Testing epoxies for use in minimum diameter fin attachment

[Neutronium95]

I'm planning on testing out a bunch of different structural epoxies, focused on surface mounted fins for high performance flights.

Right now I'm thinking that I'll make a bunch of fin cans with sections of 38mm fiberglass tubing, and 2" square fins epoxied on with some consistent fillet radius. After that I'll see how much force it takes to break each fin off, and see what can be learned. I'm not so certain about the experimental design to break the fins off yet, but I'll figure out something solid before I get to actual testing.

Right now, I'd mostly like input on just what epoxies to test. This list is what I'm thinking of for now, but I would love suggestions on things to test.

Epoxies to test:

• BSI 5 minute (gotta start somewhere)
• JB Weld (best you can get from a hardware store)
• Thickened West Systems (a classic)
• Rocketpoxy
• Aeropoxy ES6209
• Proline
• This stuff from Temu (I've heard it's surprisingly good, and it's dirt cheap)
• 3M DP420NS (Actual professional grade stuff)
• Loctite e-120-hp (See above)
• Infinity Bond EP 420 NS (Seems very similar to the 3M, but significantly cheaper)

This initial slate will probably take me a while to get through. I have all of the stuff through Proline on hand. Please do make suggestions about the methodology and if you have any other epoxies you'd like to see tested.

 

[Hobie1dog]

this article should explain things:
https://www.epoxy.com/strength.aspx...trength tests are ran,their way up from that.

 

[robopup]

That's great! If it's of interest, I have access to a 50kN test machine. It's about 300 miles away and I have to respect the owner's time, but I'm already planning to head over sometime this fall for other rocket related activity...

I'll throw a request in (although it's probably too specific). My latest thickening recipe (still being tweaked to be a bit smoother) is:
By weight:
1.0 - laminating resin, I use Adtech 820 with slow cure
0.5 - milled fiberglass
0.02 - 406
0.15 - 404

 

[cls]

... no formal tests. I used Proline on my Mongoose 98. Butt joined fins to body. Survived several Mach 1.8+ flights, plus several parachute drags on the playa.

I doubted it at first because it seemed like high solids so how could it penetrate the fin root and woven tube well enough. But roughed up with 60 grit (80 grit didn't scratch the parts!), it adhered just fine.

So, yeah, Proline with Curtis parts seems ok.

 

[Hobie1dog]

another good article which names one of the strongest epoxies:
https://www.conro.com/what-is-the-s...rong is epoxy glue,strength of 2900-4400 psi.

 

[Hobie1dog]

This is the one that was listed as the strongest epoxy available, excerpt
On July 12, 2019, DELO Adhesives lifted a 17.5 tons truck with a crane held by 3 grams of glue. This world record was obtained by using DELO MONOPOX VE403728. However, this adhesive is not commercially available.

The strongest epoxy glue you can purchase is probably Systemthree’s T-88. This two-part adhesive has a tensile strength of 7000 psi. It is well above the average strength for epoxies. T-88 exhibits outstanding adhesion and permanence on a wide variety of materials and is designed to resist adverse conditions. T-88 is endorsed by leading designers, builders and organizations.

Other strong epoxies you can consider are PC Products’ PC-7 Adhesive Paste and Loctite EA 109 High Temperature & Toughened. They are both designed to provide maximum resistance to impact, shear, cleavage and tensile loads. Both will bond to a wide range of surfaces.
:
https://www.systemthree.com/products/t-88-structural-epoxy-adhesive

 

[davidb123]

This would be some great data to have! Stoked to watch your progress. Two suggestions I have (albeit with limited experience here):

• Document what surface prep you did. Maybe another test down the road could be how much proper prep strengthens the fin joint, and what type of prep that is.
• Maybe a cool future test would be to take the top contenders, and test them immediately after being heated in an oven? The thermal effects of high mach and a motor case are hard to replicate, but that’s another area of interest besides just tensile strength.

 

[sharkbait]

This is the one that was listed as the strongest epoxy available, excerpt
On July 12, 2019, DELO Adhesives lifted a 17.5 tons truck with a crane held by 3 grams of glue. This world record was obtained by using DELO MONOPOX VE403728. However, this adhesive is not commercially available.

