Official Luthiers Forum!
http://www.luthiersforum.com/forum/

Mechanical Properties of Wood - resources
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=24029		 Page 1 of 1
Author:  David C. Hurd [ Sat Oct 03, 2009 11:50 am ]
Post subject:  Mechanical Properties of Wood - resources
About 6-8 years ago I was looking around for information on the mechanical properties of wood. One of the first things I found was a USDA Forest Products Laboratory publication, "Wood Handbook". I'm sure it's possible to find a copy through http://www.abebooks.com but it's also available in PDF format at the FPL website: http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p Chapter Four would be the appropriate one to download. And here are the Ex vs. Specific Gravity data from that source plotted up wherein each data point represents a different wood species:

Attachment:
FPLdata.jpg


In the late 1940's through the early 1950's there existed a Yale University School of Forestry publication called "Tropical Woods". I was able to obtain most of the relevant publications and plot up the Ex vs. Specific Gravity data. Again, each data point represents a single species with essentially all of them being hardwoods.

Attachment:
Yaledata.jpg


Finally, I have some data on wood species that I measured myself:

Attachment:
SGvsExDyn.jpg


Although all of the above samples were kiln or air dried to a constant humidity and only quartersawn orientations were used, there still exists a considerable amount of variability in the data. In light of these observations, testing wood before using in an instrument would seem to be a prudent gesture...

aloha,
Author:  David Malicky [ Sat Oct 03, 2009 1:58 pm ]
Post subject:  Re: Mechanical Properties of Wood - resources
Thanks for those plots, especially the last one. Yes, it looks like density is only a moderate predictor of stiffness.
Author:  Alan Carruth [ Sat Oct 03, 2009 7:18 pm ]
Post subject:  Re: Mechanical Properties of Wood - resources
Density may be only a 'moderate' predictor of long-grain stiffness in softwoods, but it's by far the best one I've found so far. Nothing else, such as grain count, or species, even comes close to being as good. It certainly is best to measure, but if you can't then density will get you closer than the usual sort of guesswork .
Author:  truckjohn [ Sat Oct 03, 2009 7:23 pm ]
Post subject:  Re: Mechanical Properties of Wood - resources
David,

Thanks for posting this.

Though there is considerable spread.... Statistics being what it is... and measuring Ex taking the time it does... and Density being so easy to measure...

If I was looking for a specific Ex stiffness range... I think it points towards the smartest way is to start within a certain Density range and then test those to find tops within the Ex range I was looking for...

Basically, it says that if you really want to find Spruce that has an Ex around 1500 KPSI ... there is no reason to waste time looking in 0.3 s.g wood.... unless you want to spend an inordinate amount of time finding that *1* outlier.

Looking for Needles in a Haystack becomes really easy when the "Haystack" is made of needles!

Thanks

John
Author:  Parser [ Sun Oct 04, 2009 9:02 am ]
Post subject:  Re: Mechanical Properties of Wood - resources
How about creep? I've only found a little bit of info regarding how much wood creeps...I think it was something on the order of 150% of the initial deflection after about 2 years (depending on the temps, humidity, etc. of course).
Author:  David C. Hurd [ Sun Oct 04, 2009 12:48 pm ]
Post subject:  Re: Mechanical Properties of Wood - resources
Thanks for the thanks! :)

Parser, I recommend the Forest Products Lab Wood Handbook Chapter 4, p. 37 (as mentioned in my initial post) for a discussion of creep in wood. It's a relatively small (1.2 Mb) PDF file that has lots of additional useful information.

As you might expect there are several factors that affect the magnitude and rate of creep. That's why I like having a graphite carbon/expoxy composite insert into the neck. Although it's generally placed near the neutral axis of the neck, this material is supposed to have negligible creep associated with it. So the neck comes up a little and stays there until string tension is released. Nice.

aloha,
Author:  Parser [ Mon Oct 05, 2009 11:13 am ]
Post subject:  Re: Mechanical Properties of Wood - resources
Are you building ukes like this or steel strings? How long have you been doing the CF thing? I assume you aren't putting a truss rod in there as well? How do the necks fair in the long run?

PRS is doing the same thing with their acoustics, and this issue was discussed while I was there. They had some steel strings that had been used for a few years that were built like this...they faired pretty well over the course of a few years. One important note is that the composite they were using was actually an aerospace grade fiber with a very high fiber volume fraction...not really off the shelf stuff. I don't know if it was overkill...or if it was...how much overkill it was.
Author:  David C. Hurd [ Tue Oct 06, 2009 10:40 am ]
Post subject:  Re: Mechanical Properties of Wood - resources
I have used this CF approach in 4-, 6-, and 8-string tenor and 4- and 6-string baritone ukuleles as well as nylon string guitars. I've probably put the CF rods in at least a hundred instruments over a 7-8 year period with no obvious issues. The CF I used was garden variety with a Young's modulus of only 10^6 psi - about the same as aluminum actually - but still sufficient for the purpose.

I worked with Josh Gordis to produce a calculation spreadsheet for neck deflection which is here:

http://www.ukuleles.com/Technology/neck.html

There's a slight error in the math which results in underestimation of the actual deflection, but the sheet is still quite useful for back-of-the-envelope calculations.

aloha,
Author:  Parser [ Tue Oct 06, 2009 7:15 pm ]
Post subject:  Re: Mechanical Properties of Wood - resources
Nice work, Very cool!

That's some real engineering there...!
Author:  David Malicky [ Wed Oct 07, 2009 2:05 am ]
Post subject:  Re: Mechanical Properties of Wood - resources
Thanks for those informative posts, all. Alan, to clarify, I actually do still measure density since it's so much faster than E. My prior post was a note (partly to myself) that I don't want to rely on it too closely. Thanks for the info on the other potential variables.
Page 1 of 1 All times are UTC - 5 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/