's quality of training in aircraft structural analysis, nevertheless I have noticed a recent trend I thought this group could help explain. Just perusing the many ads for "stress analysts" or some similar description, I have noticed that lately in addition to asking for experience with "FEA" that "hand stress analysis" experience is also requested. It might be that my training is substantially different from that of others, so I don't know--don't all FEA stress analysts know how to do hand analysis? It would seem almost obvious to me that you would need to be able to do hand stress analysis before moving on to FEA, and that hand stress analysis is still a routine part of any finite element analysis of a structure. What am I missing?





RE: Hand Stress Analysis
FEA packages, and automatic mesh generators are so good these days that there is simply no need for any engineering skill or judgment when performing stress analysis anymore.
Dump in the model and click a few buttons. Lots of pretty plots come out. If there are lots of red areas, tell the desinger to add more metal in those areas.
Repeat until no more red areas.
Why would you need hand calcs?
RE: Hand Stress Analysis
It is sad, but it is found quite often in reviewing a stinking lot of resumes that a significant number of young to middle level engineeres equate stress analysis wih building pretty FEM's.
I talk about the old school method of using a global loads FEM and then using classical hand analysis techniques to produce margins of safery based on Allowable load not some material stress level and people look at me like I am crazy.
But some things have driven the industry to this and I blame the schools in conjunction with some of the non-aerospace industries. Suspension Arms manufacured from castings with 1-4 loadcases are one thing, but a full airframe with over 1500 load cases are not FEM stress friendly. It is easy to blow a TET10 mesh on a solid model from CATIA, put a few loads on it and you have stress contours ripe for the viewgraphs.
The schools are spitting out computer wiz's who cannot free body a simple beam and cannot explain to you why you need 6 rigid body modes for a model, but boy are they fast.
Sorry, your comments touched a nerve.
RE: Hand Stress Analysis
You are right on the money. It seems that very little emphasis or training has been placed on the difference and applicability of "loads" FEM versus "stress" FEM. I do realize there are applications such as engine design which inherently needs to rely on detail FEM due to the complexity of the components involved. However, it seems that the current general trend is to build all FEM as detailed stress models utilizing plates and solids. Alot of this can be blamed on the false thinking that just because its easy to port a solid Catia model to FEM it should be just as easy to let the computer to the stress analysis.
This current trend seems to be a dangerous one for our industry as it not only tends to focus the engineer more on the task of building a detailed model rather than understanding the global applied loads and internal load paths but it also tends to skew managements perception of what it takes to perform the job. For example, in the past 15 years, it has become almost common place to use young engineers to build detailed FEM of cabinets and racks for aircraft interiors. In all of the DER reviews I have made of such models to date, I have yet to find a single one without some major mistake in load path or constraint which resulted in having to redesign it. I dont think this is entirely the engineers fault but the task of building a detailed FEM requires alot of concentration on building it leaving less time to understand load path due to the ever present pressing delivery schedule. Second, detailed stress FEM of cabinets or racks or similar structure IMHO is a complete waste of time and money and does not provide proper training for young engineers. The best way to FEM these types of structure as well as many others including full airframes are as coarse grid loads FEMs using bars, rods, shears, etc. and then perform the stress analysis using classical methods backed by test based allowables. The big rub is that it can take several weeks to build a detailed stress FEM and then many more to debug it and one is still not assured of the results. On the other hand, a coarse grid loads FEM can be built quickly and detailed stress analysis can be completed in a much shorter time. Obvisouly this all depends on the complexity of the problem and the experience level of the engineer. Unfortunately management is losing sight of this and is relying more and more on the detail stress FEM route.
Anyways, it seems that the awareness, concept and proper application of loads versus stress FEM is quickly disappearing. Just try to talk to some of the FEM sales folks and ask them if they provide GPF Balance Output or Element Oriented Forces? They will look at you as if you were crazy or from another planet and why in the world would you ever want that? Just let the model give you the stress and be done with it.
Oh well,
RE: Hand Stress Analysis
corus
RE: Hand Stress Analysis
You get to choose /some/ parameters.
I don't want to sound like a smarty, but, ultimately t sees to me, a piecewise (ie FE) solution is the only option other than interminable trial and error.
However, I agree with the general thrust here, kids of today, yada yada.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Hand Stress Analysis
I don't think any of us are saying that FEA should be done away with.
Rather, if you don't understand basic concepts like load paths, you can't be trusted to produce a useful FEM.
