Strength Degradation of Glass Resulting from Impact with Spheres

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Abstract
The nature and extent of degradation incurred by glass surfaces impacted with spheres of steel and tungsten carbide were studied. The residual strength after impact depends on the velocity, radius, and density of the projectile; on the toughness and (indirectly) the hardness of the target; and, to a lesser degree, on the preexisting mechanical condition of the surface. The damage morphology involves modification of the basic Hertzian cone crack pattern by median (radial) cracks and crushed glass at the impact site. The essential features of the degradation may be predicted by a theoretical analysis of residual strength as a function of impact velocity as derived from indentation fracture mechanics. This study accounts, in particular, for a threshold velocity for significant strength loss, above which further strength decrease is relatively slight. Small, but significant, discrepancies between observed and predicted degradation characteristics are attributed to the departure from ideal Hertzian fracture geometry and to the dynamic nature of the contact. However, it is suggested that quasi‐statically based theory may be used for estimating the strength of structural ceramics in small‐particle impact situations.

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