22 BULLETIN 556, U. S. DEPARTMENT OF AGRICULTURE. 



MECHANICAL PROPERTIES. « 



Mechanical properties are the properties of wood which enable it 

 to resist deformations, loads, shocks, or forces. Thus the ability 

 to resist shearing forces is a mechanical property of timber. (See 

 "Strength.") 



MODULUS OF ELASTICITY. 



(Seep. 14.) 



Modulus of elasticity is the ratio of stress per unit area to cor- 

 responding strain per unit length, the distortion or strain being 

 within the elastic limit. 



Numerically, the modulus of elasticity of a material is the force 

 in pounds required to stretch a sample of that material with a cross- 

 sectional area of 1 square inch to double its length, on the assump- 

 tion that the fibers would not be stressed beyond their elastic limit. 

 India rubber has a very low modulus of elasticity, while that of 

 steel is very high. It is, then, the measure of the stiffness or rigidity 

 of a substance. 



MODULUS OF RUPTURE. 



(See p. 13.) 

 PHYSICAL PROPERTIES. 



Physical properties, as the term is used in this bulletin, are those 

 properties of wood which have to do with its structure, -such as 

 density, cell arrangements, fiber length, etc. In its broad sense the 

 term physical properties includes all those properties listed as me- 

 chanical properties as well as those pertaining to its structure. 



RADIAL. 



Radial means extending outward from a center or an axis. Thus 

 a radial surface in a tree is one extending from the pith of the tree 

 outward, such as the wide faces of a quarter-sawed board. 



RINGS. 



(Seep. 8.) 



Rings are those circular markings around the center of a tree 

 section which are produced by the contrast in density, hardness, 

 color, etc., between springwood and summerwood. One ring, known 

 as an annual ring, consists of a layer of springwood and a layer 

 of summerwood. 



SHEAR. 



(Seep. 17.) 



Shear is the name of the stress which tends to keep two adjoining 

 planes or surfaces of a body from sliding, one on the other, under 

 the influence of two equal and parallel forces acting in opposite 

 directions. A force which produces shear (or shearing stress) in a 

 material is called a shearing force. 



