150 



TKNSION TESTING. 



a general way, with small differences in detail. It will be 

 useful to note any modifications Avhich are required. 



In the first place, it may be pointed out that all the 

 measurements and results which have been enumerated 

 are seldom required, especially in ordinary commercial 

 tests. In tests of this kind the quantities to be determined 

 are usually as follow : 



The maximum stress. 



The stress at the elastic limit. 



The extension on 10 in. of length. 



The extension on the 2 in., including the point of 



fracture. 

 The reduction in area per cent. 



Sometimes certain of these are omitted. The modulus of 

 elasticity is not often required for commercial purposes. Its 

 determination is most useful practice for engineering 

 students, besides helping them to a knowledge of the elastic 

 properties such as cannot be obtained in any other way. As 

 regards the effect of the material itself on the mode of 

 carrying out the test and on the subsequent calculations, 

 these will now be referred to in detail. 



Fig. 69. 



91. Mild Steel, High-grade Wrought Iron, Copper. 



The treatment of these three in tension testing operations 

 is pretty much the same. They can all be held satisfac- 

 torily in wedge grips, although in some cases the specimens 

 are turned and chased to fit screw dies. In testing strips 

 of plate a difficulty is sometimes found in getting the teeth 

 of the wedges to bite at first, especially if the metal is a 

 little harder than usual. In cases such as these the holding 

 of the wedges is greatly helped by drilling small holes through 

 the specimen close to the ends, and inserting short pins in 

 these holes. These pins may be about |in. diameter. 



