Abstract
A case-bonded solid propellant rocket grain is subjected to many stress-inducing loads during the service life, due to temperature, extended polymerization, transportation, vibration, acceleration, aerodynamic heating etc. and finally due to the operating pressure in the rocket motor. Composite propellant is a viscoelastic material whose mechanical properties highly depend on temperature and strain rate and sometimes may vary in the range of use of rocket motors for several orders of magnitude. Relationships between stresses and strains are much more complex than for the elastic material. Therefore, the stress and strain analysis and estimation of safety factor under the action of each individual load is quite complex and sometimes impossible. An even greater problem occurs when multiple different types of loads act simultaneously. An extreme case occurs in the moment of rocket motor ignition. Then, the very fast load acts due to the pressure, at which the propellant tensile strength is high. At the same time, the very slow thermal load acts on the grain, and in these conditions the propellant tensile strength is low. The vector addition of the stresses and strains due to different loads is not possible. It is also not possible to define the equivalent or resultant values of tensile strength and allowable strain. The principle of adding the current damage is applied here, similar to the model of cumulative damage. In addition, due to the large variations in mechanical properties of the rocket propellant, it is necessary to apply the methods of mathematical statistics for assessing the propellant grain reliability and service life.
Highlights
IN the theory of elasticity, under the assumption of small strains [1], in environmental conditions within the normal temperature range of use, under the uniaxial extension, there is a linear ratio between stresses and strains
The safety factor of an elastic body is defined as the ratio between the constant value of the material ultimate strength (σ m ) and maximum equivalent stress (σ 0 ), which is a resultant of stresses caused by different loads
This definition is quite simple because it implies that various types of loads and the manners that act onto the elastic body, have no effect on the ultimate strength or allowable strain of elastic material (σ m,ε m ), that remain nearly constant in all conditions
Summary
IN the theory of elasticity, under the assumption of small strains [1], in environmental conditions within the normal temperature range of use, under the uniaxial extension, there is a linear ratio between stresses and strains. The safety factor of an elastic body is defined as the ratio between the constant value of the material ultimate strength (σ m ) and maximum equivalent stress (σ 0 ) , which is a resultant of stresses caused by different loads. When the rocket motor works, pressure during the ignition and combustion is high, and the operating loads are fast They produce high strain rate and the propellant tensile strength is entirely different and very high. Its influence produces some current damage, presented as a ratio between the induced stress (and strain) and corresponding ultimate strength (or strain) This ratio is a relative value less than unity, which occupies a part of the propellant grain capacity to withstand the fracture.
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