CHAPTER I. 



THE MECHANICS OF BODIES UNDER TEST LOADS. 



1. Before entering upon a description of the appliances 

 and methods employed in the testing of materials; it will 

 be convenient and useful to discuss at some length the 

 phenomena which display themselves, or may be expected 

 to do so, in bodies which are acted upon by forces. 



In the first place it must be assumed that all bodies 

 considered are solid and homogeneous. These are both 

 only relative terms, so far as the ordinary substances of 

 daily life and commerce are concerned. A certain tendency 

 to exhibit faint signs of some of the properties of a liquid 

 often manifests itself in solid bodies, as, for instance, this 

 tendency is made practical use of in the manufacture of 

 lead pipes where the metal is compelled, by the application 

 of great pressure, to actually flow through an annular die, 

 and in this way to form a continuous pipe. Then, again, 

 as to a substance being homogeneous. By this is meant 

 that every part of the substance, even the most minute, 

 has the same composition and physical properties as every 

 other part. This is true in a general sense of most of the 

 materials dealt with in testing operations, but strictly 

 speaking, all the substances lack true homogeneity, and 

 there are always small differences in different parts, which 

 tend to manifest themselves under delicate tests. For all 

 practical purposes we can regard most of the bodies dealt 

 with in testing as homogeneous. In this chapter, there- 

 fore, we assume perfect homogeneity to exist. 



2. Elasticity and Plasticity. All bodies, if acted upon 

 by force, undergo some kind of deformation, no matter 

 what their shape, substance, or size may be. Thus, for 

 example, take the case of a solid prismatic piece of india- 

 rubber ; if one end be fixed and a weight hung at the other, 

 the indiarubber will suffer a change of shape, that is to say, 

 its length will be increased and its lateral dimensions 

 diminished ; in other words, by reason of the force applied 

 in the shape of the weight, the piece of indiarubber will 

 have undergone a deformation. The body may be of any 



