Correlation between Alternating Temperature Accelerated Conditioning and Natural Storage of HTPB Propellant

1 Introduction

During the storage of solid rocket motor grains, due to the change of ambient temperature load and the differences in thermal expansion coefficients of materials used in different structures of the engine, the grains are under alternating stress for a long time during storage. Due to the action of this alternating load, the grain material of the engine will experience cumulative damage on the microstructure, which may lead to macroscopic cracks, and eventually lead to the failure of the grain and the engine. Therefore, it is of great military significance and considerable economic benefit to study the accelerated aging method of the engine that has a good correlation with the natural storage environment for accurately predicting and prolonging the service life of the missile under the alternating temperature environment.

In order to obtain the storage life of composite propellants and engines, a reliable method is to carry out storage and related experimental researches of engines and billets in actual environments, such as the "Comprehensive Aging and Monitoring Program" and "Long-Term Life Analysis Program" of the United States. " and "Life Estimation Technology Plan", etc., it needs a long cycle and sufficient funding. Another method is the accelerated aging method. The more commonly used life estimation method in the engine design stage is the high temperature accelerated aging method. However, the high-temperature accelerated aging method also has disadvantages, that is, the degradation reaction mechanism of some polymers and composite materials at high temperature is not necessarily the same as the damage reaction mechanism caused by long-term storage at low temperature, and the results predicted by many accelerated aging tests have not undergone natural aging. The environmental conditions of accelerated aging are also different from the storage conditions of real engines. Brouwer et al. used micro-electromechanical sensors to monitor the stress-strain state during the accelerated aging process of temperature alternating, and found that the mechanical aging caused by temperature cycle is the main factor leading to the change of stress state.

In view of the above problems, the research group studied the correlation between the accelerated aging test of HTPB propellant at alternating temperature and natural storage, so as to provide a reference for engine life prediction.

2 experiments

2.1 Sample

The propellant used in the test is the HTPB propellant after 5 years of natural storage. In order to simulate the engine case, the test fixture as shown in Fig. 1 is designed for the test. During the test, both ends of the propellant specimen were bonded to the upper and lower plates of the fixture, and then four bolts and two positioning pins were used to fix the propellant specimen without stress. When the temperature of the Test Chamber rises, the propellant expands, but because the thermal expansion coefficient of the steel bolt is an order of magnitude smaller than that of the propellant, the expansion of the propellant is constrained by the clamp, which is equivalent to generating a compressive strain, which in turn induces a compressive stress. When the temperature of the Test Chamber drops, the propellant shortens, but under the action of the adhesive force of the fixture, it cannot shrink freely, quite a tensile strain is generated, and a tensile stress is induced.

2.2 Experimental scheme

(1) Take a 14 cm × 3 cm × 15 cm HTPB propellant pellet and stick it in the test fixture, fix the fixture in a sealed tank filled with dry nitrogen, and then put the sealed tank into a high-low temperature humidity and heat Test Chamber for testing. Alternating temperatures accelerate aging.

 (2) Test temperature range: - 10 to 60 ℃.

(3) The test is divided into three groups, and the temperature change rates are 10 ℃/12 h, 20 ℃/12 h, 30 ℃/12 h. The curves of temperature change with time in each cycle of the three groups of experiments are shown in Fig. 2 .

(4) In each group of tests, that is, at each temperature change rate, the mechanical properties of the propellant were tested after aging for 35 d, 56 d, and 91 d.

2.3 Experimental results

According to the data processing method of composite solid propellant uniaxial tensile test, the composite propellant alternating temperature accelerated aging test data is processed, and the tensile strength and maximum elongation are shown in Table 1. The test temperature is 20 ℃, and the tensile rate is 100 mm·min-1.

2.4 Experimental data processing

(1) Mathematical model of aging

Composite solid propellant storage aging test data processing is often selected

The following three mathematical models:

Model 1: P = P 0 + Klgt (1)

Model 2: P = P 0 + Kt (2)

Model 3: P = P 0 e- Kt(3)

In the formula, P is the performance at a certain moment, P 0 is a constant, K is the temperature-related performance change rate constant, and t is the storage aging time.

(2) Calculation method steps

在加速老化试验中 , 每个老化温度下可以获得一组老化时间 t 与性能 P 的数据 : t i , P i , i = 1, 2, ⋯ , n 。

对于模型一 , 令 X = lgt, Y = P, a = P 0 , b = K, 则式(1) 可用直线方程 Y = a + bX 表示。

对于模型二 , 令 X = t, Y = P, a = P 0 , b = K, 则式(2) 可用直线方程 Y = a + bX 表示。

对于模型三 , 方程两边取自然对数可得 : lnP =lnP 0 - Kt, 令 X = t, Y = lnP, a = lnP 0 , b = - K, 则式 (3)可用直线方程 Y = a + bX 表示。

用最小二乘法可求出系数 a, b 和相关系数 r 。

2. 5 　试验结果分析

 (1) 由表 1 可以看出 , 推进剂抗拉强度 ( σ m ) 有所提高 , 最大伸长率 ( ε ) 下降趋势明显。这和自然贮存条件下的变化趋势基本相同 , 说明交变温度加速老化过程与自然老化过程之间应存在较好的相关性。

(2) 结合贮存条件以及复合推进剂老化机理来看 ,出现伸长率变化明显而抗拉强度缓慢增大的原因可能在于化学反应使分子内生成了新的羟基 , 提高了官能团总数 , 新的交联作用使推进剂抗拉强度增大 , 最大伸长率下降; 同时由于推进剂已经自然贮存五年 , 受到环境湿气的影响 , 老化过程中处于密闭的环境 , 水分不能及时挥发 , 引起粘合剂母体水解断链 , 导致推进剂软化 ,尤其在高温时 , 这种作用更为明显; 交变的应力应变也会改变化学老化进程 , 引起推进剂内部结构发生复杂的物理化学变化 , 导致推进剂性能下降。

