165 



the diaphragm begins to move, the material at the outer edge is jerked to 

 rest by the rigid constraint there. The news of this retardation is carried 

 radially inwards by a rather sharply defined bending wave, shown in Figure 1, 

 vdiich travels at a nearly constant velocity until it reaches the vicinity of 

 the center of the diaphragm, where it appears to speed up. The material in 

 the flat central region of the diaphragm, ahead of the bending wave, continues 

 to move with almost its initial velocity until the wave reaches it. Then, as 

 the bending wave sweeps over it, the material is brought quickly to rest. 

 Finally the bending wave reaches the center, leaving the diaphragm deformed 

 into a characteristic shape, generally a surface of revolution, nearly 

 conical, with, however, a somewhat rounded apex,* 



Thickness measurements along a generator of the surface of revolution 

 show at the outer edgs a relatively large and very localized thinning, as if 

 the diaphragm has a strong tendency to shear loose froia the restraining rim. 

 Indeed, it is found necessary to round off the sharp rim somewhat at its inner 

 edge to inhibit this shearing and to obtain the type of motion of interest 

 here, as one proceeds inward ,tcward the csnt^,the thickness decreases, gradually 

 at first, from approximately the original thickness close to the diaphragm 

 periphery. Near the center, the thinning becomes greater and greater and 

 finally, right at the center there is usuall;'' observed a kind of dimple. The 

 thickness at this dimple decreases with the total amount of deformation. 



Measurements have also been made of the radial and tangential 

 strains along a radius for a great many diaphragms. These measurements show 

 that the two strains are nearly equal at each point, 



* In some of the experiments with undervrater explosions, further deformation 

 of the diaphragm takes place which, however, is attributed to causes other 

 than the initial impulse, and is therefore irrelevant to the present 

 discussion, 



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