569 



The theoretically predicted damage time curve for these conditions — ' 

 is also given in Figiire 21. However, since cavitation occiirs for this 

 case (cf . Table XVIIl) the theory does not apply here but is considered 

 more typical of most of the work where cavitation did not exist. 



The deformation time ciirve was determined also by the method dis- 

 cussed eeo-lier (See. V, 3, a) in which the depth of the charge and damage 

 gauges below the surface was varied and the corresponding effect on 

 damage, due to the rarefaction wave reflected from the surface, noted. 



The results of this experiment are shown in Figvire 22. The dotted 

 curve shows the damage and cut-off times plotted against the depth. This 

 time is calciilated on the assvmiption that a tension wave is reflected 

 from the surface and cuts off the tail of the direct wave. Acoustic 

 velocities were assiuned. The minimum time which will give full damage 

 is clearly not easy to determine with any accuracy but seems to be about 

 215 - ^ microseconds (a depth of 15 t 2 in.). This is higher than the 

 values of 138 - 177 microseconds found by the electrical contact method. 

 However, it may well be that the last few per cent of the damage require 

 a relatively long time; if so, the resiilts obtained by the contact 

 method might easily be somewhat in error. 



(d) Deformation of water-backed diaphragms . In connection with the 

 tests investigating the effect of variations in diaphragm thickness on 

 damage (Sec. V, h, a), several shots were fired using thick and thin 

 copper diaphragms clamped between the face plates of regular gauges so 

 that the diaphragms were water-backed. Thin diaphragms were placed in 

 two opposite gauges and thick diaphragms in two other gauges, all 

 supported by a steel ring. The ratio fo'ond for the damage of the thin 

 diaphragms to the damage of the thick diaphragms was about the same as 

 that found with air-backed gauges. The data are listed in Table XVII and 

 «ure ccmpaj-able with the data given in Table XIII. 



T^iOle XVII. - Damages of thick and thin copper diaphragms 



(Water-backed gauges) 



a/ Average of two diaphragms. 



b/lncrease in damage defined as difference in damage for thin and 

 thick~diaphragms expressed as percent of damsige for thick diaphrsigms. 

 Based on damages corrected for small variations of disteince from 1+8 in. 

 and 81+ in. 



18/oSRD No. 1+200, p. 1+6 (Fig. 1+); p. 55. 

 in Section TV. 



The parameter 9 was determined as 



