doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Strain gauges

I love science. Why? Because it has always answered Yes/No, black and white questions for me. I struggle with grey. I have always struggled with it. 50+ years and counting.
If we get gray, we revise the tests until we get B&W, and then a confirming.

No one should ever be saddened or upset about the discussions raised here. The science is the science, and we are all better for it.

Pressure vessel testing relies on known behavior of the vessel. Not on vessels unearthed from excavation sites.

All I meant to point out was that you can't say that because a pressure test vessel of a known construction says a cartridge creates pressures of X, you can safely fire said cartridge in some other unknown vessel, and expect the same results.

But I dare say that you can take it to the bank, that within the known margin of error of the tools being used above, that the people performing the tests are performing quality, reproducible, challengeable, defensible science.

And I am electronically applauding.


I have a particular interest because, I have 2, 32" Ithaca Flues shotguns that the chambers are 50% different in wall thickness. Well, lets just say visibly, substantially, thicker. It would be nice to have an inkling short of detonation, of what the more robust of the two could digest in terms of modern ammunition.
It was built for fowling, and it'd be nice if it could be fed modern steel shotshells.
And you can't get that answer from the tests above.

I have seen pictures of pressure guns.The shells I tested were factory made.I do not reload.I was checking their data for fun.The Pressure Trace and the laptop cost about $1500 and the gun another $150.Just another crazy project.As far as liability, the only data mentioned was obtained from the factory stuff.As far as calibration,the gauge comes with a factor printed on the package.

William, does your software calculate std dev?
It might be a nice exercise to measure half a box of store bought, and compare your findings against those of the maker.
I'll bet you are extremely close.

If you already have, that would be nice to see.

I think I missed addressing Bill's request to explain how strain gages are used in industry. So, I'll make a feeble attempt with my limited exposure to them.

I first saw a "strain gage" back around 30+ yrs ago. The type I dealt with was a 4-5 inch diameter steel cylinder with about a 1/4 inch wall and the wall was cut out, leaving little thin webs that looked like a maze, holding it all together. This was essentially a spring. Force would move the thing, either in compression, tension, or torsion. It was calibrated in a lab by applying known forces and measuring its deflection. So, in order for a strain gage of this type to be useful, you had to measure the deflection with some other device. At that time linear electronic transducers were in use. You gathered movement data, then calculated the force and came up with the stress on your part. The problem was that these things were clunky and you couldn't just stick them on something and generally, they had to be sandwiched in with the entire load path going thru them. These new little "stick-on" strain gages, essentially have no effect on the structure strength and can be put about anywhere. The load path still goes thru the real structure, but you measure the structure movement and calculate stress by knowing what your structure is made out of and the cross-section the load is going thru.

The new type of strain gage is a very convenient and inexpensive way to take a strain measurement almost anywhere.

Although structural analysis methods are becoming super sophisticated with CAD tied to computerized analytical tools like 'finite element analysis' and NASTRAN, CATIA, etc., some structures are so complex that these methods can't provide high confidence of loads(stress) in specific locations. So, testing is required to find out these unknown loads. Strain gages are placed on locations where actual stress loads are desired.

I've seen them used by the hundreds on things like the fullscale wing test on the 777 (I saw the video where they broke the wing on purpose). Engineers seeking bridge stress loads may apply these to specific areas of an old bridge to gain knowledge on an old bridge that was designed before high confidence analytical methods were developed. My wife has them applied to space vehicle components they vibration test to understand the loads in the attaching structure. They are relatively cheap and very accurate.

Clap,
Yes, it calculates SD.

So, in response Jerry's inference that this type of data is questionable or not as directly applicable to our needs, I will say; I believe it is as applicable as pressure data from a sources with a pressure gun having direct piezio transducers.

Why do I believe that? Simple, a pressure gun with a piezio tranducer like Mr. Armbruster's will give pressure. It won't tell you stress. Stress has to be calculated from the pressure and the dimensional characteristics. Strain gages measure stretch of the barrel, then you (or the computer) calculate the stress. Stress is what fails barrels. Strain gages are just one step closer to actual stress levels. In the case of the Pressure Trace that Bill and I have, we use it to calculate pressure. Then we armchair postulate as to whether or not we want to fire that load in our guns. Both methods of measurement are perfectly acceptable when all parameters are known. The trouble with both methods, for our application, is: we don't typically have all the parameters to use either data with high confidence. But both have value if used conservatively.

Simple question from a non-technical type: If the reading provided by the gauge depends on the dimensions of the metal, then how could a single strain gauge provide an accurate reading anywhere other than where it's placed? Assuming it's reading pressure in the chamber, there would not be any way to tell the pressure farther down the barrel, where the dimensions of the metal change significantly . . . would there? As I understand, that's why Bell used several strain gauges at various positions on the barrel. His tests were made in an effort to determine whether the peak pressure really does vary significantly between black and smokeless, and between fast-burning and slow-burning smokeless powders. From his results--and I think also from the earlier Dupont tests--it does not appear that there is much variation at all in the location of the peak pressure (always in the chamber), although there is some in how quickly the pressure drops.

It will take me a bit to catch up on this thread but if I remember correctly the strain gauges have to mate to the material which you are wanting to measure the deflection and the expansion is limited to say between 8% and 10% of the material. They resemble like a compressed spring and as the spring like architecture, actually parallel in theory, is expanded it changes the resistance seeing the geometry of the metal has changed and the electron lattice has been modified. When a voltage is applied to a conductor the electrons tend to wad up, with respect to their steady state position, toward one end.

Kind Regards,

Raimey
rse

Larry,
Yes, strain is measured only at the specific point the gage is mounted. For more data points, you would need to mount more strain gages...or as shown in that old drawing, more lead crusher coupons. P.S. but on a cylinder, one point will give an indication of stress distributed around that "hoop".

Whats interesting is the veiw by most here that pressure is what they want to know. Me, I want to know stress levels. I would rather to refer to steel data of various types and use known data along with the strain gage data and dimensional data from my guns to assess whether I wanted to shoot a particular loading in a particular gun. But it's certainly easier to just take general information on pressure limits of typical cartridges of a particular era and apply it to a gun that is of that era and say I'm close enough. But to be critical of a straingage measurement method, in light of the forgoing, is picking thru the pepper and ignoring much larger variables, IMO.

So, it strikes me as odd that members here will be concerned about pressure one time and dimensional issues another time, yet never put the two issues together and say "I really want to know the stress levels, because a change in either of those parameters affects stress levels and exceeding appropriate stress levels is what fails barrels (either in fatigue/cycles or overload).

Given enough effort, you can get that answer with a Pressure Trace strain gage placed at the areas in question, firing a cartridge of known pressure, knowing the dimensions of the area in question, and knowing or having a good idea of the metal properties.