Ballistic impact studies of a borosilicate glass

The study of the ballistic impact of transparent materials allows the mechanisms of penetration and failure to be observed relatively easily. For example, failure of such materials may often be seen directly using high-speed photography. In addition, if a metal projectile is used, the difference in free electron densities between target and penetrator can allow sufficient contrast for observation using flash X-rays. By comparison with metals, silica-based glasses have high compressive yield strengths but low fracture toughness. Because of their excellent rigidity and strength per unit weight, they have been used in applications such as security glazing in vehicles and blast-proof windows in buildings. The material chosen for study was the widely-used borosilicate glass, known also as 'Pyrex', which has a dynamic yield strength of over 5 GPa and an open structure which allows a degree of compaction under shock. The ballistic impact properties of a borosilicate ('Pyrex') glass was studied using mild steel rods accelerated using a light gas gun. High-speed photography at sub-microsecond framing rates was used along with schlieren optics to investigate the propagation of elastic shock waves and fracture fronts. Flash X-radiography was used to visualise the deformation of rods as they penetrated the comminuted glass normally. The rod was seen initially to dwell on the surface for at least 3 [mu]s creating a Hertzian cone-crack. Later on, between 40 and 60 [mu]s, self-sharpening of the projectile was observed as the 'wings' of the heavily deformed front end sheared off. After this event, the front of the rod speeded up. X-rays also showed that the pattern of fissures within the comminuted glass was observed to be very similar shot-to-shot. X-radiography was also used to examine the mechanisms occurring during oblique impact of rods at 45°. In oblique impact, bending of the rod rather than plastic deformation ('mushrooming') takes on the role of distributing the load over an area larger than that of the original rod diameter. High-speed photography of the rear surface of a glass block on which a fine grid had been placed confirmed that the comminuted glass moved as larger interlocked blocks. The experiments were modelled using the QinetiQ Eulerian hydrocode GRIM making use of the Goldthorpe fracture model. The model was found to predict well the transition from dwell to penetration.