STRENGTH OF MATERIALS 275 



STRENGTH OF MATERIALS 



DEFINITIONS OF TERMS 



Stress is the cohesive force by which the particles of a body 

 resist the external load that tends to produce an alteration 

 in the form of the body. It is always equal to the effective 

 external force acting upon the body; thus, a bar subjected to 

 a direct pulling force of 1,000 Ib. endures a stress of 1,000 Ib. 

 Unit stress is the stress or load per unit of area, usually taken 

 per square inch of section. For instance, if the bar mentioned 

 above is 1 in.X2 in. in section, the unit stress of the bar 

 will be 1,000^2 (sectional area) = 500 Ib. Tensile stress 

 is produced when the external forces tend to stretch a body, or 

 pull the particles away from one another. A rope by which a 

 weight is suspended, is an example of a body subjected to tensile 

 stress. Compressive stress L produced when the forces tend to 

 compress the body, or push the particles closer together. A post 

 or column of a building is subjected to compressive stress. 

 Shearing stress is produced when the forces tend to cause the 

 particles in one section of a body to slide over those of the adja- 

 cent section. A steel plate acted on by the knives of a shear, 

 and a beam carrying a load, are subjected to shearing stress. 

 Tension, compression, and shear are called simple or direct 

 stresses, to distinguish them from bending and torsion. 



The amount of alteration in form of a body produced by a 

 stress is called deformation, or strain. It may be tensile defor- 

 mation, compressive deformation, or shearing deformation, 

 according as the stress producing it is tensile, compressive, 

 or shearing. The rate of deformation, also called unit deforma- 

 tion, is the deformation of a body, subjected to tension or com- 

 pression, per unit of length. If an iron bar 6 ft. long is sub- 

 jected to a force that elongates it 1 in., the rate of deformation 

 will be 1 in.-f-72 (length of the bar in inches) = .0139 in. 



The modulus or coefficient of elasticity is the ratio between 

 the stresses and corresponding deformations for a given 

 material, which may have a somewhat different modulus of 



