404 AGRICULTURAL ENGINEERING 



Unit stress is the stress per unit area. Stresses are 

 usually measured in pounds, and areas in square inches. The 

 total stress divided by the area of cross-section in square 

 inches will give the unit stress. 



when P = total stress in pounds. 



A = area of cross- section in square inches. 

 S = unit stress. 



This rule is applied only when the total stress is uniformly 

 distributed and the stress is a simple stress. 



Elasticity. Most bodies when subjected to a stress will 

 be deformed. The amount the body is changed in shape is 

 termed the deformation. An elastic body will regain its 

 former shape when a stress is removed, if it has not been too 

 great. Up to a certain limit the amount of change in shape is 

 proportional to the stress. If the unit stress be increased to 

 such an extent that the material will not regain its original 

 shape after being deformed, the stress has passed beyond the 

 elastic limit of the material. 



Ultimate Strength. If the unit stress of any material be 

 increased until rupture or breakage occurs, the stress pro- 

 ducing the failure is the ultimate strength of the material. 

 If the failure be produced by the tensile stress, the ultimate 

 tensile strength is obtained. In like manner the ultimate 

 compressive and shearing strengths are obtained. The 

 breaking load divided by the original cross-section gives the 

 ultimate strength. 



Working Stress. The greatest stress allowed in any part 

 of a framed structure is called the working stress of that part. 

 In turn, the working strength of a material to be used for a 

 certain purpose is meant the highest unit stress to which 

 the material ought to be subjected when so used. 



