Mechanical shock question

[mathlete7]

Hi,

I have a question regarding mechanical shock, in particular velocity shock (fast change in velocity due to an acceleration pulse).

I have a structure that is exposed to a short duration acceleration pulse (~10 ms half sine acceleration pulse). For sake of example, lets say I have a square plate and I'm interested in the acceleration response of a point a little off-center. The acceleration pulse is applied around the entire perimeter of the plate. My inclination is to take the lowest modal frequency, treat this as a SDOF system and look at the acceleration response of a SDOF system to my acceleration pulse applied at the base of the system.

My question is then, at what point does one have to start worrying about the contribution of high frequency modes to the response? Is it appropriate to just do a SDOF response? Is it a function of frequency of the system and load rise time to determine whether higher order modes will be excited and significantly contribute to the response?

Thanks for any input...

 

[spongebob007]

If the first two modes are at least one octave apart, you can generally treat your structure as a single degree of freedom system.

I have found that while higher modes often do participate in the response, they usually have a small modal mass, so they don't contribute much to the overall response.