accelerate sine vibration test

[Buhl]

2 m/s^2 in 5 million cycles is the demands for a sine vibration test on a simple shaker table.
could this value bee converted in a simple way so we could perform less cycles?

im looking for some rules to accelerate this test.

 

[IRstuff]

MIL-STD-810 has an acceleration equation in the vibration method section. Note, however, that you cannot arbitrarily accelerate the stress, because at some point, your stress level will exceed some other design constraint.

TTFN
faq731-376

 

[amanuensis]

Hi,
The Basquin law gives a relationship between the number of cycles N and strain S. The product of the both terms is constant. Actually not exactly the product of N by S, but rather N*S^b=Constant. In this equation, b is a very important coefficient. Practicaly, b is fixed between 5 and 8.

Now, the reduction rule of the test duration is :

reduced acceleration = real acceleration * (Real time/ Reduced time)^(1/b)

That's it!

 

[Buhl]

Thanks guys
I found this in a letter called PIE9301T.pm5 from

http://www.ttiedu.com:

"It follows that if a test is performed with an input vibration
level of st on a test item that experiences an input vibration
level of se in its service environment, the test duration Tt
needed to produce the same damage as a service vibration
environment of duration Te is
Tt = Te (se/st)b"

This letter also mentions the b material constant and state MIL-STD-810E[7] reccomends a value of b=6 for sinusoidial vibration tests.

I just get very low values, if i use youre equation Amanuensis i can go from 5 mill cycles to 2 mill cycles just by raising the acc from 2 to 2,33 m/s^2 could this really be true?

 

[GregLocock]

rule of thumb for steel chassis components is that a 10% increase in stress halves the life, which more or less agrees with the above.

Now these equations are terrific if your system is behaving like a nice piece of steel in a torsional test in a lab, but the 64000 dollar question is how do you prove it?

Developing robust testing plans is an art form, sadly one I know little about.



Cheers

Greg Locock


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[Buhl]

Yes Greg - thank you for youre comment it sounds like i should stick to Amanuensis suggestion.


The item being tested is a cooler consisting of: alu element, steel shroud, motor bracket, motor and fan assembly.
The shaker table we use is a simple construction of a thick aluminum plate with a motor shaft connected to a adjustable mass where you can adjust the "off center" distance.

 

[amanuensis]

As irstuff said, the mil-std is a military american standard for defining mechanical (vibration and shock) environmental specifications. In my job, I use the Stanag.

Fatigue is taken into account (using Woeler curve) for reducing time of vibration test.
If you perfectly know this standard, you don't need to be an artist!

 

[GregLocock]

That's fine if you are working to a contract that specifies that standard, but it ignores physics.

Cheers

Greg Locock


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