Dynamic fracture of inorganic glasses by hard spherical and conical projectiles

In this article, high-speed photographic investigations of the dynamic crack initiation and propagation in several inorganic glasses by the impact of small spherical and conical projectiles are described. These were carried out at speeds of up to approximately 2 x 10(6) frames s(-1). The glasses were fused silica, 'Pyrex' (a borosilicate glass), soda lime and B2O3. The projectiles were 0.8-2mm diameter spheres of steel, glass, sapphire and tungsten carbide, and their velocities were up to 340 m s(-1). In fused silica and Pyrex, spherical projectiles' impact produced Hertzian cone cracks travelling at terminal crack velocities, whereas in soda-lime glass fast splinter cracks were generated. No crack bifurcation was observed, which has been explained by the nature of the stress intensity factor of the particle-impact-generated cracks, which leads to a stable crack growth. Crack bifurcation was, however, observed in thermally tempered glass; this bifurcation has been explained by the tensile residual stress and the associated unstable crack growth. A new explanation has been proposed for the decrease of the included angle of the Hertzian cone cracks with increasing impact velocity. B2O3 glass showed dynamic compaction and plasticity owing to impact with steel spheres. Other observations, such as total contact time, crack lengths and response to oblique impacts, have also been explained.