RE: Hand Stress Analysis
I'd put all the salesmen (and their bosses) of the integrated FEA/CAD products on a long plank and ask them to walk.
But, to be fair, I haven't seen these things used on a serious design. It'll happen, no doubt. It probably has happened. I'm waiting for the lawsuit, then at a guess this stupid fashion will dissappear.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Hand Stress Analysis
Maybe your problem with these people is that their analytical skills aren't good enough? It would be hard for me to imagine a situation in which someone with an engineering degree could have poor analytical abilities, ('poor' relative to the general population of college grads that is).
Sounds like a university problem and a corporation training problem. Even with all the fancy gizmos and whizbang toys engineers now have, early engineering careers still have to be apprenticeships, don't you think?
Stil, I don't think anyone gave me any insight as to the apparent recent appearance of this phenomenom. Maybe it's something like 'feature creep'--a process so gradual that no one notices the subtle changes in which structural engineers (probably the same in other disciplines) have become so tied down to their FEM and CAD tools that someone has neglected to train them to do 'back of the envelope' type analyses. It is interesting to me that corporate engineering departments appear to have noticed that the engineers they are interviewing and hiring have large enough deficiencies in fundamental capabilities such as hand stress analysis capability that they go out of their way to specify 'hand stress analysis' as a fundamental job requirement; while oh dopey, ignorant me, I naturally assume any struct. engineer I talk to could already do 'hand stress analysis.' Showing my ignorance again, I suppose.
RE: Hand Stress Analysis
In the area of personal responsibility, it is apparent to me that most engineers go to work, put in their time (which can be a 10 hour day of course), go home, and never pick up a book or read a journal article, or attend a professional meeting (most aren't even bothering to join prof. societies, much less go to meetings), etc. Personally I consider this to be insufficient dedication to the learning of and mastering of a craft such as engineering. But that's another thread for another day.
RE: Hand Stress Analysis
We've got a case where the analyst needs a week to set up and run an FEA, and only the really old geezers can do a quick estimate by hand
TTFN
RE: Hand Stress Analysis
Prost, I have to disagree with you here. We are in "business" not academa, so it is not incumbent on senior engineers to teach junior engineers; it is the responsibility of the junior engineer to learn from the senior engineer, by working hard, asking questions, looking over the shoulder and trying to understand the backwork that has been looked over by the more senior engineer.
At one time, there may have been an apprenticeship(sp), but those days have gone the way of the slide rule and drafting board.
Plus my company just recently laid off all of the junior engineers. Because it was costing too much time for the sr.'s in mentor time, and the jr's were not efficient enough.
Most engineers I know are poor teachers. The ones that teach (professors) are generally poor practisioners(sp). And now, thanks to computers, many graduate from uni thinking that they know much more than they do. And business furthers this phenomonon by giving a new employee more responsibility than they deserve... and then it's sink or swim...
Wes C.
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RE: Hand Stress Analysis
If this is the case that your company just recently laid off all of the junior engineers I am afraid they will walk the way Sony walks.
RE: Hand Stress Analysis
Now here's a little treasure hunt contest:
Who can find the most "aircraft stress analysis" courses in university curriculums? Are there any these days? FEM courses do not count. Please post links to the courses that you find.
Steve
RE: Hand Stress Analysis
And I never took an FEA course, because it was always filled....
go figure...
AND!!! why do we always stop with the Free body Diagram, when we all know it's not reall complete without the (there are several names for it) "mass-acceleration diagram".
Wes C.
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RE: Hand Stress Analysis
I cannot believe that you or anyone else thinks that university training could ever be adequate to cover all contingencies--that is, all the twists and turns an engineering career over the spectrum of all engineering careers.
IMO, you do not have any business being an engineer if you are not willing to pass on the knowledge and experience of "lessons learned" you have picked up through your career. You yourself have been the beneficiary of the experience of thousands perhaps millions of engineers, from Mesopotamia to today, and personally I would consider myself immoral for not being willing to pass that experience on to inexperienced engineers.
RE: Hand Stress Analysis
University of Arizona, Aerospace Engineering BS
Statics
Mechanical Behavior of Engineering materials
Engineering Component Design (looks like failure analysis)
Materials Selection (could have used that one myself!)
Composite materials
University of Southern California, Aerospace Engineering BS
Statics
Strength of Materials
Aerospace Structures
Stress Analysis
Theory of Structures II
(design and FEA classes not included). I have no idea whether any of those classes are filled or empty. Each school also has quite a few structures classes at graduate level.
