# Evaluation of the Mechanical Failure Criterion to Consider the Triple Base Propellant Safety Life: Application of Sustainable Renewables for Environmental Hazards

Authors: Liqian Song; Yan Wang; Dabin Liu; Hua Qian; Xiaoting Rui; M. Mehdi Shafieezadeh
Journal: Discrete Dynamics in Nature and Society
 (2022)

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## Abstract

Implementation of clean energy and renewables is essential for the consideration of environmental impact because it can be implemented in supply chain networks and sustainable management procedures for safety. In order to study the safety life failure criterion of a triple base propellant under the failure mode of mechanical properties, an accelerated aging test, an evaluation method for the launch safety test of gun propellant charge, and a compressive strength test were used. The failure criterion of mechanical properties was obtained by researching the correlation between launch safety change and mechanical property change of samples. Berthelot’s equation was used to predict the safety life of the triple base propellant. The results show that the mechanical failure criterion is “27% reduction of maximum compressive strength.” The safety life at 25°C and 75% humidity is 13.77 years.


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## Body

## 1. Introduction

As the power source of ammunition, the performance of the propellant directly affects the operational efficiency of the ammunition system [1–6]. With the rapid development of military technology, the speed of equipment upgrading is increasing faster and faster, and new propellants are emerging indefinitely [7–11]. The triple base propellant is a kind of high-energy propellant, which is widely used in large caliber ammunition systems [12–16]. Compared with the traditional propellant, its composition is more complex [17–21]. The use of new materials and new technologies leads to the degradation law and mechanism of the propellant in the long-term storage process, which is significantly different from the traditional propellant [22–27]. Triple base propellants have the characteristics of high energy, high burning rate, and low burning temperature because of the addition of nitramine azide as an energetic plasticizer [28, 29]. Due to the complex composition and variety of influencing factors in the process of storage and use, there are few research studies on its safety life [30–33] and mainly focus on the change of the content of stabilizers during storage [34–37]. Accidents such as early explosion or chamber explosion occur in the use of a triple base propellant, which is still in the safe life range [38–41]. This indicates that the safety life assessment of a triple base propellant based on the content of the stabilizer is defective. If the mechanical failure criterion can be determined by some method, the accuracy and efficiency of the triple base propellant life assessment will be greatly improved.The triple base propellant was mainly affected by the mechanical environment of the chamber in the process of use. Due to the high-speed and high-pressure combustion gas in the chamber, propellant particles will be squeezed and collide with each other during the launching process, and they will also be strongly impacted by the bottom and wall of the shell. Many fragmentations are made when the mechanical conditions of propellant particles cannot meet the requirements of the strong load in the chamber, which will lead to a sharp increase in the burning surface and the gas generation rate of the propellant. This can lead to a sudden increase in local pressure in the chamber and eventually lead to premature explosion or even chamber explosion [42, 43]. Therefore, the mechanical properties of triple base propellants are the key factors affecting the safe use of samples after long-term storage. Research shows that the change of mechanical properties is the main feature of the storage process of triple base propellants, and tracking the change of mechanical properties can directly reflect the performance of triple base propellants. But due to the lack of failure criteria, the life evaluation parameters cannot be obtained directly according to the data of mechanical properties [44].The dynamic process of propellants in the process of launching can be simulated by the “evaluation method for launch safety test of gun propellant charge” [45], which accurately judges whether the sample meets the safety requirements for launching. However, the aging life of samples cannot be obtained quickly because of the large number of test samples and long test cycles. Based on the respective characteristics of the mechanical performance test of triple base propellants and the launch safety test of propellants, whether the two can be combined to obtain the mechanical failure criterion of triple base propellants and establish the mechanical aging life evaluation method of triple base propellants is of great significance to solve the safety life problem of triple base propellants in the service process.In this study, the mechanical test and the launch safety test were carried out to explore the correlation between static mechanical property and launch safety. The obtained mechanical property change data are correlated with the launch safety test data to obtain the mechanical property failure critical point of triple base propellants. The aging life equation of propellants under a mechanical failure mode was established, and the safety life of triple base propellants is estimated based on the determined failure criterion.


