CAST IRON. 



387 



The strain of cast iron, of whatever kind, is always 

 made up of an elastic distortion and a small amount 

 of permanent set. On removing the load from a bar the 

 set does not disappear, and if the bar is again loaded in to 

 the same extent it will be found that the total deflection 

 (in the case of a beam) is less than formerly, because the 

 set has been to some extent eliminated ; and if the loading 

 is again repeated the total strain will still continue to 

 decrease. Mr. Millar experimented upon this point, and 

 published a number of figures, of which the following 

 may be quoted as representative : A beam of the ordinary 

 dimensions was loaded five times, and finally broke at a 

 load of 3,5001bs. and a deflection of 0'403in. For each 

 loading the same load was applied, viz., 2,8001bs. ; and 

 deflections were respectively for the first up to the fifth 

 loading, 0"302in., 0'282in., 0'279in., 0-278in., 0'276in., 

 and 0'273in. A load-strain diagram for a cast-iron beam 

 is given on Fig. 186. 



FIG. 186. LOAD-STRAIN CURVE FOR A CAST-IRON BEAM. 

 2in. x lin. x 36in. span. 



206. Cast Iron in Shear and Torsion. The following 

 test results are for cast iron in direct shear and torsion. 

 The testing of cast iron in direct shear is an uncertain 

 process, as it is difficult to entirely eliminate any ten- 

 dency to bending. The cast-iron shafts tested by Mr. 

 Coker and the author* were both solid and hollow, there 



* Min. Proc. Inst. C'.E., cxii. 



