Evaluation of the structural integrity of solid rocket propellant by means of the viscoelastic fracture mechanics approach at low and medium strain rates

This work analyzes the influence of the displacement rate on the tensile properties and the fracture parameters, determined through a viscoelastic Fracture Mechanics approach, of carboxyl-terminated polybutadiene (CTPB) based solid composite propellants. Low and medium displacement rates attained in an electromechanical testing machine were inspected. From the true stress-true pseudo strain curves, the Young’s modulus was not influenced by the displacement rate with value almost identical to the instantaneous modulus obtained from stress relaxation tests. The same trend was achieved for the Poisson’s ratio, with a behavior close to incompressibility. On the other hand, the yield stress increased with the displacement rate rise. Regarding the fracture behavior, a strong dependency of the resistance curves in terms of both the viscoelasic J-integral and the viscoelastic crack tip opening displacement, CTOD, on the displacement rate was proved, being greater the fracture resistance as the displacement rate increases. Moreover, a linear relationship between the viscoelasic J-integral and the viscoelastic crack tip opening displacement was demonstrated. These results were confirmed by the fracture surfaces analysis. At low displacement rate, the damage nucleation and propagation occurred in the matrix while at high displacement rates, dewetting phenomena gave rise to rougher surfaces with damage initiation and growth mainly through the matrix – particle oxidizer interface.