## 2. Experimental

### 2.1. Experimental Scheme

The experimental scheme is shown in Figure1.Figure 1
Flowchart of the experimental scheme.


### 2.2. Accelerated Aging Test

In order to shorten the test period, the samples of triple base propellants with different aging degrees were obtained by an accelerated aging test. The accelerated aging test conditions refer to the national military standard GJB 770B-05 [46].Aging temperature is as follows: 51°C, 61°C, and 71°C.Aging humidity is as follows: 75% (annual average humidity of the test site).


### 2.3. Mechanical Property Test

The precision universal material testing machine was used to test the compressive strength according to “method 415.1”, GJB770B-05.


### 2.4. Launch Safety Test

The launch safety test can reproduce the combustion and mechanical environment in the propellant chamber, simulate the extrusion crushing dynamic process of the propellant charge in the launching environment according to the chamber explosion mechanism of the propellant charge, and finally establish the quantitative relationship between the extrusion stress of the propellant and the crushing degree in the corresponding launching environment [47].


#### 2.4.1. Evaluation Process

(1)
The [48–50] extrusion crushing process of projectile bottom propellant charge during gun launching is simulated by the extrusion crushing simulation test device of propellant charge, and the broken propellant bed of propellant charge under the extrusion stress of projectile bottom propellant charge is obtained.(2)
The dynamic activity test of the broken propellant bed is carried out by using the dynamic activity test system of propellant charge, and the dynamic activity curve and the initial dynamic activity ratio of the projectile bottom broken propellant bed under real launch conditions and mechanical environment are determined.(3)
The critical initial activity ratio is determined according to the extrusion stress of the projectile bottom in the actual launching of the propellant, which is used as the evaluation standard.


## 2.1. Experimental Scheme

The experimental scheme is shown in Figure1.Figure 1
Flowchart of the experimental scheme.


## 2.2. Accelerated Aging Test

In order to shorten the test period, the samples of triple base propellants with different aging degrees were obtained by an accelerated aging test. The accelerated aging test conditions refer to the national military standard GJB 770B-05 [46].Aging temperature is as follows: 51°C, 61°C, and 71°C.Aging humidity is as follows: 75% (annual average humidity of the test site).


## 2.3. Mechanical Property Test

The precision universal material testing machine was used to test the compressive strength according to “method 415.1”, GJB770B-05.


## 2.4. Launch Safety Test

The launch safety test can reproduce the combustion and mechanical environment in the propellant chamber, simulate the extrusion crushing dynamic process of the propellant charge in the launching environment according to the chamber explosion mechanism of the propellant charge, and finally establish the quantitative relationship between the extrusion stress of the propellant and the crushing degree in the corresponding launching environment [47].


### 2.4.1. Evaluation Process

(1)
The [48–50] extrusion crushing process of projectile bottom propellant charge during gun launching is simulated by the extrusion crushing simulation test device of propellant charge, and the broken propellant bed of propellant charge under the extrusion stress of projectile bottom propellant charge is obtained.(2)
The dynamic activity test of the broken propellant bed is carried out by using the dynamic activity test system of propellant charge, and the dynamic activity curve and the initial dynamic activity ratio of the projectile bottom broken propellant bed under real launch conditions and mechanical environment are determined.(3)
The critical initial activity ratio is determined according to the extrusion stress of the projectile bottom in the actual launching of the propellant, which is used as the evaluation standard.


## 2.4.1. Evaluation Process

(1)
The [48–50] extrusion crushing process of projectile bottom propellant charge during gun launching is simulated by the extrusion crushing simulation test device of propellant charge, and the broken propellant bed of propellant charge under the extrusion stress of projectile bottom propellant charge is obtained.(2)
The dynamic activity test of the broken propellant bed is carried out by using the dynamic activity test system of propellant charge, and the dynamic activity curve and the initial dynamic activity ratio of the projectile bottom broken propellant bed under real launch conditions and mechanical environment are determined.(3)
The critical initial activity ratio is determined according to the extrusion stress of the projectile bottom in the actual launching of the propellant, which is used as the evaluation standard.


