390 



CAST IlioN. 



point there is a very slight increase of strength, but after- 

 wards, as th triiijtcratuiv is still further increased, the 

 strength continually diminishes, being only about "><> |MT 

 ccur. of its strength at the normal temperature at 1,100, 

 Mini falling to 25 per cent, at 1,400. 



20 



i 15 



| 10 



12S 



250 



375 



500 



-525 750 



Twisting moment. Inch pounds. 



I'Ki. 1>7. -LOAD-STRAIN CURVE FORA CAST-IRON TORSION Sl'ECIMKN. 



Sin. long and 0'473in. diain. 



209. The Strength of Cast Iron Thick Cylinders. In 



thin cylinders such as steel or wrought-iron pipes, and 

 steam boilers where the thickness is small compared with 

 the diameter, the tensile stress is taken as being sensibly 

 constant across the whole thickness, and the calculation of 

 their strength becomes simple ; but in thick cylinders, 

 win-re the thickness forms a large proportion of the 

 external diameter, the case is different. The tensile stress 

 in the metal, caused by the uniform pressure of an internal 

 fluid, varies continuously from the inner to the outer 

 surface, being greatest at the inner surface. The relation 

 between the dimensions of the cylinder, the internal 

 pressure, and the maximum tensile stress in the material 

 is given by Lame's formula, which is deduced purely by 

 mathematical reasoning, on the assumption that the 

 material remains perfectly elastic. The equation is the 

 Following : 



