34 MR. J. MUIR ON THE RECOVERY OF IRON FROM OVERSTRAIN. 



curve of this kind obtained from a specimen whose history will not be given. This 

 curve was more fully and carefully determined than those obtained from the specimen 

 of Diagram No. IX. ; but any of the curves in Diagram No. X. show that in the 

 earlier stages the amount of recovery is approximately proportional to the squaiv 



root of the time. 



The heating of the specimens was accomplished by immersing in a hot water-but!) 

 at the required temperature for the required time, and then cooling by at once dipping 

 in cold water. Had the specimen been aUowed to cool slowly in the air, then greater 

 recovery, due to a long and indefinite time at lower temperatures, would have been 

 obtained. 



Curve A, Diagram X., shows that at 60 C. a long time would have been required 

 to produce perfect recovery, so the specimen (of Diagram IX.) was finally put in 

 boiling water for 5 minutes. After cooling, a gradually increasing load was 

 applied, and a fourth yield-point obtained at a stress of 40 tons per square inch. This 

 test is shown by Curve No. 9, Diagram IX. 



The recovery from this fourth overstrain is illustrated by Curve B, Diagram X. 

 First GO" C. and then 80 C. were employed, perfect recovery being again obtained by 

 bringing the piece to 100 C. 



On load being once more applied to this specimen, elastic behaviour was shown up 

 to the stress of 40 tons per square inch. At the 43rd ton, creeping was detected, 

 and this load was allowed to remain on for 20 hours. Considerable extension 

 resulted, as is shown by Curve 10, Diagram IX. On now further increasing 

 the load, the yielding was found for the subsequent three half-tons to be in close 

 accordance with the elastic law. With the fourth half-ton (i.e., at 45 tons per 

 square inch) creeping was again detected. After 12 minutes, however, this creeping 

 became very slow, so another half ton was applied, with the result that local 

 extension and fracture occurred at that load of 45^ tons per square inch. This stress 

 was equivalent to rather over 42| tons per square inch of primitive area ; the total 

 elongation was about 0'81 of an inch, or rather over 10 per cent, on the 8-inch length. 



A virgin specimen from an adjacent portion of the same bar as the above, gave, 

 when tested at once to breaking, an ultimate strength of 36 J tons per square inch 

 of original area ; the total elongation in this case was 1*82 inches or nearly 23 per cent. 

 on the 8-inch length. 



It will be noticed that the distance in tons between the successive yield-points 

 shown in Diagram IX. is roughly constant, and further that fracture has occurred 

 where a yield-point (if not a fracture) would naturally have been expected. More 

 correctly, it is the distance between a yield-point and the previous maximum load 

 that is the same throughout. Thus a specimen from' the same bar as that 

 employed for this diagram, No. IX., was overstrained primarily by 27 tons to the 

 square inch, and the subsequent yielding was obtained at 33 tons. That is at about 

 4 tons higher than the second yield-point shown in Diagram IX., when the primary 

 loading was only carried to 23 tons jxjr square inch. This regularity in the raising 