## 3. Analysis of Test Results

### 3.1. Results of the Launch Safety Test

After the accelerated aging test, the broken propellant in the real mechanical environment is obtained by using the overload simulation loading device. The initial dynamic activity ratio of the broken propellant with different aging times was obtained by the initial dynamic activity ratio test. The calibration data of the test device are shown in Table1.Table 1
The calibration data of the test device.
Maximum bottom pressure/MPaInitial velocity of projectile/m⋅s−1Simulation331.0929.6Test327.4931.0Error∗1.1％0.16％∗Error value=Simulation value−Test value/Simulation value×100%.The errors between the simulated parameters of the test device and the actual launch environment parameters meet the test requirements after correction.


#### 3.1.1. Results of the Crushing Test

In order to obtain the relationship between the initial dynamic activity ratio and aging time of samples, it is necessary to conduct dynamic activity ratio tests on broken samples with different aging times and different test pressures. The first step of the whole experiment is to obtain samples with different degrees of fragmentation. The broken samples with different aging times are obtained by using the overload simulation loading device. The test results are shown in Figure2, Tables 2–4.Table 2
Test data of propellant fragmentation without storage.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1198.57.202198.510.593201.515.63Table 3
Fragmentation test data of propellant samples aged for 15 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1199.55.972203.59.123203.014.38Table 4
Fragmentation test data of propellant samples aged for 30 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1202.57.8432203.510.283202.515.36Figure 2
Broken samples of different aging times under the same pressure; (a) samples without storage; (b) samples aged for 15 days; (c) samples aged for 30 days.
(a)(b)(c)From the test results, it can be seen that the crushing degree of samples with the same aging time increases with the maximum pressure of the simulated device bed. The crushing degree of samples increases with aging time. This shows that the mechanical properties of the samples are decreased after aging.


#### 3.1.2. Dynamic Activity Ratio Test

The P-t curve shows the change of pressure in the test equipment with time and can reflect the acceleration of the initial combustion rate and the gas production rate caused by the increase of the combustion surface after the sample is broken. The dynamic activity curve can be further obtained through the P-t curve, and the initial dynamic activity ratio data and curves are obtained as shown in Figure3.Figure 3
P-t curves, dynamic activity curves, and dynamic activity ratio curves of propellants. (a) P-t curves, (b) dynamic activity curves, and (c) dynamic activity ratio curves.
(a)(b)(c)The test data of different samples are shown in Table5, Table 6, and Table 7.Table 5
Test data of the initial dynamic activity ratio of samples without storage.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01198.57.201.0242198.510.591.0843201.515.631.108Table 6
Test data of the initial dynamic activity ratio of samples aged for 15 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01199.55.971.1982203.59.121.2593203.014.381.519Table 7
Test data of the initial dynamic activity ratio of samples aged for 30 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01202.57.8431.3742203.510.281.4273202.515.361.661Composed the diagram from the test data.Figure4 shows that the dynamic initial activity ratio increases with the increase of the maximum bed pressure at the same aging time. The longer the aging time of the sample, the greater the fragmentation, and the greater the dynamic initial activity ratio, the higher the maximum bed pressure. The test results show that the maximum pressure of a gun barrel is 9.58 Mpa, and the corresponding dynamic initial activity ratios of propellant samples with different aging times are 1.057, 1.316, and 1.425, respectively. According to the data, we make a curve of dynamic initial activity ratio aging times, as shown in Figure 5.Figure 4
The initial dynamic activity ratio of samples with different aging times.Figure 5
Dynamic initial activity ratio-aging time curve.According to the “evaluation method for the launch safety test of gun propellant charge,” whenR0＜1.3, the samples are safe to launch. From the fitting equation “y=1.082+0.01227xR2=96%,” when y = 1.3, x = 17.76. In other words, when accelerated at 71°C for 17 days, the launch safety reaches the critical point.


