On the Recovery of Iron from Overstrain. 337 



" On the Recovery of Iron from Overstrain." By JAMES Mum, 

 B.Sc., Trinity College, Cambridge (1851 Exhibition Science 

 Research Scholar, Glasgow University). Communicated by 

 Professor EWING, F.R.S. Received January 25, Read 

 February 9, 1899. 



(Abstract.) 



It has long been known that iron which has been overstrained in 

 tension that is to say, strained 'beyond the yield-point, so that it 

 suffers a permanent stretch possesses very different elastic properties 

 from the same iron in its primitive condition. The material is said to 

 be " hardened " by stretching,* since the ultimate effect of such treat- 

 ment is to raise the elastic limit, and reduce the ductility of the 

 material. 



More recently attention has been called to the fact that, primarily, 

 the result of tensile overstrain is to make iron assume a semi-plastic 

 state ; so that the elastic limit instead of being raised by stretching is 

 first of all lowered, it may be, to zero.f This plasticity may be shown by 

 applying a comparatively small load to a bar of iron or steel which 

 has just been overstrained by the application and removal of a large 

 stretching load. When the small load is put on, the bar will be found 

 to elongate further than it would had the material been in its primitive 

 state ; and a slight continued elongation a " creeping " may occur 

 after the small load has been applied. If this load be withdrawn a 

 quite appreciable permanent, or semi-permanent, set will be fouud to 

 have been produced ; a set which diminishes slightly and, if small, 

 may vanish provided time be allowed for backward creeping to take 

 effect. It may also be shown that if the reapplied load be increased 

 the elongation produced will increase in a greater proportion. Thus 

 if a stress-strain curve be obtained from a recently overstrained bar of 

 iron or steel, it will show even for small loads a marked falling away 

 from the straight line which would indicate obedience to Hooke's law. 



It is the recovery from this semi-plastic state induced by overstrain 

 to a condition of perfect or nearly perfect elasticity, with raised elastic 

 limit, that is referred to in the title of the paper of which this is an 

 abstract. Such recovery is known to be effected by mere lapse of 

 time,! an d the object of the experiments described in the paper and 



* Ewing, " On certain Effects of Stress," ' Roy. Soc. Proc.,' No. 205, 1880. 



t Bauschinger, ' Civilingenieur,' 1881, or ' Mittheilungen aus deni Mecli. Tech. 

 Laboratorium in Miinchen.' An Account of Bauschinger's work is given in Unwin's 

 book on ' Testing of Materials of Construction.' Ewing, " On Measurements of 

 Small Strains in the Testing of Materials and Structures," 'Roy. Soc. Proc.,' 

 vol. 58, April, 1895. 



|. Bauschinger, ' Dingler's Journal,' vol. 224, p. 5 ; or ' Mittheilungen aus deni 

 Mech. Tech. Laboratorium in Munchen.' Ewing, both papers already cited. 



