14- 



TENSION. 



stress acting 



a 



quently the stress is greater at the circumference, and 

 gradually diminishes until it becomes zero at the axis. 



Combined Bending and Torsion is a case that often 

 occurs in practice, but which is not often applied in testing 

 operations. It occurs in such cases as the overhung crank- 

 shaft of a steam engine, where the shaft is subjected to the 

 twisting moment exerted at the crank pin, and a simulta- 

 neous bending moment due to either the weight of the 

 flywheel or to thrust of the connecting rod, or both. 



6. Relation of Stress and Strain on Elastic Bodies. 

 So far we have only referred to the various stresses them- 

 selves, and not to the effect they produce. The results of 

 stresses are strains. The word "strain" is often, though 

 "wrongly, used in the same sense as stress. 



When a bar is pulled by a load, the tensile 

 produces tensile strain, which, in this case, is an elongation 

 of the bar. Similarly, a compressive stress produces a 

 shortening of the bar as its' strain ; the strain in shear is 

 simply a transverse distortion ; that in 

 torsion a circular or spiral distortion. 

 We will consider each case separately. 



7. Tension. If a rectangular prism 

 ABC I) be subjected to a tensile load P, 

 acting along the axis, the stress induced 

 on a section of area A, at right angles 

 to the axis, will be, 



P 



The effect of this load is to cause 

 an elongation of the prism. If its 

 original length be L, and the elongation 

 be called I, then the strain is expressed 



by the ratio or fraction - T -; and the 



Li 



general relation between the stress 

 and the strain it produces is expressed 

 in Hooke's Law, which says that " the 

 extension of a bar under a tensile load 

 is proportional to the stress;" or, 



L a 



~f T 



L 



More particularly, we may say that the 

 extension is equal to the stress divided 

 by a constant quantity, or, 



L 



p 

 FIG. 4. 



