Thermal protection windows of fused silica on the space shuttle are subject to hypervelocity impacts by microscopic meteorites. Because of the extreme nature of these events, where velocities may reach 14 km/s, the resulting damage is qualitatively different from that produced under quasistatic or even normal high velocity (several hundred m/s) conditions. In particular, unusual "petaloid" cracking patterns are observed, whereby half penny-shaped cracks appear to nucleate on the base of the impact crater. These expand circumferentially (like Hertzian cracks), and then grow into the glass, curving outward continuously until they form one or more large lateral cracks attached to, but "floating" below, the crater. Also observed are smaller circumferentially-tangent half penny surface cracks located outside of the crater area. These several flaw types are characterized qualitatively and quantitatively, and correlated with crater dimensions (impact energy). Implications for window residual strength are noted.
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