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Glass surface spall size resulting from interplanetary dust impacts

The size of relatively large dynamic conchoidal fractures, i.e., surface spalls, immediately adjacent to and around interplanetary dust (IDP) hypervelocity impact (HVI) craters or pits in glass substrates is relevant to spacecraft solar cell and science instrument lens performance metrics, as well as glass pane design and safety in manned missions. This paper presents an analysis of the diameter of surface spalls in glass for the Solar Probe Plus (SPP) spacecraft, whose solar arrays and instruments must survive a 7-year mission involving significant dust interaction. Previously published data and regressions for surface spalling obtained from ground-HVI-tested and space-returned glass samples and solar cells are collated for this purpose. Analysis of the collective dataset reveals an unexpected and design-relevant finding: spall diameter, DS, obtained with dust-scale particles (diameter, dP <; 55 μm) and solar cells scales differently with impact velocity as compared with diameters obtained with “macroparticles” (dP = 400-3,500 μm) and glass monoliths. The average DS/dP obtained with dust-scale particles and glass in a layered substrate is approximately 1/5th of that obtained with macroparticles and a glass monolith. It is also found that a Ballistic Limit Equation (BLE) developed for glass HVI cratering at relatively low velocities (<; 10 km/s) can be modified for spalling and used successfully for bounding design calculations at the higher velocities considered.