THE EXPLOSIVE DISINTEGRATION OF PRINCE-RUPERT-DROPS

A high-speed photographic study has been made of the explosive disinte gration of Prince Rupert's drops. The drops were prepared by quenching molten soda-lime glass in water. The disintegration of a drop was ini tiated by exploding a small (26 mg) lead azide charge or by impacting with a hardened steel chisel on to the tail of the drop. The entire fr agmentation process was recorded at framing rates ranging from 6500 fr ames s-1 to 0.5 x 10(6) s-1. The high-speed photographic sequences rev ealed that in a disintegrating drop the crack front, having been initi ated in the tail, propagated at a high velocity (almost-equal-to 1450- 1900 ms-1) within the tensile zone, towards the drop's head. Finger-ty pe bifurcating cracks at the crack front were observed. High-speed pho tographic observations, combined with an analysis of the fragment size s, indicated that the fast-moving cracks slowed down dramatically on e ntering the surface compression zone. Sequences of high-speed shadowgr aphs also revealed that the rapidly moving crack front did not produce any strong stress waves in the drops which could have contributed to their explosive disintegration. The main features of crack propagation in a drop were found to be similar to those of the self-propagating c racks in thermally tempered soda-lime glass blocks. Measurements of re sidual stresses in the drops using an indentation technique showed tha t the surface compressive stress was almost-equal-to 90-170 MPa and th e tension zone extended across almost-equal-to 70% of the drop's diame ter at the head. Both of these values are similar to those found for t hermally tempered soda-lime glass blocks. A model based on the repeate d bifurcation of fast-moving cracks within the tensile zone in the dro p has been proposed to explain its explosive disintegration.