Abstract
Mechanical properties of viscoelastic filled polymers strongly depend on temperature and strain rate and vary for several orders of magnitude. During service life, a viscoelastic body, especially carboxy-terminated polybutadiene (CTPB) composite solid rocket propellant grain, is subjected to many stress-inducing loads. Its structural integrity analysis (hereafter: ?structural analysis?), unlike elastic bodies, is quite complex and sometimes impossible under the action of just a single load. An even greater problem occurs when multiple different types of loads act simultaneously. This study is based on a complete uniaxial mechanical characterization of a viscoelastic CTPB composite rocket propellant, made in MTI- -Belgrade, whose results were used for the analysis of the propellant grain reliability. Through an example, this paper shows a behavior of the viscoelastic propellant grain when it is subjected to extremely different environmental loads at the same time. Similar explicit examples are difficult to found in the literature, except in the form of recommended principles for analysis. It is shown that the tensile strength under the action of fast load due to the pressure may be almost 20 times greater than the tensile strength under the slow temperature load. A probabilistic approach is presented in evaluation the reliability and service life. An example is shown for a rocket propellant grain as a viscoelastic body. The presented principles of the analysis can be applied to any arbitrary viscoelastic body in other areas.
Highlights
Mechanical properties of viscoelastic filled polymers strongly depend on temperature and strain rate and vary for several orders of magnitude
The problem is even greater if the external loads, that act onto the viscoelastic body, are time dependent, which causes the additional variations of material mechanical properties
This paper discusses the complexity of determination the safety factor of a viscoelastic body due to the high dependence of its mechanical properties on external conditions, especially in the cases where different loads act simultaneously onto the body
Summary
Mechanical characterization of structural materials is the process of determining their mechanical properties, modulus of elasticity (E), ultimate strength (σm), and allowable strain (εm) in order to estimate safety factor and resistance of the body to the effects of external loads. During the ignition of the rocket motor, especially at low ambient temperature, the pressure in the motor is high, and the operating loads are fast They produce high strain rate and the propellant tensile strength is entirely different and very high. A bigger problem occurs in structural analysis of viscoelastic propellant grain when two or more different loads act simultaneously From the moment it is cast, the propellant grain is continuously under the temperature load that produces quite low strain rate. Instead comparison the constant value of ultimate strength to the resultant stress of all external loads, in order to get the total value of the safety factor, each individual load is considered separately It is theoretically possible if the linear viscoelasticity is assumed and convolution concept is applied [9].
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