DOM extended radius bars (Lars Bars) fatigue failure

[pippinmader]

So I was playing in the snow, and there was a loud thud, like I had hit a rock under the snow, not such a big deal since I recently obliterated my steering stabilizer there isn't much to damage under there. Then I hit another "rock" in a place I knew there weren't any rocks, after getting out to look I found that one of my radius arms had broken in two!!! I am running a set of Lars Bars, they use 2" DOM, 1/4 wall and are fish-mouthed to fit into the existing radius arm fronts. A close inspection showed that the DOM appeared to fail right behind the weld, it appears they failed fairly recently because the crack has very little rust on any parts. Based on appearance it appears to be a fatigue failure because the steel isn't necked down at all. It looks like the failed starting at the top and worked its way down. The failure either began because of proximity to the weld or because of a stress concentration caused by the end of the remaining portion of the original radius arms. I fixed it by inserting a 20" piece of 1.5" DOM 1/4 wall inside the existing DOM (easier said then done) and welding it back together, but of all the parts I don't ever want to break, the radius arms are right up there with steering linkage.

I am running 4 degree poly C bushings that look like they are an old Duff set, they are VERY stiff, maybe that is part of the problem? Has anyone else had trouble with this?

[duner53]

Re: DOM extended radius bars (Lars Bars) fatigue failure

It broke because how it was welded ant time you weld across a tube it will crack at the edge of the weld that has been my experience you better crack the other side

[earlybronco72]

Re: DOM extended radius bars (Lars Bars) fatigue failure

That sucks, but on a different note, I'm in truckee working on the power outages. If you see Roseville electric, I'm in the mix.

[pippinmader]

Re: DOM extended radius bars (Lars Bars) fatigue failure

It broke because how it was welded ant time you weld across a tube it will crack at the edge of the weld that has been my experience you better crack the other side

So what is the solution? I think everyone has built these radius arms about the same. I did stick weld the one that broke, and mig welded the other side that hasn't broke yet.....

Mark, I will keep an eye out for you, if you see a bronco on three wheels it is probably me Actually I am down in Sacramento until the end of the week, otherwise I would say we should get together for a beer. We didn't have power for a day and a half, I can tell you that you don't know how much you miss it until it is gone!

[earlybronco72]

Re: DOM extended radius bars (Lars Bars) fatigue failure

Sounds like I may be here until Sunday. Only about 7 hrs off ever 36 hrs. That time is for sleep. We are down at the far end of the lake tonight. Getting a bite to eat before we hit it hard.

[BRONKEN]

Re: DOM extended radius bars (Lars Bars) fatigue failure

Be safe Marc especially in those cold temps.

[crawlin68]

Re: DOM extended radius bars (Lars Bars) fatigue failure

Wow, that is scary. At least it failed while on the trial and not on the highway! I'm going to go check mine this weekend. Wonder if I should weld a reinforcement plate on the top of the arm right where the stock arm ends and the tube begins. Hmmmm.

[OG48]

Re: DOM extended radius bars (Lars Bars) fatigue failure



I had a break last summer on the rubicon. Mine was a .120 wall "custom" radius arm though. I have since upgraded to the james duff arms 5/16" wall

[71BRONCO71]

Re: DOM extended radius bars (Lars Bars) fatigue failure

Wow, that is scary. At least it failed while on the trial and not on the highway! I'm going to go check mine this weekend. Wonder if I should weld a reinforcement plate on the top of the arm right where the stock arm ends and the tube begins. Hmmmm.

heres something for ya

http://www.ruffstuffspecialties.com/catalog/R2127.html

[welndmn]

Re: DOM extended radius bars (Lars Bars) fatigue failure

Interesting!
Extended arms have been around a long time and I think I've only seen, or heard, of under 10 snapping like that.
Like Duner53 said, next to a weld is the weakest part of a splice, add in that radius arms bind in design, and that causes issues.
Still, it's not common for it to fail, I would more over bet you just got a substandard section of DOM.

[ntsqd]

Re: DOM extended radius bars (Lars Bars) fatigue failure

There are some nuances to the fabrication of these that I think should be thought about before starting.

The flanges of the radius arm should extend well past the notch cut in the tube for the web of the radius arm. Where the flanges stop is a change in cross section and a stress riser. The notch in the tube is also a stress riser. They should not sit on top of each other, they should be as far apart as is practical. It may well be that a strap should be added to the ends of the flanges and carefully blended in to extend them well down the length of the tube.

These are semi-thick sections, combined they should be pre-heated to prevent rapidly chilling the HAZ. It may well be that some post-heating is a good idea too. Sure can't hurt.

[pippinmader]

Re: DOM extended radius bars (Lars Bars) fatigue failure

Thanks for all your comments and ideas!

Regarding notching the radius arm correctly: I followed Lar's design closely as it relates to notching.

Regarding stress riser from the weld/welding accross the tube: The one that broke was stick welded using 7018 rod, possibly the larger bead resulted in more surface stress that lead to fatigue failure? Certainly heating or shot-peening would help eliminate surface stresses. The other side I pre-heated and mig welded and it appears to be fine. But for piece of mind I am ordering some of those Ruff Stuff tube reinforcements. #fingerscrossed

I did let both cool on the bench, but didn't wrap them in insulation or anything like that.

I did talk directly to Lars, he said he had only heard about a couple of these breaking and anecdotally they had all been stick welded.

I do still wonder if the additional anti-sway effect of the stiff C bushings was adding stress to the arms and was part of the problem.


Overall do you think I have to re-make the one that broke or can I get away with the fix I did with the DOM inside the broken one and additional ruff stuff reinforcements?

[ntsqd]

Re: DOM extended radius bars (Lars Bars) fatigue failure

If you wheel it hard I would make a new one and set that one aside as a spare.

The welders at the 330MW power plant that I worked at used pneumatic needle scalers to do their weld peening.

[pipelayer]

Re: DOM extended radius bars (Lars Bars) fatigue failure

I would think it would be fine after the slug and saddle.

[pippinmader]

Re: DOM extended radius bars (Lars Bars) fatigue failure

Quick update, I ordered and installed the Ruff Stuff tube reinforcers that Ryan linked. Went ahead a wrapped them all the way up the inner C and through to the weld-in Johnny-joint fitting. I also smoothed out the transition between the old radius arms and the DOM tuning. After looking at it all closely I think one of the main issues is the stress concentration caused by a few things,

1. The original radius arms are cast steel and quite stiff, compared to the DOM, so there is a stress concentration where they end
2. The radius arms end abruptly AND have a weld right across the tube
3. Welding with a big stick welder that could do that fairly large weld in a single pass leads to more thermal cooling stress

The combination of those two things lead to a huge stress concentration right there. There are a few solution, what I did was grind the ends of the stock radius arms down so the tube reneforcers could conform to the profile and also because it reduces the stress concentration. If I was building them from scratch again and didn't want to add the reinforcement you could cut the end of the stock radius arms into a point instead of straight across, again it would allow the change in material stiffness to happen over a larger area and reduce the stress concentration.