Date: 2008-10-07
By Gene Wengert
Q:We are gluing two or three pieces of wood together to
form a panel. Our joints seem very strong, but sometimes they are
very weak. We are looking for a way to test our joints so that we
can identify the factors that result in a weak joint. One person
has recommended a shear test block and sent us some info on how
to run that test. Would this be a good way to proceed?
A:When a glue joint is formed, there are two components to
its ultimate strength. One component is the mechanical strength
that results because the two pieces of wood are full of nooks
and crannies. The glue fills these spaces and then solidifies.
(Analogy: Think of a puzzle that has interlocking pieces. This
interlocking provides some strength.) If you try to slip the
one piece that has been glued past the other, the solid glue
that fills the nooks and crannies will resist this shearing
movement very well.
However, if you try to pull this joint apart (that is, put
it under tension), these small nooks and crannies are not very
strong in tension and so the joint will fail easily. What
builds up tension strength (or pulling apart rather than shear
strength) is the chemical bonding between the wood and the
adhesive. So, the second component of a strong glue bond is
chemical strength.
Unless the surface to be glued is very fuzzy or is damaged,
or if there is a lack of glue or adequate pressure, the
mechanical bonding will occur 100 percent of the time. The
chemical bonding will occur only if the wood is receptive; that
is, if the bonding sites are not oily, occupied with dust or
moisture, and so on.
(In all the years I have worked in troubleshooting, I have
seen a gluing problem related to the adhesive only twice; all
the other times, the problem is related to the wood itself.) If
I were you, I would be interested in this tension strength of
my joint, as this is what is failing when you have a weak
joint. The shear test is not what you need. Instead, use a
tension test.
Updated: Oct 07, 2008
This article appeared in FDM, May 2002. ©Copyright 2013, All Rights Reserved.