The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 11 – Laminating and Bonding Techniques 103
X
Load
8X
A High-Density Fillet
Experimentation with the
fillet can produce a joint
strong enough that failure
will occur in the plywood
rather than in the
wood-epoxy bond.
Plywood Fillet
thickness radius
High-density fillets 5
⁄32" (4mm) 5 ⁄8" (15mm)
3
⁄16" (5mm)
3 ⁄4" (18mm)
1 ⁄4" (6mm)
7 ⁄8" (22mm)
Low-density fillets 5
⁄32" (4mm) 3 1 ⁄4" (44mm)
3
⁄16" (5mm) 3 1 ⁄4" (44mm)
1 ⁄4" (6mm) 3 1 ⁄4" (44mm)
Fillet fails in tension
and starts wood
failure along
grain.
Low-density
fillet compresses
under load.
A Low-Density Fillet
Wood failure
along the base
of the joint.
Figure 11-18 Failure mode for proper sized fillet radius is
shown in Cross Section A. Failure mode for inadequate sized
fillets is shown in Cross Section B. <strong>The</strong> chart shows the radius
size required for high-density fillets to produce a wood failure
(A) rather than a joint bond failure (B). Results vary depending
on plywood quality and thickness, and filler density. Experiment
to find the proper radius for the materials you are using.
perform your own testing to determine the exact fillet
you need. This type of testing will also quickly give you
a feel for the strengths and limitations of the fillet as a
joining method. When you have gained knowledge
through use, you will find many places in your project
where fillets will both improve your structure and save
you time.
Make a test sample similar to the one in Figure 11-18,
using the material you plan to join. Take an educated
guess about the filler and fillet size needed to develop
material strength. Make a fillet on your sample and
allow it to cure for at least 24 hours or less if you apply
heat. Put the control part of your test piece in a vise and
load it. <strong>The</strong> load should be exerted a distance of about
eight times the thickness of the material, measured from
the base of the joint.
If your sample breaks clean, leaving the fillet intact, you
have achieved material strength, the breaking strength
of the material as a cantilevered beam. This is considered
strong enough for most engineering in boat
construction. If the break occurs within the fillet or the
wood fiber of the control piece, make a new sample
with a larger fillet. Repeat the test until the fillet
performs to your satisfaction.
Applying the Fillet
<strong>The</strong>re are five steps in the process of applying a fillet:
1. Fitting and marking the parts to be joined.
2. Applying epoxy/filler mixture to the joint.
3. Forming the fillet to the size and shape desired.
4. Cleaning up the fillet.
5. Sanding and coating.
If the fillet is to be a practical joining method, you must
learn to make it with both ease and speed. At the same
time, you will want to make fillets that are pleasing to
the eye so that they will not detract from the beauty
of your boat. Your first try at filleting will probably be
awkward and slow, but it doesn’t take long to develop
reasonable skills.
Fit all parts as carefully as possible. Reducing gaps
between pieces reduces mechanical loading on the filler
and improves joint strength potential. When you are
satisfied with your fit, make several surmarks up and
down the joint and across both pieces so that you can
remove and accurately replace them later. To save
mixing and preparation time, fit and mark a number
of parts and then bond them all in one large filleting
operation.