"Michael Koblic" wrote in message
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"Ed Huntress" wrote in message
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snip

It hope it will be useful to you sometime, Michael. Epoxy can be fairly
complicated, especially when you get into the elevated-temperature-cure
industrial products that we never see on the regular markets. They can
have tensile and sheer strength running 2.5X stronger than anything we
can buy in small quantities. Epoxy is great stuff but you really have to
understand its properties when you use it for hefty loads. For example,
the stronger it is in shear, the weaker it *may* be in peel.

I don't know where this scratch-in idea started, but I think I know where
it first appeared in print: in the Gougeon Brothers' first book,
published in the early '70s. (The Gougeons were the originators of the
WEST epoxy system.) They used it to fasten stainless deck hardware to
their boats.

Since those guys are highly respected empiricists -- they actually test
and try their ideas before talking about them -- it caught on in the
boating community. Then it started to show up in metalworking, with some
of my writing in the late '70s a possible contributor. d8-)

Anyway, I've used it many times, and I've done some of my own testing
(with pry bars and hammers g), and I'm sold on it. FWIW, I've gotten
mixed results with copper alloys, so I don't know about using it on them.
Sometimes yes, sometimes no.


I did a lot of reading and experimentation with epoxy a couple of years
back when I was trying to glue stones to one another. I got pretty decent
results - until you put the stone structure outside in the sun. Epoxy does
not like that!

Now I am experimenting with gluing the sundial faces to the armature. The
obvious way to attach the two is either welding or brazing, but any heat
application to a finished face can play havoc with it. I tried screwing
the two together, but there is only so many 10-32 threads you can get into
a blind hole drilled at the back of a 5 mm face, let alone the sweaty
feeling when drilling the blind hole into a finished face hoping that you
do not break through.

I had a fairly decent result with JBWeld but he key was the area of
adhesion which I made purposely quite large. That is something I would
like to change. I did a few experiments with different shapes of the
bonded surfaces (milling little pockets etc.) but the strength has been
inferior so far.

What I noticed almost without exception is that when the bond breaks the
epoxy separates almost totally from one of the surfaces whilst still
adhering to the other. There is almost never a break "through the middle"
of the bond. I make it a rule to prepare both surfaces the same way but
this suggests to me that I am failing on one of them (both are steel,
BTW). The scratch-in method may be the answer.

--
Michael Koblic
Campbell River, BC

It may be. The failure mode you're describing sounds like a basic adhesion
problem, due either to tying to bond to an oxide layer or to contamination
of the surface.

It's not unusual for epoxy to let go like that, and the stronger the epoxy,
the more likely it is. When there is a peel or cleavage load on the joint
and a tiny spot at an edge has poor adhesion, the peel will start there.
Then it peels catastrophically. That's the exact type of failure that
rivet-bonding is intended to prevent in aircraft skins. The rivets prevent
the initial lifting of an edge.

Epoxies modified with elastomers are intended to reduce that peel weakness.
Most epoxy products sold at retail have some degree of resistance to it (and
a somewhat lower shear and tensile strength as a result), but most epoxies
are more vulnerable to peel failure than some other adhesives.

So it's important to have a really clean surface to eliminate the weak spots
near the edges of the bond. On many metals, the degree of cleanliness needed
to get the best performance disappears within seconds of the metal's
exposure to air. Preventing that contact with air is what scratching-in is
all about.

Good luck.

--
Ed Huntress