Practicality of Bonded Joint Analysis 2

[ESPcomposites]

A post from another thread got me curious about elastic-plastic bonded joint analysis, such as those proposed by Hart-Smith.

I have used approaches like this in the research domain or space applications, but have yet to see them translated to the production aircraft side. I suppose as a qualitative study it could be useful, but are these models actually being used on production programs?

If using good design practice (i.e. proper taper ratios, correct joint type), would you really expect the bondline to fail if they have been properly processed? I might expect to see a bond failure due to processing problems, but the analysis cannot predict this. That scenario is the real issue in my eyes, which would make any analysis a theoretical upper limit of capability.

This is not to mention questions about adhesive properties, especially at temperature. In the end, testing and good design practice seems to be the way to go. This may then be combined with a failsafe approach which may assume a processing problem leading to a weak joint. I am not terribly confident in NDI either as poor bonds can go undetected. I have seen this happen on multiple occasions.

So I am wondering what the real world value of a high fidelity bonded joint analysis really is? Is it to just better understand the problem and make better decisions or is structure actually being certified to the analysis?

Brian

http://www.espcomposites.com

 

[blakmax]

Sorry Palomc, you are correct about the typo. The correct number is DOT/FAA/AR – TN06/57.

With regard to predicting bond strength, it is worth clarifying that Hart-Smith's equations do predict the strength of the bond, in the absence of failure of the adherends, which by his philosophy should occur first.
I was aware of the wing root design you mentioned and I regularly use the results to show the futility of inserting fasteners into bonded joints. The analysis shows that the strength actually falls and the fasteners carry a negligible load anyway.

Your point about the average shear stress method being backed up by testing is valid, provided the testing covers every combination of materials, thicknesses, service temperatures etc. However, the real danger arises in the design of bonded repairs, where the joint configuration dictated by the configuration of the structure may not match the tested configurations.

With regard to the scarf joint discussion, the point I was trying to make is that once the elastic limit is exceeded, plastic deformation occurs in the adhesive (similar to creep). That damage is cumulative and is not recovered on unloading. In a bonded overlap joint, if the elastic limit is exceeded at the ends of the joint, then the elastic trough will provide some "creep" recovery so the damage is not necessarily cumulative. This has been verified by testing (Baker, A.A., Crack Patching: Experimental Studies, Practical Applications, Bonded Repair of Aircraft Structures, (A.A. Baker, R. Jones, (editors)), Martinus Nijhoff, 1988.)

Thanks for the discussions.

blakmax

 

[ESPcomposites]

"I was aware of the wing root design you mentioned and I regularly use the results to show the futility of inserting fasteners into bonded joints."

Depends on how you look at it. In a perfectly processed bonded joint, you are correct that inserting fasteners will only weaken the joint due to the presence of stress concentrations. Clearly, the bond is a far stiffer load path and the fasteners can not react significant load up to point of a bond failure. No references are needed to come to that conclusion. Again, in a perfect joint this is what happens. If you are to assume a perfectly processed joint, then fasteners only get in the way.

In my experience, assuming you have a perfectly bonded joint may not always be a good idea. In fact, on at least one program, we actually assume the bond can and will fail due to poor processing. Damage tolerance philosophy addresses the fact that it has failed. And unfortunately some of the repairs that have been done have failed at very low loads, even at the best facilities, processed under ideal environments.

The fasteners will provide load carrying capacity in the event of the realistic scenario where the bond did not go as planned.

It will depend on the risk the program is willing to take. Military and commercial applications have different objectives and can tolerate different risk levels. But to have fasteners in a bonded joint is not necessarily futile.

All things being equal (i.e. if cost were the same, etc.), I would add fasteners since you are guaranteed a minimum strength level that may be close to your design load anyway(i.e. if you are designing to notched allowables). I would be interesting to hear other viewpoints as well.

Brian

http://www.espcomposites.com

 

[blakmax]

Brian

You are correct about the effects of processing being important. However, the solution is to firstly validate the processes by appropriate testing that actually addresses the failure mode in question, and then to specify and implement the valid processes in a rigorous manner. This is possible, because RAAF did it.

In 1992, 43% of all bonding work (mainly metal bonding but some composite patches on metal structures) at a major repair facility was repeating previous repairs which had failed. We changed the surface preparation process after it was validated using the wedge test (ASTM D3762 with more rigorous acceptance criteria). We also greatly enhanced the training given to our technicians. We also changed the film adhesive for supply reasons, but that gave us a tag so we knew if repairs were done before or after the process changes because the disbond was a different colour.

