24 Mi;. ,1. MIIU OX Till! l;i:o>VF.KY nF ll;<)N T FROM OVERSTRAIN". 



diameter. On n<>\v h-sting the specimen, no change was found in the Ix-haviour 

 of the material; Curve No. 4, which shows this test, agreeing very accurately with 

 Curve No. :t up to the stress of 35 tons per square inch. This maximum loud of 

 35 tons to the square inch (now 17 '50 tons total instead of 26 '91 tons as before 

 turning down) was kept on for 1 hour, with the result that slight creeping took place, 

 as is shown in Curve No. 4. On augmenting the load a distinct yield-point was got 

 at 37 tons per square inch. This second yield-point happened at a lower stress than 

 would naturally have heen expected, for in Diagram No. IV. the same material is 

 shown to have been subjected to a stress of over 40 tons per square inch without 

 a second yield-point having been passed. The lowness of the yield-point in the 

 present case, was probably due to an inherent weakness in the specimen, which was 

 shown by the small flaw which ran up the centre of the bar.* Owing to the specimen 

 having been turned down this weakness would exert a greater influence than when 

 the bar was of the full diameter of 1 inch. Experiments on another specimen of the 

 same material, in fact, directly showed that after turning down a yield-point was 

 obtained, at a stress lower than that to which the specimen had already been sub- 

 jected without other than elastic yielding resulting. Such behaviour was anomalous. 

 With other material which exhibited no flaw, turning down was found to have no 

 effect on the position at which subsequent yielding took place. 



The second yield-point shown in Diagram VII. having been passed, the material 

 was once more in the semi-plastic state, so to effect recovery it was placed in boiling 

 water again for 10 or 15 minutes. On cooling and re-testing, a third yield-point 

 was obtained at 4l tons per square inch, as is shown by Curve No. 5, Diagram VII. 

 The specimen was once more put in boiling water and then tested, with the result 

 that fracture occurred at a stress of 50 tons per square inch. The break was outside 

 the central 8-inch length, close to the tapering neck joining turned and unturned 

 portions. 



A short virgin specimen of the same rod as the above was tested (after being turned 

 down in the centre to a diameter rather smaller than in the last case), in order to find 

 the ordinary ultimate strength of the material. The result of this test has already 

 been given on p. 4 ; local extension set in at a stress of 39 tons per square inch of 

 original area, or about 45 tons per square inch of actual stress, and fracture was 

 allowed to occur at that load. 



The effect which a temperature of 100 C. had in hastening the recovery process 

 was further strikingly shown by an experiment On one of the specimens employed to 

 obtain Diagram V. This specimen had been allowed to rest for three and a-half 

 months, and was even then found to exhibit considerable imperfection of elasticity. 

 By heating to 100 C. for a few minutes, a marked improvement was made in the 

 elastic behaviour of this specimen. 



* See pp. 4 and 14. 



