Burst Test of the new Light Weight M-Rod air tube from JSA

[lloyd-ss]

It took some arm twisting to get me to come out of hiding and do some pressure testing of the new Light Weight air tube for the Benjamin Marauder rifle. It’s being made by Jefferson State Air Rifles in Oregon. It was my first time talking to Jacob Ensign of JSA, but Travis Whitney was involved in the mix, and he and I go back a few years.
JSA was getting ready to produce an aircraft aluminum alloy (2024T3) air tube for the Benjamin Marauder. Most people love their Marauders (and if you don’t, start your own thread, LOL) but one of the main complaints about the gun is the weight. What if you could knock a pound and a half off your M-Rod without sacrificing any shot count or pressure capability. A drop-in replacement? Well, actually, you should also switch from the steel hammer to an MDS hammer, but no big deal.
But is the light weight tube strong enough for a standard 3,000 psi fill? All the calculations showed that it was strong enough, with the proper safety factors, but nothing like some real proof.
This video and test are for informational purposes only. No guarantee or warranty but the tests were performed as shown, and you can trust me on that.

So what is being tested?
The tube is designed with a 3,000 psi maximum fill pressure, just like the stock M-Rod tube, so first, safety factors are calculated for several possible modes of failure.  Everybody thinks of a tube “blowing up”, but that is not the usual mode of failure. Also, safety factors are calculated at Yield, which means when the metal is just starting to permanently deform, and at Ultimate Tensile Strength (UTS), meaning when the metal actually breaks.
So let’s list the ways it might fail:
1.    O-ring might leak or blow out. That is a good way to fail because there usually aren’t any flying metal parts.
2.   Valve poppet might blow out.
3.   The tube might separate (the end might pop off) at its thinnest section, usually where an end plug is threaded in.  For this tube, the safety factor (SF) at Yield is 4.0 to 1, and at UTS is 5.6 to 1. So the end of the tube isn’t going to blow off.
4.   The threads in the tube or the plug might strip out (shear off). For this tube, the SF for the tube threads is 6.9 to 1 and for the plug threads, is 4.5 to 1.
5.   The tube might stretch (yield) or burst. The SF for Yield (stretch) is 2.7 to 1, and for  UTS (burst) is 3.8 to 1. That means that the tube is going to stretch quite a bit before it breaks.
6.   The bolts securing the valve might shear off. For this tube, the bolts are over 3 to 1.
7.   The heads of the valve screws might crush the bearing surface of the holes in the tube.  This tube has a SF of 2.7 to 1 for the bearing strength. This usually causes localized distortion, but then stops, unless severely overloaded.
8.   The holes in the tube might rip out from the stress of the valve screw heads. The M-Rod tube is an odd case, because the 2 side screws have lots of material behind them, whereas the bottom screw at the bottom only has about .19” of material between it and the trigger sear slot.  This SF is about 3 to 1.
Looking at these 8 factors listed above, the tube bursting doesn’t seem very likely. But maybe a problem with the valve attachment? O-Ring problem?, well, that has to do with the design, not the material.
Actually, in the first test, the o-ring on the valve body did blow out at 9,000 psi. That was the path of least resistance. That is a good thing, because nothing was damaged, and nobody got hurt. After that o-ring problem, I took the tube and valve apart and re-machined the -ring groove to a 90 duro o-ring with a back-up ring.  This is often done for very high pressure applications, but it makes assembly fairly difficult. But that stopped the leak and it was time to find the next path of least resistance. This time, it was the holes in the tube for the valve screws. At 10,000 psi, the valve body had 8,800 pounds of force pushing on it, and the 3 screw heads just plowed right through the tube wall for about 3/4”. Movement stopped when additional screw holes were exposed and let the pressure drop.
The important thing here is that it took 10,000 psi to cause a serious failure (that is 3.3 times the rated pressure), but still, the tube did not explode.
I consider this a very successful test, and shows that the tube is worthy of its 3,000 psi pressure rating.

[Privateer]

OK.
You made my head hurt!
 

Great work Buddy!
(So, have you shot that air powered bell with the Big Gun yet?)
 

[lloyd-ss]

Hey Jeff,
Man, if I blew a big hole in that gas station bell, the grand kids would come after me!  They love stompin' on that hose and making lots of ding, ding, dings.

[Privateer]

 
I'm so happy you all are enjoying that thing!
I just brought it to Kentucky to shoot it!
 

[jarmstrong]

If you would have used air instead of oil for failure test............there would have been bells ringing somewhere

Nice video

[lloyd-ss]

If you would have used air instead of oil for failure test............there would have been bells ringing somewhere

Nice video

Right you are! At 10k, the pressure wasn't increasing much because the tube was expanding.  It didn't exactly look like a python after a big lunch, but the growth was visible to the naked eye. And the growth was nicely uniform, not puffed out in one weak spot.  Lloyd

[Privateer]

He has enuff Ding dings!!

[jarmstrong]

wow
bet you get more comments on the second video, than the first one

[ezman604]

Many thanks Lloyd!!! Good to see the video and test results but GREAT to see you back!!!

[lloyd-ss]

Thanks Dez. Its good to be back!

