A Study on Criteria for Barrel Lifetime

MA Ji-sheng(马吉胜)，ZHENG Jian(郑坚)，DENG Hui-yong(邓辉咏)

(1.Artillery Engineering Department，Ordnance Engineering College，Shijiazhuang 050003，Hebei，China 2.Scientific Research Department，Ordnance Engineering College，Shijiazhuang 050003，Hebei，China)

Introduction

A barrel is an important part of a gun，and its procurement cost is very high，about 30% ～40%of the cost of whole gun.With the increase of launched projectiles，the ablation in the bore becomes more serious，and it can not be repaired.Therefore，the lifetime of barrel becomes one of important lifetime criteria of large and middle caliber guns.Usually，if the barrel is scrapped，the whole gun will be discarded.Thus，the barrel lifetime is always concerned by research，design departments and end-users［1－5］.However，in the definition of barrel lifetime，there are no definite or authorized quantitative criteria to evaluate gun lifetimes［6－9］，and this is a problem not solved in the history of gun.As the operation conditions are different for guns，their lifetimes differ greatly［10－13］.Currently，there are various viewpoints and criteria to estimate the barrel lifetime and they are selected randomly［1，14－17］.In this paper，some criteria are summarized and analyzed，and a practical criterion is proposed.

1 Analyses on Some Criteria of Barrel Lifetime

1.1 Four-criterion Method of Barrel Lifetime

1.1.1 Four criteria of barrel lifetime

1)Its muzzle velocity decreases by 10%;

2)Its bearing band is cut off，so the projectile can not fly stably;

3)Its ground dispersion area or vertical target dispersion area exceeds 8 times of specified values of firing table;

4)Its projectile’s fuse does not act or explode continuously.

If anyone of above conditions happens，it means that the barrel life ends.

1.1.2 Defects of four-criteria method

1)It can only be used in testing grounds or research departments.As the limitations of technology，equipment，field or other conditions，it can not be used by troops to check the lifetime of barrels and estimate the residual lifetime.

2)It includes four phenomena that the barrel lifetime ends，and it is difficult to predict when the phenomena happen and their sequences.As the equipment update plan can not be predetermined，once one of the phenomena happen，the gun has already lost its fighting capacity.Therefore，it has no instructive value for operation and training.

3)The specified muzzle velocity loss can not reflect the gun’s real status.For guns that the lifetime test has been carried out，it is scientific and reliable to use maximum muzzle velocity loss for determining the lifetime;but for many guns untested，the maximum muzzle velocity losses have not been given，or the given maximum muzzle velocity losses can not reflect the barrel lifetime.Exactly，the barrel lifetime is not determined by the maximum muzzle velocity loss，but by the bearing band cutting-off caused by the abrasion of the bore.The muzzle velocity losses of some scrapped guns in our army are shown in Tab.1.It can be known from the table that they differ greatly.Moreover，the muzzle velocity criterion can not be used to check the lifetime of untested guns，so the method can not be popularized in troops.

4)Usually，the four phenomena do not happen in the same time in the test or operation，anyone of them means that the barrel life ends.The corresponding relationship among them has not confirmed yet，thus，it is not scientific to use this method to estimate the barrel lifetime.

1.2 Chamber Enlargement Judgment Method for Barrel Lifetime

The maximum axial displacement Δlmaxof a chamber length gage is used to judge the barrel lifetime.The diameter measured by the gage is a value between the diameter of bearing band and the land diameter confirmed by designer，and there is not uniform standard.Therefore，Δl only represents the displacement of chamber length gage moving in the bore，and it is not the displacement of the positioning point of projectile in the bore and the volume enlargement of the chamber.It can not reflect the change of interior ballistic characteristics properly.In addition，the ellipticity of the barrel in service exists commonly，and it makes the chamber length gage move in bore difficultly;thus，the measurement error of chamber length is large，and can not express the changes of interior ballistic properties correctly.Tab.2 shows the experiment results of six guns made in different factories.

It can be seen that，for the guns made in Tianjin and Shanghai，the launched rounds are almost the same and the chamber length are almost the same，but their muzzle velocities differ by 2%.Such difference can not be accepted by users.The chamber length enlargement can not reflect changes of interior ballistic characteristics qualitatively and quantitatively，thus，the method can not be used to determine the barrel lifetime.

Tab.2 Chamber length enlargement of antiaircraft guns out of lifetime

1.3 Criterion of Barrel Lifetime Based on Number of Rounds

In the process of major gun lifetime test，not considering the differences of firing condition or the charge volume，one firing is taken as a unit.Nmax，the number of natural rounds，is the total number of rounds fired till the gun life ends.Other gun lifetimes are evaluated by Nmax.It is not correct.For example，the total number of rounds of a cannon is 780，it is much lower than the same caliber foreign gun’s lifetime listed in‘World Gun Yearbook’，the reason is that the barrel lifetime is measured by the nature rounds.In the foreign countries，the barrel lifetimes are measured by the equivalent number of rounds，that means the number of natural rounds in different conditions and different charge volumes should be transformed to the equivalent number of rounds in the standard situation.The equivalent number of rounds of the gun mentioned above is 1 500，and it is very close to the foreign number.That is to say，the numbers of nature rounds in different firing conditions differ greatly，and they can not express the barrel lifetime correctly，so the equivalent number of rounds should be used.An equivalent coefficient will be used to transform the number of rounds to the equivalent number of rounds［6］.

