Aluminum vs. Steel Cross-shaft Question

[65Stang0_1]

I'm building some upper control arms for a '65 Mustang using 1" square material for the cross-shaft. I will drilling/tapping a bolt into each end of the cross-shaft for heim joints that allow the UCA to pivot. I have two (or three) questions:
1) If I use 7075-T6 aluminum w/a 83,000 tensile strength for the cross-shaft, will it be as strong or stronger as mild steel w/55,000 tensile strength? Which should I use for this application?

2) I'm using 3/4" x 5/8" heim joints for the above, should I use 5/8 bolt or a 1/2" with bushings? I'm leaning toward the 1/2" bolts due to 1" square that I'm tapping into.

Thanks for your help.

Vic

[DeepBlue68]

I've never done any design of automotive suspension components, but one thing that may or may not be important to keep in mind is that aluminum doesn't have an endurance limit. Basically, when metal goes through repeated or fluctuating loading (and automobile suspension pieces most certainly do), it eventually will decrease in strength. Think of bending a paperclip back and forth until it breaks. It's the same thing, only it takes a lot more cycles of relatively lower stress levels in the situation you're talking about.

Steel reaches a minimum strength and then its tendency towards strength-reduction levels off. This minimum value is called its endurance strength, and it won't continue to weaken past this point once it reaches it. Aluminum, on the other hand, does not have an endurance limit, which means that it will continue to weaken and eventually will probably crack or yield. However, if the part you're talking about never sees stress levels anywhere near the levels necessary to cause that kind of failure, it might not be a major issue.

As I said, I don't design automotive parts, so I don't know if this type of thing is taken into consideration when engineers design them. I can't imagine it wouldn't be, but I don't really know. Just something you might want to look into before putting a part on your car.

Are there any automotive design engineers here that know anything about this? I've always wondered how vehicle companies design structural/suspension components for fatigue failure.

[29od1]

If you are referring to material ultimate strength above, Ftu, then the aluminum is stronger than the mild steel for this case. But, I would design to tensile yield strength, Fty, because you really want to stay in the linear region of the material and therefore I would compare Fty, as well as, shear and bearing strength of the two materials before you choose which to use.

To accurately compare which metal is better in fatigue, you need to compare S-N curves for each material.

If we assume you just look at 75ksi wrought aluminum versus 55ksi mild steel, the fatigue limit (strength) would be about 1/3 higher for the steel. Which means, for the same number of cycles, the steel can be loaded about 1.33 times higher than the aluminum to have a "relatively" infinite fatigue life (probably longer than the life limit of the car).

But for our cars, we probably won't be driving them enough over time to worry about fatigue. I'd just design to something which has at least equal or greater strength than stock unless you are planning to really beat on this car.