Localized impact damage in glass

Abstract

Glass plates were impacted at various velocities by glass, steel and nylon spheres. The impact damage was characterized by optical and scanning electron microscopy and the remaining strength was measured. When glass plates were impacted by glass spheres (1.5 mm radius), Hertz cone cracks were formed at impact velocities of 22 m s −1 or higher. The strength was degraded to less than 20% of the original value. The penetration of the damage increased with increased impact velocity, as expected. At higher velocities radial cracks and crushing were observed. Under the debris formed by crushing, the fracture surfaces appeared to have been surfaces of lateral vent cracks which formed after the radial cracks. The maximum contact radii ( a max) were greater than the Hertz crack radii ( r ∗ ). Therefore, strictly speaking, the available theory cannot be applied to calculate the penetration of the damage and strength degradation. Nevertheless, calculated values of Hertz cone length ( C) were somewhat greater than those observed in the experiments. The cone angle (α) increases with increasing impact velocity. This observation indicates that the static stress analysis, frequently applied to subsonic impact problems, may be inadequate for this purpose. Crack branching was observed frequently at the impact site. The multiple circumferential cracks in the surface appeared to form by crack branching. Also, in cases of severe impact, the conical portion of the crack branched. Impacts by steel and nylon spheres led to less interesting results. Steel caused crushing at low velocities. Nylon caused moderate strength degradation at high impact velocities, apparently as a result of small flaws which did not appear to be Hertz cracks.