TRANSACTIONS OF SECTION J, 441 



ceased, and is most decided in all ductile materials, such as 

 steel. It is confined to the portion immediately adjacent to 

 the fractured area. The local extension is, in fact, propor- 

 tional to the area of the specimen. 



Although it is usual to measure the total extension on a 

 specimen, and to express it in percentage of length, the more 

 scientific way, which has been suggested by Professors Unwin, 

 Barba, and Wickstead, is to separate the general extension 

 from the local. 



Mr. Wickstead described, at the last meeting of the British 

 Association, a method of doing this from the autographic 

 record, and recommended a column in the test-sheet of " Per- 

 centage of General Extension," in addition to the usual columns 

 — namely, "Percentage of Contraction of Area," and "Per- 

 centage of Total Extension." The local extension can then be 

 expressed by subtracting the " General Extension " from the 

 "Total Extension." 



The local extension is seen by dividing the test-piece before 

 testing, over the length of, say, lOin., into ten equal parts, each 

 lin. long : the elongations remeasured after testing will be 

 much greater on the 2in. or Sin. which include the fracture 

 than over equal lengths measured on the remaining portion of 

 the lOin. 



Plate XII. shows a bar 30in. long, divided into thirty equal 

 parts, with the remeasured lengths after testing. Here the 

 local extension is very decided. 



In autographic stress-strain diagrams, such as those pro- 

 duced by means of Professor Kennedy's apparatus, the local 

 extension is easily separated from the general extension ; and 

 when this is done it is possible to eliminate the effect of dif- 

 ferent proportions in regard to length and area of cross-section, 

 on the percentage of total extension. 



The ductility of a specimen can also be ascertained by 

 bending round a bar of given radius. But here, again, the pro- 

 portions of the test-piece exercise a decided influence on the 

 angle bent through before fracture, which measures the ductility. 

 When steel is required for boiler-construction it is usual to 

 specify, in addition to the ordinary tests referred to, that strips 

 of the plate shall be heated to a cherry-red heat, and cooled in 

 water at 80"^ Fahr., after which they should bend double with- 

 out signs of fracture. 



In order to secure a suitable material for railway-axles it is 

 usual to specify, in addition to the ordinary tests for tensile 

 strength and ductility made on specimens cut from the axle, 

 that the axle itself should be tested to destruction by a series 

 of blows produced by a falling weight, the axle being reversed 

 after each blow. The results of the drop-test — which this test 

 is called — on axles, when the experiments are carefully con- 

 ducted, give a fairly close approximation to the endurance 



