CHAPTER IV. 

 STRESS OR STRAIN. 



"STRESS" or "strain" is the name designating the ap- 

 plication of forces to a body in the same straight line but in 

 opposite directions, so that the internal resistance offered by 

 the cohesive force of the fibres or particles of which the body 

 is composed is balanced by the opposing or exterior force or 

 pressure. 



The effect of an exterior force acting upon a body to 

 change its shape, may be exerted as "tension," "compres- 

 sion," or " shear." 



If the force acting upon a body has a tendency to elon- 

 gate or stretch its fibres to the point of rupture by pulling 

 them apart, this force is termed a " tensile stress." 



If, on the contrary, the application of the force tends to 

 shorten or to compress these fibres, such force is called a 

 "compression stress," obviously "compression " and "ten- 

 sile" stresses differ only as regards the direction in which 

 the exterior force is applied or exerted upon the fibres of 

 which the body consists. 



Force applied so as to act longitudinally along any 

 " member " of a structure through its fibres, tends either to 

 elongate or to compress these fibres in direct proportion to 

 the pressure exerted, and the resistance offered to this pressure 

 by the fibres themselves is also directly proportional to the 

 tenacity and number of the fibres of which the body is com- 

 posed, as represented by its area or "cross-section." 



Beside these two stresses, there is a third, called a "shear 

 stress/' and which, as its name would indicate, is the tendency 



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