The strongest epoxy glue you can purchase is probably Systemthree’s T-88. This two-part adhesive has a tensile strength of 7000 psi. It is well above the average strength for epoxies. T-88 exhibits outstanding adhesion and permanence on a wide variety of materials and is designed to resist adverse conditions. T-88 is endorsed by leading designers, builders and organizations.

Other strong epoxies you can consider are PC Products’ PC-7 Adhesive Paste and Loctite EA 109 High Temperature & Toughened. They are both designed to provide maximum resistance to impact, shear, cleavage and tensile loads. Both will bond to a wide range of surfaces.
:
https://www.systemthree.com/products/t-88-structural-epoxy-adhesive

I don’t want to get too involved with a glue thread but there’s a lot more to it than just tensile strength, with MD and sub MD heat plays a big role also, and Cotronics 600 & 650 have tensile strengths over 11,000

 

[Hobie1dog]

I don’t want to get too involved with a glue thread but there’s a lot more to it than just tensile strength, with MD and sub MD heat plays a big role also, and Cotronics 600 & 650 have tensile strengths over 11,000

And that's why the first article link was about:

Epoxy Resin Strengths Explained​

Compressive, Tensile, Flexural, and Bond Strengths of Epoxy Resin​

 

[SolarYellow]

I would look at BSI 30 before BSI 5.

Also interested to see US Composites 635 tested.

 

[Neutronium95]

This would be some great data to have! Stoked to watch your progress. Two suggestions I have (albeit with limited experience here):

• Document what surface prep you did. Maybe another test down the road could be how much proper prep strengthens the fin joint, and what type of prep that is.
• Maybe a cool future test would be to take the top contenders, and test them immediately after being heated in an oven? The thermal effects of high mach and a motor case are hard to replicate, but that’s another area of interest besides just tensile strength.

Both of those are very good suggestions. I'll be doing my usual surface prep, but I'll probably throw in a water break test for each part to ensure consistency.

Future tests at elevated temperatures are an interesting possibility, but I'm not sure how hot the bulk material of a fin fillet actually gets on an extreme flight. The period of aerodynamic heating will be quite brief, and mostly directed at the top of the fillet, while it will take a while for the motor jeat to soak through the body tube.

 

[sharkbait]

And that's why the first article link was about:

Epoxy Resin Strengths Explained​

Compressive, Tensile, Flexural, and Bond Strengths of Epoxy Resin​

Yes the article does explain what compressive, tensile, flexural, and bond strength is, but in MD rockets that are often designed for Mach 2, 3, or 3+ heat and Tg are very important and the article does NOT address that.

 

[Neutronium95]

Yes the article does explain what compressive, tensile, flexural, and bond strength is, but in MD rockets that are often designed for Mach 2, 3, or 3+ heat and Tg are very important and the article does NOT address that.

Also, many of the epoxies I'm testing don't have data sheets. And of the ones that do have data sheets, many of them don't specify important parameters.

I also want to evaluate the epoxies on more than just the raw strength numbers. Things like how are they to work with, or if they need any special care to work well. And you can't get that from a data sheet.

 

[Hobie1dog]

Yes the article does explain what compressive, tensile, flexural, and bond strength is, but in MD rockets that are often designed for Mach 2, 3, or 3+ heat and Tg are very important and the article does NOT address that.

Yep, heat was not discussed, but the OP never mentioned testing or being concerned about the heat aspect.

 

[sharkbait]

Also, many of the epoxies I'm testing don't have data sheets. And of the ones that do have data sheets, many of them don't specify important parameters.

I also want to evaluate the epoxies on more than just the raw strength numbers. Things like how are they to work with, or if they need any special care to work well. And you can't get that from a data sheet.

Yes, a lot depends on what you are doing, sometimes you need a thin high flow epoxy to get down into places you can’t reach or to soak into a glass reinforced fillet, sometimes you need a thicker epoxy for fin fillets, a quick field repair with 5 minute epoxy or a slow set for strong bond on a leisurely build in the shop. One thing that applies across the board with all epoxies is good surface prep and accurate mixing volumes. And with Proline 4500, no matter how careful you are it gets everywhere

 

[cls]

This world record was obtained by using DELO MONOPOX VE403728. However, this adhesive is not commercially available.

... because they accidentally glued the lid to the jar, and can't open it anymore.

(oldie...)