RE: Hand Stress Analysis
I appologize for the scarcasm in my statement not being more clear. I wholehartedly agree with you that when you graduate, "noone knows nuthin!" and it is our responsibility to pass down what we have learned from the previous generation.
Unfortunately I am a bit disgruntled after the "layoff" here at my company, where they just eliminated all of the recent graduates (from '06 and '05) based on some idiot consultants efficiency-productivity metrics.
Wes C.
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RE: Hand Stress Analysis
One wonders where the consultant or management thinks 'fully trained engineers' come from? Trees? Oh, I know, India, Ukraine, or China.
I personally didn't take offense, and missed the sarcasm completely--zoomed by me like a North American X-15.
RE: Hand Stress Analysis
Unfortunately these days, companies are no longer run by engineers and the concept of providing training is solely focused on required lectures for avoiding lawsuits such as harassment training, dealing with diversity, export/import and whole bunch of others which may be needed but have nothing to do with the daily company business. In the old days, training was one of the major benefits which big companies used to bring in the best and brightest. Now days, they figure engineers should be happy just getting offered a job.
Universities of today only provide the basis for building an engineering career but not the tools to actual perform the task. The engineer himself must have enough self motivation to seek out the necessary training to excell at their job. I do think however that for the amount of money universities collect in tuition that they should be held liable for the quality and level of education they provide. As a test, try asking several professors in aircraft structures what MMPDS is or describe the difference between jo-bolts, hitigues, taperloks, etc. Far and few in between unfortunately.
I think that eventually this issue will come to a head, maybe sooner than later, and the industry will have to face the problem of fixing this. If leaders at major corporations would get together with both universities and professional organizations, a basic curricula which includes both theoretical and practical phases could be established. Such as the apprenticeship programs of long ago.
Unfortunately, I am not so sure this issue will get resolved until things come to a critical point. Industry is primarily reactive not proactive these days.
RE: Hand Stress Analysis
What would cause this issue to 'come to a head'? Catastrophic events such as the Hyatt pedestrian bridge or any one of a hundred plane crashes don't seem to cause these issues to come to head, what would?
Back to the original subject, 'hand stress analysis'--don't they teach that anymore at universities?
RE: Hand Stress Analysis
TTFN
RE: Hand Stress Analysis
I always thought Aero would be like that, but after experienceing the outsourcing on JSF I don't think that it'll be too long before ITAR is pretty much meaningless.
Wes C.
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RE: Hand Stress Analysis
TTFN
RE: Hand Stress Analysis
I do agree, Prost, that without a solid hand-calc background an engineer cannot understand the stress model he sees on the computer screen. But that experience is only going to come from us, and only if we take the time to do it.
Getting back to Crackman's comment:
"Universities of today only provide the basis for building an engineering career but not the tools to actually perform the task."
I don't think there's much we can do about the graduates produced by universities. Experienced senior engineers must cultivate a mentor-apprentice culture in their respective organizations, and resist pressure to the contrary. We may be bad as teachers, and often hesitate to stop for a moment to explain the meaning of what we're doing, but I don't see much choice. We would all be the better for it, too.
Stories of junior engineers laid off by management, graduates who can't draw the correct FBD, fatally flawed FEM's, etc. may be discouraging, but that is no excuse to throw up our hands. Quite the opposite; it is telling us what we have to do.
Electricians, airframe & powerplant mechanics, and carpenters, they all must spend time as apprentices to learn the craft and earn the ticket. Are the trades so far beneath us lofty engineers, that we cannot use a similar system (even if informally?). Doctors do residences before surgeries, and academics do post-docs before teaching. It's not like we don't have any practical examples to learn from.
I would add that I graduated from a fantastic college program that strongly emphasized analytical methods of aircraft structures. I came out of school having pounded dozens of rivets, read a dozen chapters of Bruhn, and cranked out dozens of detailed drawings. Seems it's a rarity. Since the program was cancelled by the school a few years ago, the aircraft industry in Canada is the poorer.
I know I was lucky, but most don't get such a good start. It's encumbent upon us to bridge the gap.
One last point, since I'm on a roll:
Consider how long it takes to train anyone to work well and independently as a aeronautical engineer, even if he has a good mentor. Unless he gets insanely specialized, it can take over 5 years (after appropriate schooling) for him to be ready to do his own design projects without constant supervision. Think of all the drafting, sizing, communicating, testing, negotiating, and analyzing that has to be done, and none of it can be learned overnight. Only a fraction can be learned at school.
