Ph.D. Thesis Defense by Patricia Willice Selcher

(Dr. W. Steven Johnson, advisor)

"Development of a Power-Law Crack Growth Model for a Rocket Motor Propellant Exhibiting Nonlinear Viscoelastic Behavior"


Initial attempts to characterize fracture properties in propellant involved application of linear elastic fracture mechanics (LEFM) through the use of the elastic-viscoelastic correspondence principle.  For propellant this approach is valid only in the linear viscoelastic (LVE) portion of crack growth.  For these tests, the majority of crack growth occurs in a nonlinear viscoelastic (NLVE) region.  A normalization procedure was implemented to accrue two benefits.  First, normalizing removed enough of the nonlinear effect that LEFM techniques could be applied.  Second, an effective crack length was determined as a function of time through a reduction in stiffness of the fracture sample.  Coefficients for a power-law crack growth model were then determined.

Resistance to crack growth is assessed for mode mixity, pressurization, and damage.  An error analysis was performed to determine the accuracy of the power law model.  A discussion regarding the application of this model to a motor analysis is provided.