Accelerated Life Test Specification and Method on Predicting Lifetime on Machinegun

2017 International Conference on Mathematics, Modelling and Simulation Technologies and Applications (MMSTA 2017) ISBN: 978-1-60595-530-8

Accelerated Life Test Specification and Method on Predicting

Lifetime on Machinegun

Yong-hai SHAN

_{, Xian-ming GAO}

_{and Shi-qi SHAN}

1_{Baicheng Ordnance Test Centre of China, China} *Corresponding author

Keywords: Machinegun, Barrel, Accelerating lifetime, Test specifications, Life prediction.

Abstract. According to active defining methods in national military performance test standard on fire arms, the whole lifetime shooting method is usually used in firearms life test, with a long time and consumes plenty of ammunitions. In order to reduce amount of test bullets and innovate life test methods, a lot of life tests research and data analysis on fire arms were conducted, and the main factors that influence on machinegun lifetime were decided, the research scheme of accelerating life test and of life parameters testing scheme were designed, equality between life in normal temperature and comprehensive life in normal, high and low temperature was verified, the accelerated life test specification, modeling scheme, life predicting and evaluating method on machinegun were proposed. The research shows that the amount of ammunitions using can be reduced about 70 percent in a life test on machine guns, life test cycle can be cut down drastically and the precision predicting life can be up to 90 percent above.

Introduction

The whole lifetime shooting method is usually used in firearms life test until now. It takes a long time and consumes plenty of ammunitions[1]_{. Researches show that accelerated life test is an} effective way to reduce test period and consumption[2-5]_{. As one of main parts of machinegun, the} barrel is in the worst environment of ablation, corrosion and erosion from the higher temperature and higher pressure propellant gas in the bore, and of abrasion from bullet. As to machinegun, especially, the barrel life represents the gun life to some extent[6][7].In the investigation of machinegun barrel, the accelerated life test scheme, method on modeling life and of predicting lifetime are introduced in this paper on the basis of a lot of tests, measurement and data analysis by using the methods such as test design and statistical analysis.

Analyzing Effective Factor Relating to the Barrel Life

Barrel life depends on many factors. In a broad sense, designing elements, using conditions and operating approach, etc. can affect the barrel life to some extent. It is known that the quality of structure design, manufacture technology and used ammunition of a product in the stage of approved design are all constant, however, the life of the product will considerably different because the test environment and operation approach, etc. are changeable. Result of theoretical analysis and experiment of many tests shows: the temperature rising of barrels after shooting in different test specifications are different significantly, hence the life of barrel is different obviously. The higher the temperature of barrel is after shooting, the heavier the bore is ablated, and the shorter the barrel life is. The correlation between barrel life and shooting specifications therefore can approximately convert into the relationship between temperature rising of the barrel after shooting and shooting specifications.

1) The main factors that significantly affect the barrel temperature rising(life) are environmental temperature, the amount of fired bullet during a cooling cycle(AFBCC) and the interval time of changing bullet boxes(ITCBB).

2) Burst length and interval time(2-3s) between bursts contribute little in barrel temperature

rising.

3) After firing bullet during a cooling cycle(≤200 rounds), cool the barrel in the air 3～5min,

then cool it in the water, hence, the barrel life will not be affected on the whole.

The research conclusion can be an effective guidance on choosing accelerated life test factors of machinegun and can optimize the design scheme of life test.

Designing Life Test Scheme

Test scheme designed scientifically can significantly reduce the test sample size and test consumption, and can obtain valid life parameters. According to above study, we can get such result as the main factors that affect the barrel life significantly are test environmental temperature, the number of fired bullet during a cooling cycle and the interval time of changing bullet boxes. For this reason, four schemes are designed as follow(see table 1) in order to study on the independent and interactive influence of above three factors on the life. Life parameters, for example, bullet muzzle velocity v0, firing dispersion accuracy R50, etc. were measured or calculated when every barrel(15 barrels in all, 11barrels in fact) had cumulatively fired a certain amount of bullet(about 600 rounds), and plenty of life data(omitted) (see table 2)were obtained.

