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Conclusions on Aluminum Adhesive Bonding Tests

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MalcolmW

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Jan 21, 2007
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Hello, Gary, Orion, John G.,

Here's some info on the different classes of structural adhesives. I believe I posted this or something similar a while back, however, it doesn't hurt to repeat this:

First, this is a good description of the three classes of structural adhesives:

Designfax - Adhesives + Composites A Natural Combination

Here’s a link to more than you’d ever want to know about surface preparation for adhesive bonding (comes from England, so, the terminology is a little different that what you are used to hearing). It has methods that even experts haven’t seen before… Cor! Blimey!

http://www.on-hand.com/Manuals/SurfacePreparation.pdf

Here is a listing of adhesives made by Lord Corporation, some of which have been used for assembling aluminum truck bodies (a demanding environment):

Acrylic Adhesives

And another…

http://www.sikaindustry.com/tds-ipd-sikafast3141-us_rev_4-07-05.pdf
(I believe that Sika makes all three classes of structural adhesives).

In the above link, there is reference to the LACK of chemical resistance by methacrylate adhesives to gasoline. Hmm, that could be a real concern in aluminum aircraft assembly. Also checked chemical resistance of acrylics, and found that aromatic solvents do cause damage (softening and swelling). Recognize that some gasolines do contain aromatic hydrocarbons which disqualifies methacrylate adhesives in any location that comes in contact with gasoline.

So, do NOT use methacrylate adhesives where gasoline can contact it.

Yes, polyurethane adhesives will bond to aluminum, however, they've found their greatest application to surfaces which have active hydrogen (glass reinforced polyester, cellulosics, etc.). As for bonding aluminum, yes, they will work, however, I believe that methacrylates have higher tensile strengths.

I hope that this proves useful.

All the best & fly safe,

MalcolmW
 

avior

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Hallo Gary,I'm trying to bond two 17"x17" aluminum plates with 3M AF163-2 structural adhesive. The plates are modified sulfuric acid anodized, primed with 3M EC 3960 adhesive primer within 4 hrs. after anodization , adhesive film applied and cured 250F/ 45psi 90min.Lap shear test per ASTM D1002 gives 4700-5000psi but climbing drum peel test ASTM 1781 gives VERRRY bad results (4-5 lb/in) or very good . . . 80-85 lb/in. The worst is that on the same plate from which I cut 5 test samples I may have 3 disastrous failures and 2 strong bonds . . . I can't understand what is happening. There aren't medium values . . . it fail miserably or it pass far high on the same panel.I have read Boeing, Bombardier and DeHavilland specifications concerning metal bonding and I suppose that every little detail is controlled, adhesive primer is cured, no contamination, I have plates from 2 supplier, anodized in 2 different shops . . . I already run around 40 tests pieces i don't know what to do anymore.Does anybody has heard about such contrary comportment on samples cut from the same panel?What I'm doing wrong? Does anodization process lives residues on plates?, or hydration after anodization may developed a kind of local shield so fast???
 

avior

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Cohesive failure, adhesive on both surfaces in high results.On the others samples it looks like primer doesn't stick on the anodized surface, the strip is clean, primer traces on it, most of the primer and all adhesive is on the outer strip . . . like release agent was used instead of anodize treatment (unsealed sulfuric acid anodize). What I see is that each strip that fails - has between .040-.090" cohesive failure at the very beginning and than changes to adhesive failure within next .020". I just checked bonding thickness, it seems to be .003-.005" almost uniform on each sample. Carrier I used mat and knit (knit tested in 45*) less low values using knit carrier but still are miserable failures. Low peel values are not related to strip location on the middle or side . . . There are few strips that have voids (2-3) .020-.040" diameter. (center voids no voids at extremity). I cured some panels between plates – not much differences.I have also noticed that a bit of adhesive primer may be removed from the surface using a rag with plenty of MEK after primer cure . . . . it is not full cured?
 

orion

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I'm glad to see that our own testing was not so far off in arriving at these inconsistent results. Generally, when it comes to peel strength, that tends often to be a function of the bonding agent selected. For instance, recently I've had to spend quite a bit of time searching for alternatives to the 3M product that I specified for our current projects. The epoxy formulation (DP460NS) is excellent but pricey.

