132 
PROF. W. E. DALBY: RESEARCHES ON THE ELASTIC 
§ 7. Loop Area and Permanent Set. 
The loop area increases in size as the stretching proceeds and the rate of increase 
differs in different materials. 
The question now arises : does the increase in area follow a regular law ? The 
answer is given by the curves on Sheet 1 (folding diagram). 
The co-ordinates on Sheet 1 are loop area and permanent set. Curve 1 shows the 
results obtained from a test piece of 0‘8 inch carbon steel, § inch diameter, with a 
5 inch gauge length. The slope of this curve shows the rate of increase of loop area 
as stretching is continued. The curve ends when local contraction begins. Similar 
curves are given on Sheet 1 for nickel steel, mild steel, and for iron. 
The rate of increase depends upon the time interval between the drawing of the 
loops and upon the kind of material. In irons and mild steels the influence of time 
is profound. In the alloy steels tested and in high carbon steels the influence of 
time is small. 
When the stretching of iron or mild steel is resumed after a rest, the loop area, at 
first small, increases rapidly towards the area the loops would have had if stretching 
and looping had been continued without resting. Anticipating the detailed 
description of the curves on Sheet 1, this point may be illustrated by curve 3, same 
sheet. Plates A l5 B 1} C ls D x , E l9 and F x were taken consecutively, there being no 
more time interval between the plates than the few seconds required to change the 
plates. After the mild steel test piece had been stretched to 0'2 inch it was taken 
out of the machine and laid aside for 15 days. Plate G 1} the first plate taken after 
the rest, and plates H 1} I l5 J, furnish loops of rapidly increasing area until the 
area is reached on plate K 1} corresponding to continuous stretching without rest 
intervals. 
§ 8. Loop Area and Permanent Set Curve. Curve 1 Sheet 1 (0'8 per cent. 
Carbon Steel). 
The detailed consideration of this curve will show how all the curves of the 
diagram on Sheet 1 have been derived. The capital letters along the top of the 
procession of loops seen in fig. 19 refer to the sequence of negatives recording the 
loops taken from a standard test piece of 0‘8 inch carbon steel. 
Plate A gives the record of the first application of the load to the test piece and 
its immediate removal and re-application four times. The plate therefore shows the 
elastic line and the first four loops. A scale is placed under the loops so that the 
permanent set of the primitive 5-inch gauge length can be read at any point in the 
procession of loops. For example, the permanent set at the end of the looping 
operations recorded on the sequence of plates A, B, C, D, is the distance 
Ok = 0'137 inch. 
