End Friction with Soft Plates. 
439 
1908-9.] 
of steel, /3' = 37°. So small an inclination of the lines of Liiders on com- 
pression specimens of mild steel has been seldom found by the writer ; it 
lies more usually between 40° and 45°, and sometimes even above 45°. No 
attempt is made here to explain this difference between calculated and ex- 
perimental values. 
Taking fx = /x — 0T4, the ratio of the apparent compression yield point 
to the true yield point is IT 8. The ratio of the tension yield point to the 
compression yield point is 0'76.* Hence the value of this latter ratio, found 
FIG. 5. 
17 
from experiments, should be 0* 7 6/1 T 8— -that is, 0'64. This last figure is 
certainly much lower than any obtained hitherto. 
% 
7. Effect of End Friction with Plates Softer than Material Crushed. 
In sections 3 to 6 the material forming the plates between which the 
specimens are crushed has been supposed harder than the substance under 
test. It was a common practice at one time, in making crushing tests of 
stone, to place a sheet of lead between each end of the specimen and the 
corresponding platen of the testing machine, with the object of securing a 
uniform distribution of load. Experiments soon revealed that the effect of 
the lead, instead of being beneficial, caused a great reduction in the ap- 
parent strength of the stone, amounting on an average to one-half of the 
strength obtained when the stone was in direct contact with the platens. 
The reason of this is not far to seek. As the load is applied the soft lead 
is deformed to a greater extent than the stone. When the compressive 
stress reaches a certain value — generally known as the pressure of fluidity, 
and estimated variously at f ton to 3 tons per square inch 3, 4) — the lead 
* This ratio was given previously as 0 - 70, <p having been taken as 10°. The higher value 
of tjc would seem to be nearer the correct figure, but experiments suggest one still higher. 
