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|  Perhaps
you were with me a year ago, when we discussed resonances.
We’ll repeat a little of that today, but this
time we’ll focus on the damage that is often caused
by resonances.
Let’s commence very simply.
We’ll combine a mass ...
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 with
a spring.
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|  And
create a classical SDoF or Single Degree of Freedom
system. |
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|  In
this next slide I’m holding one of my teaching
tools … a soft spring that is stretched about
6 inches by gravity pulling on a 4 ounce fishing sinker.
That is, the static deflection is about 6 inches or
150 mm. |
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Family of transmissibility graphs. Each graph
is
drawn with a different fraction of critical damping. |
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|  Let’s
imagine inverting that SDoF system and attaching it
to a shaker … a controllable source of mechanical
vibration that is suitable for vibration testing. We
used video clips last year to demonstrate that this
simple system has one resonance. At a particular frequency,
the vibratory response was significantly greater than
the vibratory input.
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That ultra-simple system has only that one resonance.
Most “real world” structures or systems
have many resonances. Consider, for example, a cantilever
beam. This animated cantilever beam is being vibrated
with a shaker at the beam’s “first mode”
bending frequency. (Some people call this the “diving
board” mode.)
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 If
this were “live”, I’d use three video
clips of a pair of cantilever beams to show you first
mode responses of that pair, first with 22 Hz shaker
sine forcing, then with 34 Hz sine forcing. Then we’ll
switch to 0-100 Hz broad spectrum random forcing, something
like road inputs to your automobile.
Click on the image to see this
video clip.
You will need Real Player to watch the video. Just click
here to download it for free.
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Click on the image to see this
video clip.
You will need Real Player to watch the video. Just click
here to download it for free. |
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Click on the image to see this
video clip.
You will need Real Player to watch the video. Just click
here to download it for free. |
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|
 What,
in the “real world,” might those three video
clips represent?
This next sketch represents a card carrier filled with
PWBs or printed wiring boards. With sine forcing, one
board at a time responds, when shaker ff matches that
board’s fn. None will deflect far enough to strike
its neighbor.
But with random forcing, all respond simultaneously.
Boards striking adjacent boards will be common. Damage
will ensue.
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Click on the image to see this
video clip.
You will need Real Player to watch the video. Just click
here to download it for free. |
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|  Plate
responses are more complex because of the extra dimension.
I wish you could see the animated responses of this
plate while a shaker sweeps upward. Nothing happens
till we excite the first resonance at 174 Hz, then nothing
until we excite the second resonance at 258 Hz, then
nothing till we excite the third resonance, at 341 Hz.
And there are more resonances, as we go higher.
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I hope you can understand that that
plate represents one of your printed wiring boards.
Yes, when vibrated, your printed wiring boards can twist
and bend in those and additional patterns.
You might be doubting me, thinking that your PWBs don’t
flex. Yes, they do. Mentally expand your cards to maybe
10 feet x 15 feet. Now can you visualize your cards
flexing as in the three animations? |
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|  Of
course the displacements on real cards are small, but
they can damage wiring on card surfaces, can damage
the attached chips, but most damage is between cards
and chips. Let’s “hunker down” and
look under a chip. Can you see the connections that
will open and close when the card flexes?
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|  One
of our courses teaches you how to predict the frequencies
at which your cards resonate, and the patterns in which
they respond. You will learn where not to put your more
delicate components.
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|  Getting
away from PWBs, and speaking in general terms about
structures, they do respond in a number of patterns.
These sketches show four of the many possible modes
in which a flat slab (an optical table) responded to
some input.
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|  The
peaks here show the frequencies at which each resonance
occurred.
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 Observe
the dramatic lessening of the peaks and the notches that
was achieved by adding internal damping.
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 Those
and about 1000 more PowerPoint slides, many of them animations
and video clips, support my 2004 Vibration and Shock lectures.
Much more theory than we’ve touched upon here. Measurement.
Analysis. Calibration. Testing. HALT, ESS and HASS. Will
you join me?
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Thanks again to B&K and to IEST! |
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