WORK DONE OX BARS. 



245 



steam-engine trial is likely to interfere with the^proper 

 action of the indicator. The principal reason why auto- 

 graphic gears ought not to be used in the place of direct 

 readings is that the accuracy of the latter is of necessity 

 .greater and their results more certain. 



33- Work done in Fracturing a Test Bar. A very good 

 criterion as to the useful properties of a ductile material 

 is the amount of. mechanical work that must be expended 

 on the bar by the testing machine in straining it up to the 

 point of fracture against the gradually increasing load. 

 This quantity is generally expressed as so many inch-tons 



3 H 



20 



25 



10 D 15 



LOADS IN TONS. 



FIG. 120. Diagram showing the Work done in fracturing a Test Bar. 



per each cubic inch of the original volume of the metal. 

 The term " original volume " is used here because, although 

 for all practical purposes the volume remains constant 

 during the test, a small variation does in reality occur. 

 The work so done may be ascertained in various ways. In 

 the first place, a diagram may be plotted from a set of 

 direct observations taken at frequent intervals during the 

 test, and the area of this curve measured ; or the same 

 thing may be done with an autographic diagram ; or the 

 work may be calculated by means of an approximate 

 formula suggested by Dr. Kennedy. 



On Fig. 120 will be seen a reproduction of the load 

 strain diagram already given, and plotted from the results 



