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Indian Bofors gun's barrel bursts during trials
A barrel of the indigenous - version of the Bofors howitzers burst during their trials in the Pokharan range, delaying plans to induct these artillery guns into the Army.
The Defence Acquisition Council had recently approved the procurement of 144 such guns by the Army which are being manufactured by the Ordnance Factory Board, Jabalpur.
During the recent trials of the gun in Pokharan, the barrel of the gun burst when the Army was conducting the user trials for approving the gun for induction in the Army, sources said here.
The OFB has now been asked to carry out a defect identification inquiry (DII) by the Defence Ministry to ascertain the reasons behind the bursting of the barrel during the crucial trials, they said.
The user trials are carried out by the Services for approving the product being tested for induction.
The gun has been performing very well so far but the mishap will delay the user trials for another three to four months, they said.
Defence Minister AK Antony has been taking personal interest in the development of this programme and has asked both the Department of Defence Production and the Army to cooperate fully for making it successful.
The gun design, provided as part of technology transfer, has been gathering dust due to the taint associated with Bofors in the light of the payoffs scam.
If successful, the project would save the country millions of rupees and give the Indian Army much needed firepower.
The Army has failed to procure even a single piece of artillery after the Bofors scandal erupted in 1980s. All attempts to procure howitzers have had to be cancelled due to one reason or another.
Indian Bofors gun's barrel bursts during trials - The New Indian Express
***************************************
Unfortunate..
However, all is not lost!
The barrel is the critical part where the steel has to be 100% pure and without impurities and the autofrettage process perfect.
Autofrettage is a process that creates an internal stress in a component, similar to the shot blasting. Autofrettage is used in most cases for the inner pressure active components without (tubes) and with cross borings (rails).
When a component is pressurized by an autofrettage pressure, which is much higher than the working pressure and is dependent on the material used, hardness, and the geometry of the component, the inner boring will be formed plastically and the deeper areas of the component wall will be formed elastically.
After relieving the autofrettage pressure from the component, the elastically formed areas try to return to their original state while the plastically formed areas prevent this process. Therefore internal stresses are created in a huge part of the wall thickness which tries to work against the stresses that arise from normal working pressure.
Under working pressure the stresses inside the component do not move between 0 and a maximal value any more, but start in the pressure area below 0 and end at a much lower maximal value.
As you can see from Figure 1, the resulting working loads (e. g. in the pipe) can be reduced considerably.
A working pressure which would have led to a failure of the component prior to autofrettage can now be used endlessly – at least for a considerably longer time – without failure. This can lead the design engineer to look for cost saving strategies (i.e. material selection, material reduction etc.).
If you want to use the same positive effects of autofrettage for bent pipes, the autofrettage has to be done after the bending. If autofrettaged after bending, the internal stresses of autofrettage will overlie or neutralize the internal stresses and the effects of autofrettage will be lost.
If you have components with cross cutting there will be an additional effect: On those areas around the drill hole there is a stress concentration. Therefore these are the weak sections inside a component. Cracks in this part are very often the reason for failure.
If you have high dynamic working pressures even the autofrettage cannot prevent a crack in these cross cuttings.
The crack will just grow up to a certain length and will then be stopped by the pressure internal stresses that are inside the wall, picture 3. The internal stresses "press" the crack together.
To receive the requested internal stresses by an autofrettage, the pressure of the autofrettage just has to be held a short time. A longer time does not increase the internal stresses inside the component, but leads to a strong widening of the inner boring.
As the pressures of autofrettage are calculated in that way that a huge part of the wall thickness is plastic, the pressure does not have to be reached with absolute exactness.
Which deviations are acceptable and which not, depends on every single case.
The autofrettage of a component in more than just one case should be avoided.
http://www.maximator-test.com/about/wp-autofrettage.php
A barrel of the indigenous - version of the Bofors howitzers burst during their trials in the Pokharan range, delaying plans to induct these artillery guns into the Army.
The Defence Acquisition Council had recently approved the procurement of 144 such guns by the Army which are being manufactured by the Ordnance Factory Board, Jabalpur.
During the recent trials of the gun in Pokharan, the barrel of the gun burst when the Army was conducting the user trials for approving the gun for induction in the Army, sources said here.
The OFB has now been asked to carry out a defect identification inquiry (DII) by the Defence Ministry to ascertain the reasons behind the bursting of the barrel during the crucial trials, they said.
The user trials are carried out by the Services for approving the product being tested for induction.
The gun has been performing very well so far but the mishap will delay the user trials for another three to four months, they said.
Defence Minister AK Antony has been taking personal interest in the development of this programme and has asked both the Department of Defence Production and the Army to cooperate fully for making it successful.
The gun design, provided as part of technology transfer, has been gathering dust due to the taint associated with Bofors in the light of the payoffs scam.
If successful, the project would save the country millions of rupees and give the Indian Army much needed firepower.
The Army has failed to procure even a single piece of artillery after the Bofors scandal erupted in 1980s. All attempts to procure howitzers have had to be cancelled due to one reason or another.
Indian Bofors gun's barrel bursts during trials - The New Indian Express
***************************************
Unfortunate..
However, all is not lost!
The barrel is the critical part where the steel has to be 100% pure and without impurities and the autofrettage process perfect.
Autofrettage is a process that creates an internal stress in a component, similar to the shot blasting. Autofrettage is used in most cases for the inner pressure active components without (tubes) and with cross borings (rails).
When a component is pressurized by an autofrettage pressure, which is much higher than the working pressure and is dependent on the material used, hardness, and the geometry of the component, the inner boring will be formed plastically and the deeper areas of the component wall will be formed elastically.
After relieving the autofrettage pressure from the component, the elastically formed areas try to return to their original state while the plastically formed areas prevent this process. Therefore internal stresses are created in a huge part of the wall thickness which tries to work against the stresses that arise from normal working pressure.
Under working pressure the stresses inside the component do not move between 0 and a maximal value any more, but start in the pressure area below 0 and end at a much lower maximal value.
As you can see from Figure 1, the resulting working loads (e. g. in the pipe) can be reduced considerably.
A working pressure which would have led to a failure of the component prior to autofrettage can now be used endlessly – at least for a considerably longer time – without failure. This can lead the design engineer to look for cost saving strategies (i.e. material selection, material reduction etc.).
If you want to use the same positive effects of autofrettage for bent pipes, the autofrettage has to be done after the bending. If autofrettaged after bending, the internal stresses of autofrettage will overlie or neutralize the internal stresses and the effects of autofrettage will be lost.
If you have components with cross cutting there will be an additional effect: On those areas around the drill hole there is a stress concentration. Therefore these are the weak sections inside a component. Cracks in this part are very often the reason for failure.
If you have high dynamic working pressures even the autofrettage cannot prevent a crack in these cross cuttings.
The crack will just grow up to a certain length and will then be stopped by the pressure internal stresses that are inside the wall, picture 3. The internal stresses "press" the crack together.
To receive the requested internal stresses by an autofrettage, the pressure of the autofrettage just has to be held a short time. A longer time does not increase the internal stresses inside the component, but leads to a strong widening of the inner boring.
As the pressures of autofrettage are calculated in that way that a huge part of the wall thickness is plastic, the pressure does not have to be reached with absolute exactness.
Which deviations are acceptable and which not, depends on every single case.
The autofrettage of a component in more than just one case should be avoided.
http://www.maximator-test.com/about/wp-autofrettage.php
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