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Old March 30th, 2012, 11:09
frijolee frijolee is offline
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Join Date: Oct 2010
Location: Tustin, CA
Posts: 367
Re: Joel's multipurpose XJ build (rocks/boulevard)

Luke, that's food for thought for sure... Thanks for the link, I hadn't seen that one. The track bar bracket at the frame rail is definitely different than what I have yet the overall shape of the bar's connection point is similar (round bore with some kind of bushing, rather than a heim joint).

Vanimal, thanks for the Rubicon Express mention too. I'll look into that as well. As to your implied question:

Originally Posted by Vanimal View Post
nice work on the tank skid, i too had wondered about making armor out of aluminum since it's lighter and typically has a higher yield strength than mild steels.
Working with aluminum is all about picking the right alloy for the job. First and foremost unless you REALLY know what the heck you're doing don't use it for structural parts if you can help it (IE I won't be making a track bar mount from the stuff) since it basically has no fatigue limit. The upside as you mention, is the weight. The density tends to run about 1/3 that of steels so you can go heavier wall (which helps bending moment of inertia) while still saving weight. The other downside of aluminum is that’s it’s expensive… I usually pay about $3.29 per lb for alum plates where steels are often only $0.89 per lb.

If you compare strengths you find:
-A36 mild steel: 36 ksi yield, 60 ksi ultimate
-5052-H32: 28 ksi yield, 33 ksi ultimate
-6061-T6: 37 ksi yield, 42 ksi ultimate
-7075-T6: 67 ksi yield, 76 ksi ultimate

If you look closely the ratio of yield (at what point we have permanent deformation) to the point it finally tears apart (ultimate strength), the mild steel will outlast the aluminum by miles. To put in another way, in an impact we want the ability to dissipate energy and the ability to stretch without breaking is great for that.

Minimum elongation at break:
-A36 mild steel: 20%-23%
-5052-H32: 12%-18%
-6061-T6: 12%
-7075-T6: can be as small at 3%, best case is 11% (varies with thickness)

This basically means that 7075 is quite strong but also super brittle. It’s a lot more likely to crack than to deform when push comes to shove.

See also the minimum bending radius (shown as a function of material thickness)

For the alloys I mentioned if I were using 1/4” plate I could bend the 5052 at 3/8” radius, 6061 I could use a 1” bend radius, but I’d have to use a whopping 2.5” radius on the 7075. Bend any sharper and it’ll just crack on you.

The other catch is welding… Once welded, aluminum is in the annealed state (meaning weakest but most flexible state possible state for the chemistry of that alloy). This is the “T0” state referenced in the chart. For the 5052 I used here, the T0 state has an yield strength of 13 ksi and an ultimate of 28 ksi. Not terrible, but still WAY down for the hardened numbers above.

In my case, I choose 5052-H32 for my gas tank skid because I thought it was more likely to need the impact resistance, but I went 6061-T6 for my belly pan because I wanted the stiffness and strength for the larger area and thought it was less likely to see blows and more likely to see scraping. I also used ribbed and riveted construction on the belly pan to avoid the heat affected zone issues, but now I’m getting ahead of myself and have a few other things I need to write up first.

Last edited by frijolee; March 30th, 2012 at 11:16.
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