Sure, you can do a backscatter on each bar. Let's add that to the chart.
Sure, you can do a backscatter on each bar. Let's add that to the chart.
HPT is a waste of time and money. It also reduces the life of the bolt. I have stated this analogy before but it is like wrecking a new car into a brick wall and inspecting the frame for cracks. It is an archaic process . The government and industry should do away with it and come up with more productive forms of standardized acceptance /quality control . MPI bolts every 5000 or so rounds would be much more productive. I got news for you guys bolts crack then they break. There is sometimes no way of telling how long a bolt will go after it cracks before it fails. Many of the bolts in your trusted guns out there are cracked.
Thank you for taking the time to give us your insight on this issue.
I think the conversation pretty much ended when you posted, unless Eugene Stoner can somehow get to post from wherever he is now.
As the issue being discussed was how to test new bolts prior to, or during assembly of the rifles, what is your opinion on how this process should go? No HPT, but still initially MPI'd? Or some other form of QA/QC?
Regards.
Mark
In my youth my father was an aircraft homebuilder and restorer. I grew up building, repairing and restoring aircraft. We rebuilt several large aircraft engines in my teen years for the Confederate Air Force and one of my tasks was Magnaflux a.k.a. MPI. I examined thousands of parts and was amazed at the cracks I found on critical parts which had been in active service. Just as Mr. Knight says, many of us are using cracked bolts and don't know it. HPT amplifies defects and cracks. Some will be below the detection limit of MPI before HPT and above the above the limit after HPT. These bolts will be condemned. Many more bolts however will have cracks that have been amplified but still fall below the detection limit of MPI. What good has that accomplished? One shot of HPT has eaten an unknown, unknowable amount of life to what good end? HPT is an ancient and crude procedure that made sense in a world where mechanical engineering, metallurgy and production methods were crude and unreliable. Shooters were killed and maimed by exploding rifles as recently as the early 1900s. Proof testing served a purpose then. Not now. I know of no test outside of HPT where a part subject to fatigue failure is subjected to deliberate overloading. If you know of such a test, please inform me.
Fatigue failure such as afflicts the AR bolt is a stochastic process, what the layman would call "random". Identical parts subject to identical conditions exhibit wide variability in lifetimes. It would be very costly to quantify the life loss due to HPT. If such a study exists I would love to see it.
It would be very interesting and not horribly expensive to run a test where a small number of bolts were subjected to repeated HPT and MPI. Wouldn't it be interesting to know, how many shots of HPT a bolt can absorb without breaking? Five shots? Fifty? Five hundred? My guess is closer to five than fifty.
Last edited by Suwannee Tim; 05-26-11 at 21:03.
Is it really that random though? I understand the regular expected life of a bolt to be pretty consistent, with the obvious outliers, but close enough that I think we could call it less than random.
I would love to see that test as well, and I would be on it being closer to 5 than 50, or even 20. I have been meaning to try an FEA analysis of the bolt when I had free time at work, but just haven't. Perhaps I should put more effort into that.
For those not familiar with fatigue, Wikipedia has here a pretty good introduction to the phenomenon.
In my experience, yes, very random. Out of several dozen exhaust valves for example, all of which had been subject to the exact same fatigue history, some would show serious cracks, some, no cracks at all.
I wonder how effective FEA is in predicting fatigue failure?
For the particular items I have used it for it is very reliable, but we have good bit of material testing data used to set the material properties. I would not be able to use C158, but I could use another steel that I have access to test data for. Run a few different cases, a typical HPT-service life case, service life only, and then HPT until failure.
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