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<DIV><FONT face=Arial size=2>Nice report, Ric.</FONT></DIV>
<DIV><FONT face=Arial size=2>Fenton</FONT></DIV>
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style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=ricb@pianostemmer.no href="mailto:ricb@pianostemmer.no">Richard
Brekne</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">pianotech@ptg.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Tuesday, February 12, 2008 1:11
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Soundboard stiffness variances
JD</DIV>
<DIV><BR></DIV>JD<BR><BR>First, let me say that I have dropped the use of CC
and RS (compression crowned and rib supported) as it gets tedious
writing all that time and time again. As the discussion centers around the
desirability or lack there of of using compression to form some of the
unloaded crown and to provide for a significant degree of crown support... I
just started using the term Compression Reliant CR. That in no way
leaves out one or the other board... just finds and utilizes their common
denominator as it were.<BR><BR>Second as for the variance of wood
strength properties (in any direction) with changes in humidity. This is
all pretty well published and plastered all over the worlds various wood
engineering / forestry department / etc websites... and is in every wood
engineering book I've picked up and paged through.<BR><BR>I own a couple
myself at this point... so I'll just quickly quote from Hoadely... tho I could
send along a couple short papers from the USFD and similar places that say
exactly the same thing in so many words.<BR><BR>
<BLOCKQUOTE>"The strength of wood increases as the wood gets drier, although
the rate of strength improvment is not directly related to the loss of bound
water (as is the case with the shrinkage rate) property. For example,
maximum crushing strength in compression parallel to the granand fiber
stress at proportional limit in compresssion perpendicular to the grain is
approximately tripled in drying from green to oven dry. Modus of
Rupture (MR) is more then doubled in the process, but the stiffness is
increased by only about half."<BR><BR></BLOCKQUOTE>Tables and graphics are
usually supplied with such paragraphs to show the relative rates of
change. In Hoadley we see that Static bending elasticity E changes at
the rate of 2% per 1 % MC, Modulus of Rupture 4% per 1% MC, and
Compression parallel to the grain 6% per 1% MC. <BR><BR>Another point
worth mentioning.. there is in reality three different E's for wood. One
for each grain orientation. Tables published usually include only the E
for static bending...which is in turn an averaged value for both radial and
tangential loading. In reality there is an E sub L, E sub R, and E
sub T. They do this because for most wood engineering problems a greater
resolution is not required. For some of the discussion here however...
it would be handy to know and understand all the twelve constants that are
needed to describe the elastic behavior of wood. The three moduli of
elasticity(E), three moduli of rigidy (G), and six Poisson ratios
(μ).<BR><BR>In anycase... I think its safe to say that stiffness in all
directions does indeed change with change in moisture
content.<BR><BR>Cheers<BR>RicB<BR></BLOCKQUOTE></BODY></HTML>