When 4130 Tubing Fails...

[Dave_G]

Here is an attached educational photo of what 4130 tubing looks like when the ultimate tensil strength is exceded. This is a front bumper attach mount from the Herbst TT that was sheared off in a test session on Dec. 21st. The truck ended up on it's nose at speed and sheared the front bumper off the car. The tubing is .095 wall 4130. Note how the tubing has failed outside of the weld area. Them boyz at Smith fab do damn good welding! ;-)

Dave

 

[Dave_G]

Here is another photo of the failed 4130 part. Note how the mounting clevis has sheared in a perfect arc outside of the weld area. Also you can see a good example of how a washer on a bolt can cary the load over a larger surface area on the other clevis as the material is sheared cleanly around the washer.

Dave

 

[Dave_G]

This is a full length view of the failed component. Note how the part has failed at both ends! The clevis material was .125 thick 4130 material.

Dave

 

[TimHayosh]

Dave, great pix!

I don't want to re-hash the "chromo vs. mild" debate, but I would like to ask a couple of questions. When I look at that part, I don't see a lot of bending. It's as if the part just snapped, like chocolate that has been frozen. Yet, the mild steel bumper on something as simple as my (old) Nine car bent severely (absorbed energy) before it finally broke.

Obviously, I have an opinion about steel vs. 4130. Can anyone supply any stories of a 4130 failure in the driver area? It's alway been a concern that rather than deflecting, the 4130 would become a spear. Is it just that the ultimate failure load of 4130 is so high that we rarely see it fail? Even the SCCA prefers mild, seemless steel over 4130 (I'm building a rally car too).

Not looking to start something! Just looking for info.

 

[Dave_G]

Tim,
I guess the simplest way to describe the difference with 4130 and mild steel tubing is that mild steel tubing will lose it's structural integrity with less loading than 4130. The mild steel tubing has a lower yield strength so it will bend before the 4130 will.

Re: It's as if the part just snapped, like chocolate that has been frozen.

Yep, that's what happens when you exceed the ultimate tensil strength of the material. Stronger alloys do less yielding before they fail but they carry higher loads than mild steel. The question here is no more complicated than do you want a part that fails at a lower load or a part that fails at a higher load? If I were building a roll cage for a car I would rather use a material that would carry the load and support the structure rather than use a material that will bend and collapse around me at a much lower load.

Best,
Dave

 

[John Bitting]

Because I do not know too much about material strengths, What causes the washer to be untouched and the material to rip around it??

 

[Dave_G]

Re: What causes the washer to be untouched and the material to rip around it??

Klaus,
Good quality washers like this one are usually heat treated and are stronger than the 4130 material. Also the thickness of the material around the bolt was increased by the washers thickness. That's why the material failed around the washer and not under it.

Dave

 

[TimHayosh]

Dave said: "Stronger alloys do less yielding before they fail but they carry higher loads than mild steel.

Exactly my point. As the steel is "yielding" is it not absorbing (using up) kinetic energy? In other words. I guess I would rather have a bent, deformed roll cage that absorbed impact than a very stiff one that when it broke, it sheared/shattered leaving many sharp and pointy bits on which I could be impaled. Am I looking at this wrong?

Again, not trying to re-ignite the chromo vs. steel thing. Just seeking informed opinions.

 

[Dave_G]

Re: Am I looking at this wrong?

Not totally Tim. Your theory about absorbing the energy is true in circumstances such as the technology in new cars. They make them fold up real easy so your body doesn't see the full impact loads. Aircraft seats are designed to collapse in a bottoming type crash to prevent spinal compression injuries. But let's get back to the issue of ultimate tensil and ultimate yield.

Let's just say for conversation purposes we pull some numbers out of the air ( I got no data books here at home...) that 4130 tubing has a yield strength of 75,000 ksi and an ultimate tensil strength of 130,000ksi. Let's pretend that 1026 DOM mild steel tubing has a yield strength of 50,000 ksi and an ultimate tensil strength of 90,000 ksi.

You crash and roll your car and generate 65,000ksi load on the material. If it's made from 4130, you don't even notice any damage. If it's made from 1026 that sucker is gonna be bent. Now you crash your car and generate 110,000 ksi load on the material. If it's 4130 you've exceded the yield strength and it's now bent. If it's 1026 you have exceded the ultimate tensil strength and it is no longer just bent, It's broken! Using 1026 tubing will shear just like you saw in the photos I posted except at much lower loads than 4130.

