140 PROF. E. G. COKER AND MR. K. C. CHAKKO : THE STRESS-STRAIN PROPERTIES 
on account of their transparency and freedom from initial stress. The sheets from 
which the specimens are made differ greatly in age, one has been in stock for at least 
eight years, most of the others have been stored one or more years. 
In order to examine the stress-strain properties of this material it is unnecessary 
to use a very delicate extensometer as the value of the modulus for direct stress is 
comparatively small, and for the purposes of these experiments a very simple form is 
employed consisting of a pair of clips attached respectively to a scale and a pointer, 
which latter slides over the scale and is kept in contact with it by suitable attachments. 
In order to examine the optical properties of the material while under stress, both 
scale and pointer are perforated to give a window opening, and thereby permit a beam 
of polarised light to be transmitted through the specimen under examination. With 
this instrument and special magnifying devices it is possible to estimate extensions 
of O'OO02 inch. 
A preliminary examination of the problem set out above may be described with 
reference to some experiments on a specimen which was originally used in 1911 for 
determining the stresses in a notched tension member.* 
Two bars, each 1 inch wide, were cut, at that time, from a clear plate of xylonite 
inch thick, and each was fashioned with notches of different sizes along the edges. 
One of these specimens has been used for the present test. A length of 6 inches was 
used lor observations of the longitudinal strains, while the lateral strains required for 
determining Poisson's ratio — = a have been measured by aid of a strain-measuring 
apparatus having a unit reading of — inches. Unless otherwise stated these 
latter measurements are in the direction of the thickness of the material. As the 
details of the measuring apparatus and the cylindrical recorder used with it have 
already been describedt they are not referred to further here. 
Longitudinal Extension .—The specimen was examined in the polariscope under a 
moderate load and it was found that the stress was very uniformly distributed over 
a length of 6|- inches, but the remaining part of the parallel portion showed signs of 
unequal stress distribution owing to the enlarged ends. It was therefore marked off 
approximately into half-inch lengths over a total length of 6 inches, the exact 
distance being read to xoVo inches. 
Young’s Modulus .—As it is convenient to start with a load of 20 lbs. on 
the specimen, a preliminary observation is made to determine the corresponding 
extension, and this value is allowed for in subsequent readings for convenience in 
plotting from a zero strain value. 
* E. G. Coker, “The Effects of Holes and Semi-circular Notches on the Distribution of Stress in 
Tension Members,” ‘ Proc. Phys. Soc.,’ 1911. 
t “Photo Elasticity for Engineers,” Gy Prof. E. G. Coker, D.Sc., F.R.S., ‘The Institute of Automobile 
Engineers,’ November, 1917. 
