Boundary Layer and Singularity Measurement in Three Dimensional Fracture Problems

摘要

Experimental methods (coarse grid and high density moire interferometry) were used to measure near tip surface displacement during the opening and growth of cracks in 'biaxial' specimens of inert propellant and pure binder in tests conducted at two different head rates (2.5 and 25.4 mm/min). Displacement data were collected from coarse grids at regular time intervals. An intense strain zone developed ahead of the crack tip in both materials. However, the size of the zone in the inert propellant was at least an order of magnitude larger than in the pure binder, shifting the high strain contours ahead of the crack tip. From such data, an idealized model for the intense strain zone in the inert propellant was postulated. Values of the dominant eigenvalue near the crack tip were computed from appropriate algorithms where the crack intersected the free surface. Frozen stress photoelastic analysis of four point bend tests were used to infer the distribution of the dominant eigenvalue through the specimen thickness for both straight front and thumbnailed cracks. Results compared favorably with an analytical result obtained by Benthem. Results indicate an absence of transverse constraint in the intense strain zone of the inert propellant which suggests the absence of a strong thickness effect upon the threshold value of the stress intensity factor. Keywords: Solid rocket propellants, Solid rocket propellant binders, Crack opening, Crack growth, Near tip displacement, Strain determination, Dominant singularity order distribution. (EDC)