STRESSES. 



13 



middle of a beam whose ends rest upon two supports. 

 Previously to the application of the load the beam was 

 straight. On the application of the load the beam is made 

 to curve downwards and a bending stress is said to be 

 produced. The result of the bending is to shorten the 

 upper surface A B, and stretch or elongate the lower one 

 C D. Consequently A B is in compression and C D in 

 tension. The greatest intensity of each stress is at the 

 outer surface, and it diminishes gradually to zero as the 

 centre is approached. This surface of no stress is called the 

 neutral surface. It will be seen that a bending stress 

 is a combination of tension and compression. 



Torsion is not, strictly speaking, a compound stress, 

 but a particular case of shearing stress. The meaning of" 

 torsion is shown on Fig. 3. A circular bar (the bar need 

 not necessarily be circular) is acted upon by two equal and 

 opposite couples, or pairs of moments, one at either end. 



FIG. 3. 



That at the left-hand end, X, tends to twist the bar in a 

 right-hand direction, while that at Y has an opposite effect. 

 The result is that the right-hand half of the bar tends to 

 twist or slide in a circular direction about its own axis, 

 with respect to the left-hand portion ; and there is a 

 torsional stress exerted across a section A. The difference 

 between this and simple shearing is that the latter 

 takes place in straight lines, in the former it is rotary, and 

 the parts of the section near the circumference have a 

 greater movement than those nearer the axis, and conse- 



