Nick Name, previous reporting elsewhere has cited Airbus as stating that the fasteners were inserted with too much force for the not-flexible-enough aluminum alloy, which thus formed cracks--but after 10 years. Since the background info also talked about the problem occurring at the aluminum/composites interface, that can make it sound as if it's the composites that failed. In any case, Airbus is not releasing much info, and not releasing any formal statements online.
Jerry, we've reported several times on on the use of composites in aircraft over the last few decades. We've also done features on fasteners and adhesives for structural apps, including aerospace. What I've been told is that composites have been adapted to component designs originally engineered for metal without rethinking those shapes, and that fasteners for composites have not been re-thought thoroughly. This was all in the rush to get composites into commercial aircraft. It's the 50% and greater proportions of composites to metal that are new--as well as all this being somewhat newer in commercial than in military aircraft--which means joints are being made between dissimilar materials where they weren't before. Also, adhesives are simply not strong/durable/reliable/etc enough for the extreme stresses involved and, as kfw2qd points out, it's tough to tell when they've failed.
Considerring the testing that has to be done -subassemblies tested to destruction, tested for some number of cycles... it would seem that the testing procedures were not correct fot this type of assembly.
It sounds like this is a problem with components inside the wing, and may not pose a risk of structural failure, but rather of wear and erosion of both the composite and the aluminum.
The problem with adhesives in some of these applications is that the adhesive may not be flexible enough, or it is extremely dificult to tell is the adhesive bond has failed. What would happen if the bond was failing in the middle of the assembly, but not near the edges? how do you check it for failure other than by damaging the adhesive to look at it?
Some of this is what every engineer has gone through at some point in their carreer - you thought you had everything nailed down, you did all the correct tests, only to find out the one thing that you were very confident in was the problem. The one property that no-one would have considerred was the one property that was the most important.
I think it was the DeHaviland Comet that had an array of holes drilled in the lower wing skin near the landing gear because of cracks that wanted to form there. By drilling the holes that section of wing skin was able to flex and no more cracks. A bigger problem was the failure of several of the comets because of the stresses around the sextant port in the top surface of the aircraft. Was one of the early pressurized aircraft and they didn't properly account for the stresses around that hole in the fuselage - several of them broke at altitude and crashed... They built a huge tank and submerged the fuselage and then presurized it under water to figure out wha tthe failure was.
An interresting aside - Jimmy Stewart played an engineer (Was a B17 Pilot WW2) in the film "No Highway in the Sky" (co-starred Marlena Dietrich ) in which metal fatigue was the cause of a catastophic failure, and crash of the Reindeer. Three years before the crash of the Comet - due to metal fatigue in the fuelage, not the tail as in the movie. They didn't want to believe him - he was a bit of an eccentric... Then as now, engineers have a problem with PR.
It has to be considered that EADS is a transnational company with plants in France (final assembly for some types as 380) and Germany. This incompatobility came from the 2 different origins of the softare packs. It was an orrigine error and it unfortunate that it manifested at this project.
I am not affraid of the hair cracks since their evolution is very well monitored and it is NOT the only plane in use which has minute cracks and requires periodically controls and repair. In fact almost all planes have this problem.
What I do not understand is the fastener introduction problem. I know the fatener type and the technology used and I do not see where the "introduction" could harm.
It would be good to have more on this subject.
As for the tests AIRBUS is extremelly careful and one cannot imagine the number and the depth of done tests. It could be possible that the loading combination in flight/landing was in the real world different from the one previewed by desighners.
As far as I know the fasteners do only "clamp" the composite between 2 metallic layers. This can be the reason the cracks did appear in the aluminum and not in the composite.
Greg, that's interresting. Any data on it online? I'd had though their expansion too would be different but not much experince with Titainium.
We used it in racing sailboats but it kept cracking, etc worse when used in the rigging, very high constant load changes, than older tech so no longer used there much.
What about the liability issues. In some ways, it's pretty amazing that they can admit there are problems that they don't fully understand, yet still keep the fleet in the air. I understand the logistics issues related to retrofitting the fleet, but still ...
titanium and carbon fiber have similar thermal expansion rates. aluminum is quite different... titanium also does not corrode when placed next to carbon fiber, aluminum must be well insulated with fiberglass or something else...
Ann, composites and metals is not a new problem at all. Been dealing with it for 40 yrs. Most aircraft were made from a combo of them.
The problem is the engineers or above can't bring themselves to do composites as they have always worked in alum, etc. So instead of doing it right, glueing, they use a bastard mechanical fasteners because they are scared of doing anything new. Sadly this goes through many industries.
Naperlou just the expansion, contraction of the Composite tubes in the metal joints from the 1000F to -200F of space temps as it comes into and out of sunlight would be enough to destroy it as their rates are so different.
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