of Steel under Mechanical Stress. 203 



sometimes so slow that it may take 30 seconds to pass up a 

 10-inch bar, in which case the pencil drawing the stress-strain 

 curve is seen to follow the wave ; in the former case the 

 pencil drops as if the bar had broken. 



When the wave starts from one end a plane in the centre 

 experiences no permanent sliding until the wave has reached 

 the plane next to it, so that while originally the resistance to 

 sliding was greater than the stress, now it is less ; in other 

 words, permanent strain (in an adjacent plane) diminishes the 

 resistance to permanent strain. 



The most reliable stress-strain curves are those drawn by 

 Prof. Kennedy's apparatus, and these show a rapid and irre- 

 gular inflexion at the yield-point (see fig. 3, Plate III.). In 

 these diagrams the record of stress is unaffected by the inertia 

 due to the load. 



These stress-strain curves indicate the condition of strain 

 in a steel bar as, by gradually increased stress, the steel is 

 converted from an elastic solid to a viscous fluid. It is inter- 

 esting to compare such curves for steel of different hardness 

 with the stress-strain curves of a gas at different temperatures 

 (see fig. 4, Plate III.). 



We have the effect of increased ^temperature J doin S 



away with the discontinuity at the < P^r f* ^ ne * ^ so 



the strong probability that in both cases the apparent discon- 

 tinuity is really a double inflexion due to a change taking- 

 place piecemeal throughout the substance, and inconceivable 

 if the substance be supposed homogeneous (cf. Prof. J. 

 Thomson, Proc. Roy. Soc. 1871, no. 130). 



[The stress-strain curves shown in the diagram are from 

 Dr. Andrews's paper (Phil. Trans, vol. clix. 1869, p. 575).] 



Thus far the strains considered have been those due to 

 relative displacements of the molecules, but it is impossible to 

 conceive that this can take place without the molecules them- 

 selves being strained, i.e. without a displacement of the atoms. 



Such a displacement of atoms, if permanent, would alter 

 the chemical and physical condition of the iron. Hence we 

 must look to the straining of the atoms to account for the 

 change that takes place when a bar is permanently strained, 

 t. e. to account for the increase in the value of p with per- 

 manent strain. 



Osmond has proved almost beyond a doubt that the so-called 

 " hardness " of steel is due essentially to the existence of an 

 allotropic modification of iron ; that the molecules of iron may 

 be either (a) soft, or (/3) hard. (€/'. Osmond's k Etude* M4- 

 tallurgiqiiesi' Paris, Dunod, 1888.) 



