The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 3 – Wood as a Structural Material 17
static strength of the small-volume specimens averaged
276 psi. <strong>The</strong> rolling shear strength of the large-volume
specimens averaged 268 psi. In this case, the size effect
appeared to be insignificant. <strong>The</strong> knockdown for the
larger volume amounted to only about 3%. However,
the size effect was bigger in fatigue. <strong>The</strong> small-volume
specimens intercepted the one million R = 0.1 cycle line
at 185.5 psi. <strong>The</strong> large-volume specimens intercepted
the one million-cycle line at only 152.2 psi. An investigative
technique that relied only on static testing would
have missed this important truth. With the so-called
secondary properties, size effect becomes increasingly
severe as cycle counts rise.
Wood Stiffness
Stiffness may be quantified by measuring deflection
under a given load. It describes how floppy, flexible,
rigid, or stiff a material or structure is. Since repeated
deflection can cause cumulative damage, resistance to
deflection is often important to the integrity of rigid
structures. In boats, adequate hull stiffness prevents the
excessive working which may result from repeated highpoint
loadings. Wood has excellent stiffness potential
and, as indicated in Figure 3-7, it is relatively light in
weight.
To illustrate wood stiffness, we conducted a simple
bending test and compared it to other materials used
in boat construction. For this experiment, we made
up samples of the following:
l. Unidirectional glass fiber with polyester resin
(50% fiber volume).
2. Aluminum (5054-H34 commercial grade).
3. Kevlar aramid fiber with epoxy (50% fiber).
4. Graphite fiber with epoxy (50% fiber).
5. White ash.
6. Sitka spruce.
7. Western red cedar.
Each sample was 24" long � 1 ⁄2" wide. Thickness was
determined by the density of each material so that all
samples weighed exactly 25 grams. In theory, the
samples represent small sections of a hull skin taken
from boats of the equal size and weight manufactured
with each of these materials.
Figure 3-7 Relative weights of flat panels built with various
structural materials. Wood/epoxy composite built with ultrasonically
graded Douglas fir and WEST SYSTEM epoxy.
To test our samples, we clamped them to a bench and
used them as cantilever beams, with weights hung from
their free ends. Figure 3-8 lists the amount of deflection
measured under uniform weight in all samples, and it
also lists the price per pound of each material. Wood
and graphite fiber/epoxy composite clearly resisted
deflection better than the others. When cost is factored
in, wood is the winner of the stiffness test. Graphite fiber
composite is almost twenty times (1985) as expensive
as Sitka spruce.
<strong>The</strong> beam test compares equal weights of materials, but
not equal volumes. A section of 1 ⁄8" aluminum would
resist deflection far better than a piece of wood with
the same dimensions, but it would also weigh about
seven times as much. Two hulls, one made of 1 ⁄8"
aluminum and the other of 7 ⁄8" wood, would weigh
about the same, but the wooden hull would be
significantly stiffer.