MECHANICAL PROPERTIES OF WOODS GROWN IN UNITED STATES. 17 



formulae are discussed in the various textbooks on mechanics and 

 strength of materials.) 



COMPRESSION PERPENDICULAR TO GRAIN. 



In the compression perpendicular to grain test, a block 2 by 2 

 inches in cross section and 6 inches long is laid upon its side and 

 pressure applied to it through a cast-iron plate 2 inches wide laid 

 across the center of the piece and at right angles to its length. Hence 

 but one- third of the surface is directly subjected to compression. 



The only strength value obtained in this test is the fiber stress at 

 elastic limit. It represents the maximum stress which can be applied 

 to the timber without injury. It is important in computing the 

 bearing area for beams, stringers, joists, etc., and in comparing 

 species for railroad ties. 



Two-thirds of the fiber stress at elastic limit, as given in the table 

 for tests on green material, may be used as a safe stress in dry interior 

 construction. 



HARDNESS. 



Hardness is tested by measuring the load required to embed a 

 0.444-inch ball to one-half its diameter in the wood. This test is a 

 modification of one originated by Janka. 1 



The hardness test is applied to end, radial, and tangential surfaces 

 of the timber. There is no consistent difference between radial and 

 tangential hardnesses and they are averaged and tabulated as "side 

 hardness." End hardness is usually greater than side hardness. 

 The quality represented by these figures is important in woods for 

 paving blocks, railroad ties, furniture, flooring, etc. 



SHEARING STRENGTH PARALLEL TO GRAIN. 



The shearing test is made by applying force to a 2 by 2 inch lip 

 projecting from the side of a block. The shearing stress is the maxi- 

 mum force required to shear off the projection divided by the area 

 of the plane of failure. 



Shearing strength parallel to the grain is a measure of the ability 

 of timber to resist slipping of one part upon another along the grain. 

 Shearing stress is produced to a greater or less degree in most uses 

 of timber. It is most important in beams, where it is known as 

 horizontal shear — the stress tending to cause the upper half of the 

 beam to slide upon the lower. It is also important in the design of 

 various kinds of timber joints. 



Only about one-eighth of the values given in the table for green 

 material (Table 1) should be used as allowable stress in horizontal 



1 "Die IDirte des Kolzes," by Gabriel Janka, k. k. Forst-und Doniiincnverwalier: Mitteilung der k. K. 

 forstliehen Versuchsanstalt, in Mariabrunn, Wien, 1906. 



91728°— Bull. 556—17 3 



