Mil Spec FoS

[cory]

Does anyone know or have the mil spec document for the Factor of Safety for designing a rifle barrel?

I'm trying to determine how thin we can safely go with this group buy barrel.

[LRRPF52]

Steel alloy type will be a huge variable with it. Soft 416R Stainless vs. CMV barrel steel are quite different when you look at the Data Sheets for them.

When I looked at the original chambering of the AR15, .222 Remington at 50,000psi Maximum Average Pressure, and thick wall hoop stress calculators, some very clean, rounded numbers popped out with a very large FoS far wider than 5.56 NATO. It's one of the main reasons why there is such a stringent alloy callout for M16 and M4 barrels.

[cory]

Thanks man, but you're no help. HAHA

The alloy is not a variable here. Putting a factor of safety onto the SAAMI Max Pressure should take care of that. Calculate the hoop stress according to that Pressure x your FoS and it should fall below the Alloy's Yield Strength in a safe design.

I've talked with Dan and I have the information I need for my calculations. (I don't know if it's proprietary for Lilja or not, but I won't share it. If Adam wants to that's fine.)

Right now I'm running my calculations with an FoS of 2. I feel like that's a little high, but 1.5 would be to low.

[Variable]

IIRC:

An original M16A1 barrel is .600" under the handguard, .625" at the gas block, and .575" in front of the block.

That being said, let me guess before you post up any answers.

Adding extra meat for the Grendel, it could probably be plenty safe at .650" under the hanguard, still .625" at the gas block, and .600" in front of the gas block. People would freak out, but it'd likely still be safe even thinner than that.

How close am I?

ETA: I'd also bet that in a 12.5" barrel w/carbine gas length it'd still be stiff enough for general purpose use.

[cory]

We could go 0.6" under the hand guard and in front of the gas block, but fluting would be out of the question!

FWIW I suspect that the barrel would be much more susceptible to barrel harmonics.

[SG4247]

I personally would not be comfortable with FoS less than 3.0 if I am holding the thing in my hand.

The first thing I thought of reading the post was - pressure vessel design - which has direct similarities to gun barrels.

Not knowing what the mil spec was, or some rule of thumb design factors from a very trusted source, I would consider reviewing some of the industry standards for FoS which you can find here:

http://www.engineeringtoolbox.com/factor..._1624.html

Hope that helps!

[cory]

I'd agree if it was a thin walled pressure vessel. The pressure vessel FoS in your link is in line with thin walled pressure vessels.

A barrel is a thick walled pressure vessel with open ends. R=Ri Po=0

Unless you only shoot bull barrels I can guarantee you, that you've shot plenty of barrels well below an FoS of 3.

[Variable]

I finally found one of my calipers, and here's one of the 10.5" Grendel barrels that Sabre was making for Alexander:



I have it a bit cocked trying to get a pic with the other hand, but it measures right at .730 under the handguard. I don't know how much (if any) parkerizing affects the measurement.

[Keep The Change]

We could go 0.6" under the hand guard and in front of the gas block, but fluting would be out of the question!

FWIW I suspect that the barrel would be much more susceptible to barrel harmonics.

What sort of FoS are you getting at that point?
What pressures are you using?

I like a FoS = 2. That's just the conservative engineering side of me talking. Engineers like it safe.

[Keep The Change]

I personally would not be comfortable with FoS less than 3.0 if I am holding the thing in my hand.

The first thing I thought of reading the post was - pressure vessel design - which has direct similarities to gun barrels.

Not knowing what the mil spec was, or some rule of thumb design factors from a very trusted source, I would consider reviewing some of the industry standards for FoS which you can find here:

http://www.engineeringtoolbox.com/factor..._1624.html

Hope that helps!

I know what you mean SG. If that FoS is against the yield stress. That means if you somehow double the pressure in the chamber, your barrel will only yield. It won't fail, just yield, probably locally. Well if the pressure is doubled, what is going to happen with that BCG as it is coming back. I think the barrel would be the least of your concerns.

If you like FoS greater than 3 when your body or life depends on it, I won't tell you the sort of FoS or Margins of Safety we go down to in the aircraft industry. You might not fly again.
LOL!!

[cory]

What sort of FoS are you getting at that point?
What pressures are you using?

I like a FoS = 2. That's just the conservative engineering side of me talking. Engineers like it safe.

I started my analysis off using an FoS of 2 at a max pressure of SAAMI Spec 52ksi, 55ksi, & 60ksi, for comparison purposes.

[Keep The Change]

So your FoS is against the properties you acquired for the Lilja barrel?
Is it written against the yield stress?

You also have other affects that will interact with your final stress as well. With barrel flex, there will be a compression on 1 side and additional tension on the opposite side where the barrel is bending.

