Impact protection of borosilicate glass plates with elastomeric coatings in drop tower tests

Drop tower testing was performed on borosilicate glass plates to determine the effectiveness of an elastomeric polyurethane/urea coating against impact and to improve our understanding of how the coating could be used to protect high voltage (HV) porcelain bushings against rifle damage [1,2]. Drop tower testing was found to be a very useful and practical tool for the prediction of failures of very brittle materials subjected to low energy impact as a function of coating thickness. The testing also improved our understanding of the impact behavior of the coatings and how to relate their low energy impact performance in drop tower tests to much higher energy impact situations, such as rifle bullet impact. By performing a series of drop tower tests on the plates with the coating thickness ranging from 1.3 to 5.6 mm, the effectiveness of the coating was determined from the critical contact force and from the critical impulse. These critical parameters were determined at the onset of catastrophic fractures of the uncoated and coated glass specimens. It was found that the coating protected the glass samples by a factor of 5.25 times with respect to the critical maximum contact force, and 9.31 times with respect to the critical impulse. Therefore, the overall coating effectiveness was estimated to be 7.28. It was also shown that the high effectiveness of the elastomeric coating can be achieved for a coating thickness as small as 1.3 mm, for a range of critical impact energies between 8.7 J and 22.7 J. Since the same coating with a thickness of 3 mm efficiently protected HV porcelain bushings against rifle impact at energies as high as 3.1 kJ [2], the very high effectiveness of the protection for small coating thicknesses could be independent of the impact energy within the range of energies considered. This therefore could be the main observation from the comprehensive work performed in this study and in [1,2].