任 萍，侯 曉，何高讓，郜 婕，何鐵山

(1．西北工業(yè)大學(xué)航天學(xué)院，西安710072;2．中國航天科技集團(tuán)公司第四研究院第四十一所固體火箭發(fā)動(dòng)機(jī)燃燒、熱結(jié)構(gòu)與內(nèi)流場(chǎng)國防科技重點(diǎn)實(shí)驗(yàn)室，西安710025)

0 Introduction

Burning pressure and external pressure are main loads for solid rocket motor in the process of working，which lead the grain to compression state．The gain’s stress analysis under this state is necessary while designing the solid rocket motor．The research of material property to get the propellant’s compression characteristic is important for analysis．

Composite solid propellant is one kind of polymer，which is composed of solid grain and shows the viscoelastic mechanical property that strongly depended on time and temperature．Tensile property is explored videly in similar material［1－3］．Such as reference［1］，tensile modulus of binded explosive is researched to get the variation of tensile modulus with temperature and strain rate．In order to research the difference between the tensile and compression modulus under the same condition，Comparative research is needed under different temperature and strain rate．

With the reference to composite solid propellant tensile modulus test standard，inert propellant is researched to probe the compression modulus．The test specimen are designed for compression test and the size of specimen are determined throuth analysis of a lot of test results and failure mode to different specimen．Comparative test of tension and compression are performed at different strain rate．

1 Brief introduction of tension and compression property test

1．1 Specimen size and facture

Specimen is factured through propellant cartoon，which is casted at the same time with the whole size solid rocket motor．Tensile specimen is dogbone tensile sample and compression specimen is square rod．In order to make the tensile and compression result comparable，compression specimen is designed with reference to the standard tensile specimen．

Compression specimen’s section is the same with the tensile specimen’s effective tensile portion’s(10 mm×10 mm)．

The length of compression specimen is adjusted approaching to tensile specimen’s effective length(h=30～50)．

Specimen’s adjust requirement:compression speed，V=0．1×h/70，1×h/70 and 100×h/70mm/min．The max load should appear at the curve of compression stress vs strain．If there’s no max load point，compression strain should reach 50%or characteristic point appear at the curve of stress vs strain(the first inflexion on the curve)．

1．2 Brief introduction of test

After the determining the specimen size，comparative modulus test of different tensile and compression speed is performed on the Instron manchine．Tensile and compression test device diagram is shown in Fig．1．In order to assure the same strain rate of specimen and make the result comparable，compression speed V=0．1×h/70，1×h/70 and 100×h/70mm/min is corresponding to tensile speed V=0．1，1 and 100 mm/min．

Fig．1 Tensile and compression test device diagram

1．3 Treatment of initial modulus

On the specified temperature and tensile speed，static load is applied on the longitudinal axis of specimen and the curve of stress vs strain is received．The initial beeline’s slope of the curve is the initial modulus．After the test of tension and compression，the curve of tensile or compression stress vs strain is received and initial modulus is treated as standard requirement．If the initial beeline of the curve is obvious，the stress corresponded with the strain of 3%and 7%is used to calculate the modulus．

2 Adjustment of compression specimen size

In order to get appropriate specimen size，compression test is carried out with speed V=100×h/70 mm/min and with the specimen height of 50，40 and 30 mm．The max load appeared on all curve of compression stress vs stain．

After the max load，specimen’s failure mode is different，which are shown in Fig．2，F(xiàn)ig．3 and Fig．4．The specimen with height of 50 and 40 mm is compressed to buckle and is broken from the middle．The specimen with height of 30 mm is compressed to destruct from impress，which is appeared on top and down part of specimen and in the direction of 45°relative to the compression direction．

The specimen test results with different specimen height are shown in Table 1．The max load，intensity and extensibility are increased with the shorten of the spencimen height．The initial modulus test results are 14．09，14．12 and 14．33 MPa with the specimen’s height of 50，40 and 30 mm respectively．The test modulus of specimen with height of 50 mm is approaching to the specimen with height of 40mm．The test modulus of specimen with height of 30 mm is higher than the spencimen of 40，50 mm．

The failure mode of specimen with height 30 mm is approaching to compression destruct and the modulus is approaching to compression modulus．The height of specimen is determined to be 30 mm and the compression test with different speed are carried out with this size．

Fig．2 Specimen compression failure mode

Fig．3 Specimen compression failure mode

Fig．4 Specimen compression failure mode

Table 1 The compression test results of different height specimen

3 Analysis of tensile and compression test result

3．1 Analysis of tensile modulus test result

The curve of tensile stress vs strain with speed 100，1 and 0．1 mm/min are shown in Fig．5．The average of test results with different speed are shown in Table 2．Tensile intensity，extensibility and modulus are all increased with the higher of the speed．The test modulus are 9．05，6．87 and 6．29MPa at the speed of 100，1 and 0．1 mm/min respectively．

3．2 Analysis of compression modulus test result

The curve of compression stress vs strain with speed 100×30/70，1×30/70 and 0．1×30/70 mm/min are shown in Fig．6．The failure mode after the max loading at the speed of 100×30/70，1×30/70 and 0．1×30/70 mm/min are shown in Fig．4，F(xiàn)ig．7 and Fig．8 respectively．The specimen in Fig．4 is compressed to destruct from impress which is appeared on top and down．The specimen in Fig．7 and Fig．8 is compressed to destruct from impress which is appeared on the top．The impresses’s angle are all the degree of 45 with the compression direction．

The average of test results with different speed are shown in Table 3．Compression intensity，compressibility and modulus are all increased with the higher of the speed．The test modulus are 14．33，10．79 and 9．41 MPa at the speed of 100×30/70，1×30/70 mm/min and 0．1×30/70 mm/min respectively．

Table 2 The tensile test result with different speed

Fig．5 The curve of stress vs strain

Fig．6 The curve of compression stress vs strain

Fig．7 Compression failure mode

Fig．8 Compression failure mode

Table 3 The compression test result with different speed

3．3 Comparative analysis of tensile and compression initial modulus

The initial tensile and compression modulus are all treated as the slope of initial beeline between the strain of 3%and 7%on the curve of stress vs strain． diagram of how to get the initial tensile and compression modulus on the curve of the tensile and compression stress vs strain at strain rate of 100/70，is shown in Fig．9．On the same strain rate，the slope of initial beeline on tensile and compression curve is different．

The tensile and compression modulus test results on different strain rate are shown in Table 4．Variations of tensile and compression modulus with strain rate are shown in Fig．10．Compression modulus is higher than the tensile modulus at the same strain rate．The increasement of compression modulus to tensile modulus is 58%，57%and 50%，at the strain rate of 100/70，1/70 and 0．1/70 respectively．

Fig．9 Calculation of tensile and compression modulus

Fig．10 Variations of tensile and compression modulus with strain rate

Table 4 Tensile and compression modulus with different strain rate

4 Conclusion

The specimen size of compression modulus is determined through a lot of test with different size．The difference between the compression and tensile modulus are probed through the tensile and compression test on different strain rate．On the same strain rate，compression modulus is higher than the tensile modulus．The rule of variations of compression modulus with test speed is the same with the tensile modulus．With the increasement of compression or tensile speed，the modulus become higher．

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