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



to falling bodies or to other conditions in which the stress is applied 

 and relieved in one-twenty-fifth of a second or less. It represents a 

 quality important in tool handles and in athletic goods, such as base- 

 ball bats. 



HEIGHT OF DROP. 



Height of drop is the maximum or last drop of the hammer. It 

 represents a quality important in articles which are occasionally 

 stressed under a shock beyond their elastic limit, such as handles and 

 vehicle and implement parts. 



COMPRESSION PARALLEL TO GRAIN. 



In the compression parallel to grain test a 2 by 2 by 8 inch block is 

 compressed in the direction of its length. Deformation is measured 

 between two collars attached 6 inches apart to the specimen. 



FIBER STRESS AT ELASTIC LIMIT. 



Fiber stress at elastic limit in compression parallel to the grain is 

 not much used because in most cases it is more convenient to use 

 maximum crushing strength, which is less variable and easier to 

 obtain. The value is important in the derivation of safe working 

 stresses for structural timber. (See also fiber stress at elastic limit, 

 glossary, p. 21.) 



MAXIMUM CRUSHING STRENGTH. 



The maximum crushing strength is the maximum ability of a short 

 block to sustain a slowly applied load. It is obtained by dividing 

 the maximum load obtained in the test by the area of cross section 

 of the block. This property is important in estimatmg the strength 

 of columns. 



Tests of the crushing strength, because of their simplicity, are 

 frequently the only tests used in studying the effect of various 

 influences or processes on strength. Crushing strength is not neces- 

 sarily representative of the other strength properties; consequently, 

 when used alone, it will occasionally lead to erroneous conclusions. 

 For instance, it was found that the crushing strength of some timbers 

 was increased 10 per cent by a certain heat treatment. Other tests, 

 however, revealed the fact that their resistance to shock had been 

 reduced about 50 per cent. 



A safe working stress for carefully selected structural timbers used 

 as columns and in dry interior construction, all exceptionally light 

 pieces excluded, is about one-third the crushing strength as given 

 in the table for tests on green materials (Table 1). If the column is 

 longer than about 10 times its least diameter, some formula should 

 be used which will take care of the increased stress which would be 

 caused by eccentric loading or by the bending of the column. (Such 



