Soundboard stiffness variances JD

[Fenton Murray]

Nice report, Ric.
Fenton
  ----- Original Message ----- 
  From: Richard Brekne 
  To: pianotech at ptg.org 
  Sent: Tuesday, February 12, 2008 1:11 AM
  Subject: Soundboard stiffness variances JD


  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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