It was also widely thought, among scientists, that within the "theoretical elastic limit" 

 the stress or unit load was directly proportional to the strain or deformation. That is to say, 

 if the stress and strain readings of a tensile test were plotted, a straight line would be observed 

 up to the "theoretical elastic limit" which would by this definition be the "limit of proportionali- 

 ty." Many scientists may be cited who state that instead of a straight line a very flat curve 

 will be obtained, if only precise enough instruments are used. In other words, the "limit of pro- 

 portionality" was found to be reduced with the use of the most precise instruments. 



A discussion of the "theoretical elastic limit" is of scientific interest only. It is unques- 

 tionable that the "limit of proportionality" is very close to actual facts. It is a point, however, 

 which is commercially impractical to obtain and of doubtful significance. So far as commercial 

 testing is concerned, the significant point which is recognized and taken account of, is the "yield 

 point," the "commercial elastic limit," or "rapidly-breaking-down point." It is this point which 

 is recognized by the Standard Specifications of the Association of American Steel Manufactur- 

 ers representing practically all of the steel manufacturers in the United States. 



It is a fact well known to steel men, that when mild or medium steel is subjected to ten- 

 sile stress and the material begins to yield plastically (that is to say, the "yield point" or "com- 

 mercial elastic limit" or "rapidly-breaking-down point" is reached) the unit load temporarily de- 

 creases which has the effect of causing the balance beam of a testing machine to drop. Thus the 

 value of the "yield point" is noted at once during a test without recourse to a chart or plotted 

 readings. This temporary drop in the unit load is noted in the diagram as a slight "kink" in the 

 otherwise smooth curve. In the case of a mild steel which has been subjected to the process of 

 cold drawing, as for example, "Steelcrete" mesh, this "kink" above referred to, does not app< 

 hence, in order to determine the value of the "yield point" in such a case, it is ner< plot 



the stress-strain readings. This procedure is characteristic of all cold drawn steels. The be- 

 havior of a piece of steel under tensile test may be read at a glance from the plotted stress- 

 strain curve and careful study of the ones hereinafter submitted is invited. 



165 



