997 
postulated (See Appendix I) that cavitation at a particular point in front of the 
diaphragm will occur only if the cavitation time @, is less than the time for the 
diffracted wave to come in from the edge of the ataphregm and that the cavitation 
time can be calculated for an infinite rigid free plate by 
e,= 9 Abn ) 
c 
wi=iey By 
in which @ is the time constant of the shock wave and 0; = m/pc, m being the rass 
per unit area of the plate, Ve and c¢ the density and sound velocity of water, 
respectively. In order to test this | hypothesis, two series of photographs were 
taken. 
In one series, shock waves were reflected from a deformable steel diaphragm, 
the center of which should approximate the motion of a free plate in the initial 
part of its motion. In the other series, a closer approximation to a true free 
plate was made by supporting a steel disk on a weak backing of cellulose acetate 
or shim brass, and in a few pictures there was no support for the steel plate. 
Figures 23, 24, 25, 26, and 27 show UERL diaphragm gages being damaged under the 
conditions given in Table I. The table shows that cavitation occurs only when Q¢ 
is less than R/c, where R is the radius of the diaphragm. In a few cases, no 
cavitation occurred under this condition, but the condition was satisfied by only 
a slight margin. In Figure 25 the cavitation region does not extend back to the 
diaphragm. This is thought to be due to the increase in pressure caused by 
"reloading" or deceleration of the diaphragm. 
The target used to obtain Figure 28 was a 0.013 in. thick steel diaphragm soldered 
over the mouth of a 6 in. pipe. Five other photographs were taken of similar 
targets in which the only experimental condition changed was the time lapse after 
impact of the shock wave from the 50 gm charge at a distance of 12 in. Cavitation 
occurred in all pictures except one in which this time lapse was approximately 8 
feec, whereas the calculated cavitation time (@,) is 6 Pee. 
Figure 29 is a drawing of the target used in the second series of photographs to 
give a closer approximation to a free plate. 
SASS] 
SBwwVeaBeBasw 
Figure 29. Target simulating free plate. 
Typical photographs of cavitation from such a target are Figures 30, 31, and 32. 
Using this target, it is found that the position of the cavitation front is very 
close to the region of zero pressure calculated by the method of Appendix I. 
Measurements were also made of the radius R, of the cavitation region and of the 
maximum perpendicular distance from the plate that cavitation occurred, X.. The 
time Va impact at which the picture was taken.te was calculated from the value 
- These values are shown in Table II, together with the corresponding time 
6/ The equation for this was developed from the theory of prcyee* I. 
tom OL, Pn 252 
Bet (B+ie " =(p-ve ws 
where 6 = 0/o, 
23 15518 
