by kennyrayandersen » Mon Mar 06, 2023 9:27 am
Whine
I wrote a big thing, and then I remember this site had a special feature where you get logged out and lose everything -- oh, how I missed that feature! So, I'll try and do it again...
Background
This is really a loaded question, and there was so much misinformation that I felt like I had to chime in. Background: I'm an aerospace structural analyst with 38 years of experience working with and analyzing flight vehicle structure (wide-body, fighters, helicopters, and tilt-rotors). I have utilized aluminum, steel, titanium, fiberglass, and Gr/Ep as well as various sandwich panels. My background is primarily in static analysis though I have done some fatigue analysis and generally understand damage tolerance. I also have some experience in welded aluminum parts, though we don't weld much, and never for primary structure -- but on occasion for systems items like ducts, reservoirs, small tanks etc.
What material?
Many of the materials we regularly use aren't readily available to hobbyists because of cost. The regular hobbyist consumer would typically use Al 6061-T6 as the high end, and for us 6061 T-3 or T-4 is the absolute least capable material we would use with most Al 6061 used would be in the T-6 condition -- anything less than that is decorative! Though it's true Steel can be stronger than aluminum, it's frequently not as most folks out there are working with mild steel. Mild steel has an ultimate strength of around 58 Ksi and a yield strength of 36 Ksi, whereas Al 6061 T-6 has an ultimate allowable of 41 Ksi. and a yield strength of 37 Ksi (Fcy). So, the least capable aluminum we use has a better yield and nearly the same ultimate strength as mild steel. Keep in mind that for most of the aluminum we use (Al 7050, 7075, 2024) they are stronger than mild steel and are only 1/3 the weight! Yes, aluminum has less stiffness, but unless your problem is stiffness or deflection driven, the stiffness doesn't enter into the strength equation (it does come into the deflection equation). When we use steel in aerospace, we use the good stuff, and sometimes the really good stuff, and none of it comes cheap! So, when we are comparing steel to aluminum, we are generally comparing mild steel against Al 6061 T-6 and there is really no comparison on which will come out significantly lighter (I could talk about bikes all day long, but suffice it to say that aluminum wins hands down in the bang-for-the-buck category (yet I ride Gr/Ep because I can!))
Welding
Al 6061 can be welded, whereas the other aluminums I listed cannot be. There are primarily 4 aluminums that can be welded Al 6013, 6061, 5052, and 7005, but none are much available for the hobbyist other than Al 6061, so I'll focus on that one. When you weld aluminum, you do end up with a heat-affected zone (1/4”-1” from the weld) wherein you lose your T-6 temper; however, Al 6061 has a quality that it age-hardens at room temperature; so, after a couple of weeks it goes from the 'W' (post welded, which is a bit better than the 'O' condition) to nearly the T-4 condition. If you can re-heat-treat you can actually get it back to the T-6 condition, which is what is frequently done. However, for a trailer chassis, it would be a real challenge to find an oven big enough to put it in to re-heat-treat; so, for practical purposes, we can use the allowables from the T-4 condition to make our strength calculations right next to the weld. Al 6061 T-4 has an ultimate strength of 26 Ksi, and a yield strength of 16 Ksi (better than wood at least!). Though that isn't great, if we are clever about it (and we are), we can put the weld areas outside of the high stress areas so that the localized loss of heat-treat doesn't affect the overall design integrity.
Fatigue
Yes, for aluminum, fatigue has to be considered, but it isn’t a show stopper. Good design practice avoids high stress concentrations (Kt), and we can also choose to locate fasteners in such a way as to avoid putting them in location where the fatigue will be a problem. Then if we set the stress level below a certain threshold, we can end up with something that pretty much has an infinite life.
Aluminum chassis design coming
I’m not sure this is the place for it, or whether I should start another thread, but I’m in the process of laying out an aluminum trailer chassis for an ultralight teardrop to tow behind a micro car like a 1963 FIAT 500, or a 2004 smart fortwo (I have one of each).
Last edited by
kennyrayandersen on Tue Mar 07, 2023 9:06 am, edited 1 time in total.
