FRACTURE OF METALS UNDER REPEATED ALTERNATIONS OF STRESS. 243 
A tensile test of the Swedish iron rod used in these experiments, carried out in a 
testing machine in the usual manner, showed a breaking strength of 23 ’6 tons per 
sq. inch (reckoned on the original area of section), with an ultimate extension of 
0'8 inch in a length of 3 inches, and a contraction of area at the break amounting to 
61 per cent. There was a well-marked yield-point when the stress reached the value 
of ] 4’1 tons per sq. inch. The diagram, fig. 1, shows the relation ot extension to load 
up to this yield-point as measured by a microscope extensometer designed by one of 
the authors. It will be seen from this that the extension remains proportional (as 
nearly as can be judged) to the load, up to a stress of about 13 tons per sq. inch. 
The value of Young’s modulus, deduced from these measurements, is 13,200 tons per 
sq. inch. 
The apjjaratus for applying repeated reversals of stress is shown in fig. 2. There a 
IS the specimen under observation. It projects from the ejid of a shaft which was 
caused to rotate, by means of an electric motor, at a speed of about 400 revolutions 
per minute. To tlie outer end of the s[)ecimen a load was applied causing a bending 
moment, dliis was done by attaching a brass cap, h, which turned freely in a steel 
ring, c, the ring being pulled downwards with a steady force which was measured on 
a spring balance. The screw coiqjling, d, allowed the load to be adjusted to any 
desired amount. The numher of reversals of stress was recorded by a revolution 
counter, e. The specimen under test was filed to a uniform I'ectangular section of 
about 0'3 inch by OT inch. Part of one of the broad faces was jjolished and was, in 
general, etched by dilute nitric acid. The specimen was inserted in the grip at the 
end of the shaft, adjusted to run true, and the desired load was applied. Alter 
making a certain number of reversals the specimen was taken out for microscopic 
examination; it was then replaced for a further run, and so on, no difficulty being 
experienced in replacing it each time m the same position as at first. 