2 A New Criterion of Barrel Lifetime

2.1 Requirements for Barrel Lifetime Criterion

2.1.1 Scientificalness

The lifetime criterion should be scientific，the detection results should be correct and credible，and it can reflect the ballistic parameters correctly during whole barrel life and can be used by troops.It can express the state of barrel life end scientifically and correctly.

2.1.2 Applicability

It should be suitable for all kinds of guns，such as cannon，howitzer，navy gun，antiaircraft gun，smoothbore gun，and so on.It can be used in each stage of research，design，production，test and operation.

2.1.3 Operability

It must be operable，and the measurement devices should be simple，durable，convenient and economic.For the guns serving in factories，testing grounds and troops，the residual lifetime should be checked at any time.

2.2 Criterion Selection for Barrel Lifetime

Based on requirements listed above，the barrel’s radical wear can be used to determine the barrel lifetime.Usually，the radical wear at specified position in bore is the radical wear at start point of land，denoted as Δd.It can be seen from the equations of interior ballistics that Δd is the main factor causing the changes of interior ballistic properties.The muzzle velocity v0，maximum chamber pressure pmand start pressure p0change regularly with the increase of Δd.The influences of Δd on v0，pmand p0for a cannon are shown in Fig.1 to 5.

Fig.1 Theoretical muzzle velocity v0

Fig.2 Measured muzzle velocity v0

Fig.3 Theoretical maximum chamber pressure pm

The radical wear at the specified point of a bore are characterized as follows.

Fig.4 Measured maximum chamber pressure pm

1) Δd determines the interior ballistic properties.For different extents of wear，the same Δd will result in the samep0.Solved interior ballistic equations，the only solutionv0andpmcan be obtained，i.e.，Δddetermines the interior ballistic properties.Therefore，the interior ballistic properties of worn guns can be expressed by the radical wear at the specified point theoretically and practically.

Fig.5 Theoretical start pressure p0

2)By use of the correlation between Δdand Δv0，the change of muzzle velocity，for guns with different wear extents，their service histories may not be concerned in assessing the barrel lifetime.The lifetime can be evaluated wholly by using the radical wear，and it simplifies the lifetime assessment greatly.

3)Δdis the mean wear at specified cross-section.The guns have usually served for a long time，and the bores have been damaged in different extents.It has been found by checking hundreds of guns，the ellipticity exist in many bores’cross-sections.For example，in the cross-section of a cannon，about 5 720 mm from the muzzle，the difference between major diameter and minor diameter is 0.33 mm;in the crosssection of a navy gun，about 5 600 mm from the muzzle，the difference reaches up to 0.47 mm.The bore’s ellipticity is common in guns in service.Considering that the bore cross-section is a circle with a diameter of the mean diameter of the elliptic bore，the circumferential plastic deformations when the projectile squash into the bore can be regarded as the same.Thus，p0will not be affected in calculation and the accuracy can be ensured，so that the theoretical muzzle velocity and bore pressure can be calculated correctly and reliably.

4) Usually，the measurement error of Δdis small.Bore erosion is not equal in different positions along axis of the bore.Generally，the wear velocity at the start of the land is the largest，in the same way，the wear at the point is the most，so the measurement error of Δdis the least.

5)Δdcan be measured statically and in the interval of firing or in the static test.Moreover，the device used to measure Δdcan be operated simply and reliably，and the troops can do the measurement very conveniently.

The features mentioned above show that the barrel lifetime can be evaluated by using Δdcorrectly，reliably and conveniently，and it is feasible to evaluate the lifetime guns in service.

In order to show the reliability of Δdcriterion，the test results for three kinds of guns are shown in Tab.3 to 5.

Tab.3 Experiment data of a kind of antiaircraft gun

Tab.4 Experiment data of a kind of navy gun

Tab.5 Experiment data of a kind of navy gun

It can be known from the tables，though the service experience， use conditions， rounds， chamber length enlargements are different，as long as Δdis the same，the mean ballistic parameters of next firing are the same，that is，the relationship between the muzzle velocity and Δdis very stable.

The conclusion is of significance to determine the barrel lifetime，because the key factor is Δd.When Δdreaches to its maximum Δdmax，the barrel life ends because the bearing band is cut off.Also，Δdstably reflects the change of muzzle velocity，and it is convenient and practical for artillery to modify muzzle velocity and determine residual barrel lifetime.

3 Conclusions

There is a strong relationship between radical wear at the start points of barrel land and interior ballistic properties;for the same type guns，if Δdis the same，the interior ballistic properties are the same.If Δdreaches to its maximum Δdmax，the barrel lifetime ends.Because Δdcan be measured statically，it can be easily obtained in the static check or daily maintenance，so it is the most superior criterion.Δdmaxof many kinds of gun can be tested and calculated theoretically.Based on this criterion，it is convenient and rapid to judge the states of guns.

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