Vips wrote:Can gurus here make any sense of this:
The shells for 155mm calibre howitzers made by the ordnance factory are designed to be fired with up to six charge modules and handle pressure up to 340 megapascals. To make the shell fly over 40km, seven charge modules are used. The OFB shells are made under transfer of technology (TOT) from South African company Dennel. Even in the TOT, it has been specified that the gun can handle service pressure of 340 megapascals, or six charge modules at the most.
This is called zone 6 ammunition, and what has been used in ATAGS was zone 7. Higher pressure due to zone 7 can lead to risks. Zone 6 is the standard world over, the sources said. OFB only makes zone 6 ammunition, which is in line with NATO standards. Even the 155x52 calibre mounted gun system developed by the OFB uses zone 6, a source said.
An official in one of the private companies confirmed that the shell was fired with zone 7 ammunition to achieve a higher charge, and rejected any claim that they had received any warning. So far, both Bharat Forge and Tata guns have fired over 1,000 rounds using the zone 7 rounds. The ATAGS barrel is designed to handle 440 megapascals of pressure.
“The pressure handling capacity is basically meant for the barrel but even the shell is expected to withstand similar levels,” the source said. A final investigation into the incident will only reveal the truth said the company official.
Thank you so much.
The shell is designed with 340 MPa steel and meant to be fired with Zone 6.
However ATAGS has fired over 2000 rounds at Zone7 safely. So shell breakup in barrel due to Zone 7 is remote probability.
And to be expected as the shell experiences compression loads from the aft of the shell when the charge is fired. And shell is supported by the barrel so it will not collpase in the barrel under buckling.
So we can safely rule out Zone 7 causing shell failure in the barrel.
OFB are being ninnies trying to blame the ATAGS team in case shell breakup is the COI root cause.
THe ATAGS barrel is made to with stand 440 MPA or a margin of 100MPa.
Lt.Gen JP. Singh elsewhere said the barrel can handle 560 MPa.
It must be noted that the guns which have till now fired almost 2000 rounds between them can easily withstand pressures up to 560 mega pascals and are the only ones to fire munitions with Zone 7.
And that the Tata gun failed at 100th round which was 5th in a rapid fire sequence versus
Whereas the BFL gun already survived 130+ rounds.
So what can we make out of this?
1) The ATAGS barrel is made with ultra high strength to handle Zone 7 charges
2) The failed gun was subject to a rapid firing sequence with Zone 7 charges. Zone 7 charge puts out quite an amount high heat needs to be understood.
3) Yet the gun barrel sheared off as the general notes. And this is at a low number of firings i.e. 100. Now he did not mention what was the expected life of the barrel. The figure 2000 rounds is nice but does not mention how many guns involved?
So whats my theory now?
I think the gun did not have time to cool down to transfer the heat from six (6th) Zone 7 charges and became soft i.e the ultimate strength at temparature was lowered due to the rapid firing of Zone 7 charges.
We dont know the steel specification but using the 560 MPa we can find typical steels.
And estimating the temperature of the gun from 5 Zone charges being fired can approximate the failure mechanism.
Thats my next investigation.
In Vietnam the 175mm cannon used to be fired in rapid mode and led to barrel failures at around 450 rounds when the proven barrel life was 1200 rounds.
And 175mm cannon was made of ultra high strength steel.
And what was found was the rapid firings heated up the barrel and created micro cracks which grew in the UHS steel and led to barrel failure.
From a Watervliet Arsennal Page :
https://www.army.mil/article/81038/wate ... t_happened
Higher Strength Steel. In general, the higher the strength of the gun steel used in a particular cannon design, the lighter the cannon can be. Also, higher strength material allows the cannon to be fired at greater pressures for improved performance and range. As with most things in life, nothing comes for free. As the strength of gun steel is increased, resistance to fatigue failure and cracking under high pressure decreases. Put another way, as strength, pressure, and performance increase, cannon life decreases, sometimes dramatically.
In order to achieve acceptable fatigue resistance, the majority of artillery cannon designs up to the 1970s used gun steel at strength levels of about 160,000 psi yield strength. Some of the "newer" cannon designs in this era, such as the original 175mm M113 Gun on the M107 Self-Propelled System, experimented with what were considered very high strength steels for the time with gun steel strength levels about 180,000 psi yield strength.
Unfortunately, what was to later become the science of fracture mechanics and related fatigue failure was not well understood or appreciated at this time, and the original 175mm M113 Gun experienced premature catastrophic brittle fracture at a very low round count. Investigation of these in-service failures greatly advanced the understanding of fracture mechanics and the relationship between strength and fatigue resistance in gun steels, essentially making Benét Lab scientists and engineers the world recognized experts in fracture mechanics of high strength pressure vessels.
In a very short period of time, the original M113 Gun was significantly redesigned to lower the strength levels of the material, while using the new autofrettage process to make up for the loss of strength. The fatigue life of this replacement design was increased at least ten-fold from the original design, and in service failure was eliminated. Although this system is no longer in service with the U.S. military, it continues to provide safe and effective fire power to NATO allies.
So our task is to find out the material strrength of the ATAGS. The key word is ultra high strength steel. It will be mentioned as an aisde in interview by the designers or in papers.
I have the original 175mm failure analysis report somewhere on my laptop.
