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Shear strength of epoxy putty

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Shear strength of epoxy putty

Unread postAuthor: btrettel » Thu Jul 01, 2010 5:20 pm

Most epoxy putties I've seen claim a shear strength of at least 700 psi. Is this approximately the shear strength I should expect when using the putty on aluminum?

I'm designing a somewhat small lightweight air chamber. Using epoxy putty to plug up the ends is the easiest construction technique I can think of, but I don't have much experience with the stuff, so I don't know if the reported values are unrealistic.

The aluminum will be clean and roughed up adequately. I intend to use a safety factor of at least 3 (I set that as the minimum for this project). Any tips would be appreciated too.
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Last edited by btrettel on Thu Jul 01, 2010 5:57 pm, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 01, 2010 5:31 pm

I have no experience with putties, but epoxy adhesives such as Araldite which I've used will happily contain 800 psi without failure if used with a properly prepared surface, case in point.
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Unread postAuthor: Technician1002 » Thu Jul 01, 2010 5:49 pm

I do know that some refrigeration repair is done with 2 part epoxy. It is used to join the aluminum ice box portion onto the copper tubing. I have used it in the past to fix leaks in a refrigerator ice box.
http://www.highsidechem.com/redepxy.html This stuff is not putty, but works well on aluminum. Gently heat it with a torch until it turns from red to a honey brown.

Only 800 PSI? check the stuff in the link in my post.
HYDRAULICALLY PRESSURE TESTED IN EXCESS OF 3000 psi ON 304 and 316 STAINLESS STEEL AND BLACK IRON PIPE.


JSR, are you proud of me. I have used epoxy. :D Maybe some day I'll use it in a spudgun.
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Unread postAuthor: SpudBlaster15 » Thu Jul 01, 2010 6:31 pm

I'd use a good consumer grade paste epoxy such as JB weld over epoxy putty. The adhesion strength of the former will typically be much higher. JB weld lists the adhesion strength of their product at 1800PSI, and the tensile strength at nearly 4000PSI. That should be more than sufficient for almost any spudgun application.
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Unread postAuthor: btrettel » Thu Jul 01, 2010 8:00 pm

Thanks for the quick replies.

I didn't know you went up to 800 psi JSR! That's impressive.

The problem is that I don't know if epoxy putty will work as well as regular epoxy. Regular epoxy has reported shear strengths often far higher than that of epoxy putty, and I think epoxy putty's reported shear strengths are optimistic. I'm filling up a pretty decent sized hole, so something already solid will be a lot easier to work with. Or so I think.

Would a 1 to 1.5 inch internal diameter tube be too big to be practical for epoxy casting? I haven't tried epoxy casting yet, but I figured that epoxy putty would be easier when there's a big hole to fill. Edit: Hmm... seems JSR has some stuff that's comparable in diameter, so I should be okay.

I've considered making a plug and using epoxy around that, but I'm not certain I'll have access to a lathe to make the plug soon.

Edit: I found some really strong epoxy putty designed for use with aluminum: http://www.devcon.com/products/products ... %20%28F%29

Unfortunately, this stuff's pretty expensive: http://www.walmart.com/ip/Devcon-Alumin ... F/13085362
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Unread postAuthor: theBOOM » Thu Jul 01, 2010 10:16 pm

Just cut out enough thick foam discs to make the plug and follow JSR's epoxy tutorial it shouldn't be too hard. Make sure the foam discs are sealed before you pour the epoxy into the hole.
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Unread postAuthor: Gippeto » Fri Jul 02, 2010 1:30 am

Would that be shear strength...or LAP shear strength?

If t'were me, I'ld drill some holes in the tube, and allow the epoxy to fill them, or better yet, place screws in the holes, and cast the epoxy around them.

More predictable as far as calculating the fail point, and the effective safety factor.

The holes should be a minimum of 1.5 x their diameter from the edge.
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Unread postAuthor: jackssmirkingrevenge » Fri Jul 02, 2010 1:41 am

JSR, are you proud of me. I have used epoxy. :D Maybe some day I'll use it in a spudgun.