### 3.2. Compressive Strength Test

In this study, the maximum compressive strength of the samples was obtained by measuring the propellant samples with different aging times. TakingF = 6200 N as the starting point (average compressive strength of the nonaged sample at room temperature), according to the original data, the curves of compressive strength with aging time were obtained, as shown in Figure 6.Figure 6
Change curves of compressive strength with aging time.It can be seen from Figure6 that the compressive strength of propellant samples decreases with the increase of aging time at different aging temperatures; at the same aging temperature, the greater the humidity is, the faster the compressive strength decreases. This may be due to the degradation and chain breakage of polymers such as nitrocellulose. The grid structure formed by nitrocellulose is damaged, and the overall mechanical strength is reduced [51, 52]. In addition, in an environment of high humidity and high temperature, the droplets condensed on the surface of the pillar will dissolve some components such as nitroguanidine, and the greater the humidity, the greater the dissolution. This will result in loosening of the overall structure of the propellant column and formation of ravines and holes on the surface, weakening the integrity of the overall structure of the propellant. Furthermore, under the action of stress, the surface layer structure is easy to deform, resulting in a decrease in the overall compressive strength.


### 3.3. Mechanical Life Evaluation Equation

MATLAB software was used to fit the compressive strength of samples at different aging temperatures. First, we define the values ofx and y, i.e., x = t (…) and y = F(…) and then output the curve. We select the classical equation in the function library to simulate the curve, and the following equation can be obtained.T = 51°C:(1)F=5173×exp−0.0007781tR2=96%,T = 61°C:(2)F=4930×exp−0.001041tR2=96%,T = 71°C:(3)F=4650×exp−0.001704tR2=95%.F is the maximum compressive strength, N;t is the storage time, d.It can be seen from the launch safety test that the sample reaches the critical point of launch safety after 17 days of aging under the conditions of the 71°C accelerated aging test. Whent = 17 is taken into (2), F = 4517N. The decrease degree of the maximum compressive strength can be determined by α = (1-F/6200) × 100%, and then α = 27.15% ≈ 27%. Under the accelerated aging test condition, when the maximum compressive strength of the sample decreases by 27%, the critical point of launch safety is reached, which is the mechanical failure criterion. According to formula (1–3), the aging time of samples reaching the critical point of failure under accelerated aging test conditions at different temperatures can be obtained based on the criterion of 27% reduction of maximum compressive strength, as shown in figure 7, Table 8.Figure 7
Life fitting curve.(4)T=95.024−18.919logtR2=96%.Table 8
Aging time to failure critical point under different aging conditions.
Aging temperature/T51°C (d)61°C (d)71°C (d)Aging time/t1748417The life evaluation equation is obtained by using the Berthelot equation of fit.T is the aging temperature, °C; t is the storage time,d.According to the mechanical failure criterion “the maximum compressive strength decreases by 27%,” and the life of the triple base propellant is evaluated. According to (4), when t = 25°C, the storage time is 5026 days, that is, the safe life of the triple base propellant is 13.77 years at room temperature (25°C).


## 3.1. Results of the Launch Safety Test

After the accelerated aging test, the broken propellant in the real mechanical environment is obtained by using the overload simulation loading device. The initial dynamic activity ratio of the broken propellant with different aging times was obtained by the initial dynamic activity ratio test. The calibration data of the test device are shown in Table1.Table 1
The calibration data of the test device.
Maximum bottom pressure/MPaInitial velocity of projectile/m⋅s−1Simulation331.0929.6Test327.4931.0Error∗1.1％0.16％∗Error value=Simulation value−Test value/Simulation value×100%.The errors between the simulated parameters of the test device and the actual launch environment parameters meet the test requirements after correction.


### 3.1.1. Results of the Crushing Test

In order to obtain the relationship between the initial dynamic activity ratio and aging time of samples, it is necessary to conduct dynamic activity ratio tests on broken samples with different aging times and different test pressures. The first step of the whole experiment is to obtain samples with different degrees of fragmentation. The broken samples with different aging times are obtained by using the overload simulation loading device. The test results are shown in Figure2, Tables 2–4.Table 2
Test data of propellant fragmentation without storage.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1198.57.202198.510.593201.515.63Table 3
Fragmentation test data of propellant samples aged for 15 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1199.55.972203.59.123203.014.38Table 4
Fragmentation test data of propellant samples aged for 30 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1202.57.8432203.510.283202.515.36Figure 2
Broken samples of different aging times under the same pressure; (a) samples without storage; (b) samples aged for 15 days; (c) samples aged for 30 days.
(a)(b)(c)From the test results, it can be seen that the crushing degree of samples with the same aging time increases with the maximum pressure of the simulated device bed. The crushing degree of samples increases with aging time. This shows that the mechanical properties of the samples are decreased after aging.


