This paper presents a study of the sequence of events by which cracks ``grow.'' A careful investigation was made of the origin, development, and characteristic pattern of level-difference markings left by rapidly moving fractures in a wide variety of materials. Characteristic markings indicating discontinuous propagation are found in fractures of plastics, polycrystalline and single-crystal materials, coal, mica, photographic film from which the emulsion was stripped, etc. Thus, the continued occurrence of essentially discontinuous initiations which comprise fracturing becomes clear. These considerations are sufficient to explain a number of well known fracture markings and to clarify what is mean by brittleness and fracture velocity.
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