220 SCIENCE PROGRESS. 



for the reduction of area in tests. The author has measured 

 the reduction of area of thousands of specimens, and never, 

 or only in the case of exceptionally badly welded wrought 

 iron specimens, experienced any difficulty in making per- 

 fectly accurate measurements of the reduced dimensions. 

 The reduction of area, in ninety-nine cases out of a hundred, 

 can, with the help of vernier callipers, be measured quite 

 as accurately as the ultimate extension ; the latter, in the case 

 of irregular fractures owing to the difficulty of fitting the two 

 halves together, cannot very often be determined with ac- 

 curacy. The objection, therefore, to measuring and record- 

 ing this factor owing to these supposed difficulties falls to the 

 ground ; in the few cases where it is impossible to do it the 

 material is usually too defective to pass muster. 



As to whether it is a fair test of the ductility of the 

 material is quite another matter, but it certainly does 

 materially help one in coming to a conclusion. 



It is often stated that the percentage of reduction of 

 area at the point of fracture rarely exhibits any relation to 

 the percentage of ultimate extension, and that for this reason 

 it is unadvisable to use it. For a perfectly plastic material 

 it can readily be shown that the percentage contraction of 

 area is directly proportional to the percentage of elongation, 

 expressed however not in terms of the original length but 

 of the final length. Now no test piece is truly plastic in 

 the latter part of the test except, perhaps, in the immediate 

 neighbourhood of the place of fracture, and therefore it 

 would be impossible for any direct relation to hold between 

 these two quantities, when the extension refers to the whole 

 bar. If, however, the percentage of elongation is obtained 

 for a short length only, which includes the fracture, say for 

 one inch of the original length at that point, and this can 

 always be done if the whole bar has been marked off in inch 

 lengths prior to the test, then it will be found that this quantity 

 is usually in close agreement with the percentage reduction of 

 area, because we are now dealing only with the portion of the 

 bar which is very nearly truly plastic. In my own testing 

 work I always have the bars marked off into inch lengths, 

 and make these measurements which I find of great value. 



