COMPLEX STRESS DISTRIBUTION IN ENGINEERING MATERIALS. 337 
VII. On the Drop of Stress at the Yield Point of Ductile Materials. 
By Professor ANDREW RosBeErtson, D.Sc. 
In the ordinary testing of a ductile material the yield point is indicated by the 
sudden extension of the specimen, and the drop of the beam of the testing machine. 
Autographic load extension diagrams also indicate that the load immediately after 
passing the yield is less than that required to initiate it. The author, in conjunction 
with Mr. Gilbert Cook (now Professor Cook), investigated the phenomenon in some 
Fia. 15. 
detail in a paper published in the ‘ Proceedings of the Royal Society, 1913.’ By taking 
special precautions to secure axial loading and to limit the strain after the initiation 
_ of the yield, it was found that the reduction of stress was of the order of 25 per cent. 
for the particular steel used. The principle of the apparatus will be clear from fig. 15.! 
The two outer bars D are provided with extensometers and the specimen under test is 
loaded through special axial loading shackles. The total load is shared by all three, 
and if the load in the centre one is reduced by reason of a ‘drop’ at yield, the load 
on the outer ones is increased. It was further found that after yield had occurred, and 
the load removed and again applied, the stress required to continue the yield was the 
or stress. This is shown in the diagram in fig. 16.” 
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