Research Opportunity: The objectives of this research are to use cohesive zone modeling to improve the characterization of flawed interfaces in solid rocket motors. Many solid rocket motors have solid propellant adhered to internal insulation that protects the case from hot combustion gases. Due to differences in thermal expansion coefficients, the propellant sometimes detaches from the insulation, and the resulting defect, called a delamination, can cause catastrophic failure of the motor upon firing. Currently, these defects are both hard to detect and hard to characterize; the characterization uses linear elastic fracture mechanics (LEFM) concepts. These concepts, developed in the latter half of the 20th century, work well with brittle materials and semi-ductile materials, but are hard to apply to extremely compliant materials such as solid rocket propellant due to assumptions about crack tip sharpness and small damage zones near the crack tip. Cohesive zone modeling (CSZ) has the potential to predict tendencies for these delaminations to propagate with predictive capability that is more robust and accurate than LEFM. To make this happen, a combination of experimental and computational (finite element) work is proposed. The cohesive zone model has parameters associated with it that can be determined experimentally, especially with the use of inverse methods. The model can then be incorporated into a user subroutine that works with existing finite element codes to predict flaw propagation. Ideally, the finite element code would be validated using more complicated specimen geometries and loading sequences. In addition, the validation process could be used to compare various aspects of cohesive zone models (e.g., the shape of the associated traction-separation law). The goal of this work would be to use a combination of experimental and computational work to optimize cohesive zone models that predict delamination propagation in solid rocket motors. A knowledge of finite element analysis and some knowledge of experimental mechanics would be very helpful. In addition, skills such as the development of finite element user subroutines and the use of inverse methods would be helpful, but can be acquired on the job if necessary.

Reference: Zhou, Qing-Chun, Ju, Yu-Tao, Wei, Zhen, Han, Bo, and Zhou, Chang-Sheng, Cohesive Zone Modeling of Propellant and Insulation Interface Debonding, The Journal of Adhesion, V. 90, pp. 230-251 (2014).

key words

Cohesive zone model; Fracture mechanics; Rocket motor; Propellant; Finite element analysis

Citizenship:  Open to U.S. citizens

Level:  Open to Postdoctoral and Senior applicants