Any Engineers out there? Engineering question heip

[alanm]

Hi all,

This is Cooper Alan's son, I have started engineering at uni and am trying to join the Monash Motorsport team. The interview requires me to solve a problem which I am having some significant trouble with given my weeks worth of engineering knowledge. Any help or guidance would be much appreciated. I have had a look at the Mhor's Circle hint however I can't wrap my head around how to use it in the question.

Thanks

[kaleuclint]

Thinking tension and shear load on the pole maximized when the wind and the 'flag' are in the same plane.  Which would be at the static bottom as opposed to the dynamic top under load (i.e. the pole should flex before failing).  Don't overthink it.

I'm only a military engineer (akin to civil) mind...

Good luck with the team, Cooper!  When I worked at Monash I used to occasionally walk through their workshop to see was happening with the Formula SAE car.

[Divano Veloce]

Its been a while, but this is how I would approach it:

1. determine second moment of pressure for the sign, this gives you torque in the tube from wind pressure

2. do a free body diagram to determine compression and bending moment in the tube (dont forget the mass of the tube or the bending from the wind pressure)

3. apply the sum of the bending, torsion and compressive stresses to an internal stress element at point x

4. draw the net stresses on a mohrs circle to determine maximum stress at point x

5. compare this maximum stress to mild steel elastic limit (assuming failure occurs with any plastic deformation)

Good luck, sounds like a great opportunity!

[Cooper75ts]

Thanks heaps for the suggestions, I'll give it a shot and work through as far as I can and go with that. Worst case I go for it again next semester until I eventually get in.

Cooper

[bazzbazz]

Before doing anything, check to see if there is a pin at the bottom that allows the pole to fall over . . . . . . like in Captain America!
(Sorry, couldn't resist)

     

Good luck & all the best.

[Citroënbender]

Gut feeling is, it's not going anywhere. Hopefully someone will post a worked solution!

[Divano Veloce]

Have a go at it, show your workings if you like and ill give you hand

second moment of pressure is wrong, find the center of pressure

[Colin Edwards]

Hi Cooper,

Never give up!

As suggested, finding the answer is less important as understanding the problem.  Given the Formula SAE perspective, predicting failure due to torsional fatigue and pure beam failure is important. 
Much of an SAE car is subject to torsional stress.  Consideration only from a beam perspective would probably be a bit too simplistic.  the Mohrs circle can illustrate pure shear and can illustrate pure bend. 

Methinks the object of the exercise is to understand and explain (graphically rather than mathematically) a combined shear force and bending moment diagram.  Given the pole is under tension at a fairly obvious location due to bend and a torsional failure of a shaft is usually at 45^o to its axis, the stress concentrator / point of failure will become very obvious. 

The length (height) of the pole is significant as is the mass of the sign.  However, no one said it would fail, just provide an opinion on where you believe it could fail.  A philosophical challenge! 

Hang in there!

[Divano Veloce]

Consideration only from a beam perspective would probably be a bit too simplistic.  the Mohrs circle can illustrate pure shear and can illustrate pure bend. 

Bending is not plotted on a mohrs circle. Bending in a beam results in compression and tension (normal stresses) across its section and transverse shear along its axis. The sum of the normal stresses (from bending, applied normal forces, element mass etc) and sum of the shear shear stresses (from applied forces and torsion) for a chosen element within the beam are plotted on a mohrs circle to determine the principle stress, peak shear stress and their orientation within the section. The task provides enough information (with some assumptions) to solve it using this approach. If I were to try I would almost certainly get it wrong.

But Colin is right,  if the SAE team want first year students, analyse the problem as best you can and apply - they won't expect a correct worked solution from someone who hasn't been trained.. I would suggest that you should try to show all the actions and the reactions on a "free body diagram", the orientation of the resultant bending from wind pressure and overhung mass would be good, where you think the highest normal stress is and if its tension or compression, and maybe the shear stress due to torsion from the wind pressure. Research second moment of area and polar moment of inertia and discuss their significance for bending and torsion in the interview.

[Divano Veloce]

Cooper, this is a good tutorial on combined bending and torsion stress analysis

http://abe.ufl.edu/tburks/Presentations/ABE4171/Combined%20Stresses%20and%20Mohr%27s%20Circle.pdf

[kaleuclint]

Before doing anything, check to see if there is a pin at the bottom that allows the pole to fall over . . . . . . like in Captain America!
(Sorry, couldn't resist)

     

Good luck & all the best.

Use to have this problem all the time, Baz.  Replacing the shear pins with hardened steel rod soon fixed those breakages...!!

[scott.venables]

Hi Cooper,

You might not remember me but you and your Dad came and picked up a Briggs and Stratton 4 stroke engine from me and my Dad nearly 10 years ago. Your dad put an ad here on the forum looking for something you could play with.  Glad to see you're continuing the fascination by studying Mech Eng!  Feel free to say hi to my Dad Hugh in the Monash Mech Eng workshop The FSAE team is great, I've been to plenty of the competitions and have had friends and my cousin go through the team.  All the best with the test!

Cheers,
Scott

[Cooper75ts]

Hello all,

I had my interview yesterday armed with all your knowlege and guidance so I was able to explain the principals behind the solution of the problem. Thank you all very much for the help and support it was and is much appreciated! I wasn't able to come up with a final mathematical answer for the problem unfortunately however I gave it a good go and delivered what I had. Turns out they don't really accept applications from 1st year students simply because we have yet to cover many of the fundamentals. So although there is still hope, it is sadly unlikely I will be succesfull this year. However I will return next year ready and eager once again to do battle with the problems they throw at me next time better prepared with a years study under my belt.

Once again thank you all for your help it definitely helped a lot not only for this problem but for the work I am doing generally within my course.

Cooper

[Cooper75ts]

Hi Scott,

I do remember going with dad to your place and seeing a couple 75 wrecks and picking up and old 4-stroke. That was definitely a little project that helped cement my fascination in mechanics. I will have to say a quick hello to Hugh when I come by him in my studies!

Cooper

[Colin Edwards]

Having an opportunity to use the Monash wind tunnel would be motivation enough for me - keep at it!
The Monash crew usually do a bit of testing at Oakleigh kart track.  Very impressive data gathering.
FSAE usually has a display at the F1GP - not far from the last corner.  Always worth a look.  Cars from RMIT, Swinburne and Monash etc.