The objectives of this research are to use experimental mechanics methods to improve the characterization of solid rocket propellants. Solid rocket propellant is composed of a high volume fraction of particles in a rubbery matrix material. The material is considered viscoelastic, and also nonlinear viscoelastic (if damage due to debonding or dewetting of the particles takes place). The complex nature of the material response is not reflected in the testing methods, however. Currently, we use simple stress relaxation tests and uniaxial tension tests to determine both constitutive and ultimate properties. In these tests, strains are measured by dividing crosshead displacement by an effective gauge length. We want to improve the accuracy of our mechanical property measurements for both small deformations and large deformations (where dewetting takes place) by employing optical methods such as digital image correlation (DIC) to better measure strain during such tests. Properties such as Young’s modulus, Poisson’s ratio, strain at failure, ultimate stress, strain at ultimate stress, volume changes during testing, fracture toughness, and stress relaxation master curves matter. An additional concept is that dynamic mechanical analysis (DMA) could be used to better characterize the viscoelastic material–the Prony series constants that we employ now come from a set of stress relaxation tests which are labor intensive test procedures–it is thought that a DMA analysis could yield useful data with less material and less effort. This work is ongoing and can be improved. The ultimate goal is better characterization of mechanical properties of propellant.

 

key words

Rocket motor; Propellant; Viscoelasticity; Stress relaxation; Fracture mechanics; Digital image correlation; Dynamic mechanical analysis;

Citizenship:  Open to U.S. citizens

Level:  Open to Postdoctoral and Senior applicants