Table 1. Designed test research scheme.

Scheme

number Barrel number temperatureEnvironmental （℃)

AFBCC

(round) ITCBB（min)

one

1

22

25×2×3 0.5

2 25×2×3 1

3 25×2×3 2

4 25×2×3 3

two

5 50 25×2×3 2

6（3) 22 25×2×3 2

7 -25 25×2×3 2

8 -45 25×2×3 2

three

9

22

25×2×1 0.5

10 25×2×2 0.5

11（1) 25×2×3 0.5

12 25×2×4 0.5

four

13（8) -45 25×2×3 2

14（12) 22 25×2×4 0.5

15 50 25×2×2 1

Table 2. Parameter test scheme.

Barrel number

Environmental

temperature(℃) AFBCC (round) ITCBB (min) Cumulative amount of fired bullet (round) Muzzle velocity ( m/s) Dispersion accuracy R50 (cm) Barrel life (round)

i Ti n_i t_i

1

i

n v_i1 Ri1

is

n

2 i

n vi2 Ri2

… … …

ir

n vir Rir

Modeling Lifetime on the Base of Test Environment

On the basis of the test result(omitted) in table 2, below model(see Eq. 1) is developed according to the rule of the characterization parameter of each barrel changing with the factor.

j iN hT gt fTN etN dtT cN bT at

n 2 2 2      

ｎ——lifetime in bound; t——ITCBB(0.5～3), min;

T——environmental temperature(-45～50), ℃; N_{——AFBCC(50}～200), round.

While there are ten unknown numbers in Eq. 1, we extract ten groups of available data from the test data. Appropriate data conversion is done in table 3 for simplifying calculation conveniently.

Table 3. Relation between stress and life.

AFBCC

Nc(N/150 rounds) Environmental temperature Tc(T/22℃) ITCBB tc(t/2minutes) Cumulative amount of fired bullet when muzzle velocity drops down to 15%ξc (n/1000 rounds)

ci

N Tci tci ci

10 , , 2 , 1 



i

Solving the equation, we can get the accelerating life model on above three factors:

7427 . 9 3013 . 12 3317 . 0 2592 . 0 0707 . 0 408 . 3 0422 . 1 635 . 4 0047 . 0 3787 .

0 2_ 2_ 2_{      }



 c c c c c c c c c c c c

c t T N tT tN TN t T N

 ₍₂₎

Substitute the variable in Eq.2 with corresponding factor value, and then predict the barrel (11 barrels in all) life. By contrast with the test life, it shows that predicting barrel life well coincides to test life, and it indicates that the predicting accuracy of the three factor fitting model is higher (see Fig. 1).

Eq.2 is founded on plenty of tests and studies, and it is a model which has universal guidance on predicting barrel life of large caliber machinegun. As long as corresponding factor values of life test are given, it can effectively predict machinegun barrel life in this condition.

Life Test Specifications and Method on Predicting Life

1) Stress screening in accelerated life test

In order to determine the stress in machinegun accelerated life test, we chose accelerating test stress combination of 15 barrels and tested(table 1). By comprehensively analyzing every datum and result in each barrel accelerated life test, it is not difficult to see that the extreme point or the inflexion point of the main life characteristic parameters v0 and R50 appear earlier (less than 500 rounds) of barrel 12, and the stability , monotonicity and change rule of the curve after the inflexion point are all better. Therefore, measuring parameters such as v0, R50 and so on, when the cumulative amount of fired bullet is 500,1000 and 1500, we can exactly describe in mathematics and predict the barrel life by using simply quadratic curve or exponential function(Fig.2). Such, accelerated life test stress should choose environmental temperature is 22℃, AFBCC is 200 rounds and ITCBB is 0.5min) [10]_.

Figure 2. The muzzle velocity and dispersion accuracy of barrel 12 vary with the amount of fired bullet.

