Compare 2 mechanical shock scenarios with different magnitude and time

[Mike J]

For example, How does one compare 100 g's for 6 milliseconds to 45 g's for 11 milliseconds. These are two different shock scenarios with different magnitudes and times, so how can I compare them? If I have something that is shock rated at 100 g's for 6 ms does this imply that it will withstand 45 g's for 11 ms?

I have tried finding a "shock equivalence" chart or a metric for comparing two shock scenarios but I haven't found anything.

I was thinking it may depend on the mass of the equipment that is being shock rated. We have:

F=ma
v=at
Power, P = Fv

An energy metric like power may give me a reasonable estimate to compare, but I am just guessing here. Is there a standard way to compare two scenarios? A chart or equation would be nice..

Thank you for the help!

 

[berkeman]

If I have something that is shock rated at 100 g's for 6 ms does this imply that it will withstand 45 g's for 11 ms?

I would think it should be okay. The higher shock in the shorter time is usually more severe. The shocks are measured with accelerometers and the shock curves are usually tuned (with damping cloths or whatever) to be half-sine in shape:

http://vibrationacoustics.asmedigitalcollection.asme.org/data/journals/jvacek/28916/061019_1_1.jpeg

 

[berkeman]

I suppose that some systems may have trouble with the extra energy in a wider shock pulse, but in my experience with shock and vibe testing of electronic assemblies, it's usually the peak amplitude of the shock that does the damage...

 

[JBA]

It is relative to the total energy transfer in some cases.

In seismic applications, the duration relative to the response profile (natural frequency) of a structure is critical.

As for the human body, see: https://www.quora.com/What-is-the-most-g-force-that-a-human-can-resist

 

[Mike J]

http://vibrationacoustics.asmedigitalcollection.asme.org/data/journals/jvacek/28916/061019_1_1.jpeg

View attachment 194719

Thank you for the reply. This is a helpful visualization.

 

[Mike J]

I would think it should be okay. The higher shock in the shorter time is usually more severe. The shocks are measured with accelerometers and the shock curves are usually tuned (with damping cloths or whatever) to be half-sine in shape:

http://vibrationacoustics.asmedigitalcollection.asme.org/data/journals/jvacek/28916/061019_1_1.jpeg
View attachment 194719

Would the shock magnitude - the 100 g's - correspond to the peak value or the RMS value of the sine wave?

 

[JBA]

This raises an issue I have not previously considered of the actual shock profile vs the effective profile vs the rigidity of materials involved in the shock contact, i.e. will high modulus material collisions tend to be more of a saw tooth wave vs low modulus materials resulting in more of a sine wave effect.

 

[berkeman]

Would the shock magnitude - the 100 g's - correspond to the peak value or the RMS value of the sine wave?

It is my understanding that the number is the peak of the half-sine shock waveform (at least in mechanical shock testing of assemblies at our local test labs). The waveform is directly off of the accelerometer that is attached to the falling assembly.

 

[urish111]

hi,
if we look at a half sine shock and try to integrate it we would get a value of velocity that can be observed like energy.
so same shock amplitude (A) let's say, 10g@1ms and 10g@100 at different integration times will have different energy.
I think the greater the integration time the more area you get that is more energy from the ∫f(A)dt ...
hope that helps.

 

[urish111]

It is my understanding that the number is the peak of the half-sine shock waveform (at least in mechanical shock testing of assemblies at our local test labs). The waveform is directly off of the accelerometer that is attached to the falling assembly.

can it be that from both 10g@1ms and 10g@100ms we would get same amplitude of equivalent force?

 

[dannad]

The area under the shock curve is the total energy, so for saw tooth shock 10g 20 msec. is equal to 20g 10 msec. (have the same energy). However the force if not equal. I'm not sure it is possible to compare two different shocks. I'll be happy if I wrong.

 

[onatirec]

Not sure how these half-a-decade old threads get revived from the grave like this...

Total energy doesn't really tell you much (what if it were 1g for 200ms? 0.1g for 2s?). Shocks have a frequency component which must be compared to the resonances of the system being tested to understand the severity of response.