(3) 温度变化速率在加速老化过程中对推进剂性能变化有较大影响。由表 1 可以看出 , 温度变化速率( 变化周期为 3 ～8 d)较低时 , 推进剂力学性能变化影响不是很明显 ; 当提高温度变化速率 ( 变化周期为1 ～3 d)时 , 推进剂力学性能变化较为明显 , 抗拉强度明显提高 , 最大伸长率明显下降。

(4) 由于推进剂抗拉强度变化规律不明显 , 取最大伸长率进行相关性分析。将最大伸长率三组数据每两组都分别拟合 , 运用最小二乘法求得的直线方程Y = a + bX 的系数 a 、 b 和相关系数 r 见表 2, 其中 b 为伸长率变化速率之间的相互关系。由表 2 可以看出三组数据之间相关性较好。

3 　相关性研究

3. 1 　贮存条件相关性研究

 (1) 推进剂放进充满干燥氮气的密封罐内进行加速老化。一般发动机贮存时都是在发动机及贮运发射箱内充满干燥的惰性气体来隔绝水分和氧气。本试验在密封罐内充满干燥氮气有效地防止了空气中水分和氧气的影响。

 (2) 推进剂粘接于特制的试验夹具中 , 模拟真实发动机壳体。在本试验中将推进剂无应力粘接于试验夹具中 , 随着温度的变化会在推进剂内部产生交变的应力应变 , 来模拟实际贮存过程中发动机壳体内的应力应变。

3. 2 　温度载荷相关性分析

在加速老化过程中 , 由于加速老化的试验条件与自然老化的环境条件毕竟有所不同 , 温度变化范围、速率的选择对试验结果的准确性起着重要的作用。温度变化范围过大容易导致推进剂化学老化机理发生变化 , 引起更为复杂的化学反应 , 范围过小 , 老化时间又太长。温度变化速率过快容易导致传热过程达不到平衡 , 温度引起的应力响应远远滞后于温度的变化 , 由温度交变引起的应力应变也会改变老化进程 , 容易造成粘合剂母体主链的破坏。变化速率过低同样会导致老化时间延长。高温加速老化一般维持在较高温度下进行 , 与自然贮存过程中温度周期性变化的实际情况缺少很好的相关性。交变温度加速老化方法在温度变化过程中加快温度变化速率 , 加大温度幅值范围 , 温度加载尽量模拟自然贮存过程中温度的变化规律。

在进行与自然贮存条件下环境温度载荷的相关性分析时 , 选取具有代表性的南海某地区气温资料与随机一组试验温度数据进行研究 , 1981 ～ 2000 年南海地区某阵地仓库月平均气温见表 3 。

为使温度数据具有可比性 , 准确地分析加速老化试验与自然贮存温度变化规律之间的相关性 , 对时间单位进行无量纲化处理。由于试验温度变化周期对应于自然温度变化周期 , 试验温度值的选取也对应于自然温度变化值 , 在对温度值变化趋势进行比较时 , 对时间单位无量纲化可以使时间轴一致 , 不影响温度值变化趋势。将温度数据进行曲线拟合得到温度随时间变化曲线 , 如图 3 、图 4 所示。可见 , 加速老化试验中温度变化趋势与自然贮存温度变化趋势非常吻合。按照前面所述相关系数的计算方法 , 将每组试验温度值与自然贮存温度值进行相关性分析 , 得到相关系数分别为 0. 9611, 0. 9577, 0. 9588 。

3. 3 　力学性能变化相关性分析

During the long-term natural storage of solid rocket motors, the mechanical properties of the propellant grains will continue to decline, eventually leading to engine failure. Our unit has carried out long-term monitoring of a certain type of solid rocket motor for many years, and has successively dissected several motors with different storage years for testing. The average values ​​of tensile strength and maximum elongation of propellants during different storage periods. It can be seen from Table 4 that the tensile strength of the propellant does not change significantly with time, and the maximum elongation decreases significantly.

In order to analyze the relationship between accelerated aging and the rate of change of elongation during natural storage, the rate of change of elongation under different aging conditions was obtained by data fitting. Considering that the change of the maximum elongation rate under natural storage conditions in Table 4 takes years as the time unit, in order to ensure the unity of time units, after converting the aging time into years, the data of elongation changes with time were simulated. combine. The coefficients a, b and correlation coefficient r of the linear equation Y = a + bX obtained by the least square method are shown in Table 5, where b is the change rate of propellant elongation under corresponding aging conditions.

It can be seen from Table 5 that the change rate of the maximum elongation of the propellant is different at different temperature change rates, and the higher the temperature change rate, the greater the elongation decrease rate. According to the relationship between the change rates of elongation, the change rates of elongation under three different accelerated aging conditions were 9.2 times, 13.3 times, and 19.7 times that of natural storage, respectively, that is, alternating Temperature accelerated aging for 1 d is equivalent to natural storage for 9. 2 d, 13. 3 d, 19. 7 d, respectively.

4 Conclusion

 (1) The change trend of mechanical properties of H TPB solid propellant in the process of accelerated aging at alternating temperature is basically consistent with the change trend of properties in the natural storage process.

 (2) The rate of temperature change has a great influence on the rate of change of propellant properties during the accelerated aging process.

 (3) There is a good correlation between the accelerated aging at alternating temperature and natural storage, and it can be used as a new method for propellant life estimation for further research.