The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 3 – Wood as a Structural Material 15
for our wind turbine blades. Our tests, which are
described more fully in Appendix C, centered on ultrasonically-graded
Douglas fir veneers laminated with
WEST SYSTEM epoxy, with and without synthetic fiber
augmentation. <strong>The</strong> discussion of our results which
follows is limited to wood/epoxy composites; details
about other laminates made with wood and glass
aramid and graphite fiber reinforcement are provided
in Appendix C. No similar comprehensive data exist
for solid timber, but values are usually lower.
Figure 3-3 shows the resulting fatigue curves in tension,
compression, and reverse axial tension to compression.
Wood is considerably stronger in tension than in
compression at low load cycles. At ten million cycles,
however, their capabilities are very similar and the
two fatigue curves move closer together.
We have designed blades for wind turbines up to 400'
(121m) in diameter and built blades that are 651 ⁄2'
(20m) long. Veneers are only 8' (2400mm) long and
must be joined in some fashion in order to build larger
rotors. For this reason, all of our test samples have
included either staggered butt or scarf joints—built-in
manufacturing defects. In one group of tests, we used a
12-to-1 slope scarf joint between mating veneers and
in a second series we induced the much more serious
flaw of three butted joints. While the difference
between the scarfed and butted specimens, logarithmically
plotted in Figures 3-4 and 3-5, was significant, it
was much less than we anticipated. In any other
material, this type of induced defect would likely cause
a much more significant reduction in fatigue capability.
<strong>The</strong>se were tests for longitudinal mechanical properties,
measuring fatigue when loads were applied parallel to
the grain of the wood. Just as important are secondary
properties or cross-grain material capability. Trees have
very simple load paths, with most loads longitudinal
to wood grain direction, but boats and other complex
structures do not. In these, loads will vary from fiber
direction, so some understanding of a material’s ability
to carry loads radially and tangentially is needed for safe
design.
All unidirectional composites exhibit substantially less
strength across their fibers than parallel to them. Wood
laminates are generally about five times stronger in
tension parallel to the fiber direction than tangentially
Figure 3-3 Laminate fatigue properties. Maximum strength
adjusted to 12% wood moisture content vs. total cycles for
BG-1 Douglas fir/epoxy laminate with 12:1 slope scarf joints
with 3" stagger, 31.8 in 3 test volume, parallel to grain load
direction, at room temperature.
Figure 3-4 Compression 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.
to the fiber direction. Dense high-strength fiber bundles
such as glass or carbon have a much worse problem in
this regard: their cross-fiber strength may only be a very
small fraction of longitudinal capability. It is our view
that a majority of composite material failures in the
marine field are in these secondary properties, and it is
also in these areas where limited or no fatigue data exist.