440 REPOKT — 1891. 



Form and Dimensions of Test-pieces. 



The forms and proportions adopted by the author are illus- 

 trated on Plate XI., in order that the results obtained might 

 be comparable with the greatest number of English, German, 

 and French results. 



It is well known, however, that the form and dimen- 

 sions of the test-piece have a very marked effect upon the 

 results obtained in testing. See Hackney, "Forms of Test- 

 pieces," Proc. Inst, of Civ. Eng., vol. Ixxvi. High per- 

 centages of elongations may be obtained from short or 

 thick test-pieces : long and thin test-pieces give much lower 

 percentages of elongation for the same material. 



The tests intended to govern the quality of the material 

 for a particular purpose will next be considered. 



In testing wrought-irou and steel intended to be used 

 in engineering construction it is at least necessary to deter- 

 mine the strength and ductility. The ductility is usually 

 ascertained by measuring the percentage of elongation in the 

 manner already described, or by the percentage of contraction 

 of the fractured area. The contracted area is measured most 

 conveniently by means of micrometer callipers. The strength 

 alone, as first pointed out by Mr. Kirkaldy, is no indication of 

 the quality of the material. " A high breaking-strengtii may 

 be due to the iron being of a superior quality, dense, fine, and 

 moderately soft, or simply to its being hard and unyielding. 

 A low breaking-strength may be due to looseness and coarse- 

 ness in the texture, or to extreme softness, although very close 

 and fine in quality. The contraction of area at fracture forms 

 an essential element in estimating the quality of the specimen, 

 and by comparing the breaking-strength with the contraction 

 of area at fracture the respective merits of various specimens 

 can be correctly ascertained." 



The contraction of area can generally be measured with 

 sufficient accuracy in round specimens, but in the case of flat 

 specimens, especially very broad, thin strips, it cannot be 

 measured with sufiicient accuracy ; and when the fracture is 

 oblique, as is often the case, the difficulty is increased. The 

 contraction of area is also largely influenced, as stated by Pro- 

 fessor Unwin, by local conditions of hardness and homogeneity 

 at point of fracture. 



It is in consequence of the difficulty in measuring accu- 

 rately the contraction of area at fracture that many competent 

 authorities have advocated its omission in specifications of tests 

 of materials, in favour of elongation ; but here, also, a difliculty 

 exists. The elongation consists of two parts — namely, general 

 and local. The general extension in a specimen continues so 

 long as it offers increased resistance to the force producing it, 

 and is proportional to the length of the specimen ; but the 

 local extension commences after the general extension has 



