408 



Mr. R. C. Nichols on the Resistances 



And although experiments prove that rupture does not 

 actually take place until the tensile strain is such as would 

 correspond, if the elasticity continued unimpaired, to a tensile 

 stress of at least 60 or 70 thousand pounds, the actual stress 

 producing rupture, and which cannot therefore have been 

 exceeded, is not more than a third of that amount. The 

 experiments of Hodgkinson show a tensile strength for various 

 samples of cast iron varying from 13,404 to 21,907 lb. Mr. 

 Barlow's "experiments on tensile strain" give a stress of 

 15,747 to 22,035 lb. These experiments unfortunately do not 

 show the amount of strain. 



The annexed diagram (fig. 5) will then represent approxi- 

 mately the tensile strain and stress in units for strain of one 



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a 











i 



9 



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 & 





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Scale Jn 



rS/ress 



.. 





J, 









V 



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% 



P 







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Sr«-lt 



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Maximum 



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Fig 5 



inch for one thousandth part of elongation, and in units for 

 stress of ^th of an inch for every thousand pounds. This 

 assumes the modulus of elasticity to be 20,000,000, which is 

 about that indicated by the deflexions of a bar 5 feet long 

 with moderate loads in the experiments of Mr. Barlow. 



The excess of strain beyond its normal proportion to the 

 stress may be termed the overstrain. 



An unfailing indication that overstrain has taken place 

 and that the stress has not maintained its proportion to the 

 strain, is given when on the external forces causing the strain 

 being removed the original condition is only partially restored 

 and part of the strain remains as a permanent set. In the 

 experiments of Mr. Barlow, already described, after the load 

 of 14,746 lb. was removed, the distance between the lowest 



