Compression Question

[Delwin D Fandrich]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: August 28, 2003 7:55 AM
Subject: Re: Compression Question


> I see you are two steps ahead of me in one direction here Ron. And of
course I
> agree that I have seen this in pianos, and thats half of why I asked. The
other
> half is that I'm still trying to get ahold of exactly how this shrinking
and
> expanding works.... especially in the face of a compressioned peice of
wood.

Forget about wood for a day or so. Go out and buy yourself a piece of sheet
Styrofoam. Something about 10 or 12 mm thick. The rigid kind. Now, play
with it a bit. Notice that you can bend it slightly and nothing happens.
Bend it a lot and it breaks--don't do that. You can push against it just a
bit and the stuff will compress slightly but bounce back when you take your
finger away. Now, go out to your shop and cut a slice of the stuff exactly
100 mm wide--measure it to be sure--and put it into your vice. Crank on the
vice until you can measure 99 mm between the jaws. You have now compressed
the Styrofoam by about 1%. Release the pressure and measure the width of
the piece. It will probably be back to about 100 mm. (No, I don't know the
compression strenghth of Styrofoam, I'm guessing a bit here.) Back in the
vice, this time take it down to 98 mm. Release the pressure and measure
again. At some point you will find that the Styrofoam panel will no longer
bounce back to 100 mm but to some reduced width--this is compression set.
Now take a piece of spruce and repeat the process. In both cases you are
crushing things and causing them to deform. So slightly you can't readily
detect the crushing by eye, but it has taken place all the same. The same
thing is happening with compression set over time. In this case it is
caused by something called creep--the physical deformation of a material
resulting from stress and time. Or time deformation under load. In the case
of the Styrofoam and the vice you just accelerated the process.


> Something about the fact that a panel having assumed a certain set of
> dimensions at a given EMC tells me that it shouldnt be able to get
smaller
> then those same dimensions at that same EMC.

Why not? You've permanently altered the shape and physical characteristics
of the wood fibers (or the Styrofoam) by crushing them, however slightly.


>
> Ok... so compressing the panel as described doesnt stop a panel from
> shrinking as its moisture content drops... tho it does to some degree
inhibit
> that shrinking capacity.

No it doesn't inhibit "that shrinking capacity." At least it wouldn't if
wood were a perfectly uniform or consistent material. Assume that an
unrestrained and undamaged 1,000 mm wide panel made of perfect spruce will
change dimension by, say 1.5%, from a low of 4% MC to a high of 14% MC. Now
take the panel through your torture test. After some years of being held
under compression it will have physically altered its
dimension--compression set. Back at 4% MC our original 1000 mm wide panel
might now be only 985 mm wide. But, when taken back up to 13% or 14% it
will still expand by something close to 1.5%, or about 999.8 mm.

In reality, of course, wood is not a perfect material--if it were we
wouldn't be having this discussion. But some regions of every soundboard
panel are more susceptible to compression damage than others. Some are
going to want to expand by less than 1.5%, some more. There are the areas
of the panel that are going to sustain much more than their fair share of
deformation under load. It is here that compression damage in the form of
compression ridges will appear first and most. Once wood fibers have been
damaged to the extent that it becomes obvious to the eye, then yes, it's
shrinking capacity--more properly, it's tensile strength--will have been
compromised to the extent that the panel is susceptible to cracking. But,
as has been repeatedly stated in the past, it is not the crack that is the
problem. The crack is mearly a symptom. The problem is the fiber crushing
throughout the region of compression damage.


>
> I suppose this means that the cells would be even tighter
> pulled together as origionally they were when first dried out to the same
> EMC. I guess its this last that causes my present head scratching.

No, I wouldn't say "pulled together." The swelling and shrinking process
remains pretty much the same. It is the wood fibers themselves that have
changed. Their shape has been physically and permanently altered.

Del