Up until I retired in 2007, there were three bond failures out of over 4500 repairs, and in each case we could determine where the technician had varied from the process specifications. So we reduced the failure rate from 43% to 0.06%.

We never used any fasteners.

So I suggest that this is not a design issue at all, and I also suggest that the best solution is not to just accept processing deficiencies, fix them.

Regards

blakmax

 

[ESPcomposites]

blakmax,

I can see your point. But is that not specific to one type of scenario? For example, bonded joints may be used in different capacities. Here are a few I can think of.

High volume production - I think this is the scenario you are talking about? Provided you are considering a specific joint, you can identify all the problems and achieve a high success rate. As you mentioned, this may not be achieved right away, but rather through modifications to the processes.

Low volume production - Perhaps it is not worth the cost to generate a "bullet proof" process. Instead, fasteners are used as a fail safety mechanism.

Repairs by OEM - Repairs can be very challenging if done on existing structure because you have limited control. You can't use an autoclave, you may not be able to achieve uniform and correct pressures due to structural deflections, existing structure can create heatsinks that lead to nonuniform temperatures, different part thicknesses have different optimal processes, etc. Almost every scenario may be different and the failure rates may be high, even at the best facilities with the best personnel.

Repairs in the field - Perhaps the most challenging of all since there may be many entities attempting the repair with all of the same problems the OEM would have had.

Some of these scenarios beg the question of why even use a bonded joint. Perhaps it is a customer desire/requirement. Perhaps you hope to have the increased strength of a bonded joint, but you absolutely require to have some capacity in the event of a failure.

I agree that a bonded/bolted joint is awkward in some sense, especially since the two load paths cannot work together. I have never designed one like this myself, but I have seen it done plenty of times. The common argument against is that fasteners reduce strength in a perfect joint. The argument for is that you can be assured of a minimum strength level in the event of a bond failure. Each situation and engineering problem is unique. All I am saying is that I don't know if it is the general case that adding fasteners is futile.

Brian

http://www.espcomposites.com

 

[blakmax]

Thanks for the response Brian. Our scenario is quite general repairs on mainly metallic sandwich structure, but the occasional repair to non-sandwich structure. In effect, no two repair scenarios are the same. We have on quite a number of cases applied bonded composite patches to active fatigue and stress-corrosion cracks.

You list a number of factors of fundamental importance to repair integrity. The way we approached these was to identify the threat and then take steps to eliminate or mitigate those threats. To establish the outcomes, we found it necessary to scrap many OEM methods and to develop our own internal publications and then train our technicians to that standard.

There are three absolutely critical issues which must be addressed; surface preparation, temperature management and managing the repair environment.

Sorry, I am out of time to finish this message. I will expand on the issues within the next few days. The temperature management issue is probably the least understood of all aspects.

My concern is that many operators who use bonded and bolted joints take the approach of indifference to bonding procedures because the bolts will keep the patch in place.

 

[ESPcomposites]

Yeah, we can go on forever on this topic. There is just so much information and things to consider.

You also mentioned sandwich structure and overlay patches, which I had not considered previously. In general, I find both of those scenarios to have relatively high success rates. Sandwich structure usually has thin facesheets so overlaps can be large. Processing is easier since thermal gradients and gas entrapment are less severe. If your sandwich structure is there for usual stiffness requirements, then damage is not likely to show a dramatic change in the structure's requirements (unlike strength). Similar type of arguments can be made for patching localized damage. I think these are good candidates for bonded repairs. Even if a failure were to occur, it is not likely to cause the loss of aircraft (generalized here).

Thick composite scarf joint repairs are a different animal, with a whole different set of risks. They present more problems and may require strength to be restored. I was referring more to this, but failed to state that.

Good point about the operator's, I did not consider this. The reality is that both processes are operator dependent (though to a lesser degree for bolts). If there is a potential that they focus less on the bond, then the objective may be lost.

I don't think we need to get into the details of processing, etc. though. I think we both have pretty good grasp on it.


Brian

http://www.espcomposites.com

 

[blakmax]

Fair enough, Brian. I was concerned that we were getting off the topic of the thread. If anyone wants to follow it up, they should read DOT/FAA/AR – TN06/57.

Thanks for the discussions.

Blakmax

 

[ESPcomposites]

Blakmax,

Good discussion. Thanks for the input. I will have a look at the references too.

Brian

http://www.espcomposites.com