994 
III. RESULTS OF EXPERIMENTS 
1. Cavitation 
Inasmuch as damage theories depend on the presence or absence of cavitation, it was 
considered important to investigate experimentally the more important factors influencing 
the production and decay of cavitation. The various divisions of this problem will be 
discussed separately. 
(a) 
(b) 
Minimum tension necessary to cause cavitation. -- While theoretical considerations 
lead to the expectation that very high negative pressures should be necessary to 
start cavitation in extremely pure (nucleus-free) water, it can be demonstrated 
that if bubbles or particles of radius n, are present, cavitation should appear 
at P = -28/T, where s is the interfacial tension between water and the particle. 
Inasmuch as seawater contains numerous suspended particles, some of considerable 
size, cavitation might be expected at small negative pressures (tensions). 
In order to determine the minimum tension required for cavitation in seawater, 
the following experiment was carried outs A weak shock wave impinged on an air- 
backed cellulose acetate diaphragm 0.02 in. thick and 6 in. in diameter. Ata 
known time after impact a photograph was taken and the position of the cavitating 
region noted. From the curves given in Appendix I, the tension was estimated 
at the minimum distance from the diaphragm at which cavitation occurred, since 
the magnitude of the tension is a direct function of the distance from the 
diaphragm, this value gives the minimum tension necessary for cavitation. It is 
possible that lower values can be found under other conditions. 
Figure 19 shows cavitation in front of such a surface when a 10 gm charge of 
loose tetryl was fired 24 in. from the diaphragm. The peak pressure 24 in. from 
this charge is estimated to be 2700 lbs./in.%, The charge used in making Figures 
20, 21, and 22 was a three-foot piece of primacord stretched in a straight line 
perpendicular to the diaphragm at its center. For Figure 20 the closest end of 
the primacord was 8 in. from the surface of the diaphragm, in Figure 21 it was 
10 in. and in Figure 22 it was 20 in. The peak pressures p, in the shock fronts 
at the target are estimated to be 1100, 900, and 450 lbs. 2 respectively. The 
letters A and B mark the positions of the primary and reflected shockwaves 
respectively as observed in the original negatives. It will be noted that cavita- 
tion is visible in all four pictures. 
The theory of Appendix I has been used to estimate the pressures in the water in 
front of the diaphragm. This simple theory assumes that the shock front is planar, 
that the diaphragm acts as an incompressible free plate of infinite extent, and 
that the region ahead of the cavitation front is unaffected by the presence of the 
cavitation. 
Cavitation is observed at least as close to the diaphragm as 1/8 in. From Figure 
131 (Appendix I) it is estimated that the pressure at this point (X = 0.04) never 
fails below P = 0.1 or p = -0.1 pg. In the case photographed in Figure 22 this 
means, according to this theory, Phat the tension =p near the diaphragm never 
exceeded 45 lbs./in.2. We therefore conclude that cavitation in seawater can 
oceur at 45 lbs. /in.2 (or even less) on the basis of this interpretation of the 
experiment. Since the maximum tension possible in this experiment on any theoret- 
ical basis is 450 lbs./in.2 we conclude that this value represents an upper limit 
for the required tension. These values are estimated relative tensions, from which 
about 18 lbs./in.~ must be subtracted to correct for atmospheric plus hydrostatic 
pressure, 
Criterion for cavitation in front of a steel dia . ~" Kirkwood 5/ has 
aS 
on of M S tures an Underwat ° ¥. » John 
G. Kirkwood, OSRD=-1115, Serial No. 450, December 9, 1942. © 
20 15518 
