TESTING OF TIMBER. 341 



TIMBER 



178. The tests to which a specimen of timber may 

 be subjected are : 



Tension, generally along the grain ; 

 Compression, both along and across the grain ; 

 Cross -breaking or bending ; 

 Shearing, across the grain. 



Tension. A tensile test of a bar of wood is not 

 altogether satisfactory, considerable difficulty being 

 experienced in holding the piece so that there is little 

 crushing of the ends and no shearing. If the ends 

 are simply enlarged, as in a metal-plate specimen, it is 

 probable that the parallel portion will, be sheared out 

 of the enlargement. One of the most satisfactory ways 

 of forming the specimens so as to go in ordinary wedge 

 holdefs is to make the piece parallel to begin with, and 

 then diminish the cross section of the middle to a small 

 extent only, the smaller section being approached by 

 very gradual curves. 



Compression Tests are generally carried out on cubes 

 or short cylinders of the wood, care being taken that the 

 ends which come in contact with the compression plates 

 are quite parallel. If the ends are rough, millboard may 

 be used as a bedding material. 



Cross- breaking. This is easily carried out, and is 

 probably the most satisfactory kind of test to which 

 it is possible to subject timber. In testing a beam of 

 timber on knife edges care must be taken to interpose 

 small pieces of thin iron or steel plate between the 

 specimen and the knife edges, to prevent penetration of 

 the knife edges arid the consequent damage to the 

 material. 



A very convenient way of expressing the strength of 

 a timber beam is to give the number of pounds required 

 to fracture a beam lin. broad, lin. deep, and 1ft. span, 

 assuming the truth of the ordinary beam formula; in this 

 way beams of diiferent sizes may be compared. 



The modulus of elasticity of timber can easily be 

 determined by making use of one of the appliances 

 mentioned in paragraph 104, or by applying an extenso- 

 meter to a tension specimen. 