### 3.1.2. Dynamic Activity Ratio Test

The P-t curve shows the change of pressure in the test equipment with time and can reflect the acceleration of the initial combustion rate and the gas production rate caused by the increase of the combustion surface after the sample is broken. The dynamic activity curve can be further obtained through the P-t curve, and the initial dynamic activity ratio data and curves are obtained as shown in Figure3.Figure 3
P-t curves, dynamic activity curves, and dynamic activity ratio curves of propellants. (a) P-t curves, (b) dynamic activity curves, and (c) dynamic activity ratio curves.
(a)(b)(c)The test data of different samples are shown in Table5, Table 6, and Table 7.Table 5
Test data of the initial dynamic activity ratio of samples without storage.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01198.57.201.0242198.510.591.0843201.515.631.108Table 6
Test data of the initial dynamic activity ratio of samples aged for 15 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01199.55.971.1982203.59.121.2593203.014.381.519Table 7
Test data of the initial dynamic activity ratio of samples aged for 30 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01202.57.8431.3742203.510.281.4273202.515.361.661Composed the diagram from the test data.Figure4 shows that the dynamic initial activity ratio increases with the increase of the maximum bed pressure at the same aging time. The longer the aging time of the sample, the greater the fragmentation, and the greater the dynamic initial activity ratio, the higher the maximum bed pressure. The test results show that the maximum pressure of a gun barrel is 9.58 Mpa, and the corresponding dynamic initial activity ratios of propellant samples with different aging times are 1.057, 1.316, and 1.425, respectively. According to the data, we make a curve of dynamic initial activity ratio aging times, as shown in Figure 5.Figure 4
The initial dynamic activity ratio of samples with different aging times.Figure 5
Dynamic initial activity ratio-aging time curve.According to the “evaluation method for the launch safety test of gun propellant charge,” whenR0＜1.3, the samples are safe to launch. From the fitting equation “y=1.082+0.01227xR2=96%,” when y = 1.3, x = 17.76. In other words, when accelerated at 71°C for 17 days, the launch safety reaches the critical point.


## 3.1.1. Results of the Crushing Test

In order to obtain the relationship between the initial dynamic activity ratio and aging time of samples, it is necessary to conduct dynamic activity ratio tests on broken samples with different aging times and different test pressures. The first step of the whole experiment is to obtain samples with different degrees of fragmentation. The broken samples with different aging times are obtained by using the overload simulation loading device. The test results are shown in Figure2, Tables 2–4.Table 2
Test data of propellant fragmentation without storage.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1198.57.202198.510.593201.515.63Table 3
Fragmentation test data of propellant samples aged for 15 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1199.55.972203.59.123203.014.38Table 4
Fragmentation test data of propellant samples aged for 30 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPa1202.57.8432203.510.283202.515.36Figure 2
Broken samples of different aging times under the same pressure; (a) samples without storage; (b) samples aged for 15 days; (c) samples aged for 30 days.
(a)(b)(c)From the test results, it can be seen that the crushing degree of samples with the same aging time increases with the maximum pressure of the simulated device bed. The crushing degree of samples increases with aging time. This shows that the mechanical properties of the samples are decreased after aging.


