Big Red here with a question from "Dewey Oxburger"...
So, TAKE FIVE!
"Big Red, sometimes on fixed loops I see a little crack on the edge of the helmet right between the loops, do you know what causes them?
Great to hear from you Ox,
Yes, residual stress triggered by a notch effect.
As knowledge was gained about what caused the M-1 helmet to age crack, it was discovered that all post production age cracks were caused by the residual stress left in the helmet from the severe cold working it underwent during the three shaping operations, initial draw, trimming and visor spanking.
It was determined that the stress left behind in the helmet ranged between 80,000 and 90,000 psi which in and of itself almost exceeded the stress limits of the steel. A helmet cracked when any given area under high stress was triggered by either a defect in the steel or a manufacturing defect left behind by the production process.
Most age cracks were edge cracks as they formed at the edge of the helmet and worked their way into the helmet body. It was determined that highly stressed areas, like the visor, could be triggered by a notch left on the edge by a nick in the trimming die. Age cracks could present as a body crack as well if conditions were right. Body cracks formed within the body of the helmet and did not extend to the edge of the helmet. These cracks formed when highly stress areas in the body of the helmet were triggered by small steel defects like a decarburized stringer or a grain boundary issue. Because the engineers working on the cracking problem understood the list of steel issues that could trigger a body crack, they did not call them out individually choosing rather to refer to the cause generically as a “notch effect”.
When production began on the M-1 helmet, experimentation and innovation were occurring simultaneously which meant nobody understood why they were experiencing breakage or why age cracks were developing. After Abraham Hurlich and his team of Metallurgists at the Watertown Arsenal Laboratories investigated the issue, they determined the root cause was residual stress in the edge of the helmet triggered by a notch effect created by poor welding technique.
What they discovered was the physical size of the spot weld being used was too small and that the duration of the heating cycle was too short.
Basically, spot welding the two feet of the chin strap loop was just enough to relax the stress in a small radius around the weld nugget. This formed an extremely high stress concentration in the steel between the welds where the intense heat of the weld followed by the rapid cooling of the short heat cycle created brittle martensite or simply, the welding process created a notch effect right between the loops. Over time the continued decomposition of austenite into martensite would weaken the steel enough that the concentrated stress between the feet of the loop would cause the helmet to crack.
The solution was to use a larger size spot welding electrode along with a longer heating cycle. This technique allowed the entire area between the loops to be heated while the cycle time permitted a slower cooling period, which allowed the carbon to settle back into austenite thereby preventing the formation of an age crack.
Big Red Says!
FIVE'S OVER - MOVE OUT!