pijesident's adijke;<s SECTIOX u. 621 



niid Osmond. Tliey have shown that the markings known as Liider's 

 lines make their appearance in phices on poHshed specimens long 

 l^efore even the propoi-tional limit is reached, and these lines are due 

 t.) the fact that plastic yielding has taken place at the spots where 

 they show. 



If after straining the bar to some point beyond C we entii-ely 

 remove the stretching force, and then, after an intei"val of rest, start 

 afresh and gradually strain the bar as before, we find that now the 

 yield point has been raised, this increase of the yield point in tension 

 being accompanied, as Bauschinger showed, by a lowering of the cor- 

 responding point in compression. The character of the metal is thus 

 somehow altei-ed by its being strained bej^ond its yield point ; it has 

 no longer the same qualities, but it requires a much higher stress 

 than before to bring it to the plastic stage, and it has become, as 

 it is usually somewhat loosely termed, harder. This particular ex- 

 periment is an illustration of a very general law, apparently applic- 

 able to all the metals, that whenever a metal is strained beyond its 

 elastic limits by any sort of stress its power of resisting that kind of 

 sti'ess is thereby increased. We may get an interesting demonstra- 

 tion by marking a small test piece with a punch, then tiling off the 

 mark and polishing the metal. If the test piece is now subjected to 

 compression the mark wall reappear soon after the plastic stage is 

 reached. The stress-hardened parts offer a greater resistance to 

 plastic flow, and do not in consequence lose their polish so readily 

 as the other portions of the test piece. 



We see practical applications of the principle in the processes 

 of hammering, rolling, and -wire-drawing of metals. The tensile 

 strength of Swedish iron, for instance, may be raised from 20 tons 

 per square inch in the bar form up to 80 tons per square inch in a 

 thin wire by drawing it through dies. It appears to be an equally 

 general law that whenevier a metal has l^een "hardened" in this w'ay 

 its plastic quality may be restored by heating, the temperature to 

 which the metal has to be raised in order to effect this restoration 

 being far below that necessary to put the metal in a molten con- 

 dition. This, again, seems to apply to all the metals. Thus a strip 

 of silver foil may be made very hard and springy by hannnering, but 

 after heating to 260 degrees C. it loses all its spring, and becomes 

 quite soft. Mr. J. Muir* made some experiments with steel that 

 had been hardened by tensile overstrain until it could be loaded up 

 to 50 tons per square inch without a yield point being reached. 

 His specimens were then subjected to a series of tests after being 

 heated to various temperatures. He found that whilst a temperatvu-e 

 of 310 degrees C. produced no softening of the material, 360 degrees 

 C. lowered the yield point to 47 tons ; and 500 degrees, 600 degrees, 

 and 700 degrees C. lowered the yield point to about 40, '55, and 

 •30 tons per square inch respectively, the original yield point of the 

 material before hardening by overstrain being 36 tons ]ier square 

 inch. Similarly gold, copper, and magnesium, which have been 

 hardened by hammering, can be rendered quite soft by raising them 

 to temperatux-es ranging from 250 degrees to 300 degrees C. The 



Proc. R.S., of .London, vol. 67. 



