4 Wilson, Stress and Strain in Copper Bars. 



p 

 The values oi e and —{i-\-e) have been calculated 



for each bar, and are shewn in the appendix (Tables I. to 

 V). The value of n has been obtained by finding the slope 

 between each pair of points in the test and taking the 

 mean of the results, and has also been checked graphically 

 from the diagrams. 



The corresponding values of C have been calculated 

 from the assumed values of n, and are shewn below. 



Test. 



n. 



c. 



1 



•535 



38-08 



3 



•535 



38^97 



4 



•503 



36-165 



5 



•526 



37-863 



6 



•519 



37-470 



Means. 



•524 



37"9i 



The relation is therefore j'^= 3 /pi ^'s^^- 



The maximum variation of n from the mean value 

 being 4% below and 2% above, whilst the corresponding 

 variation in C is 4'6% below and 2"8% above. 



If, instead of plotting a stress-strain curve for any bar, 

 the extensions are plotted as abscissae and the corres- 

 ponding loads as ordinates, it will be found that, after the 

 elastic limit is passed, the curve so obtained falls away 

 from the straight line, the strains increasing much more 

 rapidly than the loads. At some point in the curve the 

 loads reach a maximum value, after which further exten- 

 sion is accompanied by a diminished load. 



This maximum value of the load, when divided by the 

 original area of the cross-section of the bar, is called the 

 maximum stress, and it is simply stress per unit of 

 original area instead of stress per unit of actual area. 



