STRENGTH OF WROUGHT IRON. 



351 



irons as shown in the following table, it will be seen that 

 the ratio varies from - 76 to 0'87. 



Calculating the coefficient of torsional strength by the 

 usual torsion formula which assumes perfect elasticity 

 up to the point of fracture from the ultimate twisting 

 moments, it will be seen that in all cases the value is 

 much larger than the shearing strength obtained from 



30 



25 



2O 



- 15 



10 



\ 



o 



10 



2O 3O 



Load in Tons. 



FIG. 173. LOAD-STRAIN CURVES FROM MR. GROVER'S TENSION .TESTS. 

 A UNCOMPRESSED ; B COMPRESSED AND ANNEALED. 



direct tests. This discrepancy is caused by the false 

 assumptions made. That the condition of a ductile 

 material approaches that of perfect plasticity at the point 

 of fracture in torsion a condition involving a uniformity 

 of torsional stress at every point of the section of the 

 bar is shown by calculating the coefficient of torsional 



