U8 



MEASUREMENT OF PRESSURES 



wave pressures is the fact demonstrated by Hilliar (47) that the natural 

 period of the large gauges with soft lead crushers used by Abbot must 

 have been of the order 400 to 800 microseconds. As a result, the effec- 

 tive pressure acting on the crusher in the course of its deformation is a 

 kind of average over comparable intervals (a more exact description of 

 the actual mechanism of deformation is given later in this section). 

 Times of this order are however comparable with shock wave durations 

 of even larger charges. Exactly what the gauges used by Abbot meas- 

 ured is therefore highly uncertain, but it could not have been any kind of 

 approximation to the initial pressure. That the measurements made 



DEFORMABLE 

 PELLET 



PRESSURE-^ 



PISTON I 



ANVIL 



(a) PRINCIPLE OF (b) SECTION OF 



CRUSHER BALL CRUSHER 



GAUGES GAUGE 



Fig. 5.1 Principle and construction of crusher gauges. 



by Abbot have no fundamental significance of this kind is perhaps suffi- 

 ciently well indicated by his finding pressures varying with weight W 

 and distance R from an explosion roughly as W^-'^/R^-'^, results which 

 are neither consistent with similarity nor in agreement with later work 

 giving laws more nearly of the form W^-^^/R^-'^'^. The method is, how- 

 ever, of interest because it is the predecessor of fundamentally more 

 useful gauges based on the same principle which will be considered in 

 more detail. 



Although cylindrical crusher pellets were used both by Abbot and 

 Schuyler and by Hilliar in a classic series of measurements described 

 later in more detail (see section 5.2), a more recent modification de- 

 veloped by the Naval Ordnance Laboratory, U. S. Navy (78), employs 

 5/32 inch or 3/8 inch diameter soft copper spheres as the "crusher" ele- 

 ment, the construction of the gauge being indicated in Fig. 5.1(b). A 

 considerable amount of work has been done to analyze the response of a 

 gauge of this type to various types of applied pressures.^ Calibration 

 tests on deformation of the copper balls by impact of a piston travelling 



^ A number of such calculations are given by A. B. Arons, to be issued as a 

 NavOrd Report by the U. S. Navy Bureau of Ordnance. 