 

[G's Monster Rocket]

Both of those are very good suggestions. I'll be doing my usual surface prep, but I'll probably throw in a water break test for each part to ensure consistency.

Future tests at elevated temperatures are an interesting possibility, but I'm not sure how hot the bulk material of a fin fillet actually gets on an extreme flight. The period of aerodynamic heating will be quite brief, and mostly directed at the top of the fillet, while it will take a while for the motor jeat to soak through the body tube.

Not a lot of tech data available on the one I use most often for fin fillet, but Total Boat Thixo in both the slow setting and fast setting have worked well on my HPR projects. I've used it on everything from reinforcing balsa fin surface mount up to 4" bluetube with 3/16" G10 fibreglass fins. Nozzles for automixing at 2:1 ratio, from either West Systems or Total Boat make applying the fillet bead pretty simple coupled with a 1" tongue depresser for fin radius. Surface prep was lightly roughed up with 80 G, cleaned with 70% IPA then dried before applying Thixo. I use it also for bonding the full length of the fin root to the motor mount tube, thru the fin slot in the sustainer body (if the fin can isn't pre-built). The only drawback I've found is the high count of nozzles I go through. Still easier than getting liquid epoxy all over the place trying to drip or place it on the motor mount for fin bonding. Been there, tried that a ton. Or building dams to hold the liquid epoxy in place while it cures. Just my $.02 worth.

 

[JohnCoker]

I love the idea of doring testing like this.

Do we know that straight-line force is a relevant test? Are you planning to break the off to the side or along the body tube? What's the failure hypothesis you are testing?

I've done head-to-head tests of a few epoxies in the past:
http://www.jcrocket.com/adhesives.shtmlBut the question of relevance still remains; how does this compare to transportation, in-flight and landing stresses.

 

[Neutronium95]

Do we know that straight-line force is a relevant test? Are you planning to break the off to the side or along the body tube? What's the failure hypothesis you are testing?

I'm planning on applying force perpendicular to the face of the fin. I'm mostly thinking about the load situations of hitting a high angle of attack at high speed, and having a hard landing on the playa.

It's probably not the best way to tease out the exact parameters of any epoxy in particular, but I think that it'll be close enough to a real world application that it should be a useful indication of relative strength at least. I'm mostly just focused on surface mount fins on high performance rockets for this.

 

[Rschub]

I would like to see the results of an actual layup compared to any epoxy fillet. Say a small fillet to reduce the bend angle, and then 1" BID cloth on each side of the fillet for the length of the fin.
T-88 is good stuff.

 

[Back_at_it]

I've never done a 38mm minimum diameter but I can tell you that T-88 epoxy when used correctly is will rip the tube away before failing. I had a failure to eject come in ballistic and hit something solid. The rear fins tore a large section of the LOC tube off on impact but the glue joint held strong. When I got home I tied to pull the remaining three fins off the tube and could not apply enough force to get them off without taking the tube with it. I eventually used a Dremel and cut them off.

When using epoxy it's important that it soaks into the materials that you apply it to. Generally this means sanding the joining surfaces. Be sure the root edge of the fin is clean and remove the outer layer of the body tube where the fins will attach. Be sure to remove a large enough area to allow for the fillets as well.

I then warm my epoxy and all of the parts being joined. Join the parts then use a heat gun to get the epoxy to flow and soak into the materials. Repeat the process with the first round of fillets and you should be good to go.

 

[jderimig]

Interesting about T-88, I will have to try it. It is very easy to exceed 7000 or 11000psi stress. An invisible missed flaw due to our deteriorating eyesight (bubbles, cracks, scratches) in the bond can easily raise stress concentrations >3x. S-Glass has a tensile strength 700,000psi. A small reinforcing glass tape from fin to airframe, done correctly, makes the strength differences between good epoxies moot.

 

[yvanthesaxophonist]

Do you have a strategy for your additive choices/densities/consistency? I'm sure you could do this whole experiment just on the West Systems additive line, so just wondering what your scope is.

 

[watheyak]

I've sent 3M DP-460NS to Mach 2.7 with just normal sized fillets and it survived just fine. REK carbon tube and Dragon Plate fins. It's be interesting to see the difference between that and the 420.

I'd be interested to see how Loctite EA-9394 measures. It's my current epoxy crush. High temp, toughened and a pain in the butt. I'd donate what's left of my set for testing. I also have some half used sets of the 460, but they're pretty old.

