17 



Figure 1 - Approximate Shock-Wave 



Pressure in the Water at 50 Feet 



from 300 Pounds of TNT 



The ordinate represents the pressure In open water 

 as it would be recorded by a gage so small as to cause 

 no appreciable modification of the pressure field. 



Figure 2 - Oscillogram Showing the 

 Pressure in the Water at 17 Inches 

 from a Charge of 1 Ounce of Tetryl 



The pressure rises almost instantaneously to 

 a peak value of 3400 pounds per square inch. 



come to be related almost in the way in which they would be related if the 

 water were incompressible. Any effects that may be produced by the tail of 

 the pressure wave constitute those effects which are sometimes ascribed, not 

 to the pressure wave, but to the expansion of the gas globe. 



During subsequent recompressions of the gas globe, secondary pulses 

 of pressure are emitted. The character of these is not yet certain. The 

 theory of an oscillating spherical gas globe indicates that the time graph of 

 the pressure in the secondary pulses should be roughly symmetrical about the 

 point of peak pressure, without any shock front, and should be weaker and 

 much broader than the initial shock wave. See Reference (10). 



PRIMARY SHOCK WAVE AND AN AIR-BACKED PLATE: 

 A TYPICAL SEQUENCE OF EVENTS 



The analytical results will now be described for the case of a 

 shock wave falling upon one of the plates of a ship's shell, or for a corre- 

 sponding test on model scale. The wave will be assumed to fall normally upon 

 the plate, and both wave and plate will be assumed to be sensibly plane. The 

 action can be divided into two distinct phases, which will be discussed in 

 order. 



Primary Shock Phase 



In the cases considered here, the time required for an elastic wave 

 to traverse the thickness of the plate is so short that it fnay be neglected; 

 the plate can be treated, therefore, as a two-dimensional structure with a 

 certain mass m per unit area. 



Before the beginning of the explosive action, the elastic stresses 

 in the plate will be in equilibrium with the difference between the hydro- 

 static pressure in front of the plate and the pressure on the back face. 



