100,000 



90,000 



80,000 



EVOLUTION OF THE MODERN HEAVY GUN 153 



So important is the relation between cast iron, wrought iron, 

 and steel that it may be well to illustrate this by the use of a dia- 

 gram due to Professor Merriman. Assume that short rods of these 

 materials, each of the same length and one square inch in cross- 

 section, are subjected to great stretching force by the use of a test- 

 ing machine. As this force increases up to the elastic limit of six 

 thousand pounds, the cast-iron rod becomes elongated proportion- 

 ally. It breaks suddenly when the stress reaches twenty thousand 

 pounds. At this limit of tenacity the rod has been increased in 

 length less than one per cent, as shown in the diagram. The 

 wrought iron becomes lengthened at a less rapid rate, reaching 

 its elastic limit for a stress of about twenty-five thousand pounds. 

 In each case, up to the elastic limit, if the stretching force be re- 

 moved the rod will recover its former length and condition. On 

 further increasing the stress, the wrought iron stretches at a 

 more rapid rate, and bears a stress as great as fifty-eight thousand 

 pounds. If now the force be withdrawn the iron remains in its 

 deformed condition, the lengthening being about twenty-two per 

 cent. On again apply- 

 ing the stress there is 

 further rapid length- 

 ening up to twenty- 

 five per cent, this 

 yielding causing a de- 

 crease of stress till 

 the rod breaks at a 

 limit below fifty-eight 

 thousand pounds. 

 The elastic limit and 

 the breaking limit are 

 thus widely different. 

 In the case of steel 

 the elastic limit is 

 not reached until the 

 stress becomes fifty 



thousand pounds. Its elastic limit is thus double that of wrought 

 iron. Further increase of stress now causes the steel to increase 

 its rate of stretching, and permanent strain results. Its breaking 

 limit, one hundred thousand pounds, is nearly double that of the 

 wrought iron, and is reached when the yielding attains fifteen per 

 cent. This is not much more than half of the twenty-five per 

 cent of yielding of the wrought iron. 



The figures just given are only averages. Cast iron has been 

 made with a tenacity in excess of forty thousand pounds, while 

 that of steel may vary in different specimens from sixty thousand 

 to three hundred thousand pounds. This wide range shows that for 



a 



u 70,000 



< 60,000 



m 50,000 



40,000 



3 

 o 



a 



a 30.000 



20,000 



10,000 



10 15 



Per Ceut. of Elougation 



20 



25 



Curves showing Tensile Strength of Timber, Cast Ikon, 

 Wkought Ikon, and Steel. 



