A study of near tip phenomena for cracks in a particulate composite

An experimental investigation using grids with a frequency of 125 lines/in. (5 lines/mm) was performed on inert propellant and pure binder at two different global head rates of 0.1 in./min (2.5 mm/min) and 1.0 in./min (25.4 mm/min). From the extracted data, displacements, strains, and dominant eigenvalue for displacement were calculated. An idealized model was used to explain the high strain zone ahead of inert propellant that caused severe blunting at the crack tip.

Using the available algorithms and three dimensional photoelasticity, the dominant stress singularity order values were calculated in a four point single edged cracked bend specimen with both straight front and thumbnailed cracks. The free surface values are the same as for the inert propellant and in good agreement with analytical values.

A boundary layer is observed in the singularity order which extends towards the mid-plane of the specimen. This region is about twenty percent of the distance from the free surface to mid-depth of the fractured body.

The slow and fast head rates alter the global behavior of the specimen as well as the density of the displacement and strain contours. However, the near tip mechanisms are not altered.