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Transactions. 



rapid rate than the theory of applied statics and dynamics. 

 As the theory developed, cumbersome full-sized experiments 

 were replaced by those on models, when it became evident 

 that if the physical properties of the material were accurately 

 known experimental work would in ordinary cases be unneces- 

 sary 



Machines had been made for the proof testing by " pulling " 

 of chain cables, suspension-links, &c, and only trivial modi- 

 fications in design were required for the production of smaller 

 appliances suitable for testing specimens of the material it 

 was proposed to use. A period of refinement brought the 

 sensitiveness of such machines up to 1 in 20,000, and instru- 

 ments capable of measuring extensions as small as 2 ooVoo m - 

 were constructed. With these it was ascertained that on being 

 progressively loaded a bar of iron or steel is practically elastic 

 up to a point of loading known as the " elastic limit," and 

 imperfectly elastic up to a further point known as the " yield- 

 point " ; at this point it suddenly breaks down, extending 

 rapidly without further loading, then hardens, and the ex- 

 tension increases with — but more rapidly than — the load, being 

 partially " elastic," but for the most part " plastic," until the 

 point of maximum load is reached, when the material begins 

 to flow, contracting rapidly until it fails at a lower total load 

 than the maximum, but at a greater stress per unit, of the now 

 reduced cross-section. 



This behaviour is most readily shown by means of a dia- 

 gram, similar to that drawn by the autographic recorder with 

 which most testing-machines are equipped, in which the co- 

 ordinates represent the extension of, and the corresponding 

 load upon, tiie tested piece. 



In such a test the ultimate and 

 elastic strengths of the material are 

 obtained, together with its extension 

 and contraction of area ; the latter 

 are measures of its ductility. The 

 test usually required by engineers is 

 of this description, and must satisfy 

 certain specified values. 



In quite the early days of tensional 

 testing it was noticed that metal 

 which had given excellent results on 

 test often failed under a compara- 

 tively low load in service, and that 

 such failure occurred when the load, 

 or some portion of it, was a five 

 one. It might naturally have been 



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