Drop shock calculation according to MILSTD810F

[danny-buber]

Hi,

I have to calculate trop shock for packge according to MILSTD810F TABLE516.5-VI.

I have :
-product weight (91kg)
-distance of fall (61cm)
-fragility factor G=15

I need

-find the static stress
-find the right thickness for foam (with known density)
-find Face area

I DONT FIND THE Formulas to calculate it.

please help me with this!!!

 

[3DDave]

There aren't formulas. Having gone through exactly this same problem, you need to do the engineering analysis, just like any other structure, but there isn't reliable information to base it on.

Things to look for - they don't seem to require testing at all temperatures and most foams are polymers which change stiffness with changes in temperature. So maybe it passes the actual drop test and fails in the field.

It's not the density of the foam that matters but the compressibility - which I never found good information on because it is hugely non-linear. There is often information about how much deflection will occur for a particular load, but that's only at one point; many foams get progressively more resistant to deformation with increased deformation.

I would recommend picking a foam that will survive all the other requirements and then making a mock-up of the same weight and add an accelerometer and begin dropping and cutting and gluing to get a feel for what is required. Trust me, not only will the result be more reliable than an analysis, it will be cheaper and quicker to get to the point where the item passes the test.

 

[danny-buber]

Thanks for the replay...

I have a hard polypropylene plastic case, with product inside (91kg).

I need to protect the product with foam (I dont have problem with this- I can use different types with needed information).

I want to find the right thickness of the foam I need to use (mabye it can used us spring calculation) .To make sure that I will pass MILSTD810F TABLE516.5-VI standard .

I got the -product weight (91kg)-distance of fall (61cm)-fragility factor G=15, as I mentioned.. mabye I need to look on that as a free fall with spring?

 

[IRstuff]

If you have samples of the foam, I would test them directly, rather than trying to come up with a model with untraceable parameters.

TTFN (ta ta for now)
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[3DDave]

Starting with mgh = 1/2kx^2; and F=ma = kx. So you know that k = ma/x.

91 kg * 15G * 9.81 m/s^2 = 13390 Newtons; so the k factor is 13390 N/x.

h is .6 meters, g is 9.81 m/s, and the mass is 91 kg so mgh = about 600 N-m.

Therefore 600 N-m = 1/2*13390 N/x * x^2; x = 0.09 meters is how much a perfect spring needs to deform and the spring rate is 13390 N/0.09 m or 150000 N/m.

And you still need to test the foam.