442 



Strength of wet or green wood is much more sensitive to changes of 

 speed than is dry wood. At the speed adopted for official tests a change 

 in speed of 50% may ordinarily be allowed without causing a variation in 

 strength of over 2%. 



2. Temperature. — Since wood is a more or less plastic substance it is 

 sensitive to changes of temperature. Tiemann's 3 experiments show that 

 soaking certain species iu water at normal temperature does not affect 

 their strength. It appears, however, that warm water has a marked weak- 

 ening effect. The extreme condition is when wood is made pliable by boil- 

 ing. Some woods are no doubt more sensitive than others to the effect of 

 temperature of the water in which they are immersed. In recent tests 

 made in winter weather on red oak (Quercus Rubra) ties at Purdue Uui- 

 versity, ties taken from the temperature of the storehouse (about 25° F.) 

 were from 9 to IT per cent stronger than those tested at the temperature 

 of the laboratory (about 70° P.). Probably this marked difference in 

 strength is to be found only in case of green or wet wood. The rupture 

 work is not affected to the degree of the ultimate strength. Hickory seems 

 specially sensitive to change of temperature. It is concluded that the 

 ordinary temperature variations of the air of a laboratory are not im- 

 portant, but that the temperature of the storehouse may render it neces- 

 sary to warm the wood. In fact, the effect of a given factor on the 

 strength of timber, or difference of strength of two species, may at times 

 be entirely masked by variations of temperature of timber at the time 

 of test. 



3. Moisture. — The effect of moisture on the strength of wood has 

 been thoroughly investigated by Tiemann. 4 His material was small test 

 pieces uniform in moisture content throughout the cross-section ; and he 

 determined the distinct "fiber saturation"' point, above which increased 

 moisture content did not affect the strength of timber and below which 

 there was an increase of strength. Previous experiments, yielding a con- 

 tinuous moisture strength law, were apparently made with "case-hardened 

 material." 



3 Bulletin 70. Forest Service, 1906. "Effect of Moisture on the Strength and 

 Stiffness of Wood," by H. D. Tiemann. 



* Bulletin 70. Forest Service, 1906. "Effect of Moisture on the Strength and 

 Stiffness fo Wood," by H. D. Tiemann. 



Circular 108, Forest Service, 1907. "The Strength of Wood as Influenced by 

 Moisture," by H. D. Tiemann. 



