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Glass Crack Rate

Materials science text books say that many glass cracks at speeds like 5 km/s. That means if you have a 5 km-long piece of glass (that is 3.1 miles), and you create a fracture down the middle of that glass, it will take only 1 second to reach the other side. That is 15x faster than the speed of sound in air. As for mechanism, the American Ceramic Society states, "A wave moves in one of three ways: as a primary (longitudinal) compression wave, a secondary (transverse) shear wave, or a surface (Rayleigh) wave. Primary waves are the fastest, followed by secondary waves, and finally surface waves. When a crack propagates, there are multiple different waves at work, each moving as one of these three wave types. In a perfectly brittle material stressed in tension, you could see crack propagation occurring at—or above—the speed of sound. But materials are rarely perfectly brittle and stressed in tension. It is more likely you will see crack propagation ranging between 0.01 to 4 km/s , depending on strain-energy distribution." (ceramics.org) The crack rate here measures very closely to what Dan and Gav of the Slow Mo Guys measured (~1.5 km/s). https://www.youtube.com/watch?v=GIMVg.... We also measured some slower cracks. Camera was v2512 at 512x512 at 330 kfps and 13 us exposure time.