Compression Question

[Richard Brekne]

Delwin D Fandrich wrote:

> > 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.
>

First off.. thanks both Del and John for the << homework >> as it were. I see
the styrofoam example well enough, and understand Compression and compression
set well enough to follow that far with it. Its this above thing that causes me
a bit of confusion. But that's clearing up now as well..  I guess its just
difficult to follow a couple of the implications of this shrinkage not changing
in the face of compression damage bit... for example... it sort of means that
compression set (damage) effects the minium (lowest EMC) size of the panel.....
I suppose that shouldnt be a moment of confusion for me.. but it was... still
bothers me a bit.. but I'll sort it out sooner or later :) Its like you can
hypothetically speaking squezze the thing down next to nothing and it will
still shrink that 1-1.5 % given a drop 8 % drop in EMC.

Its easy enough to see that if you force the thing to take on constrained
dimensions at 13 % EMC that it would naturally have at 4.5 % then it wont
expand any more then that any time you bring it back up to 13 %... but that
compression set more or less just reduces the panels size and otherwise does
not alter the net change in size for change in EMC.... well all three of you
guys say its so... and so does the book... so its obviously so... but... grin..
it still bothers me..

Dont worry :):)  I'll get it in the end.

Thanks
RicB



>
> >
> > 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.
>

Ok..... and this susceptiblity to cracking...... that can happen to an
unconstrained panel... I mean at some point the tensil strength can so weakened
that an increase in EMC will just start pulling the fibers apart ?


>
> >
> > 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.

Yes... I believe I see what you are saying... poor choice of words on my part
there... they've been  pressed closer together then their stress level
tolerates at some point... (in my torture test the 13-14 % EMC level) and so
take on the smaller shape/size... none of  which bears on the degree of
swelling and shrinking with change in EMC the panel otherwise will display,
yes ?? .....  until the compression has become so severe that tensil strength
has also been comprimised... and at that point an increase in EMC  will crack
the panel ... also yes ??

>
> Del

--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html
http://www.hf.uib.no/grieg/personer/cv_RB.html