Steven Fahey, CET
RE: Hand Stress Analysis
Another factor is that semi-professionals are pushed into computer analysis. In Michigan wooden roof trusses are common in construction, and we have had some spectacular roof failures, even in state buildings.
RE: Hand Stress Analysis
RE: Hand Stress Analysis
The design requirement flowdown for the transducer would include the vibration profile of the mounting. And generally, the component in question can indeed operate in such an environment. However, fatigue failure data is usually very sparse, so an engineer would ask for data from the manufacturer and not get any and march forward because there's no red flag.
TTFN
RE: Hand Stress Analysis
Since my moniker indicates a bias for hand-calculations, I have to add another perspective as to why ads for "stress analysts" would explicitly request experience in hand-stress analysis.
My generation started out with the slide-rule in High School, progressed to calculators and Hollerith punch-cards in College and Universty and (if lucky) we gained limited experience with FEA packages in industry. Stress reports and aircraft drawings were prepared with a pen or pencil, and right or wrong, engineers were expected to perform simple stress checks to ensure parts were strong enough for their intended purpose. Detailed stressing was passed to more experienced engineers (if required).
Today's generation are better equipped to use computers to design, analyze and draw components, and they accomplish these tasks much quicker than the previous generation. What I have observed, however, is that younger engineers and designers often lack the ability to check their own work. Frequently I am asked, "Is this strong enough?", and my reply is "How strong does it have to be?" Then I use the opportunity to teach about loads, material properties, manufacturing techniques, Free-Body Diagrams and design (airworthiness) requirements.
Depending on the age of the person who wrote the job ad, they may need someone who is equipped with other tools to check their work. Not all stress analysts have been properly instructed in their trade, and these tasks are sometimes pushed down to design draftsperson who have never been formally trained in stress analysis, and are simply running FEA packages attached to their CAD programs.
Depending on financial constraints, not all organizations can afford the licence fees associated with multiple copies of FEA packages, and many are still using legacy data (hand-calculations) on a daily basis.
All engineers have a responsibility to share what they have learned with the next generation, and vice-versa. I have mentored a new hire in my department with challenging work assignments, have shown him when hand-calculations are sufficiently conservative, and he in-turn has shown great patience in educating me on how quickly he can use computers (and free downloads) to solve more difficult problems. We teach each other...and learn together...
Hand calcs are still needed, and authors like Bruhn, Peery, Niu, Megson, Flabel have left a wealth of information behind for those willing to learn....
RE: Hand Stress Analysis
Anyways, back to the hand stress analysis requirements for jobs. I think one of the reasons for this too other than those mentioned by HANDCALC is that when reports get approved it gets very difficult to document computer runs and results. I know for a fact that the initial Raptor stress reports had to be completedly redone because when the USAF reviewed them they were full of nothing but Excel spreadsheet results and found unacceptable. The USAF requirement included hand stress calculations for verification purposes of all computer models, calculations, etc. This is true with the FAA world as well. So, many agencies use the hand stress calculation as a validation tool and therefore companies need to require it.
RE: Hand Stress Analysis
corus
RE: Hand Stress Analysis
Those of you have not read Petrowski's book "To Engineer is Human" will enjoy the chapter "From Slide Ruler to Computer (or Forgeting How it Used to Be Done).
RE: Hand Stress Analysis
The above is like reading about some alternate universe to me.
BUT, I used to work in research and my senior ME kept insisting we FEA everything. Never mind early attempts at said FEA produced failure loads 10x the actual measured loads- it was FEA all the way. Seemed he was responding to industry "trends". Have you seen a copy of Machine Design lately? There are consistently multiple articles and many more "product info" sections on FEA in there and also in most similar trade magazines. There's usually an ad for the latest and greatest FEA package on the facing page.
Just a thought here: there is money, BIG money, to be made on FEA by the vendors. Might they be influencing the industry with this type of "information"?
RE: Hand Stress Analysis
This might be related, it might not be. A number of years back, the accrediting agencies and prof. societies began pushing universities to include a full year of "design" class work for undergraduate engineering students. Since the push to make engineering a 5 year degree has petered out, that means that requiring a full year of anything that isn't there now is a zero sum game--if you suddenly demand 8 more classes in design (vs. the 2 that was in the normal curriculum at the time), and you still need the degree completed in 4 years, what do you delete? I asked some professors about that 5 years ago, and they said they looked at the individual classes and decided there was considerable overlap. I didn't have a clever response at the time, but now thinking it over, I would have asked them this--isn't there quite a bit to be gained by the repetition? For myself, I know that I had to take quite a few math classes, some material from which was repeated in various engineering classes. I also know that such repetition helped me remember that math both for my future graduate degrees and my work. I also know that repetition in hand stress analysis techniques also continues to help me in my current work.