## 3.1.2. Dynamic Activity Ratio Test

The P-t curve shows the change of pressure in the test equipment with time and can reflect the acceleration of the initial combustion rate and the gas production rate caused by the increase of the combustion surface after the sample is broken. The dynamic activity curve can be further obtained through the P-t curve, and the initial dynamic activity ratio data and curves are obtained as shown in Figure3.Figure 3
P-t curves, dynamic activity curves, and dynamic activity ratio curves of propellants. (a) P-t curves, (b) dynamic activity curves, and (c) dynamic activity ratio curves.
(a)(b)(c)The test data of different samples are shown in Table5, Table 6, and Table 7.Table 5
Test data of the initial dynamic activity ratio of samples without storage.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01198.57.201.0242198.510.591.0843201.515.631.108Table 6
Test data of the initial dynamic activity ratio of samples aged for 15 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01199.55.971.1982203.59.121.2593203.014.381.519Table 7
Test data of the initial dynamic activity ratio of samples aged for 30 days.
No.Weight/gMaximum extrusion stress of the propellant bed/MPaInitial dynamic activity ratio/R01202.57.8431.3742203.510.281.4273202.515.361.661Composed the diagram from the test data.Figure4 shows that the dynamic initial activity ratio increases with the increase of the maximum bed pressure at the same aging time. The longer the aging time of the sample, the greater the fragmentation, and the greater the dynamic initial activity ratio, the higher the maximum bed pressure. The test results show that the maximum pressure of a gun barrel is 9.58 Mpa, and the corresponding dynamic initial activity ratios of propellant samples with different aging times are 1.057, 1.316, and 1.425, respectively. According to the data, we make a curve of dynamic initial activity ratio aging times, as shown in Figure 5.Figure 4
The initial dynamic activity ratio of samples with different aging times.Figure 5
Dynamic initial activity ratio-aging time curve.According to the “evaluation method for the launch safety test of gun propellant charge,” whenR0＜1.3, the samples are safe to launch. From the fitting equation “y=1.082+0.01227xR2=96%,” when y = 1.3, x = 17.76. In other words, when accelerated at 71°C for 17 days, the launch safety reaches the critical point.


## 3.2. Compressive Strength Test

In this study, the maximum compressive strength of the samples was obtained by measuring the propellant samples with different aging times. TakingF = 6200 N as the starting point (average compressive strength of the nonaged sample at room temperature), according to the original data, the curves of compressive strength with aging time were obtained, as shown in Figure 6.Figure 6
Change curves of compressive strength with aging time.It can be seen from Figure6 that the compressive strength of propellant samples decreases with the increase of aging time at different aging temperatures; at the same aging temperature, the greater the humidity is, the faster the compressive strength decreases. This may be due to the degradation and chain breakage of polymers such as nitrocellulose. The grid structure formed by nitrocellulose is damaged, and the overall mechanical strength is reduced [51, 52]. In addition, in an environment of high humidity and high temperature, the droplets condensed on the surface of the pillar will dissolve some components such as nitroguanidine, and the greater the humidity, the greater the dissolution. This will result in loosening of the overall structure of the propellant column and formation of ravines and holes on the surface, weakening the integrity of the overall structure of the propellant. Furthermore, under the action of stress, the surface layer structure is easy to deform, resulting in a decrease in the overall compressive strength.


## 3.3. Mechanical Life Evaluation Equation

MATLAB software was used to fit the compressive strength of samples at different aging temperatures. First, we define the values ofx and y, i.e., x = t (…) and y = F(…) and then output the curve. We select the classical equation in the function library to simulate the curve, and the following equation can be obtained.T = 51°C:(1)F=5173×exp−0.0007781tR2=96%,T = 61°C:(2)F=4930×exp−0.001041tR2=96%,T = 71°C:(3)F=4650×exp−0.001704tR2=95%.F is the maximum compressive strength, N;t is the storage time, d.It can be seen from the launch safety test that the sample reaches the critical point of launch safety after 17 days of aging under the conditions of the 71°C accelerated aging test. Whent = 17 is taken into (2), F = 4517N. The decrease degree of the maximum compressive strength can be determined by α = (1-F/6200) × 100%, and then α = 27.15% ≈ 27%. Under the accelerated aging test condition, when the maximum compressive strength of the sample decreases by 27%, the critical point of launch safety is reached, which is the mechanical failure criterion. According to formula (1–3), the aging time of samples reaching the critical point of failure under accelerated aging test conditions at different temperatures can be obtained based on the criterion of 27% reduction of maximum compressive strength, as shown in figure 7, Table 8.Figure 7
Life fitting curve.(4)T=95.024−18.919logtR2=96%.Table 8
Aging time to failure critical point under different aging conditions.
Aging temperature/T51°C (d)61°C (d)71°C (d)Aging time/t1748417The life evaluation equation is obtained by using the Berthelot equation of fit.T is the aging temperature, °C; t is the storage time,d.According to the mechanical failure criterion “the maximum compressive strength decreases by 27%,” and the life of the triple base propellant is evaluated. According to (4), when t = 25°C, the storage time is 5026 days, that is, the safe life of the triple base propellant is 13.77 years at room temperature (25°C).


