- 5 - 599 



the mode of dishing leading to the same final shape of dish will make a large difference In the 

 final energy absorbed, always provided that the energy is absorbed in the same way by plaitic 

 stretching and thinning of the diaphragm. It is conceivable, though somewhat unlikely, that 

 the plastic wave mechanism involves a large absorption of energy in bending since each element 

 is first bent in one direction and then back ^gain as the wave passes. Such a source of energy 

 absorption is not allowed for in our theory based primarily on plastic stretching which certainly 

 predominates In the case of dishing under static load. Since the energy absorption due to 

 stretching depends on the square of the dished volume we should need a large contribution from 

 any such bcr^ling mechanism to account for the discrepancies shown in Table 2. 



FIna'ly we have made throughout the customary assumption that the pressure-pulse varies 

 exponentially in time whereas we know that this is not accurately the case in practice, the pulse 

 being rather of an initial exponential form followed by a tail decaying more slowly. In 

 particular we have, in the absence of direct information, derived the formula for" n given in 

 equation (8) from the associated maximum pressure and impulse formulae, the latter of which 

 depends somcuhat on the tail of the wave. For the initial part of the wave n should be higher 

 than assumed and this would tend to improve agreement, more especially for the intermediate 

 charge weights where the pulse is neither so short that impulse is the deciding factor no so long 

 that maximum pressure is the main parameter. The fact that the discrepancy in Table U is greatest 

 for the intermediate charge weight of 100 lb. suggests that part of the error at least is due 

 to this lack of accurate representation of the pressure-time for the incident pulse. In view 

 of this it seems desirable to carry out controlled experiments in which copper diaphragms and 

 piezo-gauges are both used so that the observed pressure time curve can be used direct in the 

 present theory, i.e. in place of the assumed exponential term on the right-hand side of equation 

 (A, 37). Any further refinements of the theory are better deferred pending a better check on 

 the present theory obtained from such experiments. 



References . 



(1) Diviion 8, N.D.R.C. Interim Report 1J.E.16 



(2) Report S.R.E./A.C.S.83 by Admiralty Computing Service. 



