The ballistic and mechanical properties of a composite solid propellant (CSP) grain deteriorate due to physical ageing, cumulative damage and chemical ageing during its long storage period. In this research work, the chemical (natural) ageing of CSP has been explored which occurs due to the reactive state of the CSP ingredients under the influence of ambient temperature. In literature, isothermal ageing has been adopted to study the thermo-chemical ageing of CSPs. It does not account for the diurnal & seasonal variations in the ambient temperature and the reported diffusion limited inhomogeneous degradation of large propellant grains/ blocks indicate the significance of these variations on the CSP ageing. Moreover, the shape/ size of a grain and thermal diffusivity of the CSP influence the diffusion-driven natural ageing and can also contribute to the heterogeneous ageing in the grain/ sample. Therefore, a novel cyclic-thermal ageing methodology has been developed to address the above mentioned issues. In addition, an inexpensive and non-contact type thermal diffusivity measurement technique suitable for CSPs and low flash/ melting point materials has been devised, since there are only few techniques which could be effectively employed for CSPs. Thermal diffusivity of the CSP is required for the heat transfer analysis which verifies homogeneous ageing in the grain/ sample.
Accelerated ageing (cyclic & isothermal) of an AP/Al/HTPB based CSP (86% solid loading) has been conducted for the equivalent natural ages of up to 5 years wherein the ambient temperature variation during 24 hours is reduced to equivalent time periods of 90, 153, 181 and 214 minutes by increasing the mean temperature using Arrhenius relationship and heat diffusion analysis. The maximum temperature was restricted to 65 oC to ensure natural degradation processes during the accelerated ageing. Physical, tensile and creep properties of the aged specimens are obtained and compared with those of unaged specimens to observe the changes caused by the ageing. The changes indicate higher deterioration under the cyclic ageing as compared to the isothermal ageing. The results also affirm that the temperature rate of the ageing stimulant has a significant influence on the degradation rate of the CSP, besides the mean temperature of the ageing cycle. Above signifies the importance of the diurnal variation during the natural ageing. Therefore, the influence of diurnal variation of the ambient temperature should be included while designing the accelerated thermo-chemical ageing methods for propellants. The developed cyclic thermal ageing method effectively addresses this.