Insane Drop Test

[SalamiVice]

I have no idea if this is the right place to put this.

I have a drop to preform on an emergency beacon. The unit weights 5.1 lbs fully assembled, must be subjected to a 500g 4 ms test. I need to create a bracket to hold this, and have this bracket deform enough to absorb enough energy that the beacon will only see no more than 90g's for those 4 ms. Its a half sine curve impact.

So I am having problems with CosmosWorks and its drop test analysis as the results do not seem to make sense.

is there anyway to simulate this through a static condition?

 

[FeX32]

I don't recommend approximating and dynamical analysis with a static one. My personal experience indicates that the best way to perform dynamic impact tests is to conduct an experiment.
These can be relatively simple. All you need is a simple DAQ system and one accelerometer. (I have done this before for an SAE competition)



Fe

 

[SalamiVice]

Fex32:

only problem i see is that the SAE rules imply a 300kgs car with a deceleration no grater than 20g's...We did that too...

But I have no idea or the capabilities to get the acceleration or velocity out of this object.

The only lab that does this charges me 8,000 per test, and I want to have some idea of confidence before going.

 

[FeX32]

Oh ic.
Well in that case I would recommend LS-DYNA. I have not had any experience with cosmos.
LS-DYNA is an industry leader in impact testing.
I still don't recommend that you try to approximate a dynamic test with a static one.
Since you're into SAE read what they have to say about that...



Fe

 

[gwolf2]

I think its a good idea to get some sort of feel for how the structure will perform before the test. If the test fails you could spend a lot of repeat money if you don't really know how to fix it.

I think you need to define first whether you are going to absorb the energy by plastic deformation of the bracket or by elastic natural frequencies of the bracket.

If you are going to absorb energy plastically then maybe you can do this with a large displacement, plastic material model and run it statically. Measure the energy absorbed if the FE package prints that out, or come up with some sort of force-displacement curve. Then use this curve as a nonlinear spring and do an energy based hand calc. You might need to do a speed of sound check for the bracket material to make sure that the loads have time to propogate through.

If you are going to react the load elastically and hope that the bracket natural frequencies isolate you enough from the shock then I see no alternative to a full transient analysis. You may not need an explicit code like DYNA if can assume a simple contact - a 4 ms half sign base kick might do the trick for you.

I hope this helps, I have only limited experience of this type of analysis.

gwolf

 

[zekeman]

I doubt that they expect you to simulate the 500g shock, but to accept the results of an analytic assessment. I think you have to demonstrate that the mass will be subjected to less than 90g's, but not for the 4 ms duration of the shock.
I would solve it by writing the ODE

mx"+k(x-y)=0
where
x= mass displacement
y= frame displacement
y"=500sinwt for t<4ms
m= beacon mass
k=bracket spring rate
wn=sqrt(k/m)
Now get
m(x"-y")+k(x-y)=-my"
by subtracting my" from both sides
The maximum of k(x-y)/m is the max acceleration of the mass
call x-y=u the differential displacement,
mu"+ku=-y"=Am*sinwt
u"+k/m*u=A*sinwt
the general solution for u is
A/(w^2-wn^2) *sinwt+B*sinwnt
since u(0)=0 and u'(0)=0
A*w/(w^2-wn^2)+B*wn=0
Finally, the general solution:
A/(w^2-wn^2)*sinwt-A*(w/wn)/(w^2-wn^2)*sinwnt
since the max force on the mass is ku/m from above ,I get after substitution
Awn^2/(w^2-wn^2)one peak of the high frequency term and Aw/wn/(w^2-wn^2), the 2nd peak (low frequency term) Allowing wn<<w, the dominant term , the second, becomes
A/wnw). For A=500g and the allowed 90g I get
A/wnw*wn^2=Awn/w
500(wn/w)=90
wn<(90/500)*w=sqrt(k/m)=sqrt(k*g/5.1)
From your data I get w=628 rad/sec.(for a 5 ms pulse)
k<(90/500*628)^2*5.1/32=2036lb/ft=169lb/inch
Please check the math on this.
You could make it softer within geometry limits.

 

[GregLocock]

Agreed, use a SDOF model to get the necessary force characteristic then a handcalc to size the absorber.

A very quick estimate gives about a 3 inch stroke shock absorber (of whatever form). Doesn't sound very insane to me. Most reliable one use shock absorber is a steel tube using plastic deformation in hinges. You can hand calculate them, and the plastic load in them is testable on a static rig.



Cheers

Greg Locock

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

 

[SalamiVice]

Guys thank you all for the input, it has definitely helped. I managed to do a combination of hand calc and a full drop test simulation running Cosmosworks.