In evaluating possibly twenty or thirty formulations from at least four vendors, it became pretty clear that you have a choice, either strength or toughness. Systems that formed strong bonds (high shear strength) and were characterized by good temperature resistance tended however to have relatively poor peel strengths. Toughened systems with excellent peel strengths however tended to deliver lower shear strength properties and a more substantial degradation in strength at elevated temperatures.

But relating to your experience, a few years back I conducted several test series in order to evaluate candidate bonding methodologies that could be used within the homebuilt industry. The investigation looked at epoxies as well as two lines of Methacrylates. In both cases there were several products that showed excellent promise so a follow-on set of tests were conducted in order to see whether we could control the bond to the point where we could specify reasonable procedures for the homebuilder to follow and expect that builder to achieve a predictable and safe range of bond values.

In short, our conclusion at the time was no. Despite consistent materials and prep practices, controlled bond lines and cures, and consistent subsequent tests, we continually ran into the same circumstances you describe above: Some of the samples bonded so well that in pulling them apart we actually failed the aluminum. But unfortunately, about the quarter to a third of the coupons showed a sudden and catastrophic failure, resulting in a peel strength on the order of a few pounds per lineal inch.

Despite a significant amount of interest by several individuals and companies, including one of our current customers, I'm still not comfortable enough with the results to baseline a flight product on any bonding process that involves aluminum materials.

Also, regarding the MEK, I think its a pretty good solvent for most materials so its removal of your primer is not too surprising.
 

GESchwarz

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Avior,

You're going to have to do some root cause analysis and corrective action. There are numerous process variables that you'll have to identify. By experimentation and experience you will become familiar with what does and what does not work on a consistant basis. Until you can get consistant results with your coupons you can't really go forward in production.

Just as with my post cure riveting, your post cure cutting may be too much for your adhesive to survive due to localized vibration or other relative motion between the two plates. How much elasticity does AF163-2 have? .003"-.005" may be a bit on the thin side. I believe that the thinner the bond line, the more concentrated the strain becomes.

I looked up the data sheet for AF163-2. It looks like a promising material. As I indicated earlier, the single distinguishing factor between my best adhesive and the 5 others was it's elasticity. The Partite 7350 could be carved smoothly with an X-Acto knife; the other 5 would chip in a brittle fashion in the path of the blade, epoxies and methacrylates alike.

Adding primer into the equation adds a whole lot more variables. Without primer, you just have the bond between the adhesive and the metal. With the primer you have the primer to metal bond, the primer cohesive strength, and the primer to adhesive bond. Wow, that's a lot more things that can go wrong. And the annodize process itself may also be a source of variation. The less steps the better, especially for the homebuilder.

Happy testing. We all can benifit from what you learn and share.
 

GESchwarz

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In support of points brought up by Orion...

Peel is the achilles heel of adhesives, so just don't go there. You do this by good joint design.

Achieving maximum strength shouldn't be the goal. I'm sold on the toughened adhesives over the high strength ones. Reliability is the goal. High strength can be achieved by maximizing bond surface area.

Peel is all about Substrate Deformation. When the substrate deforms under tensile load, it stretches the adhesive beyond its elastic limit and causes delamination at the bondline itself. That is the stress riser of adhesives. Design your joints so that you don't have stress risers, just as we do in aluminum fabrication; we know the weakness, so we must design accordingly. That's why I think it's improtant that I go with the PVC foam ribs rather than the traditional formed aluminum ribs...I'm getting away from the stress risers.
 

orion

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BTW, the line of adhesives that will replace the 3M product I'm using comes from Magnolia Plastics, Inc. (Magnolia Plastics Home Page - High-Performance Epoxy Systems Since 1957). The formulation I narrowed in on is their 6168-A/B. I still haven't been able to get to the hangar long enough to assemble the bond test coupons however, based on my discussion with the rep, and the data available in the spec sheet, this will do the job as well as the 3M product, if not better, but at about a 30% cost savings.

