COMPLEX STRESS DISTRIBUTIONS IN ENGINEERING MATERIALS. 295 
less in the plastic stage. Thus in a nitro-cellulose model, of the form used by 
the Engineering Standards Committee, it is observed that the varied colour 
effects, which mark the penetration of complex stress within the gauge length, 
tend to recede towards the enlarged ends as the material passes into the plastic 
state, and in general at fracture the line of greatest colour variation is concen- 
trated near to the section where the curved contours meet the parallel part of 
the gauge length, so that it seems possible in ductile material that nearly the 
whole of the parallel part of the gauge length ultimately comes into a state of 
uniform stress. 
Discontinuities in Materials Subjected to Impact Tests. 
The practical necessity of ascertaining the ability of materials to withstand 
suddenly applied loads has led to the introduction of shock tests of various 
kinds, and these are generally arranged in such a manner that fracture of the 
specimen shall occur, generally, with a single application of a load applied by 
a falling weight or swinging pendulum. Such tests are essentially different 
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from static tests of materials, and in general classify them in a different manner, 
for it is quite possible to have materials which give similar results under static 
loading and yet behave very differently under impact tests. 
Although there cannot be any doubt of the importance of impact tests in 
affording an element of value in determining the properties of a material to 
resist stress, yet there is a considerable difference of opinion as to the exact 
nature of the information afforded, and this is hardly surprising when the 
number of variables is considered which enter into the problem. 
The various mathines designed for impact testing differ greatly in the manner 
and swiftness with which the lead is applied, in the rigidity of the frame and 