2) Checking the equivalence of normal temperature life test

Current machinegun life test needs two cycles on three kinds of temperature, i.e. normal temperature, high temperature and low temperature. The paper, however, presents a new way to complete the test just on normal temperature(22℃). Is the machinegun barrel life test on normal temperature equivalent to that on multiple temperatures (normal temperature, high temperature and low temperature)? For this reason, plenty of tests were carried out, and the rule of maximum temperature rising(barrel life) on the barrel outer wall on different temperature was investigated in those test. The result shows: the barrel temperature after shooting on normal temperature(adopting different shooting specifications) well coincide to weighted average of that on normal temperature, high temperature and low temperature, and the barrel life on normal temperature approximately equals the comprehensive barrel life on normal temperature, high temperature and low temperature. Therefore, machinegun life test can be carried out just under normal temperature.

3) Shooting specifications of life test

According to above analysis, machinegun accelerated life test specifications are determined as: use 1 machinegun with 2 barrels(the old standard needs 2 machineguns with 2 barrels for each), environmental temperature is 22℃(the old standard needs two shooting cycles on normal temperature, high temperature and low temperature), AFBCC is 200 rounds(it is 100 rounds in the old standard), ITCBB is 0.5min(it is about 2 min in the old standard), burst length is 25 rounds(it is 10～15 rounds in the old standard), interval time between bursts is 2～3s, cool the barrel in the air for 3 min after shooting during a cooling period, then cool it in the water to normal temperature. Check static parameters of the machinegun , measure muzzle velocity and firing accuracy of the barrel before every barrel shooting and when the cumulative amount of fired bullet for every barrel is up to 600 rounds. The cumulative amount of fired bullet for every barrel is 1800 rounds(it is 3000 rounds in the old standard). Using the Accelerated life test specification presented in the paper,

8400 1 2 1800 2 2

3000      _{bullets can be saved in one machinegun life test, i.e. ammunition} consumption is reduced by 70%.

4) Method on predicting life

From Eq.2, the barrel life njs_{under the condition of accelerated test is 2610 rounds in using} above accelerated life test scheme(i.e. AFBCC is 200 rounds(here, quantify it to 200/150, i.e. 4/3) , environmental temperature is 22℃(here, quantify it to 22/22, i.e. 1), ITCBB is 0.5min(here, quantify it to 0.5/2, i.e. 0.25))；however, the life under normal test conditionnzc _{(i.e. AFBCC is} 100 rounds(here, quantify it to 100/150, i.e. 2/3) , equivalent environmental temperature is 22℃(here, quantify it to 22/22, i.e. 1), ITCBB is empirical data from test(here, assume it is 2min, and quantify it to 2/2, i.e. 1)) is 4483 rounds. Hence, the acceleration coefficient under comprehensive stress is:

can be converted to normal stress(i.e. AFBCC is 100 rounds, equivalent environmental temperature is 22℃, ITCBB is 2min) result nzc_{(see Eq.4).}

js

zc n

n 1.72 ₍₄₎

According to GJB3484-98，as for large caliber machinegun barrel:

   

     



 

 

 _



10

min 0

50 50 0

5 . 2 50

50

% 85

t

R R v v

js

n

n n R R

n n v v

n

In the equation：

v_{——muzzle velocity, m/s;}

0

v _{——initial muzzle velocity, m/s;}

50

R _{——dispersion accuracy at 100m, cm;}

0 50

R ——initial dispersion accuracy at 100m, cm;

t

n_{——the amount of ellipse hole created by current group of bullets.}

It is suggested to obtain firing dispersion accuracy by using fixed gun rack or under the standard shooting condition so as to eliminate effect of other factors besides the barrel self, and ensure consistency of shooting condition.

The machinegun barrel life is predicated being up to standard if the lives of the two barrel allocated to the tested machinegun all reach the required value of the tactics and technology index. Otherwise, the machinegun barrel life is not qualified if any life of the two barrel does not reach the required value.

Conclusion

On the basis of comprehensive analysis and study, the paper identifies the main stresses which are decisive to machinegun barrel life, proposes the method on modeling machinegun barrel life under three test stresses and model, and presents scheme, modeling method, predicting life method on machinegun barrel accelerated life test. The research result lays theoretical and trial foundation for innovating machinegun life test method.

References

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