Good questions for stress engineers

[compositestress]

Hi

I often have to interview stress engineer applicants and decided to put together a list of problems that I could use to test the applicants knowledge.
I asked my collegues for their suggestions and was suprised by the limited response I received - surely it cant be that difficult to come up with a question/problem that tests an applicants knowledge - everyone must have a favorite!!
So I have decided to appeal to those of you out there that are experts in asking the technical questions that quickly determine if the applicant really knows his/her stuff.
Your help will be appreciated.

 

[rb1957]

i would like to discuss statically indeterminate structures with them ...
as a lead in i ask them to derive the bending moment diagram of a simply supported beam (the discussion would then follow as how would the solution change if the beam was redundantly supported, what would that phyiscally look like ?, why would we do that (complicate the structure) ?, how can we solve it ?) ...

unfortunately the vast majority can't tdo the simply supported beam, but they no doubt can whip up impressive looking FEMs ... sigh

 

[berkshire]

compositestress (Aerospace)
As a related question to the one posed by rb1957. how about buckling/crippling loads on two identical push rods in compression, one made from 4130 low alloy the other from 1020 mild steel.
B.E.

 

[SuperStress]

Back when I used to work interior structures, I used to show applicants an inboard-facing open-faced cabinet and ask them to freebody the reactions for 9g-forward crash loads. It was a good way to see if they understood shear center.

Now, I'm happy if I can find anybody that can give me a balanced freebody for just about *any* kind of structure! :-( Doesn't anybody teach load paths anymore?

rb1957 is right though - I've sure seen a lot of very fancy, expensive (and ultimately incorrect) FEMs!

SuperStress

 

[Kwan]

I was on a day long interview about a year and a half ago and was grilled by many stress guys. Almost everyone asked me to freebody a shear clip. I feel a stress engineer could do this in their sleep. I asked some of my interviewers how many people can't FB a shear clip. They said I would be surprized.

Other good questions would be:
non-linear buckling (how dou you check if you are non-lin)
how do you calculate crit stress for non-linear buckling?
what does IDT mean?
what's a fitting factor and when do you use it?
How do you calculate crippling for two nested angles?
How do you calc crippling on .050 by 1" strip? haha
Draw a pic of a angle and then a "T" shear clip (include fasteners).
what's a tear strap?
Explain the difference between a shear crit and a bearing crit joint.
How do you figure out the load distribution in a joint with more than two fasteners (assume bearing crit and ultimate load). haha first ask about only two fasteners.

I can go on all day but time to drink another beer. Hope this helps.

 

[crackman]

A dozen or more years ago, I was in an interview and was given 10 problems and 10 minutes to complete them in. They were certainly not very difficult as of course 10 minutes is not a whole lot of time but they were very practical and tested to see if one knew the fundamentals. Plus the added thrill of the pop quiz gave some real excitement to it. Not that our industry works on unreasonable deadlines eh?!.

Anyways, some were fairly straight forward and if my memory recollects they were generally as follows: 1) a group of cross sections and you had to identify the one with the largest Ixx, Iyy, etc., 2) then another selection where you had to pick the one that was most torsionally stiff. 3) a cross section for which you had to calculate by hand what the area, ybar, Ixx, Iyy were. 4) a joint where you had to identify the fastener with the highest load transfer. 5) Identify an equation (it was Cozzone's plastic bending formula. 6) then id what portion of the stress strain curve the formula applied to. 7) a simple lug analysis. 8) bearing versus shear critical joint. 9) this one was a bit tough but no numbers to calculate - a picture of a wing box with an offset (in both x and y) shear load and you had to identify which axial members had the highest tension and highest compression and which spar webs had the highest shear. Basically one need to figure out in their head the resulting bending moments and torsions. 10) last but not least a VM diagram for a simply supported beam but with a little twist, an added angled vertical member with an applied resultant load and moment.

After the test, I was told my score was 9.5 out of 10 and I was a bit disappointed when I found out I got a small kink in the M diagram wrong. The guy giving me the test was surprised at this and told me I was the first one (out of about 15 interviewees) to get more than 50% right. That was more than a dozen years ago. Unfortunately, I dont think the industry has a luxury to do this otherwise no one would get hired today.