 

[Neutronium95]

I've never done a 38mm minimum diameter but I can tell you that T-88 epoxy when used correctly is will rip the tube away before failing. I had a failure to eject come in ballistic and hit something solid. The rear fins tore a large section of the LOC tube off on impact but the glue joint held strong. When I got home I tied to pull the remaining three fins off the tube and could not apply enough force to get them off without taking the tube with it. I eventually used a Dremel and cut them off.

The T-88 epoxy seems to have been mentioned a few times. I'll add it to the list.

Do you have any experience with using it on fiberglass rockets?

 

[Titan II]

"but I would love suggestions on things to test..."

Hysol 9462

 

[boatgeek]

I've used T-88 a few times on fiberglass projects. It works really nicely, though the resin part tends to crystallize if you store it below ~65F. A warm water bath cures that. It's also in an odd viscosity spot well thinner than thixotropic but noticeably thicker than laminating epoxies. It cures slowly and gets significantly stickier as it cures, so you may not be able to mold a specific radius fillet. It might just be a leveled out triangle cross section.

 

[eggplant]

I'm sure you already saw, but I ran a similar (but limited) experiment, using just DP420, mostly to compare the strength of a fincan with T2T to one without. One important detail you'll need to get right to get valuable data out of your experiment is to size the fillets such that the failure point is the fillet itself. In my experiment, the failure was actually the tube I glued the fins to delaminating. I guess some of the weaker epoxies on your list might not have this problem, but to be able to differentiate between the stronger ones you might want a smaller fillet than I went with, which should move the failure from the tube to either the fillet or its adhesion to the tube (though for an extreme flight I'd rather have a fillet sized such that it isn't the weakest link)

 

[manixFan]

I'm sure you already saw, but I ran a similar (but limited) experiment, using just DP420, mostly to compare the strength of a fincan with T2T to one without. One important detail you'll need to get right to get valuable data out of your experiment is to size the fillets such that the failure point is the fillet itself. In my experiment, the failure was actually the tube I glued the fins to delaminating. I guess some of the weaker epoxies on your list might not have this problem, but to be able to differentiate between the stronger ones you might want a smaller fillet than I went with, which should move the failure from the tube to either the fillet or its adhesion to the tube (though for an extreme flight I'd rather have a fillet sized such that it isn't the weakest link)

View attachment 607600View attachment 607601

Hmm, that would certainly seem to suggest that if the failure point is the tube itself with correctly sized fillets, then the epoxy isn't the 'rate limiting step' in fin attachment? Would testing 'flight size' size fillets and determining if the fillet fails or the tube does, tell us what we really want to know – that the epoxy/fillet is at least as strong as the tube itself?

Personally, I would love to see the results of a brittleness test regarding fin attachment. In my mind, the most useful epoxy would be one that has not only excellent adhesion but also toughness to absorb landing shocks without fracturing along the root. Maybe it's all the same strength and testing?

I'm not an engineer, and I try not to play one on TV. But I do like to build rockets that go fast and have the fins stay on!

Thanks all for working on this, I'm looking forward to seeing the results.


Tony

 

[Neutronium95]

I'm sure you already saw, but I ran a similar (but limited) experiment, using just DP420, mostly to compare the strength of a fincan with T2T to one without. One important detail you'll need to get right to get valuable data out of your experiment is to size the fillets such that the failure point is the fillet itself. In my experiment, the failure was actually the tube I glued the fins to delaminating. I guess some of the weaker epoxies on your list might not have this problem, but to be able to differentiate between the stronger ones you might want a smaller fillet than I went with, which should move the failure from the tube to either the fillet or its adhesion to the tube (though for an extreme flight I'd rather have a fillet sized such that it isn't the weakest link)

View attachment 607600View attachment 607601

I hadn't considered that. Right now I'm thinking that I'll go with my usual rule of thumb for min diameter fillets and use a fillet radius about a quarter that of the tube diameter. Trying out a few different sizes in the beginning definitely couldn't hurt. Having a fillet that delaminates the tube underneath it is definitely my goal for flight hardware. At a certain point, I imagine I'll hit a tier of epoxy that's good enough, and will provide a strong enough joint. Finding out where that point is will be the interesting part.