234 APPLIED SCIENCE 



bodies. This force must be overcome in order to change the 

 condition, form, or size of the iron bar, or to break it into 

 parts. When the iron bar is supporting a load, the resistance 

 which the bar offers to the pressure or pull of the load that 

 tends to overcome the force of cohesion is called a stress. If the 

 load is not very great, the particles of iron may be separated 

 while the iron is supporting the load, but they will return 

 to their original position as soon as the load is removed. 



The elasticity of different substances varies. The degree 

 of elasticity of the various materials is found by measuring 

 the forces required to produce equal changes in four pieces 

 of the same material of like dimensions. In case the load 

 is very great and the particles of iron are separated to such 

 an extent as not to return to their original positions when 

 the load is removed, the structure of the iron is more or less 

 broken down. This is very clearly shown by the change in 

 appearance of polished surfaces of a metal in a stressed condi- 

 tion. The bright surface suddenly becomes dull when the 

 stress exceeds the amount which affects the permanent struc- 

 ture. Another example of stress is seen when a large casting 

 is lifted by a crane or derrick. The chains supporting the 

 casting are then said to be "in stress" or " stressed." 



282. Different Kinds of Stresses. Stresses may be di- 

 vided into the following five classes according to the action 

 of the force producing them : 



(a) Tension (pulling stress) usually called tensile stress. 



(6) Compression (crushing stress) usually called compres- 

 sive stress. 



(c) Shearing (cutting stress). 



(d) Torsion (twisting stress). 



(e) Flexure (bending stress). 