I might just be 'out there--Does anybody else think that the full year of design classes requirement has contributed to the big holes that are apparent in current undergraduate structural engineering curricula? I can imagine other applied mechanics areas are suffering, such as fluid mechanics.
RE: Hand Stress Analysis
A good structure curriculum should have the traditional methods. It's okay to also include FEM. But the FEM should be compared to hand analysis and testing. This way the graduate would have valuable information regarding the strength and weakness of each method.
RE: Hand Stress Analysis
WOW! It appears Kwan agress with you, but are there any other recent graduates who can attest to this, or is this just the group's perception? Was my college experience an abnormality (Eng Mechanics with aero emphasis, about 6-8 stress courses, only one FEA)?
RE: Hand Stress Analysis
YoungTurk--what university?
RE: Hand Stress Analysis
RE: Hand Stress Analysis
The hand calculations are not that difficult. If you know the basic rules, the stress/stran relations, and how to use the propere literature (bruhn, niu, etc.) you can do a lot of work manually, even without needing to go to the computer.
I have all the software I need, but sometimes it's just faster to give a general result/estimate using hand calculations (to be backed with further analysis).
RE: Hand Stress Analysis
Is it possible that these job postings are making a differentiation between actual on the job experience and class instruction? I could see that being a much smaller group as many of the people I went to school with went on to non-stress related fields, even though the engineering mechanics and astronautics degree I got was oriented toward just that type of job.
Pyro- Glad to see I'm not the only one with similar experience.
RE: Hand Stress Analysis
First, while I got my BS in the mid-late 80's (and masters in the 90's), the bulk of my "hand analysis expertise" was learned "in the field" and by amassing my own library of methods over time. Most of what I got from the universities was more theoretical than practical.
My current gig is at a place that "worships" Patran/Nastran. However, as I come up to speed with that software, I can see that it does a pretty good job at predicting material behavior. Furthermore, with the addition of PCL scripting, one could use it to augument their hand analysis capabilities.
So, it will never replace the ability to do a "sanity check", but I don't poo-poo it as much anymore.
--
Great Spirits have always encountered violent opposition from mediocre minds
-- Albert Einstein
RE: Hand Stress Analysis
We have recently hired some recent graduates that are very bright. I believe they’ve received a good education, but I think they’re going to get most (if not all) of their practical knowledge for performing hand calculations and verification testing here in the real world. Our company has neither a formal mentoring program nor structured classroom program, but I’ve told them that I’ll help them as much as I can, nevertheless I can only do so much. I think a decent structures course would be good to start. I’ve heard Flabel and Lake City has a decent class. Can anyone comment or recommend others?
RE: Hand Stress Analysis
To help you train new structural engineers, we've made some stress liaison engineer notes and solutions available. I've been going through these notes, mentioned in thread
thread2-153152 and they seem pretty good. You might want to consider going to that thread and obtaining the notes with the instructions there.
RE: Hand Stress Analysis
The problem was that at that point, I of course had no job experience and so the class didn't benefit me as much as it would if I took it again now, after having been a stress analyst for some years. Now I have a much better feel for what I should be looking at and how structures behave.
In my job I use FEA for almost every analysis, and in my company we like to do full-blown 3D models with solid elements whenever possible. Whenever I can I also do a hand calculation as a sanity check. Many people have disparaged FEA here, and there is no doubt it can be done wrong or done needlessly, but I also must point out that we often get very good agreement between our FEA predictions and strain gage readings, at least for static tests.
We often encounter (usually older) people who don't believe in FEA or in fact in the value of engineering analysis at all. I can see where they are coming from in some cases, and in others it just seems like plain ignorance on their parts. In fact, we have used FEA to design some advanced features for our product that are generating increased sales.
RE: Hand Stress Analysis
If your referring to the ABDR notes and solutions from crackman - yes, a very nice course with some good fundamentals - thank you. Nevertheless, in order to get the full effect from the course, it appears that McDonnell handbook 339 should be used in conjuction with it. Unless you work for Boeing, you'll not have access to this text.