An extension of the time–temperature superposition principle to non-linear viscoelastic solids

Publisher Summary This chapter discusses designing of the solid propellant grain. The design of propellant grains involves a number of techniques because of the nature of propellants, the geometry and architecture of propellant grains, and their operation modes in rocket motors. There are two main types of grain architectures: free-standing grains and case-bonded grains. Free-standing grains are introduced into rocket motor cases after being manufactured. Case-bonded grains are bonded to the motor case during the casting and curing steps of the propellant grain manufacturing process. There is not a single, well-defined, procedure for selecting a free-standing grain architecture or a case-bonded grain architecture for a given rocket motor, except when one of these two architectures is obviously the most appropriate for a specific reason. Selection of a propellant for designing a given grain is based on numerous criteria. A certain type of architecture energy and burning-rate criteria, structural integrity considerations, smokelessness and safety considerations make a propellant family. Each of the propellant families covers a certain range of properties, and it is necessary that the properties of the selected propellant allow design and manufacture of a grain satisfying all the requirements.

Solid Propellant Grain Design

Publisher Summary This chapter focuses on the structural analysis of propellant grains. During their entire service life, propellants are subjected to stresses, which may cause cracks in the propellant grain or separation between the propellant and the inhibitor or the liner. During firing, there are a number of possible consequences from one of these structural failures. An analytical method allowing the determination, a priori, of the structural integrity of the propellant grains is, therefore, often considered better. The principle is simple and based on determination of two values for each loading condition. These values are the induced stress or strain resulting from induced loads in the propellant grains and the allowable stress or strain. The induced stress or strain in a propellant grain is the maximum stress or strain developed in the propellant. Each type of propellant has its own specific mechanical characteristics. The methods to determine their behavior are identical, and the influence of various parameters is also the same for all propellants. The propellant capability is the induced maximum stress or strain necessary to cause failure of the material.

Structural Analysis of Propellant Grains

L'article presente une methode d'etude mecanique de chargements moule-colles en propergol double-base composite (en abrege DBC) soumis a des cycles thermiques. La methode proposee pour determiner les contraintes et le coefficient de securite est basee sur le comportement non-lineaire et la compressibilite du propergol. Le comportement non-lineaire est obtenu d'apres des essais de traction. Les equations d'equilibre sont resolues numeriquement en decoupant l'epaisseur de propergol en plusieurs couches. Les non-linearites affectent essentiellement le module et on utilise un critere multiaxial et l'equivalence temps-temperature. On calcule a chaque pas de temps et pour chaque couche la temperature, le temps reduit, le facteur non-lineaire, le coefficient de Poisson, et le dommage (selon le concept de Farris). Differents types de chargements (bloc a canal central etoile, a mamelles, finocyl) ont ete soumis a differents types de cycles thermiques. La comparison entre prediction et experience est convenable meme pour des enchainements complexes d'allongement et de temperature.

Etude mécanique de chargements moulé‐collés en propergol doublebase composite par Une méthode viscoélastique non‐linéaire

Mechanical design of case bonded CMDB grains by a non linear viscoelastic method

This paper discusses a method for the mechanical design of composite modified double base (CMDB) casebonded grains subjected to thermal cycling. The proposed iterative method for stresses and margins of safety calculations takes into account the nonlinear viscoelastic behavior and compressibility of the propellant. The propellant behavior is derived from tensile testing. The nonlinearities mainly concern the modulus. The equations of equilibrium are numerically solved by sharing the grain web into many layers. The temperature, reduced time, nonlinear factor, Poisson's ratio, and damage (Farris's concept) are calculated at each step of time for each layer. Different grain shapes (star-shaped, wagon-wheel, finocyl inner bores) have been used in experiments with various types of thermal cycles. The comparison between prediction and experiment is acceptable even for a very complex strain-temperature history.

G. Dubroca

Papers

Etude mécanique de chargements moulé‐collés en propergol doublebase composite par Une méthode viscoélastique non‐linéaire

Mechanical design of case bonded CMDB grains by a non linear viscoelastic method

Nonlinear Viscoelastic Design of Case-Bonded CMDB Grains