188 JOHN C. KOCH 



6. Stress is the internal force which, when a body is acted 

 upon by external forces, resists the change of position of the 

 molecules of the body and tends to preserve their original 

 position. Stress, like force, is measured in pounds (or tons, 

 kilograms) . 



7. Deformation is the change of shape which is produced by 

 the action of equal but opposite forces and is measured in units 

 of length — inches (feet or millimeters). The units, inch and 

 pound, will be used throughout the following discussion. 



8. Unit-stress is the stress per square inch of cross section = 



total stress P 



total area of cross section A 



alteration in length d 



Unit-deformation is the ratio 



original length e 



9. Compression (fig. 1, a,h, c) is the stress which tends to keep 

 adjoining planes of a body from being pushed together by the 

 action of two equal and opposite forces acting toward each 

 other. If a vertical post of uniform cross section is supporting a 

 load of 1000 pounds uniformly distributed over the surface of 

 the top, for equilibrium there must be an internal force or com- 

 pression in the post, equal to this load, or a compression of 

 1000 pounds. 



10. Tension (fig. 1, d) is the stress which tends to keep adjoin- 

 ing planes of a body from being pulled apart by the action of two 

 equal and opposite forces acting away from each other. If 1000 

 pounds is suspended from a rigid support by a steel bar of uni- 

 form cross section there will be in every section of the bar an 

 internal stress produced by this load : for equilibrium this stress 

 or tension must be equal to the external load — 1000 pounds. 



11. Shear is the term applied to the stress (internal force) which 

 tends to keep two adjoining planes of a body from sliding one 

 past the other, under the action of two equal and parallel forces 

 acting in opposite directions. These forces that produce shear 

 are called shearing forces and are usually very close together so 

 that their action is similar to that produced by a pair of shears, 