And while I'm in the test phase, I'll also take a look at a number of aluminum coupons - the published aluminum bond strength data is quite good so it may be a good candidate material for the application.
 

bmcj

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In support of points brought up by Orion...

Peel is the achilles heel of adhesives, so just don't go there. You do this by good joint design.

Achieving maximum strength shouldn't be the goal. I'm sold on the toughened adhesives over the high strength ones. Reliability is the goal. High strength can be achieved by maximizing bond surface area.

Peel is all about Substrate Deformation. When the substrate deforms under tensile load, it stretches the adhesive beyond its elastic limit and causes delamination at the bondline itself. That is the stress riser of adhesives. Design your joints so that you don't have stress risers, just as we do in aluminum fabrication; we know the weakness, so we must design accordingly. That's why I think it's improtant that I go with the PVC foam ribs rather than the traditional formed aluminum ribs...I'm getting away from the stress risers.
Can you reduce the threat of peel by designing your adhesive joints as a 3-layer sandwich instead of a 2-layer lap joint? That way, your bonds would be constrained to shear loads, similar to the way a clevis pin is loaded when placed through a forked clevis.

Bruce :)
 

orion

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The problem though is that even with that type of bond joint you still have exposed edges. It is those edges that are susceptible to peel either due to vibration, service loads or environmental exposure. The only way to address the issue is through the practice of using chicken rivets along all exposed edges. This provides a physical lock for the bond so even if a minute amount of peel does initiate, it cannot progress due to the presence of the rivets. Install these with a squeeze tool (as opposed to a rivet gun and bucking bar) and only after the bonding agent has had the time to fully cure (wait about two to three months).
 

avior

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I'm measuring primer thickness using elcometer 345 probe. On flat, bare metal panels primer tickness is between 0.15 mils and 0.17mils, that is actual paint gun setup. On anodized panels I can't measure the thickness because of anodized layer, even to make "0" is impossible, the error is around 0.05mils. Anyway I'm using the same paint gun setup and I hope I have the good thickness. What is killing me is the fact that primer may be wiped with solvent after primer cure cycle, not all but most of it . . . if insist. On other hand the humidity in paint booth is never more than 20%, I don't have equipment to maintain humidity between spec's limits (40-60%).I don't know why but I have doubts about rinsing after anodization . . . they are using tap water to rinse my panels, the practice is to use demineralized, deionized or distiled water. Does chlorides, or minerals in tap water my live traces on my panel surface, or may react with remanent anodizing salts during rinsing?By the way I tested with similar peel results Cytec BR127 primer with FM73 Cytec adhesive . . . ?!?!?!?!?!
 

GESchwarz

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Avior,

I am certainly not an expert on the processes you are working with but I can tell you that because you have so many more steps that what I've been using, you are adding that many more variables than can result in failure. Anytime you are outside spec limits or using materials, such as tap water, you are going to get poor results, guaranteed.

Orion,

On the subject of joint design, the idea of going with a foam rib is to distribute the strain across the width of the bond line. Whereas with a conventional formed aluminum rib, the type of loading across the bond line width would vary from the formed radius to the trimmed flange edge. So the formed rib is like a "C" channel, and the foam rib is like an "I" channel. True, you do have stresses at the edges, but I think the stresses at the two corners of the C where the web meets the flange is higher than at the trimmed edges. My thinking is that by locating the web at the centerline of the bond that I an reducing the peel risk. I have been thinking of a hybrid rib that uses high density foam rib caps, bonded to an aluminum rib that has no formed flange. The foam rib cap would be triangular (45-90-45 degree) in cross section with a slot cut bisecting the 90 degree, penetrating about 2/3 the way down towards the opposite side, which is the bond surface to the skin. The slot would then be filled with adhesive, and the aluminum rib web would be inserted to complete the rib assembly. These hybrid ribs would be fabricated in a female form block to insure uniform shape. I believe a rib of this configuration may be ideal for bonded skin construction. The aluminum rib webs provide all the rigidity of standard aluminum ribs and the PVC rib caps provide excellent bond lines that conform very well to minor variation in mating pressure and gap width.

