ON THE MECHANICAL PROPERTIES OF STEEL. 



221 



A very slight variation in the observed deflection at the commencement of 

 the experiments before the bar had got its natural set would increase the 

 difficulty of ascertaining the correct permanent set corresponding to very 

 limited strains. We all know when a bar is a little bent we can make it 

 straight by hammering or by pressure, but the probability is that the first 

 form is the natural disposition of the material. 



This principle is adopted in the calculations, as the elasticity of a bar is 

 impaired when the deflection decidedly exceeds what the law of deflection 

 would give. After the elastic limit is passed the deflections increase in a 

 geometric progression, whereas up to that limit the deflections are in propor- 

 tion to the strain. 



One of the marked peculiarities of steel as compared with iron is, that the 

 strain corresponding to the elastic limit approaches more nearly the breaking 

 strain. Hence will be found the comparative high value of the constant G, 

 or the unit of pressure determined for the bars. A load of one-third the 

 breaking weight has always been considered a safe rule, but it is only con- 

 ventional ; but there is something still wanting relative to the point of strain 

 corresponding to the injury done to the material, as the inference drawn 

 from the Tables indicates that the strain producuig the permanent set had 

 not seriously affected the soundness of the bars. This is a question of con- 

 siderable importance, and requires further investigation, which I hope to 

 accomplish at some futm-e time. 



SECOND SERIES OF EXPERIMENTS. 



TENSILE STKADr. 



Elongations 



Exp. I. — Bar of Steel from Messrs. Brown & Co., Shefiield. 



taken on 8 inches length. Mark on bar, " B 1." Diameter of specimen 

 •77 inch. Area -4656 square inch, lleduced diameter after fracture 

 •77 inch. Area ^4656 square inch. 



Results. — Here the breaking strain (P,) per square inch of section is 

 68,404 lbs., or 30-53 tons ; and the corresponding elongation (l.^) per unit of 

 length is -0056. By formula (13). — The work («) expended in producing 

 rupture =191. 



