INTERACTION BETWEEN BENDING FRACTURES AND EMITTED STRESS WAVES
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An experimental investigation of the interaction between the brittle fractures which are observed in glass beams in flexure, and the stress waves generated by these fractures is described. A number of different experimental techniques have been employed. These are (a) The use of resistance strain gages to detect and record the waves generated; (b) High speed photography with a Kerr-Cell framing camera. This camera was also used to take high speed photoelastic patterns of the stress distributions set up during fracture; (c) Direct measurements of the duration of the fracture process by measuring the interval between the breaking of two electrical contacts on opposite sides of a glass specimen. It was found that the fracture started at some point on the side of the specimen which was under tension, and rapidly traversed about 70% of the cross section; it then slowed down as it further penetrated the zone which was initially under compression. When the bar was sufficiently long, i.e. when there was no interference from reflected pulses, the crack was found to take a sharp turn and travel slowly (i.e. at velocities very much lower than the wave velocities in glass) in a direction parallel to the axis of the beam. Fracture was finally completed when the compressive longitudinal pulses generated by the fracture returned from the ends of the beam as pulses of tension, and reached the fracture plane. This last stage could be lengthened either by using very long rods or by suitably changing the acoustic conditions at the ends of a rod. It is shown that the shapes of the emitted pulses which are observed some distance from the fracture plane can be adequately predicted by the assumption used earlier for tensile fractures, namely that as fracture progresses, the emitted wave corresponds to that expected from the relief of the stress present in the fracture cross section before fracture started. 1977.
