234 STRENGTH OF MATERIALS 



strength varies from 50,000 lb./in. 2 to 150,000 lb./in. 2 , a good average 

 being about 95,000 lb./in. 2 . 



The metal is so imperfectly elastic that Hooke's law does not 

 strictly hold for any range of stress, however small. The modulus of 

 elasticity in tension varies from 15,000,000 to 20,000,000 lb./in. 2 , and 

 in shear from 5,000,000 to 7,000,000 Ib./inA On page 232 is given 

 a table of the tensile strength of various samples of cast iron of 

 different chemical compositions. 



184. Strain diagram for cast iron. The strain diagram <>l 

 iron in tension, shown in Fig. 151, illustrates clearly the fact that 

 the metal is very imperfectly elastic. No part of the diagram is a 

 straight line, and no elastic limit is shown by the curve. Tin 1 maxi- 

 mum load in this case was 34,750 lb./in. 2 . The curve was drawn 

 from data given in the Water totvn Arsenal Report, 18 !).". From the 

 results of four hundred and fifty tests of cast iron in tension, com- 

 pression, and cross-bending, Kirkaldv found the average coinpn- 

 strength to be 121,000 lb. in.-, the tensile siivngtli li.1,000 lb./in. 2 , 

 and the cross-bending modulus (see Article 65) 38,000 Ih./in.-. 



I :. l.VJ shows a strain diagram of cast iron in compression. Like 

 the tension diagram, this shows no \vcll-detined elastic limit and no 

 constant modulus of elasticity. The maximum compressive strength 

 in lli is case was 50,000 lb./in. 2 . 



When tested in compression as a short block, cast iron has a 

 characteristic fracture, shearing along a plane making an ang! 

 about 30 with the vertical. This differs by 15 from the theoretical 

 angle (4."> ) of maximum stress for such cases. 



185. Cast iron in flexure. The most extended series of tests ever 

 made on cast iron in flexure was made by .1. W. Keep on bars \ in. 

 square and 12 in. long. From these tests, the average strength was 

 found to be 450 lb., giving a modulus of rupture of 64,800 lb./in. 2 . 

 A good average for the modulus of rupture for ordinary comm< 

 cast iron would he between 36,000 lb./in. 2 and 42,000 lb./in. 2 . 



186. Cast iron in shear. The strength of cast iron in shear \ 

 from 13,000 lb./in. 2 to 25,000 lb./in. 2 . Tests are made in the ordinary 

 torsion machine. The fracture in this case is the characteristic frac- 

 ture of brittle materials in torsion ; that is, instead of shearing off in a 

 plane at right angles to the axis of the test piece, as is the case with 



