Underwater Shock

[StuartTHannah]

Need some help understanding the reality of underwater shock. Have done some Modal analysis on the subject above and come up with a number of conclusions/assumptions can someone set me straight.

I am aware that when an explosive device goes off underwater a pulsing gas bubble is created which subjects any vessel within proximity to a shock wave of certain frequency - Normally modelled as a damped sinusoid.

The vessel and any equipment within will then respond to this energy input but what is actually occuring does :
a) The energy input primarily affect only the equipment of similar natural frequency (or harmonics of that frequency), with subsequent effect on other parts of the vessels strucuture
b) Is the energy input to every item onboard and set up their own responses e.g. ringing.
c) Some other explanation

 

[GregLocock]

I disagree with this statement : "I am aware that when an explosive device goes off underwater a pulsing gas bubble is created which subjects any vessel within proximity to a shock wave of certain frequency - Normally modelled as a damped sinusoid."

A shock wave produces many different frequencies - the spectrum looks like shaped broadband noise, typically with a fairly gentle high frequency roll off at a frequency related to the shock duration.

It is directly analagous to an impact test in experimental modal analysis.Think about it - a shock wave sounds like a thump, not a pure tone.

Simple linear systems tend to respond at their first natural frequency, as a damped sinusoid. Perhaps that is what you meant.

If I am right then (b) is the nearest approximation to the right answer.




Cheers

Greg Locock

 

[StuartTHannah]

Thanks Greg,

I agree with your view on the above, however my initial statement was purely based upon my experience (which isn't a lot) of working with Military standards. When performing a dynamic analysis naval vessels,a shock input, normally a specific frequency damped sinusoid, is used as an input and the response calculated from this. Again not knowing for certain, I have assumed that this would primarily affect items of a conincident natural frequency.

Stuart

 

[GregLocock]

Perhaps they use a sinusoid related to the fundamental frequency of the ship's hull. This would seem a reasonable starting point for assessing the shock resistance of the ship'ds contents, which I have been led to believe is an important parameter in the design of both the ship and its equipment.

If the sinusoid is very heavily damped then it approximates to an impulse anyway. Cheers

Greg Locock

 

[StuartTHannah]

The input excitations taken from naval specs were derived from actual tests carried out in the 60's/70's, measured by placing accelerometers around the vessel. Can I ask would you expect given this situation to see a sinusoid with the natural frequency of the ships hull. Furthermore could you explain the significance of high damping and impulse. Sorry if this seems basic but I am quite new to the field.

 

[GregLocock]

Yes, absolutely. The fundamental (lowest) mode of a structure is the one most likely to be strongly excited by an impulse. The hand-waving explanation is that the response of the structure is a summation of modes to match the input force matrix (in both a time and space, sense), and that you need more low frequency stuff than high frequency to match a pulse. OK, that was awful, how about: imagine a flexible beam. You poke the beam with your finger (anywhere). The most natural response is the simple half sinewave and that is a good approximation to the actual deflected shape. More technically read the literature on modal summation and superposition.

By an impulse I meant a positive going spike of arbitrarily short time span. The bandwidth of the impulse is strongly related to the inverse of its length (ie a 1ms impulse will have a bandwidth of /about/1 kHz irrespective of the precise shape of the pulse). A heavily damped sinusoid can be considered to be an impulse of time t/2, followed by a weaker negative impulse t/2 later. The first one dominates, because the sine wave is exponentially decaying, rapidly. The point is, even though you 'think' it is a sine wave, it still has energy content at almost all frequencies. Bear in mind that this is a very heavy level of damping, with a time constant of the order of 1/f. Cheers

Greg Locock

 

[IRstuff]

What little experience I've had with MIL-S-901D suggests that it's not anywhere close to a damped sinusoid. The middle and light weight tests are designed around the "hammer" test, which involves swinging a massive weight to impact the test fixture upon which the UUT is mounted. This is followed by the impact shock of the test fixture hitting the motion snubbers.

The heavyweight test involves the "barge" test, where actual explosives are detonated around a barge upon which the UUT is mounted. I don't have the Navy paper in front of me, but the measured stimulus is also not a damped sinusoid.

In all cases, there is no official definition of the input disturbance, so I'm not sure where the damped sinusoid is coming from, unless it's the response of your test fixture to the input shock.

TTFN

 

[GregLocock]

Exactly. The test fixture is the hull, the damped sinusoid is applied to the stuff bolted to the hull.
Cheers

Greg Locock

 

[vanstoja]

In my experience with shipboard equipment shock tests, there has been no attempt to prescribe or control input frequencies in the design of test fixtures. Usually, the g-load inputs to the equipment in vertical and horizontal directions have been specified for design purposes and sometimes a pretest shock analysis is done to determine the equipment's ability to pass the shock test. Test foundation designers try to simulate the design g-loads but not always successfully. In several barge tests, either the foundation failed before the pump or the equipment was subjected to high shock overloads in the direction not adequately accounted for by the foundation designer. Another problem with barge tests is their varying test sites which bring in bottom reflection effects dependent on site geology, eg. a shallow,marshy bay in Washington vs a deep water-filled stone quarry in Virginia vs the end of a pier in Chesapeake bay. Often it is not the initial shock ramp that is the worst but rather some delayed reflected wave response that needs to be addressed. vanstoja