Strengths and Frailties of wood
Prunus serotina or
Black Cherry – Specific Gravity .50 (dry), MOR = 12, 300 lbf/in2
From a structural standpoint, wood is pretty amazing stuff. Under a microscope, you can see that wood is made up of many spindle-shaped cells. These Cells are mostly filled with air (or water when it becomes humid or wet) but the walls of these cells are made up of long helical chains of cellulose. You can see Cellulose in a purified form as cellophane - Wrigley’s Spearmint gum used to come in cellophane wrappers.
Once for ounce, wood’s tensile strength is
greater than steel. It is easy to
machine and takes a wide variety of adhesives.
Compared to man-made materials with similar strengths (Kevlar or exotic
metal alloys), it is inexpensive. That
said, there are number of foibles that a builder
should be aware of.
Wood is non-isotropic,
that is its strength and other physical properties are not constant it all
three dimensions. It is fairly easy to
split a stick of wood along the grain with a hatchet. It takes a lot more energy to split wood
across the grain. Because of this, it is
important to orient the grain in specific directions on highly stressed components.
Wood is a natural material, and defects
like knots, splits and shakes can limit its strength. Wood is an organic material – remember it is
made up of these empty cells that will shrink and swell as the ambient humidity
changes. Almost all of this shrinking an swelling action is across the grain.
Over time, wood will slowly deform. Take a look at an old roof beam or joist and
you will see a definite curve downward.
It will try to run away from the load.
A great book on harps is Armstrong’s The Irish and Highland Harps
(Preager, NY, published in 1969). Mr. Armstrong describes a dozen or so ancient
Celtic harps, depicting (in vivid detail) their demise as they slowly collapsed
under the tension of their strings.
Armstrong hypothesizes continually on successive repairmen’s attempts to
forestall the harp eventual structural demise.
The
harp builder can take a number of steps to mitigate the weaknesses of
wood:
- Select
clear grained stock
- Ensure
that it is dry. A lot of
woodworkers use expensive moisture meters.
I’m saving that money for a surface sander. I make do by patronizing suppliers that
I trust or buying green wood from a local sawyer and storing it for a few
years.
- Consider
lamination for the neck and other parts under high stresses.
- Use
designs and joinery that allow for Expansion/contraction
- Look
on structural failures as a tuition – a chance to improve your design or
building skills
Picea sitchensis
or
If you really want to get technically educated
on different woods and their mechanical properties, the U.S. Forest Services Products
Laboratory has published the Wood
Handbook. Chapter 4 is a
particularly useful reference on the mechanical properties and testing
procedures used for lumber.
Mechanical
Properties
Commonly Used Harp Woods
Dried to 12% MC
|
Common Body Woods |
|
|
|
|
|
|
Density |
Stiffness |
Hardness |
Strength |
|
Cocobolo |
1.00 |
|
|
10,067 |
|
Wenge |
0.87 |
2.46 |
1,630 |
19,500 |
|
Bubinga |
0.71 |
2.48 |
2,690 |
22,293 |
|
White Oak |
0.68 |
1.78 |
1,360 |
15,200 |
|
Sugar maple |
0.63 |
1.83 |
1,450 |
15,800 |
|
True Mahogany |
0.59 |
1.50 |
800 |
11,500 |
|
Black Walnut |
0.55 |
1.68 |
1,010 |
14,600 |
|
Black Cherry |
0.50 |
1.49 |
950 |
13,250 |
|
Bigleaf
Maple |
0.48 |
1.45 |
850 |
10,700 |
|
|
|
|
|
|
|
|
|
|
|
|
|
Common Soundboard Woods |
|
|
|
|
|
|
Density |
Stiffness |
Hardness |
Strength |
|
Douglas Fir |
0.48 |
1.95 |
710 |
12,400 |
|
|
0.40 |
1.57 |
510 |
10,200 |
|
Spanish Cedar |
0.40 |
1.44 |
600 |
11,500 |
|
OG Redwood |
0.40 |
1.34 |
480 |
10,000 |
|
Western Red Cedar |
0.32 |
1.11 |
350 |
7,500 |
|
Eastern White Pine |
0.35 |
1.24 |
380 |
8,600 |
Density = Specific Gravity
Stiffness = Modulus of
elasticity, Million lbf/square inch
Hardness = Side Hardness, lbf
Strength = Rupture Bending
Strength, psi
Sources
Chapter Four of the
http://woodworkerssource.net/onlinewoods/
Back
to the Structural Analysis of the Folk Harp