812 



THE ELASTIC LIMIT. 



Commercial Limit, perhaps better called the Yield Point 

 in order to prevent ambiguity. In the report of the 

 French Commission on Testing, three limits are defined as 

 follow : 



Elastic Limit is the unit stress beyond which a 

 portion of the deformation remains as permanent set. 



The Proportional Elastic Limit is the point where 

 the deformation ceases to be proportional to the load. 



The Apparent Elastic Limit is the point where the 

 deformations increase rapidly without any increase in the 

 force exerted. 



7 8 9 IO II 1.2 13 Tows 



FIG. 159. LOAD-STRAIN CURVE FOR MILD STEEL, SHOWING ELASTIC LIMITS. 



In order to render the useful elastic limit more definite, 

 Professor Johnson has thus defined it : - 



The Apparent Elastic Limit is the point on the stress 

 diagram of any material, in any kind of test, at which 

 the rate of deformation is 50 per cent, greater than it is 

 at the origin. This point is determined by first finding 

 a line whose inclination has a tangent half as great again 



