178 Prof. W. C. Unwin. On the Yield Point of lion [Dec. 13, 



II. " On the Yield Point of Iron and Steel and the Effect of 

 repeated Straining and Annealing." By W. C. UNWIN, 

 F.R.S., Professor of Engineering, Central Technical 

 College, City and Guilds Institute. Received December 4, 

 1894. 



A bar is subjected to a gradually increasing tension, and the 

 extensions for a series of loads are measured. If the loads are 

 plotted as ordinates and the extensions as abscissas, a load-strain 

 curve is obtained. In the case of many materials the load-strain 

 curve is fairly continuous (figs. 1 and 2). But with certain qualities 

 of iron and steel there is a remarkable irregularity in the curve 

 (fig. 3). Up to the elastic limit a the strain is proportional to the 

 stress, and Oa is a straight line. Beyond a the strains increase 

 faster than the stresses. At some point, 6, termed the yield point, a 

 very large and abrupt permanent elongation is produced. The bar 

 continues to elongate for some time without increase of load, or even 

 apparently with some decrease of load. At some point,' c, the 

 material is so hardened that further deformation can only be pro- 

 duced by increase of load, and a continuous load-strain curve, cdf, is 

 obtained. 



In some tests of very long iron and steel bars, by a Committee of 

 the Institution of Civil Engineers,* the extensions for equal incre- 

 ments of load were carefully measured, and load-strain diagrams were 

 plotted. These diagrams showed for the first time the abrupt 



FIG. l. 



C/fST /RON 

 10- " 



fNCHCS. 



* Experiments on the mechanical and other properties of steel, made at H.M. 

 Dockyard, Woolwich, 1870. 



