Soundboard stiffness variances JD

[Richard Brekne]

JD

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.

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.

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.

    "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."

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. 

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 (?).

In anycase... I think its safe to say that stiffness in all directions 
does indeed change with change in moisture content.

Cheers
RicB
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