Chalo said:
One difference that steel has compared to aluminum, is that after repeated loads, aluminum will fatigue. Steel has an indefinite fatigue life, which means it has enough strength to withstand repeated load cycles, and hence, it lasts longer, or is preserved.
That's not quite it. Steel has a fatigue
threshold, which is to say that below a certain amount of stress, no fatigue cracking will occur. But every steel frame experiences forces above this threshold, if it is ridden. A frame would be annoyingly bulky and heavy if it were built to stay under its fatigue threshold all the time.
Sorry, but that interpretation does not gel with the engineering principles for steel and Al that I practiced for 40 years.
Chalo said:
Correspondingly, because aluminum has a low density, it's easier to build in a way that lowers stresses enough to avoid low-cycle fatigue entirely, and moves high-cycle fatigue into the millions or billions of cycles range before failure. That's why we can have all-aluminum B-52s and airliners that are are several decades old and have been working all that time. Heck, there are still DC-3s running regular scheduled service, and that plane was built when metal fatigue was poorly understood. Few if any steel tube framed aircraft can claim that kind of longevity-- I'm guessing none, in terms of flying hours.
Guess away.
Do you realise that every commercial passenger and transport aircraft has a schedule of maintenance that in addition to per flight, daily, weekly, monthly/400 flying hours has extended inspections:
A-check: every 600 flying hours: 6 engineers and 12 hours. Includes all the checks performed above plus more.
2A-check: every 99 days. 3 shifts of 30 engineers per shift; All of the above plus more including " partial strip down of structure and engines for detailed inspections, replacement of worn components"
4A-check: every 190 days. 4 shifts of 30 engineers per shift. All of the above plus more including " Detailed inspections of flying controls, structure and engines."
C-check: Every 18 months. 6-7 days of 3 shifts and 30 engineers per shift. All of the above plus more including "Detailed inspection and repair of aircraft structure, engines, components, systems ".
C2-check: Every 3 years. 10-12 days; 3 shifts of 30. All of the above plus more.
D-check: After 8 years and the every 6 years. 25 days/3 shifts 30+. "Involves major structural inspections including attention to corrosion.
Aircraft is virtually dismantled, repaired and rebuilt as required, with systems and parts tested and repaired or replaced as necessary. Opportunity taken to carry out major modifications as required Corrosion prevention and control tasks carried out."
Around 20 years ago I was contracted to KLM at Schipol Airport. After working straight through a long weekend fighting and finally putting out fires on the systems; I and 4 others were rewarded with a guided tour around under and through a 747 heavy "in the green"*; undergoing its D schedule.
*So called because of the green color of the anti-corrosion coating on the entire internal Al structure of the plane; As we walked through the body -- you cannot believe how huge they are internally when all the seats, internal paneling and insulation, windows, headliners et al has been removed -- there were entire chunks 3 or more meters wide and high of missing paneling in the sides and roof prior to total replacement.
Every single pop rivet and its holes in both the external skin and the internals spars are ultrasound tested. If any of the hundreds of rivet holes around each panel shows signs of fatigue hardening, the entire panel is replaced. If the corresponding spar hole also shows any of the same signs, it (and all the other panels that attach to it) are replaced. The attention to detail, along with the logistics, work rates and time pressures are mind boggling.
The particular jumbo I walk through was having it second D check (14 years), and over 30% of the skin, 12% of the spar structure and all 4 engines were being replaced. When you read that a plane flew commercial service for 40 years, less than 20% of what left the factory is still original. That's mostly the landing gear (steel or titanium), wiring, flap actuator mechanisms (steel) etc.
Pretty much every scrap of aluminium in the plane will have been replaced at least once, and much of it more than once. Aluminium is used in planes because of its low density; no other reason.
Which is why Boeing and Airbus are moving to replacing it with CF composites as fast as they can. Its not that much lighter or stronger; and its far more expensive to construct, but its fatigue life is so much greater that is doesn't need to be replaced. It is projected that the carbon fibre wings on the A350 XWB with last the entire 30+ years service life of the aircraft.