👍 Thanks wingman

For those that are curious - https://www.smithsonianmag.com/air-space-magazine/what-determines-an-airplanes-lifespan-29533465/

That's pretty much what I was getting at, good find. My I was concentrating on aerospace for my engineering degree. It was sidelined several years ago but I picked it up again recently.

How do you guys put those GIFs in your comments? Haha.

just copy+paste the URL

No it's not soft. To be precise, aluminum is very rigid. (That's why when aluminum wheels fail, they crack, whereas steel bends).
It has TWICE the strength to weight ratio of steel.

However ...
#1 - the material costs a lot more than steel
#2 - aluminum requires different, more expensive forming, welding and construction techniques.
#3 - the frames would have to be much bigger to achieve the same strength.

#4 - it fails catastrophically. A single impact in the wrong place can compromise the strength of the whole frame.

Look at the thickness of aluminum bicycle frames versus steel.

Yeah, certain alloys exceed the strength of steel by large margins per unit weight, but your run-of-the-mill aluminum alloys are pretty soft and ductile. We did experiments in materials science comparing the ductility of steel, aluminum and PVC by stretching samples with one of those hydraulic machines. A36 steel was the most rigid (it broke rather than stretching much), PVC was the most ductile, it never really broke cleanly. It just stretched. Haha. Aluminum was right in between. It stretched a lot more than steel (it's called actually called necking, but 99% of people think of necking in other ways when they hear that word, lol). The fact the bike frame is larger also points to aluminum being softer than steel. The larger diameter tubing is compensating for its softness by basically spreading out the load over a wider area.

Aluminum alloy wheels crack rather than bend because they're a high strength alloy, putting dissimilar elements in the aluminum introduces distortions and such in the crystal structure. Rather than bending and shifting atoms within the metal, like a soda can will, for example, the non-aluminum atoms in high strength alloys resist the motion of the bulk aluminum until it cracks under the load, rather than bend. You can make aluminum extremely rigid, but it comes at a price, which is reduced ductility. Forging also commonly used to strengthen (stiffen) aluminum. That distorts the atoms in the structure without introducing dissimilar elements, simply by creating obstructions to atoms' movement. Bending, smashing, etc. on the metal without breaking it causes distortions in the crystals if it exceeds the yield strength. Those distortions strengthen the metal, but it also reduces its ductility and the stuff tends to crack.

I'm off my soap box now, haha. This was my favorite part of my engineering classes.

well of course you wouldn't use pure aluminum for structural parts. It's almost always alloyed. Same as iron.
Structural (mild) steel owes much of its durability to the fact that it's ductile

True, it also tolerates practically an infinite quantity of loading and unloading cycles, as long as you don't exceed a certain limit. Aluminum alloys don't share the same property, airplanes have a useful life of so many flight hours before they gotta be retired. It's tens of thousands of cycles, but still. Nobody would keep those logbooks for their car. Haha.

I thought of the tailpipe I put on on my truck a few years ago, it's aluminized steel. That would stop frame rust, but it's also probably expensive to do.

there are some very old airplanes still around. 1960's, 1970's.
They're still fit to fly and they're made of aircraft aluminum.