142 PROF. E. G. COKER AND MR. K. C. CHAKKO: THE STRESS-STRAIN PROPERTIES 
It will be observed that there is no very pronounced elastic limit, and that the 
curve is nearly straight up to 150 lbs. load (1900 lb./in. 2 ), which latter value may be 
taken as the elastic limit of the material. There is a “ semi-permanent” set of 
0 001 inches for each repetition of load, and a pronounced recovery between successive 
loadings especially with a short period of rest. 
Measurements at Higher Stresses .—The spring balance used to measure the 
moderate loads in the above observations had a maximum capacity of 200 lbs., but for 
the higher stresses required, a balance recording up to 500 lbs. was necessary, 
the observations being made in a similar manner with readings on the magnified 
scale up to 400 lbs. load, and after this coarser readings were taken with the 
telescope. A maximum load of 476 lbs. (6000 lb./in. 2 ) was reached, but as the 
extension then increased very rapidly it was not possible to keep the load at this 
maximum value, moreover, as the stressing frame was of rather limited capacity for 
large strains, the test could not be carried to fracture, although a total extension of 
1*211 inches was obtained. The observations also showed that the permanent 
extension was very uniform from section to section. 
The condition of the material has in fact some resemblance to that of a mild steel 
which has been overstrained and allowed to rest. This is shown by a subsequent 
experiment in which the loading was repeated and the stress-strain properties examined 
anew. It was then found that the elastic limit of the material was still approximately 
at 150 lbs. load, corresponding to a stress of 1900 lbs. per sq. inch, but the modulus E 
had now risen to 502,000, measured in pound and inch units, as a result of the 
overstrain. The relations of load to extension for both conditions are shown in the 
accompanying fig. 2, but as the scales of load and extension are the same for both 
experiments the curve for overstrained material lies below that for unstrained material. 
It may be observed that the material possesses, in a marked degree, the property of 
contraction when the load is removed even when very much overstrained, and in this 
case when the full load of 300 lbs. was removed the semi-permanent extension was only 
0*006 inch, and half of this disappeared with a few minutes rest. 
Observations of lateral strain were also made with a suitable extensometer at several 
sections of the test bar, and their mean value for 100 lbs. load showed a strain of 
0*00144, corresponding to a value of m = * = 2*45 where u is Poisson’s ratio. 
(7 
The value of E is high as later experiments show, and this may possibly be due to an 
ageing effect, as in process of time the material appears to undergo some change, 
especially if the cut surfaces are not highly polished. This may probably be ascribed to 
the escape of a small portion of the volatile constituent of the material. It is also 
worthy of remark that the usual method of polishing appears to produce a thin outer 
layer which is harder than the interior, and this also has the effect of raising the value 
of E in thin specimens. 
The effect of removing this thin layer of hard material has been under observation 
