SIMILAR BARS. 



299 



area. The last mentioned is a good and sound criterion 

 of a metal's ductility, and is independent of the length 

 of the specimen. 



156. Similar Test Bars. Another way of getting over 

 the length difficulty would be to adopt a certain 

 fixed ratio of length to diameter for all test pieces. 

 The following test results for a set of bars of 

 various sizes having a constant ratio of length to 

 diameter, and of the same metal, made by M. Barba,* 

 show that for similar specimens of the same material the 

 extensions will be practically constant : 



TESTS OF BARS OF MILD STEEL. 



These and other tests show that constant results may 

 be expected from similar bars of the same material. 

 Unfortunately, however, it is not possible to take ad- 

 vantage of this, chiefly because it would, in most cases, 

 involve too great an expenditure of time and money in 

 machining the bars to the required proportions. In 

 plates, for instance, it is convenient to adhere to one 

 size and to machine the specimens in batches, and for 

 short specimens, such as those cut from tyres, in order 

 to make them of the proper proportions it would be 

 necessary to make the diameter far too small. 



157. Relation between Elongation and Reduction in 

 Area. It has already been pointed out that, assuming the 

 volume of the bar to remain constant, and so long as it 



* Hackney on " Test Pieces," Min. Proc. Inst. C.E., Vol. Ixxvi. 



