RAISING THE LIMIT. 



31 3"- 



as the tangent of the early part of the diagram, and then 

 finding where a line of this inclination just touches the 

 strain diagram. This point will be the Apparent Elastic 

 Limit. 



On Fig. 159 are plotted the strains for a test given. 

 Here Y is the yield point or commercial limit; J is the 

 apparent elastic limit obtained according to Johnson's 

 rule ; A is the proportional elastic limit of the French 

 Commission ; while the true elastic limit, which was not 

 determined in this case, will be slightly nearer the origin. 



So far as commercial testing is concerned, the only 

 elastic limit which is recognised and taken account of is 

 the yield point. It is near the true limit, and can be 

 determined with far greater ease and certainty without 



5 10 ' 15 20 25 



FIG. 160. DIAGRAM SHOWING THE RAISING OF THE ELASTIC LIMIT 



the employment of instruments of precision r and is 

 sufficiently accurate for all practical purposes. 



164. Raising the Elastic Limit by Stress. It is well 

 known that the result of overstrain on a test bar is to 

 change the load at which the elastic limit occurs, and any 

 further repetition of this overstraining will produce a new 

 limit. The meaning of this will be clear from a, con- 

 sideration of the diagram on Fig. 160. 



