The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 14 – Hardware Bonding 131
side, created by drilling a fastener hole with a bit
about 1 ⁄4" (6mm) larger in diameter than the fastener.
Drill the wood to the proper depth, fill the hole with
epoxy, coat the shank and threads of the fastener
with epoxy, and set it in.
Oversize holes and epoxy interfaces between metal
fasteners and wood fibers become increasingly
important with fasteners which are larger than 1 ⁄4"
in diameter. This is particularly true in situations
where the potential of a bolt, machine screw, or
threaded rod must be maximized.
3. Bond the contact surface of the fitting to wood
fiber with epoxy. Bonding the contact surface of
a fitting can contribute a great deal to load distribution
over maximum wood surface. Proper bonding
of a 21 ⁄2" diameter pad eye, for example, can add
up to 7,000 pounds of extra shear-load resistance,
assuming Sitka spruce with the load parallel to grain.
We are primarily interested in improving the shearload
capacity of hardware installations. It’s difficult
to make significant shear-load improvements by
bonding fasteners, but when a fitting is bonded to
the wood on which it sits, it has far more capacity
in shear than the fasteners used to attach the item
have by themselves. We bond hardware mainly to
improve shear strength, the main weakness of lowdensity
woods.
<strong>The</strong> Design Process for Fastener Bonding
Hardware bonding is a predictable process that allows
a designer or engineer to determine in advance how a
Annulus of thickened
epoxy. Use drill bit 1 ⁄4"
larger than fastener size.
Wood screw
in standard
pilot hole.
Wood screw
in oversize
holes.
Figure 14-1 Four different bonding techniques for small
fasteners shown in cross section.
1 ⁄8"
Hardware
Fastener
Machine screw
in oversize hole.
Blind threaded
rod in oversize
hole.
bonded fastener or fitting will fail if it is overloaded.
This is important because it is desirable to have
warning signs before a catastrophic failure occurs.
<strong>The</strong> information presented in this section provides
the formulas and data necessary to properly design
a wood/epoxy/metal joint whether it is stressed in
tension, shear or a combination of the two.
Fasteners Stressed Primarily in Tension
With epoxy-bonded fasteners, the idea is to balance the
three parts (fastener, epoxy, and wood/hole surface area)
to obtain optimum performance. <strong>The</strong> key information
needed is the tensile strength of the fastener, the shear
strength of the epoxy, and the withdrawal resistance of
the wood or backing block.
<strong>The</strong> fastener dimensions and tensile strength should be
available from various handbooks. We use a conservative
shear strength of 800 psi for the WEST SYSTEM
105/206/404 epoxy mix. <strong>The</strong> wood performance values
are available in timber handbooks and vary by grain
direction (that is, whether the fastener is loaded parallel
or perpendicular to the grain of the wood). Typically,
one or more of the above factors is fixed, and the others
must be adjusted to fit a pre-existing condition.
As mentioned above, we typically use 800 psi as the
ultimate shear stress for the filled epoxy mix. This is
a conservative value based on long-term fatigue data.
At this stress level, the epoxy is the limiting component.
<strong>The</strong> interface between the fastener threads and the
epoxy is the typical initiation point for fatigue failure.
This is good. <strong>The</strong> fatigue failure mechanism for
epoxy—cracking and gradual destruction of the
matrix—gives some warning. A failure at the
wood/epoxy interface or the loss of the bolt head
is usually catastrophic.
<strong>The</strong> U.S. Forest Products Laboratory uses a formula to
estimate the ultimate withdrawal load for a drift bolt or
pin from the side grain of seasoned wood:
p=6,600G2DL
(p=ultimate withdrawal load in pounds, G=specific
gravity of wood @12% moisture content, D=diameter
of hole in inches, L=length of hole in inches.)
This formula gives a good estimate of the wood’s resistance
to withdrawal of the epoxy plug and can be used
to determine the size of hole. <strong>The</strong> actual performance