On another subject...

My load testing is showing that AN -3 rivets tend to always tear through .020" 2024 T3 skin, while AN -3 rivets tend to shear in half when joining .032" 2024 T3 skin. I don't have any .025" in 2024 T3 at the present time. These tests were performed against Partite 7350 methacrylate joints. A comparison between the adhesive joint (of .75" width) and the riveted joint showed that an adhesive joint of 5/8" in length is stronger than two AN-3 rivets. An adhesive joint of 1 1/4" in length is not as strong as three AN-3 rivets. So that should give you an idea as to how strong this adhesive is. I have never had a Partite 7350 joint perform unexpectedly bad. Another important thing to point out is that these "failures" were debonds as opposed to cohesive, that shows that even in the weaker of the two possible failure modes, these joints are stronger than two AN-3. The material thickness was .032 and in both cases the coupons were curled indicating that the failure was not a brittle fracture. All good news.

A debond failure is not necessarily a result of a poor prep. My experience shows that anytime there is deformation of the parent material (the metal) the failure will result in debond, as opposed to a cohesive failure. All my cohesive failures occured when the parent material thickness were greater (.050", .065") to the point that they did not deform under load.
 
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MalcolmW

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Bonjour, Monsieur Avior;

Je ne parle pas Francais bien, so, je escrit en Anglais...

Anodized surfaces are not the best for adhesive bonding. It is better to have a 'passivated' surface with either CHROMATE or PHOSPHATE anions. I have provided a procedure for treating aluminum previously in this thread, and have also provided a link to a very comprehensive set of procedures for preparing substrates for adhesive bonding. See: http://www.on-hand.com/Manuals/SurfacePreparation.pdf

So, first, please consider using a chromate cleaning / passivation process. Second, do not use tap water to rinse after cleaning / passivation - DO use distilled / deionized water. If too costly, consider using rainwater (discard first five minutes of flow - this is the roof rinse water).

Lastly, do consider taking advantage of the information posted by Mr. Schwarz (who has shared the results of his efforts with all of us), especially in his conclusion that methacrylate adhesives offer a good compromise in performance qualities. Yes, I know from professional experience that epoxies have and are used for hi-performance bonding in aircraft and race cars, however, the practicality for a home-builder must be considered, and Mr. Schwarz has provided much good information on his testing and using methacrylate adhesives. He has found they provide a substantial bond between aluminum, without extensive or expensive surface preparation.

Also, I would recommend using some rivets (even pop rivets) - maybe one in five or less (from the riveting schedule) - as locators or jigs in the adhesive bonding process, and definitely during the adhesive application / curing part of the operation (not post bonding). My experience (professional) is that lap joints, properly bonded, do not experience 'peel' failure. This is especially true if the joint design is focused upon minimizing surface drag (surface skin is butt-joined & bonded to a member / rib underneath).

Orion, a 'chicken rivet' isn't necessary IF the joint design is done properly. Rivets are stress risers and can accelerate / concentrate fatigue failure unless used in quantity to spread the stresses. As for applying pressure to the 'lap joint' during the adhesive 'set up' time, consider the use of elastic straps, or bags filled with water (spreads the pressure uniformly).

Even a layer of mastic beneath a riveted joint will both keep out water and spread the stresses ('deconcentrate' stress) and increase reliability. Unfortunately, it adds weight to the aircraft.

