MODEL INVESTIGATION OF EXPLOSIONS IN PRESTRESSED MEDIA

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Abstract
Seismic effects of explosions and rupture propagation in prestressed two‐dimensional models (Plexiglas, aluminum) as well as anisotropy produced by the stress field are investigated. An explosion in a prestressed medium releases a portion of the stored strain energy by one or more of the following mechanisms: (1) formation of directional cracking, especially in brittle materials, (2) release of strain energy in the elastic zone outside the cavity, and (3) rupture propagation. Phenomena associated with all of these mechanisms were observed in the present investigation. Explosions in prestressed Plexiglas produce cracks in preferred directions, the intensity of which increases with the applied stress. Explosions in prestressed aluminum sheets do not cause fracturing but rather plastic deformation about the explosion. Straight and branching modes of moving cracks initiated from explosions in prestressed Plexiglas can be explained on the basis of stress distribution ahead of the crack tips. Observed radiation patterns resulting from explosions in prestressed media indicate asymmetrical radiation fields which are a direct consequence of strain energy release for the case of aluminum and by the combined effects of directional cracking and energy release in the elastic zone for the case of Plexiglas. Explosions in prestressed media generate shear waves. The observed S‐wave magnitude increases sharply with the level of the existing stress field for a given amount of strain energy release. It is concluded that this phenomenon is attributable to the effective conversion of energy release to seismic radiation at high ambient stress fields. In other words, the effectiveness of S‐wave generation is governed by the rapidity with which the existing strain energy is released. A definite anisotropy effect was observed in prestressed models, but this effect is not large enough to affect wave propagation in the range of the tensile loads applied.

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