THE ELASTIC LIMIT. 311 



163. The Elastic Limit in Tension. Some ambiguity 

 exists as to the correct meaning of this term. The words 

 " elastic limit " are used to describe several things which 

 are not quite the same, and some confusion has in conse- 

 quence arisen. 



In its original and true meaning, the elastic limit of a 

 material is that stress beyond which the strain will not 

 wholly disappear on the removal of the stress. In other 

 words, when the stress imposed produces the smallest 

 possible amount of permanent set, the material can no 

 longer be regarded as truly elastic, and the limit has been 

 passed. 



This is a rather more rigid definition than saying, as 

 it is often done, that the elastic limit is the stress beyond 

 which the strains cease to be proportional to the stresses. 

 By this latter definition, if a stress-strain curve is plotted, 

 and the straight portion produced, the point where the 

 curve departs from the tangent is taken as the true elastic 

 limit. But the accurate determination of this point is 

 dependent, to a great extent, on the degree of precision of 

 the instrument with which the extensions are measured. 

 An extensometer reading accurately to ten-thousandths 

 of an inch will detect the limit sooner than one only 

 reading to thousandths. 



The first definition given above is that of the true, or 

 what is often called the scientific elastic limit. This 

 is to distinguish it from the more commonly used or 

 commercial elastic limit, which is a point in the loading 

 a little beyond the true limit, where a very large increase 

 takes place in the strain, with little or no increase in the 

 stress, producing a distinct "jump" or step in the plotted 

 curve. This point has also been called the "yield point" 

 and the "breaking down point," to signify the apparent 

 sudden breaking away or yielding of the substance. As 

 previously stated, the commercial limit or yield point is 

 roughly determinable by noting the sudden drop of the 

 testing machine lever when it occurs, or better, by placing 

 a pair of dividers on a marked length of the bar, and 

 noting when one point leaves its mark. 



These two limits are the ones commonly made use of, 

 namely, the Scientific or True Elastic Limit and the 



