Significance of Shock Duration 1

[johnfitzgerald]

I have an optical instrument that I am drop (shock) testing to determine a spec to give to a packaging engineer who will design the shipping container. What significance does the shock duration have? If I still achieve the same g level does it matter? Is it just a matter of the effects of larger displacements that go along with longer durations?

 

[GregLocock]

Shock duration is vitally important, since it controls the bandwidth of the excitation. Crudely, the upper limit of the frequency of excitation is given by the inverse of the duration of the shock pulse (eg, if the pulse lasts 2 milliseconds then it'll excite up to 500 Hz).

So, for a given peak g level a shorter pulse puts less eenrgy in anyway, spread over a greater bandwidth. Even if you normalise for energy input in the event then it is still putting less energy in per unit bandwidth.

Now, at some point this argument does fall apart, since if you make the pulse long enough, but keep the total energy input the same, you minimise the damage.

It's actually an impedance matching problem - a sufficiently fast pulse does no damage, a sufficiently slow pulse does no damage, it's the bit in between where real life operates.





Cheers

Greg Locock

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

 

[IRstuff]

Alternately, you can look at it from the perspective of the dissipated KE from the drop. Since the total KE is constant in the test, the duration of the impact determines the maximum acceleration and hence shock, i.e., if you had bubblewrap around your object, it would cushion the impact (increase duration) but lessen the acceleration. Conversely, if you surrounded the object with stainless steel, you'd have a short duration, but violent acceleration.


Therefore, if you are trying to protect against a drop, you need to determine the maximum KE and the maximum allowable acceleration and that will determine the duration required to keep the acceleration below the critical level.

TTFN

 

[EricZhao]

Just a supplementary to the previous excellent answers. MIL-STD 810F Method 516.5 for shock testing might be a good reference.