166 



Sur.-nises Concerning the Motion 



Ue may no'.v draw certain inferences from the previously described 

 observations and make certain surmises as to the details of the motions 

 and as to the mechanisms involved. Tliese surmises themselves have not yet 

 been verified experimentally, but they seem physically reasonable in the 

 light of present knowledge and in the light of the conclusions to which they 

 lead. 



At t'le initial instant the diapb-ragm material has a uniform normal 

 velocity component v; the radial velocity component is certainly zero. 

 However, the restraining effect of the rim must be felt quickly by the 

 diaphragm material adjacent to it. This is considered to have two effects. 

 First, the presence of the rigid edge constraint is quickly made laioivn to the 



interior portion of the diaphxagm. The nev/s is no doubt tranc:nitted by an 



inward 



elastic stress wave traveling^at a high "sonic" velocity. V/e ma;'- surmise 



that this wave leaves the interior of the diaphragm in a state of plastic 

 flow so that a non-zero radial velocity distribution may be superimposed on 

 the uniform normal velocity, vrtiich itself remains practically unaffected. 

 Elementary estimates indicate that this presumably takes place in a time 

 which is very short compared to the time taken for the complete deformation 

 of the diaphragm. Second, a plastic bending wave is generated near the edge 

 and is propagated inward more slowly. It is believed that one function of 

 this wave is to inhibit the tendency, as evidenced by the thinning at the edge, 

 of the diaphragm to fail in shear. 



As the bending wave sweeps inward over material flowing outward in 

 a radial direction, the material loses most of its velocity, both its radial 

 component and the original component, v, normal to the initial plane. It is 

 surmised that little or no impulsive thinning or other form of plastic working 

 is associated with this action; that is, the bending wave in effect simply 



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