The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 3 – Wood as a Structural Material 21
bending strength may easily be doubled and, in some
woods, tripled. <strong>The</strong> reason for this is the actual strengthening
and stiffening of cell walls as they dry out. Not all
strength properties are changed in such a dramatic way.
Figure 3-10 gives a tentative average for several species
of wood of the change of various physical properties
per 1% decrease in moisture content. By multiplying
the values in the table, you can see that reducing the
moisture content of wood results in substantial increase
in its physical properties.
<strong>The</strong>se data are averages for several species of wood and
therefore are not suitable for engineering work. We present
them only to illustrate the fact that up to a point,
dry wood makes a stronger boat. <strong>The</strong> percentage increases
in strength are the average for the entire range,
from 25% to 0%. <strong>The</strong> first test, for instance, shows that
in fiber stress at proportional limit in static bending, the
increase in value is 5% for every percentage point moisture
content is reduced below 25%. One hundred
percent is the reference value for 25% moisture content;
225% becomes the value at 0%. It is incorrect, however,
to conclude that the value is a straight line function of
moisture content and that it would be 105% at 24%,
110% at 23%, and so on. It’s most likely that in most
Static Bending Increase
Fiber stress at proportional limit 5%
Modulus of rupture, or crossbreaking strength 4%
Work to proportional limit 8%
Work to maximum load or shock-resisting ability
Impact Bending
1 ⁄2%
Fiber stress at proportional limit 3%
Work to proportional limit
Compression Parallel to Grain
4%
Fiber stress at proportional limit 51 ⁄2%
Hardness, end grain 4%
Hardness, side grain 21 ⁄2%
Shearing strength parallel to grain 3%
Tension perpendicular to grain 11 ⁄2%
References: “Strength and Related Properties of Woods,” Forest Products Laboratory,
Forest Service, U.S. Department of Agriculture Technical Bulletin No. 479.
Wood Handbook: Wood as an Engineering Material, Forest Products Laboratory,
U.S. Department of Agriculture Handbook, No. 72.
Figure 3-10 Physical properties. Average increase is value
affected by lowering the moisture content 1% from fiber
saturation point, approximately 24% in most woods.
species of wood the percentage increase in the value is
higher as the moisture content is first lowered from 25%,
then lower as it approaches 0%.
While the effects of moisture on maximum load-to-failure
static properties are substantial, for most marine applications,
the designer should not be overly concerned. Most
marine structures are subjected to stress levels well below
maximum static one-time load-to-failure conditions, and
the effect of moisture on lower load levels in long-term
fatigue is a more pertinent issue. Recent scattered and
preliminary <strong>Gougeon</strong>-developed fatigue data indicate
that the detrimental effects of higher moisture content
may diminish or even disappear at 10 million cycles
and beyond. <strong>The</strong>se early indications require further
substantiation before the extent of the phenomenon
will be accurately known.
Wood/Epoxy Composites
Wood is strong, lightweight, stiff, and resistant to fatigue.
Some of its shortcomings as a material for engineering
result from inherent defects such as knots and grain
irregularity, but most are related to moisture. By using
wood as a reinforcing fiber, which may be bonded into
many shapes and forms with epoxy, we are able to make
the most of its structural advantages and overcome its
limitations.
<strong>The</strong> primary goal of incorporating wood in a composite
with epoxy is to provide its fibers with maximum practical
protection against moisture. When it is able to
resist violent seasonal fluctuations in moisture and its
moisture level is stabilized at lower levels, wood
maintains good physical properties and dimensional
stability.
Wood/epoxy composites may also, when properly
engineered, provide a means of homogenizing the
defects and variations of lumber and of increasing its
strength in compression. Chances of failure of structural
boat members may be greatly reduced by using beams
laminated from a number of pieces of thinner wood
instead of a single board, particularly when grain alignment
within a laminate is manipulated to best receive
predicted loads.
We seal all wood surfaces, those which come into
contact with water as well as those which come into