46 
FIRST BUBBLE MINIMUM 
JUST TOUCHES PLATE | 
AT THIS DISTANCE ——>y 
AFTER FIRST 
BUBBLE PULSE 
AFTER PRESSURE PULSE 
DEFLECTION — in 
ADDITIONAL eens 
DUE TO FIRST 
BUBBLE PULSE if 
° 5 10 25 
CHARGE DISTANCE — i 
Pig. 21 - Central plastic deflections of box model target plate 
Fig. 21 shows the results. Geduced fran the displacement/time records of the 
central deflection for a small-scale box model involving rectangular air— 
backed target plate 1 ft6 im by 1 ft subjected to the explosion of 
4 oz P.A.G charges. The curves shown refer to permanent deformation 
excluding transient slastic contributions to the deflection It is 
seen from fige21 that as the charge distance decreases below 23 in. the 
additional deflection due to the first bubble pulse at first increases 
faster than the deflection due to the initial pressure pulse and the two 
contributions become approximately equal for a charge distance of about 
19 in. This distance corresponds to the critical case in which the 
attraction of the target is such that the bubble arrives at the plate 
when it is of minimum size and the pressures are highest # For closer 
charge distances the bubble contribution to central deflection in fact 
decreases and tends to become small canpared with that due to the pressure 
pulsee This decrease of the additional deflection produced by the bubble 
is probably due to two main causées:- 
(4) as the original deflection due to the pressure pulse increases, the 
pressure necessary to stretch the plate still further will also 
increase. 
(2) at the closer distances the bubble arrives at the target plate 
while it is still fairly large and the presence of the plate tends 
to prevent the bubble from contracting to the same extent as it 
would in the absence of the target; this larger minimum volume 
implies lower bubble pressures# In support of this explanation, 
it should be noted that for the critical charge distance, the 
damage due to the bubble pulse is concentrated near the centre 
of the plate. The final overall damage to the target as measured 
by the mean deflection of the plate over its area does not show 
any pronounced bump at the critical distance corresponding to the 
bump in the upper curve of fig. 21. The bubble pulses subsequent 
to the first bubble pulse contributed little to the final damage. 
*This is similar to the result that the sea-bed tends to suppress 
the bubble pulses, 
