WATER 



AIR 



r - - -. 



— • • 



- « 4 



.• •• 





PLATE 



TIME 



Figure 18. Reloading of an airbacked plate. 



The impact of the shockwave accelerates the plate to a high velocity. This 

 takes place within a very short time, i.e., near the origin of the plot in Fig. 18. The 

 pressure in the water subsequently decreases rapidly with time and cavitation sets in 

 near the plate. At this moment, the plate has a higher velocity than the water particles 

 which follow; — each cavitated particle being slower than that ahead of it. 



When the plate is retarded by the elastic forces of the material, the cavitated 

 water particles catch up. They transmit their momentum to the plate and form a layer 

 of uncavitated water which moves with the plate. This so called spray reloading 

 occurs during the first phase of the deflection. The energy of this reloading stems from 

 the shockwave. 



When the plate has lost its velocity (first maximum of the deflection curve in 

 Fig. 18) that portion of water which moved with the plate has also come to rest. 

 But the water farther away from the plate is still moving and is pushed ahead by the 

 expanding bubble. These two masses of water are separated by gaps, namely the 

 cavitation bubbles. These are quickly closed and the moving mass impinges on the 

 resting one. This produces a closure shock and the second reloading. Its energy stems 

 from the bubble energy. By this means the bubble may transmit up to twice as much 

 energy to the plate as the shockwave. 



This type of reloading occurs only with thin plates. It is not expected to occur 

 with more rigid structures, such as a submarine hull, because no or only weak cavitation 

 [10, 20] will occur there, and cavitation is necessary to liberate the bubble energy ioi 

 the reloading. 



The Response of Cylindrical Shells to a Shockwave 



The response of an elastic cylindrical shell to a pressure wave has been treated 

 by Mindlin, Bleich [21] and others, Fig. 19. Here the acoustic approximation and the 

 neglect of bubble effects is quite realistic. Hydrodynamically seen this is a straight 



343 



