518 



Transactions. 



This formula may be always used when the statical breaking- 

 strength (t) of the material is known, and gives, in terms of the 

 ratio of the least (L min.) to the greatest (L max.) load on the 

 piece, the limiting stress (a) per unit of cross-section. 



In applying the formula to arrive at the safe working-stress 

 (b), a factor of safety (say, 3) should be adopted. Further, 

 this formula fits intermediate experimental results, and if the 

 minimum stresses be plotted as abscissae and the corresponding 

 ranges of stress as ordinates the points lie upon a parabola. 



LBS. PER 

 SQ. IN 

 60,000 



AO.OOO 



SO.OOO 



20.000 - 



Fig 2. 



Rept. 



Alt. 



For some years after the publication of Wohler's researches 

 it was believed that the problem of the working- strength of 

 iron and steel had been satisfactorily solved ; but the frequent 

 failure of material of presumably stable elastic limit within the 



