paging mike lopez: epoxy thickness for good bonding (internal reinforcement)

[wallymann]

lets say i have a tube with 22.2mm ID that i want to reinforce by bonding in another tube inside of it.

what should the OD of the reinforcement tube be such that the epoxy is thick enough to bond properly?

is 0.6mm enough gap for the epoxy, meaning the OD of the inner reinforcement tube would be 21.0mm? if the epoxy matters, it'd probably be DP420.

[prototoast]

0.6 is a little thick. General guidance is 0.2 - 0.3 mm. With 1.2mm difference total, you'll have a hard time keeping the inner tube concentric, and will probably have a weak bond, which may or may not be a problem depending on your application.

[wallymann]

Originally Posted by prototoast

0.6 is a little thick. General guidance is 0.2 - 0.3 mm. With 1.2mm difference total, you'll have a hard time keeping the inner tube concentric, and will probably have a weak bond, which may or may not be a problem depending on your application.

suitable filler can help with that, yes?

[Mike Lopez]

I agree that 0.6mm is a bit much but probably not an issue depending on your application. The data sheets for epoxies like this generally quote 0.1 - 0.5mm.

Their test data us usually developed at the thinner end, say 0.1 - 0.2mm because that’s where they have the best shear values. Doesn’t mean that 0.5-0.6mm will fail but the mechanicals won’t be as good.

On long parallel bonds like you’re describing a little more gap is probably better cause it’s easier to keep the goo in place as you slide one part into the other.

These materials are fairly forgiving on the bondline thickness but do require proper surface prep for good adherence. Abrade the surfaces and clean well with acetone.

What is it that you’re trying to do?

[Mike Lopez]

Originally Posted by wallymann

suitable filler can help with that, yes?

I would not try to build up the gap with “filler” as it has very poor mechanicals. The DP 420ns would be much stronger on its own. Even at a very thick bondline.

[wallymann]

Originally Posted by Mike Lopez

What is it that you’re trying to do?

reinforce a steel steerer-tube exteded with a butt-weld.

not exactly perfect, but it seems a bonded-in reinforcement will provide some insurance to that butt-weld.

[Mike Lopez]

Originally Posted by wallymann

reinforce a steel steerer-tube exteded with a butt-weld.

not exactly perfect, but it seems a bonded-in reinforcement will provide some insurance to that butt-weld.

I'll buy in on the not exactly perfect sentiment.

Seems like there's been a couple of threads on this sort of thing and I don't recall a simple butt weld being on top of the list. Could be wrong about that but an additional reinforcement certainly wouldn't make it any weaker if that's the route you choose to go.

It does present a bonding challenge though trying to bond a sleeve in that deep in a tube and be certain you haven't lost most of your adhesive in the process of insertion. Chamfer the leading edge, use lots of material on both parts before assembly, make a mess, and hope enough stays in the joint to do it's job.

This is a case where excessive clearance in the joint actually helps.

[wallymann]

Originally Posted by Mike Lopez

...use lots of material on both parts before assembly, make a mess, and hope enough stays in the joint to do it's job...

thats what i had in mind!

[dddd]

Not sure what was meant by "insurance", but I'm thinking that the priority here is to add bending stiffness, thus sparing the welded area from flex-induced fatigue.
Perhaps also preventing any possibility of yielding or cracking when you hit that once-in-a-lifetime pothole at speed (depending highly on the resultant material properties of both the weld and adjacent zone).

I've not heard of one of these kind of (albeit uncommon) welds failing, but with a larger rider and/or with the weld being closer to where the butt ends, better safe than sorry as the bending stresses are higher there.
You would need a stepped reinforcement tube to provide reinforcement near the end of the butt, say 35mm or closer(?). How close to the butt is this weld?

You can adjust the resin's viscosity using heat before inserting the reinforcement.
Be aware that you will likely be creating a huge plug of resin below the insert, which may or may not drain out (heat will assist this). So I would attach some kind of "handle" to the insert to control it's depth while I kept the fork vertical, so as to let that resin "plug" drain out (or as I perhaps extracted it from the bottom using a screwdriver blade or other tool).

Resin along the inside of the upper half of the steerer will be easiest to remove before it hardens, perhaps easier than drilling it out later.

Lastly, use of a high-grade material for the insert will preclude the possibility of the insert breaking during use and thus creating additional stress concentration at the weld.

[dddd]

One more thing, perhaps an old, cr-mo steel mtb handlebar could provide a suitable donor material for the sleeve, assuming no stepped outer surface is needed, and assuming that the handlebar material slips freely through the inside of the steerer.
This assumes that a good weld quality was used and that was suitable to the steerer material.

I think the handlebar could easily withstand the degree of flex that the steerer will ever see, while adding a good measure of stiffness (20% or so?) to the welded area. No need to over-do this imo and I wouldn't make the sleeve much more than three inches long.

I don't see the bond resin thickness as a big issue if the tube is 3+ inches long and the resin is high-modulus material with minimal air gaps.