GLASS‐FRACTURE VELOCITY*

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Abstract
Using an electric spark lasting less than one‐millionth second as a light source, cracks in glass have been successfully photographed during their propagation. A steel ball driven against the glass with a spring initiated fracture and simultaneously actuated an adjustable electrical time‐delay circuit, which in turn set off the spark at the required instant. By the use of two sparks, the velocity of propagation of cracks in glass was found to be constant and almost exactly 5040 feet per second—the same velocity found by Schardin and Struth. In general, the tips of the cracks form a nearly perfect circle at any particular instant, though some individual photographs show nonsymmetry.

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