TENSILE STRENGTH OF RIVET IRON. 



421.' 



to from twenty to twenty-five times its original length ; whilst plates, such 

 as we have selected, never come under the hammer, and seldom exceed six 

 or eight times the length of the original shingle after passing through the 

 rolls. 



On comparing these results with those of a similar quality of iron, viz. 

 S.C. ^ bar iron, experimented upon at Woolwich Dockyard, it will be found 

 that a corresponding and progressive increase of strength is equally appa- 

 rent as in the above experiments; that increase, however, arising from a 

 different cause, namely, the repeated fracture of the bars as exhibited in the 

 following Table: — 



From the above it will be seen that the mean strength of the bars was 

 24 tons, whilst that of the rivet iron was 28 tons per square inch, at a tem- 

 perature of 60°, and that the former attained its maximum strength of 

 29 tons from repeated breakages, whilst the latter reached a strength of 

 37 tons by an increase of temperature up to 317°. These are curious and 

 interesting facts, exhibiting a parallel increase of strength, in the one case 

 resulting from repeated strains, in the other from increase of temperature. 



The foregoing Table indicates a progressive increase of strength, notwith- 

 standing the reduced sectional area of the bars. This fact is of considerable 

 importance, as it shows that a severe tensile strain is not injurious to the 

 bearing powers of wrought iron, even when repeated to the extent of four 

 times. In practice, it may not be prudent to test bars and chains to their 

 utmost limit of resistance ; it is however satisfactory to know, that in cases of 

 emergency those limits may be approached without incurring a serious risk 

 of injury to the ultimate strength of the material. 



It is further important to observe, that the elongations are not in propor- 

 tion to the forces of extension ; thus in the bar F, the elongation of a bar, 

 5i inches long with 33*25 tons, is 10*5 inches, giving an elongation per unit 



105 

 of weight and length = „^.^- g. ='0058, whereas an additional weight of 



2*25 tons produces an elongation of 1"25 inches in 36 inches of length of 



1-25 

 bar, giving an elongation per unit of length and weight= ^.^^ gg ='0154' ; 



that is, the elongation in this case is about three times that in the former. 