Anyways, just a suggestion for the types of thought provoking problems one could provide to new engineers in training.

Good luck everyone.

James

 

[Jordonlaw]

I always like engineers to have a practical background as well as a theoretical one. To me its no use to simply know the maths if you don't have some understanding of design, manufacture and problem solving. Accordingly I use some of the interview to ask the person about projects they've been involved in and problems/solutions they obtained; that way you'd also catch out the FEA-only jocky. If I were to interview a designer I'd ask about load paths etc. I feel a more rounded enginner or designer is better than a purely theoretical one.

 

[737eng]

My situation is a little different. I have been a liaison engineer for approx 10 yrs, got into it right out of school and have been designing structural repairs and providing technical support to maintenance since then. I got my Structures DER several years ago and have been designing, substantiating and showing compiance for major repairs and some altrerations (few STC's under my belt) for most of my career. I am now at a point in my career where I want to get out of liaison and get into stress and really want to get a better understanding of fracture mechanics, fatigue, Damage tolerance, etc.... and eventually be able to competently accompilsh a DTA. I have just accepted a position with a company where I will be doing stress and have been really looking forward to getting started. I felt pretty confident, but now I am starting to get a little nervous, Of all the posts and questions above I could answer most and was familiar enough with the rest of them to know which book to go to to be able to find the info I need. I have been using Bruhn and Niu for years, I took the BAC Structures Part I and II class as well as Airbus's Stuctural repair class. I am familiar with Flabel's book and plan on purchasing it. I have no experience with FEM's and all my analysis has been hand calculations.
Any advice from any of you seasoned stress guys on how to better prepare myself for this new position, additional books I should obtain, classes to take, etc.......

 

[GregLocock]

How much do you know about FEA theory?

It sounds like you've got a great background to be a useful FEA engineer, as opposed to a node-pusher.

One way to reduce your feelings of trepidation might be to get to grips with an FEA program now - there are plenty of free demos out there which will give you a reasonable insight.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[737eng]

The only background I have in FEA using a FEM and software (not classical hand calculations) is my college days. I believe we used COSMOS and Varimetrix. If I remember correctly you break everything down into rods and nodes and then have to assume fixity in so many degrees of freedom etc.... I honestly let the other students build the model and use the software, I found it easier to do everything by hand. I will search the net and see if I can find a few free demos, I am also going to contact my new future employer and find out exactly what software they are using.

What does IDT mean?

 

[SuperStress]

737eng,

I assume they are referring to "Intermediate Diagonal Tension"... beams designed such that the web buckles (by design) at a load below limit load.

See p. 483 (and on) in Niu for an explanation of shear resistant, pure diagonal tension, and intermediate diagonal tension beams.

As for your preparation in your new job, if you've been using Bruhn and Niu for years, and have taken (and understood) the Boeing classes, I think you are well on your way to being a good stress analyst already.

Depending on the size of your new company, you may or may not be tasked with much FEA. Most larger companies have dedicated FEM analysts, and you will just be responsible for extracting internal loads from someone else's model, and doing a competent hand analysis to determine stresses and margins of safety.

Rather than focusing on learning a finite element code, I might suggest spending some time polishing up your skills on stability and other non-linear hand analyses (things not usually part of liaison engineering). Lots of people can do P/A, but not as many really understand lateral-torsional instability, for instance.

Good luck on your new path!

SuperStress

 

[yassermahdy]

Hi,

It depends of course of what level you are looking for (Entry, intermed or Senior) but here are some ideas:

- simple section properties (a rectangle, a tube, ..)
- Finding the reaction at one support for a statically determinate problem. (Ex. a simple landing gear layout)
- Let them talk about their understanding of strain fatigue versus stress fatigue.
- What does damage tolerence analysus do?
- Got three lay-ups of a composite material with some material properties and loads, calculate stress at each layer.
- Which reference would you go to for stress concentration factors, for stress formulaes and which one for equations on web/stringer/skin comnined stress?

Things like that ..

Hope was helpfull

 

[sdsurfer]

I think it would be a good start with asking new engineers to start with FBD of a structure, define safety factor, predict a failure in truss/plate when 2 forces are pulling on opposites sides. Another one would be to ask stress difference between metals and composites.