180 Prof. W. 0. Unwin. On the Yield Point of Iron [Dec. 13, 



If at some point, d, the load is removed and again imposed, then in 

 the reloading a curve, edf, is obtained without any marked yield 

 point. The material is physically a different material, when it has 

 been strained beyond the yield point. The new curve is similar to 

 the curves figs. 1 and 2, and unlike the primitive curve Oabcd, fig. 3, 

 for the material. In the case of a steel which will slightly harden if 

 heated and suddenly cooled, the load-strain curve is Oacdf, if it is 

 tested unhardened, and very similar to edf if it is hardened before 

 testing. 



Fig. 4 shows load-strain curves for two pieces of the same steel 

 unhardened and tempered by heating and sudden cooling. 



FIG. 4. 



/o- 



NORMAL 



MILD STEEL 



As the yield point only occurs in materials which have been rolled 

 or hammered in a plastic state, it has been supposed to be due to 

 some purely mechanical condition of constraint produced by pressure, 

 which could be broken down if the stress reached the yield point. 

 But any purely mechanical condition of constraint, such as is pro- 

 duced by cold hammering, is entirely removed by annealing. On the 

 other hand, annealing the bar before testing does not alter the yield 

 point. If anything, it makes it more distinctly marked. Further, if 

 the bar is strained to the point d and the load removed, or if it is 

 hardened by sudden cooling, and the bar then annealed and again 



