PROCEEDINGS* 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 3 MAY 15. 191 7 Number 5 
THE LAWS OF ELASTI CO- VISCOUS FLOW 
By A. A. Michelson 
DEPARTMENT OF PHYSICS. UNIVERSITY OF CHICAGO 
Read before the Academy, November 14, 1916 
When a solid is subjected to a strain beyond the ^Elastic Limit/* its 
behavior may be summarized as follows: 
First: The application of the stress results in a rapid elastic yield, 
which if inertia be negligible is practically instantaneous. If the stress 
be now removed, the specimen returns to its former position.! 
Second: This is followed by a slower yielding whose rate, if the stress 
is not too great, diminishes with time, and which ultimately attains a 
constant value which may be zero. 
If the stress be now removed the specimen returns almost instanta- 
neously to a point short of its original position, and then continues at a 
much slower rate and ultimately comes to rest at a point short of its 
original position. 
If the stress is too great, the slow yield may increase until rupture 
occurs. t 
The following may be considered as a provisional attempt to formu- 
late the behavior of substances under stress by the simplest expressions 
which have been found to satisfy all the essential requirements. 
* The term 'elastic limit' is very vague and should be replaced by limits which may be 
characterized as follows: a. The first limit is that within which the specimen returns in- 
stantly to its original zero. Beyond this limit, if stress be instantly removed, the specimen 
promptly returns to a position short of its original one, which may be designated the 'new 
zero.' b. The second limit is that beyond which the specimen does not return to its original 
position or to the 'new zero,' even after a long time. c. The third limit is that value of 
the stress which produces rapid yielding or rupture. 
t In many cases the time interval between application and release of stress cannot be 
made sufficiently short for complete instantaneous recovery. 
I Rupture may occur in consequence of such slow yielding, or, it may be practically in- 
stantaneous. In the former case, the result is due to separation of the viscous coupling; in 
the second to the snapping of the spring. 
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