TESTING MACHINES. 47 



CHAPTER II. 



TESTING MACHINES. 



19. Every material of engineering whose strength is of 

 any importance has, at some time or another, to resist force. 

 The general intention, in all kinds of testing, is to so deal 

 with small specimens of the material in question that they 

 may be subjected to stresses similar to those which they 

 will be expected to withstand when forming parts of actual 

 structures. 



The stresses imposed on parts of structures are, or 

 should be, safe stresses that is to say, stresses which do 

 not in any way produce deformation or deterioration of the 

 material. In the testing machine, however, the treatment 

 is as a rule different. In commercial testing, at all 

 events, most specimens are tested to destruction, that is to 

 say, stresses of such magnitude are applied that complete 

 failure of the material takes place. This failure may take 

 the forms of rupture, crushing, splitting, or crumbling, 

 according to the nature of the material and the kind of 

 stress imposed. In some tests, especially in connection 

 with educational and research work, the forces applied are 

 only such as will cause ejastic deformations, in which case 

 they may be repeated as often as may be desired. 



The reason why specimens of materials are tested to 

 destruction is that only by doing this can those qualities 

 of the material be made to show themselves, which will 

 enable the observer to gain a sufficient knowledge of the 

 really important strength properties of his material. It is 

 not sufficient to know, for instance, what is the maximum 

 safe load that may be applied to a wrought-iron tension 

 bar without producing permanent deformation, but more 

 important than this is a complete knowledge of the 

 uniformity, the ductility, and the nature of the internal 

 structure of the iron ; and such information as this can 

 only be obtained by actually breaking the bar or testing it 

 to destruction. 



When a specimen is to be tested, three functions have 

 to be performed by the appliances used. These are as 

 follow : 



(a) The specimens must be firmly held, and such loads 

 as may be desired applied. These must be steady loads 



