q 
a? 
- 
% 
reached 16°5 tons per square inch. When 
+ square inch the load was almost entirely 
removed, and then again immediately 
until the stress was about 24-4 tons per 
— 
"BEHAVIOUR OF MATERIALS IN TESTING MACHINE 177 
of this material when tested in torsion is shown in Fig. 252. It 
: will be seen that the surface of the oblique fracture is a screw surface, the 
ABC Development of outside surface 
ABC, ” » Uuside ” 
Scale Half Full Size 
Fig. 252. 
edges of which are helices. The developments of the outside and inside 
surfaces of the specimen after fracture are shown to the right in Fig. 252. 
168. Hardening Effect of Overstraining.—When a ductile material 
__ like mild steel is loaded beyond the yield point, and therefore given a 
_ definite permanent set, and then unloaded, it is found that when the load 
_ is again applied the yield point is higher than before. This is well 
_ illustrated by Fig. 253, which is a stress strain diagram (the stress being 
the nominal stress) for a bar of mild steel re 
tested in tension. The first loading pro- g* A) 
duced a decided yield when the stress S 
the stress reached about 21°5 tons per 
point was found to be at about 22-2 tons 
‘per square inch. The load was increased 
applied, with the result that the yield } 
Square inch, when it was again almost Extension a hes r 
_ entirely removed. A third application of Fic. 253 
the load showed a yield point at about Mer 
_ 24°7 tons per squaré inch. The load was then continued until fracture 
occurred. 
Compared with a test of the same material, in which the bar was 
broken with one loading, the maximum and breaking loads were found 
to be higher, but the final elongation was less in the bar tested with 
interrupted loading than in the other. 
4 