So on the tension side of the barrel when it is flexing, there is a hoop tension and a tension acting front to back where the barrel is flexing. Now I suspect those tension stresses due to bending to be fairly small but looking at some super slow motion footage of guns being fired, I don't think they would be insignificant.

[cory]

So your FoS is against the properties you acquired for the Lilja barrel?
Is it written against the yield stress?

You also have other affects that will interact with your final stress as well. With barrel flex, there will be a compression on 1 side and additional tension on the opposite side where the barrel is bending.

So on the tension side of the barrel when it is flexing, there is a hoop tension and a tension acting front to back where the barrel is flexing. Now I suspect those tension stresses due to bending to be fairly small but looking at some super slow motion footage of guns being fired, I don't think they would be insignificant.

Yes and Yes.

Originally I ran both radial stress and hoop stress. Radial Stress was significantly lower than the hoop stress, so I've since disregarded it.

[Keep The Change]

I wish we had some deflection data so we could determine any bending stress in the barrel as it whips. Of course the worse bending would likely occur close to the chamber where your barrel is the thickest.

So if you are primarily looking at GP area and such, the bending stresses should be pretty low. But your FoS being to yield, I would have no worries with a 2.

Typically what we do in aircraft structure is take a limit load, in your case it would be your 50 ksi pressures. Then multiply that times 1.5 to make that load 50% higher. Then right a margin of safety against the Ftu of the material if it is a tension load.

So yes the margins of safety might get really low in airplane structure, but then again we have some padding in that number because we increased the load by 50% of the limit load or design load.

So that would make SG4247 a little more comfortable on an airplane.

[SG4247]

I wish we had some deflection data so we could determine any bending stress in the barrel as it whips. Of course the worse bending would likely occur close to the chamber where your barrel is the thickest.

So if you are primarily looking at GP area and such, the bending stresses should be pretty low. But your FoS being to yield, I would have no worries with a 2.

Typically what we do in aircraft structure is take a limit load, in your case it would be your 50 ksi pressures. Then multiply that times 1.5 to make that load 50% higher. Then right a margin of safety against the Ftu of the material if it is a tension load.

So yes the margins of safety might get really low in airplane structure, but then again we have some padding in that number because we increased the load by 50% of the limit load or design load.

So that would make SG4247 a little more comfortable on an airplane.

Spent a million hours on airplanes, restored a couple, witnessed three serious crashes, and still don't really like flying!

I will not ride on fatigue testing machine either, ie helicopter!

(Unless my life depended on it)

[SG4247]

I'd agree if it was a thin walled pressure vessel. The pressure vessel FoS in your link is in line with thin walled pressure vessels.

A barrel is a thick walled pressure vessel with open ends. R=Ri Po=0

Unless you only shoot bull barrels I can guarantee you, that you've shot plenty of barrels well below an FoS of 3.

Is just a reference, notice thin wall vessels were 3.5-6!

So, let's see some reverse calculations. Maybe the numbers would tell us what kind of FoS a high pressure cartridge is likely designed to, working backwards from known dimensions, materials and pressures?

[Keep The Change]

Sounds like we need to chat.
What planes did you restore?
I love aviation even though I'm not a pilot and probably never will be.

I'm with you on the helicopter. I'm not getting on one intentionally. That is a very good and appropriate name for it also, "fatigue testing machine".

[tackdriver]

Sorry this is off subject Cory.

Isn't the "fatigue" factor why such machines (helicopters) have to go through phase or "check up" every 500hrs when in service ?

[Keep The Change]

I'll assume your 500 hr check as gospel, because I don't really know the inspection intervals. Also there could be different phase of checks during a longer period of time. For airplanes, there are checks that are more frequent in some areas because they maybe high stressed and high fatigue locations based on the airframe analysis.

Fatigue is critical in airplanes as well. It is the cause of just about all structural failures in the industry. However the harmonics and vibration of choppers makes them susceptible to fatigue and they are basically built a lot more "rinky dink" than airplane.

Fatigue can be very hard to predict because you need to have accurate load data or load cycles for the aircraft, accurate stress analysis, and accurate calculation of stress intensity factors at fillets and joints. These 3 groups of data need to be accurate in order to accurately predict fatigue. If there is doubt in the accuracy of any of these groups, conservative assumptions are made.

Then you test and see how good the analysis was.

The F-35 fatigue test article cracked a bulkhead in half the time it was supposed too. It was the bulkhead that carried the most loads between the wings also.
A lot of times these tests open up areas of concern that may not have been seen during the analysis phase. Sometimes the analysis shows to be conservative in other areas. It's all why testing is necessary.

Good job in instigating me to hi-jack the thread, TD.
Sorry Cory

[tackdriver]

I am not an engineer so this is just an interesting read for me. I do fond it intresting Lilja has the consumer crunching #'s. I would think that liability would lie with them and they wouldn't produce anything less than what their #'s concluded was safe. I apologize about the aircraft talk!