Branching of a fast crack in polymers

Crack branching in Araldite d and Homalite-911 was studied using a modified Cranz-Schardin type high speed camera which provided simultaneously taken shadow patterns and specimen-focussed images. Fracture parameters including crack velocity a, stress intensity factor K_d and specific crack extension resistance R^{{six pointed black star}} were measured in the course of crack propagation. No definite critical values could be determined with one of those fracture parameters for the onset of crack branching. Instead, the product R^{{six pointed black star}}a, i.e., energy per unit crack width per unit time, was found to have a critical value for the branching.