Experimental Evaluation of Three Dimensional Effects in Cracked Motor Grain Geometries

Authors: C. W. Smith; VIRGINIA POLYTECHNIC INST AND STATE UNIV BLACKSBURG

Abstract:

A series of experiments utilizing a refined frozen stress photoelastic analysis were conducted on a generic motor grain geometry consisting of six equally spaced fins in order to study the feasibility of measuring crack growth and stress intensity factor (SIF) distribution along the grown flaw borders of internal surface cracks under internal pressure and corresponding corner singularity orders where the cracks intersected fin surfaces. Subsequently, these methods were applied to scale models of a real motor grain. Parallel studIes using a coarse grid method were conducted on cracked inert propellant and pure binder biaxial models in order to study the influence of hard particles on crack opening and growth and to characterize the material behavior of simulated motor grain over an environmental range typical of transport and storage of rockets. Uncracked specimens were used to study stress relaxation and using acousto-ultrasonics, damage development was measured. Results showed that (1) crack geometry and SIF distributions were generally three dimensional but, in special cases, could be predicted with simple two dimensional models, (2) near tip opening and growth mechanisms were the same in both pure binder and inert propellant but that a non-linear blunt- growth-blunt phenomenon, occurred at all temperatures except -65 deg F where the blunting was suppressed, simulating dosely linear elastic fracture mechanics behavior, (3) measured corner singularity orders were comparable to theoretical results in the photoelastic models, in the simulated propellant models, displacement values were greater for blunted than for sharp crack tips, (4) damage accumulation was best described by an energy content parameter in the acousto-ultrasonic method. (AN)

Limitations:

APPROVED FOR PUBLIC RELEASE