Since i actually have to test a single unit at 500g's on all 6 axis and then another 100 g 12ms on all 6 axis, I have secured the funds to run one test preliminary.

So i will drop this at 500g 4 ms, on the weakest axis with a bunch of accelerometers and see how far off my mathematical skills are.

 

[FeX32]

I am glad you worked an approximation out.
I hope you don't expect to get exactly what the experiment will get.



Fe

 

[SomptingGuy]

This thread reminds me so much of my 3rd year university design project. Catching ejected, spent fuel rods, subject to a G limit and delivering them to another bit of the handling system.

School?

- Steve

 

[SalamiVice]

not a school project. its a Commercial aircraft emergency location beacon.

 

[zekeman]

Quote"So i will drop this at 500g 4 ms, on the weakest axis with a bunch of accelerometers and see how far off my mathematical skills are."
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I'm curious as to how you can presume to get precisely 500G's @4ms, and if you could why? And why all the accelerometers?
And having asked that, I find it preposterous that anybody would or should ask you to do this since if you are to prove the shock isolation scheme in this case , all you need to do for the drop test is make one basic obvious assumption and that is that the mounting system natural frequency, wn=Sqrt(k/m) is much lower than w= pi/tau=3.14/.004= 785 radians/sec
Since the system is subjected to the 1/2 sine wave shock of 500G's @ 4msec, the velocity of the frame just prior to impact is determined as the integral of the deceleration pulse ( a consequence of momentum change) which comes to
2Ag/w =2*500*32/3.14*.004= 40.7 ft/sec
and the height of the drop is
h=V^2/2g=40.7^2/64=25.9 feet
Without any fancy math, if you mount it elastically, the problem is solved by first equating the kinetic energy stored in the 5.1 lb mass, m to 1/2*F^2/k
where
F is the maximum spring force on m
k= spring constant
Thus
1/2*m*V^2=1/2*F^2/k
reducing to
V^2=(F/m)^2*m/k
V=F/m/wn
Now from above, V=2Ag/w
Substituting
wn=F/m/2Ag*w
Now F/m is 90g and A=500g, I get the stiffest mount,( equivalent wn) to be
wn=90/2/500*785=70 rad/sec
If there are no errors here I would as a first test make a reasonably rigid frame and support a dummy 5.1 lb mass according to wn<70, attach accelerometers to the base and the dummy for the first test and if the peak deceleraton is not quite 500g , it would by OK as long as the area under the acceleration time curve is close to 40.7ft/sec and the main pulse width is not too far from 4 msec.

 

[SalamiVice]

@zekeman: The reason for this test, is I am required by the FAA to actually go out and test these type of shocks on a single working unit to be able to get certified. believe me if it was up to me I would not even bother, as the airplane structure where this gets mounted to has will deform more than enough to absorb my 500'g shock. And on the other hand 100% of these beacons will NOT work during an accident since the antenna gets cut off the beacon.

East/West will do the test for us, they have the capabilities to produce a 500g 4ms impact. and i will most definitely post pictures of the event.

on the other hand thank you for your suggestion, i will go through it and see what i can get from it.

 

[zekeman]

quote "@zekeman: The reason for this test, is I am required by the FAA to actually go out and test these type of shocks on a single working unit to be able to get certified. believe me if it was up to me I would not even bother, as the airplane structure where this gets mounted to has will deform more than enough to absorb my 500'g shock. And on the other hand 100% of these beacons will NOT work during an accident since the antenna gets cut off the beacon."
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I guess this is another example of our hard earned taxpayer dollars at work. And we need more F22's as well.

 

[gwolf2]

> I guess this is another example of our hard earned taxpayer dollars at work.

I view it as a form of wealth recycling, most industries have their equivelent.

 

[zekeman]

"I view it as a form of wealth recycling, most industries have their equivelent."
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Economists used to call it the " broken window theory", from the 1930's depression, I believe.

 

[IRstuff]

You incorrectly assume that the airplane is supposed to survive even a single 500 g shock.

The objective is to ensure that every production part hereafter will survive an actual crash shock, making allowance for human fallability, assembly errors, wear and tear, end of life, etc. The FAA knows that you'll cherry pick the qual unit, or coddle it differently than the production parts, so the qual units have gobs of safety margin.

Heaven forbid they only qual a brand new part to the minimum actual crash shock, or other environmental limit. If they did so, miracles like the Gimli glider would never occur, and failure of a single engine would cause the plane to drop out of the sky.

We pay a lot for the safety margins, becaue we're unwilling to accept fatalities in even the rarest statistical sports.

TTFN

FAQ731-376