The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

16 Fundamentals of Wood/Epoxy Composite Boatbuilding
Figure 3-5 Tension fatigue. Maximum stress vs. total cycles
for Douglas fir/epoxy laminate with 12:1 slope scarf and butt
joints with 3" stagger, 31.8 in 3 test volume, parallel to grain
direction, room temperature and 12% wood moisture content.
Figure 3-6 Tangential fatigue, wood/epoxy laminate.
Size effect in laminated veneer
During the second half of the 1980s, <strong>The</strong> U.S. Department
of Energy funded an extensive GBI test program
to investigate various properties of epoxy/Douglas fir
veneer laminate. <strong>The</strong> research included static and fatigue
measurements of two secondary properties: (1) radial
cross-grain tensile strength and (2) rolling shear strength.
Radial refers to the through-the-thickness dimension,
perpendicular to the plane of lamination. Radial tensile
stress tends to pull the veneer layers apart. Rolling
shear refers to the application of opposing forces,
parallel to the plane of lamination but perpendicular
to the wood fibers. <strong>The</strong> opposing forces are slightly
offset to generate shear, such that wood fibers in
adjacent slip planes have a tendency to twist and roll
over each other.
Both secondary property studies featured two populations
of test specimens made from the same parent
laminate, one with small-volume specimens and one
with large-volume specimens, in order to gauge size
effect. Size effect refers to the often-overlooked phenomenon
that causes physically small test specimens to
demonstrate unrealistically high strength with respect
to that of actual, real world structures made from the
same material.
<strong>The</strong> test sections of the 28 small radial cross-grain tension
specimens were 1.5" in the grain direction, 0.5"
wide and 2.0" (20 veneer layers) thick. <strong>The</strong> test sections
of the 35 large radial cross-grain tension specimens
were 6.0" in the grain direction, 4.0" wide, and 12.0"
(120 veneer layers) thick. <strong>The</strong> large/small specimen
volume ratio was therefore 192:1. <strong>The</strong> average crossgrain
tensile strength of the small-volume specimens
was 393 psi. <strong>The</strong> average static strength of the largevolume
specimens was only 280 psi, a knockdown of
about 29%. <strong>The</strong> laminate moisture content of both
specimen populations was the same, ranging from 5 to
6%. In fatigue, the size effect turned out to be much
more pronounced. <strong>The</strong> S-N (Stress-Number of cycles to
failure) curves showed that for R = 0.1, the small specimens
could reach 10 million cycles with a peak stress
of 275 psi. (R is the ratio of the minimum stress
divided by the maximum stress. A ratio of 0.1 means
the maximum stress is 10 times the minimum.) For
the large-volume specimens, the S-N slope was much
steeper. A typical large-volume specimen could reach 10
million cycles only if the peak stress was 134 psi or less.
This was convincing evidence that size effect should
determine the design allowables for large, fatigue-driven
wood structures.
As for rolling shear, the specimen stressed volume ratio
was 160 in3 vs. 2.5 in3 , or 64:1. <strong>The</strong>re were 29 smallvolume
specimens and 14 large-volume specimens,
all made from the same parent material. <strong>The</strong> laminate
moisture content was just over 6%. <strong>The</strong> rolling shear