Effect of Cyclic Loads on Structural Damage

of Rocket Propellant Grain
 

Nikola Gligorijević
Vesna Rodić
Bojan Pavković
Jovica Bogdanov
Saša Živković
Zoran Bajić

Rocket motor propellant grain, cast or bonded into the motor chamber or solid coating, is exposed to various loads during its service life. Rocket propellant is a viscoelastic material, whose mechanical properties strongly depend on temperature and strain rate. Application of cyclic loads, primarily thermal, onto a rocket motor on the field stock, may cause an undesirable mechanical damage of the grain. Over time, after long storage, it may grow up and lead to the grain failure. One of the ways for evaluation of this phenomenon is to determine a cumulative damage law. In this example, this law has been evaluated for hydroxy terminated polybutadiene (HTPB) composite rocket propellant by exposing a number of specimens to different long
term stress levels. Using this law, it is possible to calculate a probability of failure and time distribution of the structural reliability of the grain in order to determine the useful life. Besides, the analysis has shown that there is a possible correlation between tensile strength of the propellant and the cumulative damage law. This fact can be used instead of carrying a longterm experiment for the exact determination of the law

 

Key words: rocket motor, propellant grain, mechanical properties, structural analysis, cumulative damage, service life.