Well, that's enough for now. Gary, Orion, I thank both of you for your generous & willingness to share your knowledge with us folk who haven't done as much as you. You make this site a pleasure to visit.

D'accord, Avior, J'espere c'est suffice.

All the best & fly safe,

MalcolmW
 

lr27

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I thougth I heard someplace that anodizing could be very good for bond strength if it was left a bit porous.

I don't remember if I saw it here, but one trick I've read about is to glob on some epoxy and sand right through it, so that the surface doesn't have time to go through any changes before the epoxy grabs it.
 

GESchwarz

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Is "a bit porous" a reproducable characteristic of the process? I don't think so.

The thing about tricks is that they can be tricky, reliability wise. Sanding throught wet epoxy sounds like a really messy operation that could also introduce lots of porosity and contamination, and would seriously eat into your working time. Hand operations like that would be difficult to verify and would thus introduce the probability of a high degree of variation, where uniformity is essential..one week spot could be a stress-riser for the rest of the joint.

Yes, it may have worked for some guy in a tight spot, but there is no data to show really how well it worked or for how long. It's just crazy. Certainly an insane way to go about building an airplane.

This is why kit manufacturers really shun offering new processes that require a certain degree of discipline in order to assure success for the homebuilder.
 
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lr27

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I understand what you're saying about process.

For all I know, there are particular, specified types of anodizing, and some are consistently more porous than others. But then of course it has to be done by someone who is trustworthy and consistent. I'm not suggesting that someone go to the anodizer and tell him to do it that way, but if there's a mil spec or something and a known supplier...

The sanding idea, I think, is from Strojnik. He runs hot and cold on the subject of bonding aluminum. He ostensibly tries to steer people away, but then he tells you how he does it (or would do it) himself. I understand that his experiences were successful. I think there's an article in Sport Aviation where he actually loads a wing to destruction after it was sitting around for several years outside. It still exceeded design loads before failure.

If I understand correctly, if you glue to epoxy within a day or so (assuming room temp. cure), the bond is very good. So perhaps if one was doing the sanding, one could do that one day, leaving a very thin layer, and finish the job the next day. Unfortunately, I can't seem to find the original reference and there are probably a bunch of details I'm forgetting. There's probably a way to make the sanding consistent, but I don't know what it is.

Strojnik mentions that Boeing has a procedure with anodizing at some particular voltage in phosphoric acid.

Scuba gear (modified with exhaust hose outside the room, of course), sandpaper, and a room full of dry argon?? ;-p
 

GESchwarz

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Thanks for the clarification. There are several ways to skin a cat, I'm just trying to keep it simple, repeatable, and reliable. If I can do that, then I bet others can too.

Scuba gear in a room full of argon? Holy Smokes Man, that's way off at the deep end! :eek:
 

lr27

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It strikes me that some company somewhere has a procedure which if religiously followed is not too horrible and produces a nice job. Certainly lots of commercial aircraft have had bonded metal, including the Tiger. (I'd say Grumman Tiger if not for all the other companies that have manufactured it over the years.) I mention that because I see a couple of them every time I go to my local airport.

hmmm... pun may have been unconsciously intended...
 

MalcolmW

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Hello, Ir27;

I believe that this thread has more than one procedure for adhesive bonding of aluminum. Even with my experience (mil spec adhesives for military aircraft), I find the practical aspects of the work done by Gary S. to be very relevant as far as homebuilt aircraft are concerned. Yes, with the right equipment (probably not feasible for the home builder), I believe epoxy bonding would prove superior to other classes of adhesives. The real key to strong, durable bonds is surface cleaning and passivation.

It seems to me that Gary's work has pointed in the direction of an adhesive that should perform well when used in a home workshop. His work is not based upon conjecture or vague memories of what someone might have done. His testing has produced real data under conditions which he can reproduce.

This thread has a wealth of information on adhesive bonding of aluminum, and it's available to all, freely shared by Gary and others, for which they should be thanked.

All the best & fly safe,
MalcolmW
 
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