MECHANICS OF MATERIALS 67 



All fibers above it are subject to compression and all fibers 

 below it to tension. In a beam having a rectangular cross 

 section and made of the same material throughout, this 



Fig. 94. Showing the tendency to slide along the 

 longitudinal elements of a flexed beam 



neutral point is just halfway between the top and bottom. 

 In beams of unsymmetrical sections or ones that do not have 

 the same kind of material throughout, such as reinforced 

 concrete, the neutral surface does not fall exactly halfway 

 between the top and bottom, its position being determined by 

 the shape of the beam and the properties of the materials. 

 The degree of stress increases from zero at the neutral 

 surface to a maximum compression at the top and a maxi- 

 mum tension at the bottom. 



If the load on the beam is placed at the center or is evenly 

 distributed both ways from the center, then the most dan- 

 gerous section falls directly at the center of the beam. If 

 the loading is other than this, then the dangerous section is 

 at some other point, depending on the position of the load. 

 It can be shown that, wherever this section is, there is no 

 shearing stress at that point. 



Torsion. Figure 95 shows a case of torsion or twisting 

 such as occurs in shafting. The stresses which are present 

 are shear on any transverse section, and tension on the fibers 

 running spirally in one direction, compression on fibers 

 running spirally in the other direction. Brittle materials 

 fail in the tension fibers, fibrous materials split apart, 

 and plastic materials shear off squarely under such con- 

 ditions. * 