The problem with racing organizations frowning on 4130 in the cockpit has more to do with how it is welded than anything else. If you don't stress relieve the weld areas on 4130 it becomes hard and the fatigue life of the welded joint is severely limited. This can be eliminated by heating the welded area to a dull orange glow and cool slowly. Most fabricators don't bother with this since it is time consuming. If you look closely at the pictures I posted you will notice that the weld did not fail. Just the material outside the weld. That's because the welded area is harder than the rest of the structure because it was not stress relieved after weld.

Best,
Dave

 

[TimHayosh]

I see!
(Ref paragraph 3 of the previous post..)
So, the material will deflect if the force is between the yield and the ultimate tensile. If the force exceeds the tensile, it will break.

Is the trick then to find a material that has the highest margin between yield and tensile yet still have very high ratings in each category? This would seem to be the material that would stand up to the highest loads, yet absorb the most energy before breaking.

I will assume that the work has been done, and 4130 is the material that fits all of the above and is most cost effective and available.

With all of the above stipulated, I will edge ever closer to the waters of "The Great Alloy Controversy".

How do each of these materials hold up to long-term, repetitive high and low frequency vibration? My only interest is in use of the material in an off-road racing car. Is weight a factor? Suspension damping? Torque? I am not looking for a "which is better" argument, only what are the characteristics of each under the conditions we racers subject them to in a long-term scenario.

 

[Dave_G]

Re: So, the material will deflect if the force is between the yield and the ultimate tensile. If the force exceeds the tensile, it will break.

BINGO!!!! And once the material has exceded the ultimate yield it usually takes a permanent set.

Re: Is the trick then to find a material that has the highest margin between yield and tensile yet still have very high ratings in each category?

BINGO AGAIN!!!!

Re: I will assume that the work has been done, and 4130 is the material that fits all of the above and is most cost effective and available.

Yep. Rest assured all the aerospace companies figured it out over 60 years ago. No reason to go through the trouble to reinvent the wheel.

Re: My only interest is in use of the material in an off-road racing car.

It should work great. The biggest factor that keeps people from using 4130 is it's cost. It's more expensive than 1026 DOM.

Re: Is weight a factor?

No difference to speak of except that you can use a thinner wall 4130 tube to do the same job as a thicker walled 1026 tube. That's when you got to get busy doing you engineering home work. ;-)

Best,
Dave

 

[Tyson]

I had a question and I figured that this thread would have a good answer. I was curious what type of metal and/or thickness MDR, Whiplash, and SCORE require for roll cages and bumpers? I hope to be able to race in MDR this season and possibly SCORE in the future, and I want to know what type of metal I should use in my Toyota, trying to future think this so I don't have to set it up, then cut it off and start over in the future. Any thoughts.

Thanks

You may beat me in the flats, but watch out in the whoops.

 

[Kritter]

I am sure somebody will correct me if I am wrong but 1.75" .120 wall mild or 1.5" .095 4130 Ithink are the rules.

Kris

"Revenge is best served cold"

 

[EQuin]

Hi Dave,

Thanks for sharing all this excellent info. Would thicker walled 1026 tube (.125) be of the same strength (both yield and tensile) as thinner walled 4130 (.090)? Also, does overall diameter of the tubing have an affect on strength (yield and tensile)?

Take care,


Ed Q.

 

[jeff]

The material requirements are different for the different weights and body styles. If you plan on racing you need to purchase that sanctioning bodies rule book, something that they generally include when you become a member. If you want to run SCORE, BITD, MDR, I'm pretty sure the minimum material allowed in cage construction for your Toyota is 2" .120

Aloha

 

[mike_hinson]

The SCORE Rule Book has minimum wall thickness and diameter requirements in the rollcage section of the CR Rules by vehicle weight.

 

[Tyson]

Jeff: You said the sanctioning bodies usually include the rule book when you become a member. How do you become a member?

You may beat me in the flats, but watch out in the whoops.

 

[Dave_G]

Tim,
Here is some engineering data from one of my material suppliers....

1026 DOM Tubing
80ksi tensil
70ksi yield
10% elongation

4130 CDS/ Normalized
90ksi tensil
70ksi yield
20% elongation

Elongation is what you are refering to in how much the material will bend or deform before it fails. As you can see the percentage of elongation is higher in 4130 than 1026.

 

[JasonHutter]

Yota 602, to become a member of SCORE, all you have to do is get on their website, they either have a form there that you can print out and send in with some $$$. Or get their phone number off of their site and give them a call. They usually ask you what you will be racing, 4wheel vehicles, or 2wheel vehicles. (4 is $75, and 2 is $50 I think)

Jason

 

[TimHayosh]

Thanks for the material data Dave. I'm surprised that the difference wasn't more dramatic. Also surprised at the difference in the elongation; favorable to 4130!

Again, thanks.