The binary adhesive gospel is spreading :D

Gippeto wrote:If t'were me, I'ld drill some holes in the tube, and allow the epoxy to fill them, or better yet, place screws in the holes, and cast the epoxy around them.


I was about to suggest the same, using long screws also reinforces the plug allowing a lower thickness to be poured.
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Unread postAuthor: PVC Arsenal 17 » Fri Jul 02, 2010 4:48 pm

If you can afford it, buy a large pipe cutter to accomodate the tubing you're going to use and make the end cuts with it. When these cut, they collapse the edge along the circumfrence of the cut just slightly to form a lip on the inside of the tube. Typically this lip would be reamed out to eliminate flow restrictions, but in your case it might serve as an additional safety feature to help ensure that the epoxy plug stays put. If not that, you could achieve a similar lip-forming effect through other means.

Image

In addition to that and roughing up the inner surface, you can use a punch to form dimples protruding from the inside of the tubing where epoxy is to be applied. There are several ways you can prepare the piece so that the finished product is stronger. If I had to do this, those are among the three I would try.



I know you want to avoid welding for several different reasons, but if you're not successful with the epoxy and happen to find a metal plug suited for the application, you might want to try Alumiweld rods. It's sort of like soldering with aluminum. I've never had much success with them, but I've heard of others who claimed to...
"Shear Strength: 18,000 to 33,000 pounds per square inch depending upon joint design."
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Unread postAuthor: ramses » Fri Jul 02, 2010 11:41 pm

The specified tensile strength of 15 hour JB weld is something like 3800 PSI.
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Unread postAuthor: btrettel » Sat Jul 03, 2010 8:17 am

I started thinking about putting screws in there to reinforce the plug after it was mentioned (Thanks, by the way). The problem with that approach is that I plan to use use an aluminum tube with an 0.035" thickness. That might be too thin for screws to work well, but I'll go look in a book about stress concentrations to make sure it's okay.

Just to be clear, I'm interested in the shear strength (not tensile; shear is different) of the interface between the aluminum and the epoxy or epoxy-putty. The ASTM test for this uses a lap joint in tension. Gippeto has the right idea.

Image

I've read that the shear strength value obtained for this test might only be valid for comparison purposes because of the affect of the total surface area of the bond, but it's the best value I have to go with.

PVC Arsenal 17, I have more of a problem with heat treating the aluminum again than welding or soldering. I'm using 6061-T6 because it's cheap. If you heat it up sufficiently, the area that was heated will be close to 6061-O, which is far weaker. You can turn it back into 6061-T6 through heat treatment. 6061-O should still be okay for my purposes, but I'd rather have the higher safety factor.

I'll also see if I can get a hold of a large pipe cutter... I certainly don't want to buy one for this project, but I should be able to find one.
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Unread postAuthor: btrettel » Mon Jul 05, 2010 5:36 pm

I've decided to buy some Devcon epoxy from McMaster-Carr for convenience's sake. There's some data about it here: http://www.devcon.com/products/products ... &sc=Market

This says it's good for casting, but the viscosity of the mixture is comparable to that of chocolate syrup. Is that what I should expect? I thought "low" would be lower, but this should be manageable.

Also, I'm evaluating whether or not I should add the screws. I might need to do some FEA to make sure the stress concentration doesn't affect my safety factor too much. :wink:

I'm angry I hadn't done this before!
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Unread postAuthor: jackssmirkingrevenge » Mon Jul 05, 2010 10:54 pm

The stuff I use will be similar to water unless it's quite cold, still a "syrup" will still pour and fill any cavities. Putting it in a bain marie should help make it flow better (and mix better) if room temperature is a little low.

I would go with the screws, just in case ;)
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Unread postAuthor: Technician1002 » Mon Jul 05, 2010 11:19 pm

jackssmirkingrevenge wrote: Putting it in a bain marie should help make it flow better (and mix better) if room temperature is a little low.


I've never heard it called that before. I learn something new every day. In the US, they are called double boilers.
http://en.wikipedia.org/wiki/Double_boiler
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Unread postAuthor: jackssmirkingrevenge » Tue Jul 06, 2010 12:08 am

Joys of being European ;)
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