Summary

Composite solid propellants (CSPS) are widely used as fuel for solid rocket motors (SRM), where the binders of CSPS hold all solid particles (sush as ammonium perchlorate-AP, cyclotetramethylene tetranitramine-HMX, aluminum-Al) together to produce particle composites, and in addition to acting as the primary fuel, It also provides the necessary mechanical properties. However, CSP particles are constantly endure thermal and mechanical loads during storage, transportation, and operation, which may lead to the development and propellant of cracks in the propellant particles.These cracks can result in the formation of additional combustion surfaces within the propellant particles, thereby increasing pressure and causing combustion instability in the SRM.

Therefore, in 2023, the journal “Polymer Testing “published a study conducted by the Aerospace Engineering Department of  Madras Polytechnic University on the behaviour of composite solid propellants under biaxial tensile loads.The aim of this study was to investigate  the mechanical response of CSP particles under complex loading such as storage, transportation and operation,in order to assess the structural integrity of the various components within the grains after exposure to such conditions.

Based on cross-shaped specimens, biaxial tensile tests with two loading ratios of 1:1 and 0.5:1 at different loading rates and temperatures were designed. The mechanical responses of materials under different loading conditions were compared with uniaxial tensile tests, and the dependence of loading rate and temperature on the yield stress and initial modulus of materials under biaxial working conditions was summarized. According to different mechanical responses, energy dispersive spectroscopy (EDS) was used to perform fracture imaging and elemental mapping on the fractured surfaces for mechanism analysis.

 Preparation of Test Pieces and Experimental Design

On a low-speed computer numerical control drilling machine, the CSP plate with a thickness of 20 mm was processed into a cruciform sample shape with a copper-titanium non-sparking cruciform cutter, and the center of the sample was thinned.

    Fig. 1.  Sample Preparation Process and Sample Size

The biaxial tests were performed on a BISS (ITW, Illinois, USA) biaxial testing machine with a load of 5kN, using a single charge-coupled device (CCD) camera facing the front surface of the cross-shaped sample to capture DIC images. Biaxial tests were performed at three displacement loading rates of 1mm/min, 50mm/min and 1000mm/min. Images were captured at 2 frames per second and 50 frames per second respectively at low loading rates. At the displacement rate of 1000mm/min, the sample damage occurred very quickly. Capture images at 1000 frames per second using a high-speed Photron FASTCAM SA4 camera. The DIC technology can accurately measure strain and displacement fields up to 650 ℃. The influence of the observation window of the environment chamber and the light source outside the environment on the measurement of DIC strain and displacement field was studied through the rigid body translation experiment on the cross-shaped sample at 20 ℃.

Fig. 2. Biaxial test machine and its pneumatic grip

Test result

The test results indicate that the stress-strain response under biaxial loading is non-linear and significantly affected by the displacement loading rate. In all loading cases, the slope of stress-strain response and yield stress in the linear region increase with the increase of displacement loading rate. No significant changes were observed in the stress-strain response and yield stress slope in the linear region as the temperature changed from 20 ° C to 55 ° C. However, as the temperature increases from 20°C to 55°C, a significant decrease in the failure strain is observed.

 Fig. 3.Stress–strain response in the X and Y directions during biaxial tensile tests at displacement rate ratio (X:Y) of 1:1 at 20 ◦C

Fig. 4. Stress–strain response in the X and Y directions during biaxial tensile tests at displacement rate ratio (X:Y) of 0.5:1 at 20 ◦C

Fig. 5. Stress–strain response in the X and Y directions during biaxial tensile tests at 55 ◦C up to failure at displacement rate ratios (X:Y) of (Left) 1:1 (Right) 0.5:1

At the same time, compared with the uniaxial test, the yield stress of equal biaxial test is 25% - 47% higher when the loading ratio is 1:1. However, the failure strain under equal biaxial load is 50%-70% lower than that under uniaxial load.

Fig. 6. Stress–strain response during uniaxial and equi-biaxial loading at various displacement rates and at 20 ◦C (24 ◦C for uniaxial)
(Left) Up to failure (Right) Zoomed view up to 50% strain

Mechanism study

To explain the reasons for the differences in the above test results, energy dispersive spectroscopy (EDS) was used to conduct fracture surface imaging and elemental mapping on the fractured surface.

Fig. 7. Fractography of uniaxial and equi-biaxial tested samples at 20 ◦C (24 ◦C for uniaxial) and at various displacement rates of (a) 1 mm/min (b) 50 mm/min (c) 1000 mm/min

EDS technology can effectively observe the distribution of components in CSP particles, and the fracture mechanism of CSP is usually the matrix tearing of AP particles at a low strain rate and the fracture of AP particles at a high strain rate, while chlorine exists only in AP particles, so the distribution of AP particles can be effectively obtained by observing the chlorine diagram.

Fig. 8. A typical micro-graph and corresponding chlorine map for various displacement rate ratios (X:Y) and temperatures at 1000 mm/min
(a) 1:1 and 20 ◦C (b) 1:1 and 55 ◦C

(C) 0.5:1 and 20 ◦C (d) 0.5:1 and 55 ◦C

    After analyzing the fracture sections, the following failure mechanisms were identified:

1. Compared to the uniaxial fracture surface, the voids in the biaxial fracture surface are smaller in size and depth. This is because the void formation occurs faster, and the multiple-site damage caused by void formation and coalescence results in a lower failure strain during biaxial testing. Micrographs of the biaxial fracture surfaces under different displacement rates and displacement rate ratios did not show significant differences. The fracture surface at 55°C is smoother than that at 20°C. The failure strain at 55°C is much lower than that at 20°C, leading to fewer detached solid particles and a smoother surface.

2. It can be seen from the micrograph and corresponding chlorine diagram that in uniaxial and biaxial tests of various displacements, the AP particles in the CSP of nitrate ester plasticized polyester base did not fail even at the highest displacement rate of 1000 mm/min.

Conclusion

1. The stress-strain response under biaxial loading is non-linear and depends on the displacement loading rate. Under all loading conditions, the slope and yield stress of the stress-strain response in the linear region increase with the displacement rate. No significant changes were observed in the stress-strain response and yield stress slope in the linear region as the temperature changed from 20 ° C to 55 ° C. However, as the temperature increases from 20°C to 55°C, a significant decrease in the failure strain is observed.

2.compared with the uniaxial test, the yield stress in equibiaxial loading testing  is higer by 25% - 47% . However, the failure strain under equal biaxial loading is lower by 50%-70% compared to that under uniaxial load.

3. Energy dispersive spectroscopy (EDS) was used to conduct fracture imaging and element mapping on the fracture surface. A large number of holes with larger size and depth are observed on the fracture surface of the uniaxial loaded sample compared with the sample with equal biaxial loading. CSP particles remain intact in all conditions.

Original text
Ranjan R, Murthy H, Bhowmik D, et al. Behaviour of composite solid propellant under biaxial tensile loading[J]. Polymer Testing, 2023, 124: 108054.