Photoelastic Investigation of Fragmentation Mechanisms. Part I. Borehole Crack Network,

摘要

The detonation of an explosive in the vicinity of a free face produces a complicated fragmentation crack network. Experiments have shown two distinct generation sites exist for cracks making up the fragmentation network--borehole wall sites and sites at flaws located away from the borehole. This report considers the crack network originating at the borehole. The details of borehole crack initiation,propagation and branching were studied in two-dimensional polymeric models. Through the use of dynamic photoelasticity and high speed photography,it was found that the reflected stress waves greatly influenced the borehole crack network and aided in the overall fragmentation. The major contribution of the reflected stress waves is in the initiation of a circumferential crack network between the borehole and the free face. This circumferential crack network is assisted by re-initiating previously arrested cracks and by branching and the reorientation of existing propagating radial cracks. The fracture characteristics of the polymeric material used in the fragmentation tests is thought to be similar to those of fast fracture in rock. Thus these results are felt to be qualitatively applicable to fragmentation blasting in rock.