46 FARM MACHINERY 



slab and the whole rolled into plates (Fig. 27). A soft- 

 center steel may be made by carbonizing a plate of mild 

 steel by a process much the reverse of malleable making. 



STRENGTH OF MATERIALS 



All materials used in construction resist a stress or a 

 force tending to change their form. Stresses act in three 

 ways: (i) tension, tending to stretch; (2) compression, 

 tending to shorten ; and (3) shear, tending to slide one 

 portion over another. 



75. Tension. Material subjected to a stress tending to 

 stretch it, as a rope supporting a weight, is said to be 

 under tension, and the stress to the square inch of the 

 cross section required to break it is its tensile strength. 



76. Compression. Material is under compression 

 where the stress tends to crush it. The stress to the 

 square inch required to crush a material is its compressive 

 strength. 



77. Shear. The shearing strength of a material is the 

 resistance to the square inch of cross section required to 

 slide one portion of the material over the other. 



78. Transverse strength of materials. When a beam 

 is supported rigidly at one end and loaded at the other, 

 as in Fig. 28, the material of the under side of the beam 

 is under a compressive stress, and that of the upper part 

 is subjected to a tensile stress. The property of materials 

 to resist such stresses is termed their transverse strength. 



79. Maximum bending moment (B.M.) is a measure 

 of the stress tending to produce rupture in a beam, and 

 for a cantilever beam (i. e., one supported rigidly at one 

 end, Fig. 28) is equal to the load times the length of the 

 beam (W X L). The maximum bending moment de- 

 pends upon the way a beam is loaded and supported ; 

 thus with a simple beam loaded at the center and sup- 



