Strength of bent aluminum

[Evenglischatiest]

Hello all.

I'm in need of a little help on a structural problem. I'm a machinist, and I've been asked to make a part that I don't think will be safe. It involves a bent aluminum rod, which will need to hold a man's weight, on a moving vehicle, in an awkward position. (pages of lawyereese prohibit me from saying exactly what it is, but it's nothing fancy) I can calculate the strength of the initial rod, and it will do. What I'm worried about is the strength after it's bent.

Can anyone help me calculate the strength loss when cold bending 6061T6 rod? The specifics are: 13/16" rod, bent 45 degrees, with about a 6" bend radius.

Thanks in advance for your help.

 

[Ron]

It would be better if you were at T4 or lower, but since you are not, you have a couple of choices...

1. You can anneal the aluminum rod, bend, then re-temper
2. You can bend it and check for cracking.

Bending, obviously causes strain hardening, which has a greater susceptibility to crack on the outside radius. I would suggest die penetrant testing after bending and maybe even radiograph a few. Ultrasonics would not give accurate results in this case.

Your bend radius is soft enough that you probably won't have an issue. If there will be welding on this rod, remember that welding 6061-T6 will significantly lower the allowable stress. Also, if the "man's" weight will cause bending on the rod, you might want to check the stresses from the bending. Axial stress is small for the load, but if the bend causes the load to be in any direction other than axial you'll induce bending, likely on the inside radius.

 

[Evenglischatiest]

Ron,

Thanks for the input.

We already have one prototype bent, and it has small but clear stress fractures all the way around. The guy who wants these parts doesn't want to deal with heat treatments. (he's a carpenter, with no understanding of metals) I'll see if I can talk him in to more testing, and trying one in T4. So far, I've had no luck trying to talk him in to using steel.

Unfortunately, the full load is cantilevered 5" past the bend, hanging at about 60 degrees from vertical. And the vehicle will be traveling off road, guaranteeing frequent shock loading.

What has me worried is that this is intended to be a commercial product, not a one-off. So I need to do genuine fatigue life calculations, if only to satisfy my own conscience. And that bend puts it beyond my limited knowledge of structural analysis.

 

[msquared48]

Aluminum is very susceptible, far more than steel, to fatigue loading and you are apparently very aware of that fact by your post.

That being said, if this rod is supporting expensive equipment, you may want to logically present the failure scenario to the contractor with the cost (timeline included) implications to his project, and compare that to the cost of doing it right.

He might re-think the problem.

Mike McCann
MMC Engineering

 

[Ron]

Agree with Mike. Further, it sounds as though you have stresses in the rod that are not accounted. Perhaps you could provide a sketch with the directions of the loads and the attachments.

You might have to go to another aluminum alloy, perhaps a 7000 series.

If this product has any life safety implications, make sure you point out the repercussion of failure. I do failure analysis for a living. Some of those involve personal injury and death. You client does not want to be pulled into a personal injury or death lawsuit as a result of his product...it ain't purty.

 

[Evenglischatiest]

Well, in the interest of safety, I'll just ignore the non-disclosure paperwork. He wants me to make bicycle seat posts. Bent ones, like the 80's BMX bikes had. No reputable company makes them anymore, and he thinks there's a big market. I'm trying to convince him that everyone stopped making them because they're not safe.

At this point in time, simply telling him no would bankrupt me. Too much other business depends on keeping him happy. So I'm trying to talk him out of it. But he's uninterested in my gut feelings, and wants hard numbers. Frankly, I doubt he'd understand those numbers, so I'm considering making them up. But I'd rather do it right.

 

[Ron]

Post a sketch and let's see where we can go with this.

 

[BAretired]

How about reinforcing it like the attached?

BA

 

[LittleWheels]

I remember layback seatposts. AFAIR, they were always steel, usually 4130, with a 22.2 or 25.4 mm external diameter (22.2 mm seatposts are pretty rare nowadays). I wouldn't trust an aluminium layback at all.

Don't forget the potential damage that the seatpost clamp is going to do to the alu seatpost, right where there is the greatest moment. Few BMX bikes have nicely reamed seat tubes and the slot is likely to create some nice scratches.