## 4. Discussion and Conclusion

### 4.1. Discussion

This study shows the correlation between mechanical properties and launch safety. The combination of the two test methods has solved the problem that the triple base propellant lacks the mechanical failure criterion and cannot be evaluated from the perspective of mechanical properties.The test results of propellant launching safety show that under the same chamber mechanical environment, the extension of aging time will lead to the increase of triple base propellant fragmentation, the sudden increase of the combustion surface, the acceleration of the initial burning rate, the acceleration of local pressure, and the increase of the dynamic initial activity ratio. The mechanical test shows that the maximum compressive strength of the triple base propellant decreases with aging time; at the same aging time, the higher the aging temperature is, the faster the maximum compressive strength decreases.The change laws of these experimental phenomena are related. Combined with the data of the 71°C accelerated aging test and the results of the propellant launching safety test, the mechanical failure criterion of the triple base propellant is calculated as “the maximum compressive strength decreases by 27%.” Of course, this result may not be accurate and specific, but this method to determine the mechanical failure criterion of the propellant is feasible. In the follow-up study, more accurate and instructive life evaluation results can be obtained by increasing the number of tests and reducing the error.


### 4.2. Conclusion

The method of combining launching safety tests with mechanical property tests solves the problem that it is difficult to determine the mechanical failure criterion of triple base propellants and provides a strong basis for the establishment of a life evaluation system for such propellants.(1)
The relationship between the aging time and initial dynamic activity ratio in the process of accelerated aging of triple base propellants at 71°C can be expressed by the equation:y=1.082+0.01227x (R2 = 94%). When the dynamic initial activity ratio is 1.3, the aging time is about 17 days.(2)
The mechanical failure criterion of the triple base propellant is calculated as “the maximum compressive strength decreases by 27%.” According to the Berthelot equation, the safety life of the triple base propellant at room temperature (25°C) is 13.77 years.


## 4.1. Discussion

This study shows the correlation between mechanical properties and launch safety. The combination of the two test methods has solved the problem that the triple base propellant lacks the mechanical failure criterion and cannot be evaluated from the perspective of mechanical properties.The test results of propellant launching safety show that under the same chamber mechanical environment, the extension of aging time will lead to the increase of triple base propellant fragmentation, the sudden increase of the combustion surface, the acceleration of the initial burning rate, the acceleration of local pressure, and the increase of the dynamic initial activity ratio. The mechanical test shows that the maximum compressive strength of the triple base propellant decreases with aging time; at the same aging time, the higher the aging temperature is, the faster the maximum compressive strength decreases.The change laws of these experimental phenomena are related. Combined with the data of the 71°C accelerated aging test and the results of the propellant launching safety test, the mechanical failure criterion of the triple base propellant is calculated as “the maximum compressive strength decreases by 27%.” Of course, this result may not be accurate and specific, but this method to determine the mechanical failure criterion of the propellant is feasible. In the follow-up study, more accurate and instructive life evaluation results can be obtained by increasing the number of tests and reducing the error.


## 4.2. Conclusion

The method of combining launching safety tests with mechanical property tests solves the problem that it is difficult to determine the mechanical failure criterion of triple base propellants and provides a strong basis for the establishment of a life evaluation system for such propellants.(1)
The relationship between the aging time and initial dynamic activity ratio in the process of accelerated aging of triple base propellants at 71°C can be expressed by the equation:y=1.082+0.01227x (R2 = 94%). When the dynamic initial activity ratio is 1.3, the aging time is about 17 days.(2)
The mechanical failure criterion of the triple base propellant is calculated as “the maximum compressive strength decreases by 27%.” According to the Berthelot equation, the safety life of the triple base propellant at room temperature (25°C) is 13.77 years.



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*Source: 8395116-2022-09-22.xml*