Formation of a near-surface fracture zone in a contact explosion

This paper describes optical recording studies of explosive fracture on the surface of colophony specimens. It is shown that the existence of a free surface results in formation of a conical fracture front, which considerably increases the volume of the dynamic fracture zone. On the other hand, the free surface effect is responsible for spallation in the surface layer. Simple estimation of the spall layer thickness using measured blast wave parameters is in good agreement with experimental values. A comparison of the experimental results with data on the structure of meteorite craters shows similarity in structure between the fracture zones.