has been presumed that the reader of this pamphlet is familiar to a certain extent 

 with the characteristic behavior of steel under tensile test. In order that the im- 

 portance of the conclusions in this pamphlet may be emphasized, it will be recalled 

 ^ at w ^ en a stee ^ s P ecmien is tested in tension to destruction it passes through two 

 well defined and significant stages. During the first stage, the elongations or de- 

 formations are comparatively small and increase approximately proportionate to the load. 

 During the second stage there is a plastic yielding of the material which is attended by gieatly 

 increased elongations amounting at fracture to many hundred times the whole elongation oc- 

 curring during the first stage. 



Between these two stages there is a well defined point marked by a sudden increase of el- 

 ongation which is easily noted when the readings are plotted on a chart. (See Curves, pages 168 

 and 169.) This point is generally termed in the literature of the steel industry, the "yield point" 

 or "commercial elastic limit;" more accurately called, in foreign texts, the "rapidiy-breaking-down 

 point;" sometimes erroneously spoken of as the "elastic limit." At this noint total failure does 

 not occur, but the warping of the structure which follows ruins it for practical purposes. In 

 the case of a steel which is to be used for reinforced concrete, this point is of great importance 

 as actual failure occurs immediately after it is reached. 



It was at one time widely thought among scientists, that steel was perfectly elastic up to 

 a point called the "elastic limit," which we will here call the "theoretical elastic limit" (a point 

 near the "commercial elastic limit" or "yield point" above mentioned). By "perfect elasticity" 

 in the steel was meant that after having been stressed, it would recover its original length if the 

 load were released; that is to say, at the "theoretical elastic limit" a permanent set took place. 

 It is now known, however, that a permanent set can be detected soon after the load is applied, 

 if only instruments precise enough are used. 



