TENSION TESTING. 149 



of figures. For the purpose of comparing tests of similar 

 materials the use of diagrams is invaluable. In order to 

 plot these diagrams, faint-ruled paper, divided into inches 

 and tenths of an inch, should be used. For anyone to go 

 through the process of measuring up a sheet of paper every 

 time a diagram is to be plotted is wearisome in the extreme, 

 and not at all necessary. On Fig. 68 are plotted two dia- 

 grams of the above test. The larger one on the right is a 

 complete diagram of the test, and is called the load-strain 

 diagram. The horizontal measurements represent loads 

 as indicated by the scale ; and extensions are plotted 

 vertically to the scale shown. The first part of the diagram 

 is approximately a straight line almost coinciding with the 

 base line, and represents the period below the elastic limit 

 when the extensions are relatively small, and can only be 

 measured with an extensometer. After the load of 12 tons 

 has been passed, the extensions, which up to now have been 

 sensibly regular, begin to increase, and the line begins to 

 curve upwards more and more rapidly as the load is 

 increased. This rapid upward tendency ceases at 14 tons 

 load, and the curve settles down to a uniform curve, the 

 extensions increasing constantly. 



The last measurement of extension, while the bar is in 

 the machine, is made at a load of 21 tons. The maximum, 

 load is reached at 21 '9 tons, and the specimen breaks at a 

 lower load than this, namely, 18*4 tons. The last point on 

 the curve, which here comes backwards, is at this load of 

 18'4 tons, the extension being the total elongation, 3'06in., 

 measured after fracture. 



The second curve, to the left hand of the diagram, is 

 the elastic portion of the above curve plotted to a much 

 larger vertical scale, 100 times the first. It will be seen 

 that this is an almost straight line for the greater portion, 

 and curving very rapidly upwards after a load of 12 tons 

 has been passed. This load of 12 tons represents the 

 true elastic limit, the commercial limit being taken at 13 

 tons, after a distinct set has taken place. 



In the curves shown on Fig. 68 the actual points plotted 

 are represented by the centres of small circles, and a smooth 

 curve has been drawn through these. 



90. Remarks on Tension Testing in General. The case 

 which has just been described in detail may be taken as 

 being typical of most tension tests. The tests of other 

 materials, and of specimens of other shapes, are similar in 



