Ballistic fragmentation confinement of coated brittle transformer bushing models

Pressurized borosilicate glass cylinders were used to (1) simulate experimentally high-voltage transformer bushing behavior under high-velocity impact from an air gun, (2) investigate fragment dynamics to estimate the impact range of airborne fragments, and (3) evaluate the efficiency of an elastomeric coating on ballistic damage initiation and fragment confinement. Using high-speed cameras, the velocities and directions of ejected fragments from the cylinders were determined along with fragment distribution symmetry. Drop tower testing was added to independently verify the air gun results on the cylinders through a different test under similar impact energies using flat samples of the same glass. Elastomeric coatings were also applied to both the cylinders and flat samples to increase their resistance to high energy impact. It has been shown that internal pressure in the glass cylinders plays a major role in the failure modes during high-velocity impacts. This pressure affects velocities, directions, and symmetry of fragment distribution. The fragment impact range investigation turned out to be inconclusive due to highly erratic behavior of the fragments after the blast. Most importantly, it has been ascertained that fragment dynamics are drastically altered by elastomeric coatings, producing a high level of fragment confinement both in the drop tower and gun tests. By extrapolating the air gun and drop tower data out to high power rifle energy levels, unique predictions of the critical coating thicknesses to prevent the initiation of ballistic damage and to confine fragments in borosilicate glass cylinders and C-120 porcelain bushings were achieved for the first time.