[Motorhead]

Lloyd,
Thank you for the test & data ... always good to know  ( or at least figure where ) a pending failure may happen.
being few have miens above 4500 psi it is good to know where the limits are.

[oldpro]

Many thanks Lloyd!!! Good to see the video and test results but GREAT to see you back!!!

Amen. Definitely better here with your presence and of course skills.

[makoda]

so after buying one of these I got wondering how strong they are and was glad to see how it does.  but how does it compare with the factory tubes?

[subscriber]

Lloyd,

What about metal fatigue?  Aluminum is notoriously poor in fatigue resistance when cyclically loaded well below yield stress, compared to steel: 

https://www.asminternational.org/documents/10192/1849770/05224G_Chapter14.pdf

A comparison
of the S-N curves for steel and aluminum
is shown in Fig. 14.4. Note that steel not only has
a higher fatigue strength than aluminum, but it
also has an endurance limit. Below a certain
stress level, the steel alloy will never fail due
to cyclic loading alone. On the other hand,
aluminum does not have a true endurance limit.
It will always fail if tested to a sufficient number
of cycles.

[lloyd-ss]

Lloyd,
What about metal fatigue?  Aluminum is notoriously poor in fatigue resistance when cyclically loaded well below yield stress, compared to steel:

The 2024T3 alloy is one of the "aircraft alloys" and one of the properties it was developed for was its excellent fatigue resistance, along with 7075.
The fatigue question keeps popping up whenever aluminum tubes are mentioned, but that argument is misapplied in this case, for at least four reasons.

First, is the selection of the correct alloy, and 2024T3 is a fatigue resistant alloy.
Second is the low loading of the tube, with UTS being more than 3 times greater than the applied stress.
Third is the low cyclic rate of loading and unloading, specifically hundreds of cycles verses tens of thousands, or greater, cycles for the fatigue testing.
And fourth, although anecdotal, is the somewhat common use of aluminum pressure tubes in airguns, particularly more expensive airguns, without incident.

So again, I do not believe that fatigue in a properly designed, and used, 2024T3 air tube is a concern.
Lloyd

[lloyd-ss]

Lloyd,
Thank you for the test & data ... always good to know  ( or at least figure where ) a pending failure may happen.
being few have miens above 4500 psi it is good to know where the limits are.

Thanks Scott.  Sometimes we just have to trust the physics and our calculations, but it is always nice to have a real-world test to prove that our calculations are correct. I've tested steel and aluminum tubes, and every time, the tubes were as good, or better, than predicted by the material specifications.

[FuzzyGrub]

When I saw the 1st video, I was wondering if there was any distortion in the tube near the three valve screws.  I appreciate JSA having the tests done and for Lloyd performing them.  Lloyd has been GTA's defacto expert on this subject.   Thx again, and ordered a Gen 2 tube w/gauge hole this AM.   

[CraigH]

Lloyd,

Thank you for these tube tests.   I watched the first and only today viewed the second.   I have often thought about "work-hardening" of tubes when being exhausted of all pressure and re-pressurizing.   Appreciate the comments on fatigue and parameters that effect tube fatigue.

[lloyd-ss]

One thing about material testing is that you are only testing a single sample of the material, so the results you get aren't really exact, but you hope they fall into an "average" range. In a way, its a little like a one-shot group.   Even when looking up the actual material specifications, you can find variations. There can be variations in the spec for material shape and size, and also what is listed as the minimum and the typical, so there is some judgement involved.  Also, knowing what the material will be used for has some bearing on how the specifications should be applied.

But as I always say, the purpose of these destructive tests is NOT to find out at what pressure the part fails,  but rather to prove that your calculations were correct. And again, the calculations were correct.

An interesting thing  about the 2024T3 aluminum alloy is its elongation, or how much it will stretch before it breaks. For this alloy it is about 10% stretch, or more.

During this testing, I measured the diameter of the tube several times as I pressurized and depressurized the tube. The 5 minute video is actually edited from about 4 hours of testing and video.  The tube started at about 1.245" dia.  I say "about", because the diameter varied slightly along the length of the tube and it was not exactly straight or round, but it was well within the manufacturing specifications. So, starting at 1.245" dia, at 9,000 psi, the tube measured about 1.249". after releasing the pressure, the diameter was about 1.247". That is the text book definition of "yield." The material has stretched, and has not returned back to its original diameter. And that isn't necessarily a bad thing.  During processing, metals are rolled and stretched and stress relived, etc, and it is a very exact science, specific to a particular material and shape. Sometimes, one of the final processing steps is stretching the material, usually to improve the strength. This is usually indicated by the temper designation of the material, the "T" at the end of the alloy number.

But back to the diameter measurements.
Started at 1.245" dia.
At 9,000 psi dia was about 1.249".
Dropped pressure to zero, and dia measured 1.247".
Raised pressure to 10,000 psi and dia was about 1.300".
Dropped pressure back to zero, and dia returned to about 1.280-1.290"

I find that interesting , but I know that some folks might not really care , and that is fine, too.

[makoda]

are the steel tubes able to go up to even higher pressures even?  I have wondered how much of a safety factor guns like the huben have where they are filling to 4500 or are we pushing the metals more than should be done.