MECHANICAL PROPERTIES OE WOODS GROWN" IN UNITED STATES. 21 



after which the deflections increase more rapidly than the increase 

 in load. 



A timber stressed beyond the elastic limit will not resume its 

 original form immediately upon the removal of the load. 



ELASTICITY. 



Elasticity is the property (possessed by most materials) of chang- 

 ing form with the application of force and recovering at once upon 

 release from the force. 



In any elastic material the amount of compression or deformation 

 is proportional to the force applied. 



Air and other gases under compression are elastic. The most 

 commonly recognized elastic material is rubber. Timber is elastic 

 within comparatively narrow limits. 



The term "very elastic" as applied to wood is indefinite, because it 

 may mean that the force required to produce a given deformation is 

 great and the recovery sudden as in an ivory ball (see "Modulus of 

 elasticity"); or that the amount of distortion to the elastic limit is 

 great, as in a rubber ball, or that the wood possesses high elastic 

 resilience, a combination of the two properties. (See "Elastic resili- 

 ence" or "Work to the elastic limit.") 



FIBER SATURATION POINT. 



(See p. 11.) 



Green wood usually contains water within the cell walls and "free" 

 water in the pores. In drying, the water in the pores is the first to 

 be evaporated. The fiber saturation point is that point at which no 

 water exists in the pores of the timber but at which the cell wails are 

 still saturated with moisture. The fiber saturation point varies with 

 the species. The ordinary proportion of moisture— based on the dry 

 weight of the wood — at the fiber saturation point is from 20 to 30 

 per cent. 



FIBER STRESS AT ELASTIC LIMIT. 



(See pp. 13, 15, and 16.) 



Fiber stress at elastic limit is the stress obtained in a timber by 

 loading it to its elastic limit. It is the greatest stress the timber 

 will take under a given loading and immediately return to its former 

 position. 



FLEXIBILITY. 



Flexibility is that quality which renders a material capable of 

 being bent without breaking. Thus, green timber is more flexible 

 than dry. 



GREEN. 



Green is the condition of timber as taken from the living tree. 



Immediately upon being sawed from the tree lumber begins to 

 lose moisture and otherwise change its condition. The rapidity of 

 these changes is determined by the species, humidity, and circu- 

 lation of air, heat, etc. 



