TENSION TESTING. 135 



such as those cut from large masses of metal, are 

 sometimes turned with enlarged ends, which are intended 

 to fit specially shaped dies, or they are made with 

 the ends screwed so as to fit a pair of .screw dies. 

 These two forms are shown respectively at F and G. Cast- 

 iron specimens should be made with enlarged ends. These 

 are the chief forms used for test specimens of ductile 

 materials. 



74. Making a Tensile Test. It will now be well to des- 

 cribe in detail the several operations necessary in the 

 carrying out of a simple tensile test, and for this purpose 

 an example will be taken of the test of a round bar of 

 mild steel, 18 in. long, 1 in. diameter. A full test will be 

 discussed, that is to say, one in which all the available 

 information is obtained. 



75. Preparing the Bar. After the bar has been cut to 

 the required length, which should allow a clear length often 

 inches for measuring purposes between the shackles, as 

 well as sufficient length at the two ends for holding 

 purposes, altogether about 18 inches, it should be marked 

 off' in inches from end to end, these inches being 

 indicated by centre-punch marks. By thus marking the bar 

 from end to end the extension on any length, including the 

 point of fracture, whether the fracture has occurred in the 

 part between the shackles, or, as sometimes happens, inside 

 the wedges, can be readily measured. The inch division 

 marks must be accurately set out, as upon these depends 

 the accuracy of the measurements of extensions after 

 fracture. 



76. Initial Dimensions of the Specimen. Before the 

 specimen is put into the machine, or as soon as it is in 

 place, its lateral dimensions must be ascertained ; the longi- 

 tudinal dimensions at the beginning of the test being 

 determined by the centre-punch marks which have been 

 placed on the bar. In the case of a round bar, it is the 

 diameter that is required ; when the bar is flat the width 

 and thickness are to be measured. The lateral dimensions 

 are rarely quite the same at every point throughout 

 the length, and it is, therefore, necessary to take the 

 measurements in several places and take an average. For 

 making these measurements a vernier caliper, reading to 

 a TTTffTjth f an inch, should be used. 



77. Balancing the Machine. Before finally tightening up 

 the wedges, it is most important that the machine be care- 



