482 



As it is of the utmost importance that steel should be uniform in 

 strength, if the greatest tensile strength when tested exceed the least by 

 more than 15 per cent., the limiting stress shall be reduced. 

 Percentage of variation in tensile 



strength 16, 20, 30, 40, 50, or more. 



Percentage by which the limiting 

 working strength should be re- 

 duced (arbitrary) l^, H, 22|, 37^, 50. 



(b) Ductility required in oi'der that any material may safely be sub- 

 jected to the limiting working stress (a), to vary with the proportion in 

 which the stress is caused by a varying or live load ; and may be deter- 

 mined from 



a = 50 (1-0) 



in which h = percentage of contraction of area of fracture under tensile 

 test, and f (as in the Launhardt-Weyrauch Formulae) denotes ratio of 

 constant to total load. 



Materials which do not exhibit the ductility required by the conditions 

 under which they are strained are only to be subjected to a stress &=a 7.-, 

 in which h is the admissible stress in a material of which the actual con- 

 traction of area is A, used under conditions of loading for which the 

 required ductility is S, and A; is a coefficient derived from the empirical 

 formula 



Table of Values op I: 



2. In cases where the material is subject to tension and compression 

 alternately. 



