14 Transactions of the Society. 



II. — Microscopic Study of Strain in Metals. 

 By F. EoGERS. 



Read December \Wi, 1906. 



Plates IV. and V. 



CoMPARATn''ELY few workers have devoted themselves to the 

 interesting study of the effects of stress upon metals by means 

 of the Microscope, yet important work has l^een done by Ewing, 

 Eosenhain, and Humfrey, by Stead, and by Stanton, in this country, 

 and by Osmond, Fremont, and Cartaud, in France ; the English 

 workers having studied more especially the effects found in the 

 ordinary forms of metals, and the French, with admirable pains, 

 have worked upon abnormally large individual crystals of iron, 

 and, in general, have regarded the subject from the crystallographic 

 standpoint. 



The author's studies about to be described have borne more 

 particularly upon the fatigue of metals which is brought about by 

 submitting them to alternating stresses. Previous to his work, all 

 that was definitely known was due to Ewing and Humfrey, who 

 found that in Swedish soft iron the process of fatigue consisted of 

 a slipping backwards and forwards of some portions of the metal 

 over other?, along crystal cleavage planes, the grinding action 

 gradually diminishing cohesion until fracture occurred. 



In ordinary structural steels, which consist of a conglomerate 

 of ferrite and pearlite, the effects are in some respects of a similar 

 nature. Not only, however, are the effects in pearlite distinct from 

 those in ferrite — owing to the probable absence of crystalline 

 orientation in pearlite — but, on account of the local support given 

 by the pearlite grains, the effects in the ferrite of steels are different 

 from those in pure iron. Further, it will be seen that each varia- 

 tion of carbon content, and manv variations of heat treatment of 

 the steels, affecting as they do the proportions, arrangement, and 

 nature of the two constituents, have a profound influence upon the 

 manner in which repeated stresses gradually cause disruption of 

 the metal. 



It is almost natural at first glance to suppose that the incipient 

 cracks in steel would tend to select a path through the weaker 

 constituent, ferrite, avoiding pearlite as much as the arrangement 

 of the two constituents in a particular sample admits. This con- 



