grounded27,
The alloy AND thickness of the rib feet was insufficient for the application. I am saying that if 7449 is a good alloy for this application there was not enough for it. If it was not it is obviously the wrong material for the job. The bottom line is that Airbus was going for weight savings in this monster of an aircraft and is now paying for a poor decision.
Thank you for responding and answering my questions.
First, I am reposting the video of the A-380 wing assembly process as I think it is significant. Particularly, at the times of 7:24 & 8:00, stop and look at the brackets, also known as rib feet. For each wing, there are 2,000 of these. Each wing has 62 ribs, 38 are metallic and 24 are carbon composite. The wing panels, some are very long, are creep formed to the desired contour of the wing surface at the location they will be fastened to. The rib feet of the top of the structure is designed knowing it will be in compression during actual service. The rib feet on the bottom of the structure that you cannot see are designed to be in tension and I would bet they would be different, more robust. The alloy, 7449-T7651 is a new alloy that contains no chromium unlike other 7XXX series alloys or older aluminum alloys. The T7651 condition stands for an over tempering heat treatment. This is done to maximize improvement of corrosion resistance during service. This comes at some sacrifice of other properties. What is important to understand is the significance of the assembly orientation and the possible stresses put on the rib feet that would not be realized in service.
Airbus has indicated the following:
Justin Dubon, an Airbus spokesman, said the company had established an inspection and repair procedure in coordination with the safety agency to address the cracking, which he said was linked to unforeseen stresses placed on the wings during the manufacturing process and not to a design problem.
“Both types of cracks have been traced to the manufacturing process, and those practices have now been changed,” Mr. Dubon said. All of the A380s’ wings are built at an Airbus plant in Broughton, Wales.
The first cracks were found late last year on the wing of a Qantas A380 that was being refurbished after experiencing a spectacular midair engine explosion in 2010. Airbus deemed the cracks — which extend from a bolt hole in the bracket — to be “noncritical” and advised airlines to inspect and replace the parts during routine scheduled four-year maintenance checks. The first A380s entered service four years ago.
This statement does make logical sense. The first cracks found, extending out of the bolt hole were, IMHO, coming from sharp corners. In a compressive tensile state, they would not grow very fast and would not necessarily be of grain boundary nature. Their rate of growth could be predicted and a time set for when repairs could be carried out without jepordizing safety.
The other cracks are not on the feet per-se, but on the shape below the feet in between the rib structure and the feet. These are more serious and IMHO, would be grain boundary cracks that could extend quickly depending on grain boundary length. With the orientation of the feet and the wing panel during assembly, it may be possible to unevenly stress this area depending on the force being supply to press the panel to the feet verses the weight of the panel and the sequence of bolt insertion and fastening.
Since we don't know the location of the troubled rib feet, and if the troubled feet are repeatable from wing to wing, aircraft to aircraft, it is hard to know for sure. But IMHO, the assembly process does have something to do with this, more so than thinking the ribs are under-designed. We will see as more information evolves.
TD