7/29/2018 9:19 AM
Edited Date/Time: 7/29/2018 12:05 PM
You need to understand aluminum completely before you judge it. You can most certainly use aluminum axles safely in many apps IF you use the proper type of aluminum. I have done it and some people running the newer bikes in recreational road racing have aluminum axles, however, they are running 7075 aluminum and I use 7068 aluminum which is much stronger, and both of these grades of aluminum are much stronger than 6061.
"Most aluminum alloys lack such a fatigue limit, so they accumulate fatigue damage steadily under stress cycling. This is why airframes, wings, etc. have a "lifetime" or service limit."
Yes, aluminum has a fatigue limit, however, you will likely be dead and buried long before an aluminum axle reaches that limit in this type of application. Kaiser aluminum does fatigue tests on their aluminum and the test on the 7075 is ten million cycles. Also lot of force must also be applied to aluminum before it appreciably contributes to the fatigue of the metal, and this force is fairly high. For example, if you bolt the end of a 1" round bar of aluminum to a vice, leaving 12" of the bar exposed, then apply 1 lb of force to the exposed end of the bar and do it a bazillion times, the axle will still not have fatigued enough to fail. The greater the force applied, the sooner a failure will occur.
You also state that airplanes are made of aluminum, and my guess is that you have flown in one before, and that if you did, I'm also guessing that you didn't worry about how many fatigue cycles it had been through, but I could be wrong. Irregardless, I for one would much rather have a part break from fatigue when I drop from 10 feet in the air instead of when I drop from 35,000 feet in the air.
7068 aluminum is also much stronger than what aviation typically uses, although all aluminum still has a fatigue limit.
There are also cars that have aluminum frames. The 2013 and earlier BMW M6 is one of them, plus, many motorcycles, including many motocross bikes have aluminum frames and aluminum swing arms and aluminum handlebars, and aluminum engines, aluminum wheels and aluminum sprockets etc, yet people don't seem to be concerned about these parts breaking in half from fatigue.
They have also been making connecting rods and rocker arms out of aluminum for racing engines for around 40 years, and many street racers have over 100,000 miles on their rods and rocker arms.
Steel can and does also occasionally fail. I had many steel motocross bike frames develop cracks and also cracked many when I worked in the R & D department of Kawasaki Motors.
If one is concerned about an aluminum axle reaching its fatigue limit and failing, they can simply replace it periodically, like after every 5,000 hours of riding etc.
Axles can also be made out of "Giga steel" (alumasteel) but that is not easy to obtain at this time.