Mil. J. MUIR ON THE RECOVERY OF IRON FROM OVERSTRAIN. 



ami readings taken, from which the part of Curve A lettered abc was plotted. 

 This shows that, while a considerable proportion of the lail was being removed, 

 contraction occurred quite elastically, a straight line being first obtained at an inclina- 

 tion giving the Young's modulus for the material. Latterly, however, as more load 

 was withdrawn, the retraction became more rapid, and after all load had been removed, 

 creeping was observed to continue in a marked fashion for a few minutes, as is shown 

 by the horizontal line cd. The load was now increased, and the curve def obtained. 

 At the maximum stress slight creeping occurred, and then the load was once more 

 removed, and curve a'b'c was plotted from the extensometer readings taken. This 

 curve differs distinctly from abc (obtained on first removing the load), the material 

 behaving less elastically during the early part of this second removal of the load. The 

 curve defa'b'c, however, represents an approximately cyclic state, which illustrates 

 the imperfectly elastic condition of the material of specimen A at the time in question. 

 When such a cycle due to hysteresis in the relation of extension to load is performed, 

 work is done on the specimen, and the energy so spent is no doubt dissipated as heat. 



Specimen B, which had been resting for forty days free from load, was next put into 

 the testing machine, and the load was gradually applied. The result of the first 

 li lading is shown by the part of fig. B, Diagram V., lettered /3y. This curve is 

 straight for a considerable portion (^8) the material at first approximately obeying 

 the elastic law but latterly greater extension occurred, and at the highest load 

 creeping continued very obviously for a short time. The load was then removed, and 

 curve 8e obtained. This curve resembles closely the part a'b'c of Curve A, not the 

 part abc. Specimen B was next reloaded, and curve '/3'y' illustrates the manner of 

 yielding. This curve differs from a/3y just as a'b'c differed from abc. A cyclic state 

 has now been attained, and the cycle a' / 6'y'8e closely resembles that got with 

 specimen A, which had been allowed to rest under high stress. The readings for 

 curves def&nd a'^y' of these cycles were compared in the table given above. 



It will have been noticed that it is not only the cycles ultimately obtained which 

 are analogous in the two figures of Diagram V., but that, if one of the figures, say A, 

 be turned upside down, then the three curves of that figure closely resemble the three 

 curves of the other figure, B. Considering in particular the curves first obtained in 

 the two cases, viz., abc and a/?y, it will be seen that they consist of two parts. There 

 is first a range of almost perfect elasticity, then an elastic limit is passed, and greater 

 extension or retraction obtained, according to the curve in question. The breaking 

 up in the structure of the material, which occurs after this elastic limit has been 

 passed (by a decreasing or an increasing load, as the case may be), is probably 

 analogous to the much greater breaking up which occurs on the passage of a yieldr 

 point. 



In Diagram No. VI., there is illustrated the effect, on an overstrained specimen, of 

 keeping an intermediate load acting for some time ; and it will be seen that the 

 process of recovery tends to produce an elastic range about the position of continued 



