APPEARANCE AND CHARACTERISTICS. 27 



that it moves over a graduated arc when the beam bends, and thus 

 shows the amount of bending. 



The test is begun by loading the beam with about one-twentieth 

 of the probable breaking load and noting the deflection. The load is 

 then increased by a certain increment which is recorded with the cor- 

 responding deflection, and the process continued until the beam breaks. 

 The results of tests on beams of various sizes are reduced to a unit 

 basis, so that direct comparisons as to strength and stiffness can be 

 made between pieces of different sizes of the same or of different 

 species of wood. 



In computing the results, the breaking strength is represented by 

 11 modulus of rupture," the stiffness by " modulus of elasticity," the 

 load the material will carry without taking a set by " fiber stress at 

 the elastic limit," and the ability to withstand shock without taking 

 a set by " elastic resilience." 



Tests in compression parallel with the grain are made by crushing 

 the specimens endwise as they stand upright on the platform of the 

 testing machine. In the case of compression perpendicular to the 

 grain, the tests are made by placing a piece of metal 2 inches wide 

 across the test specimen as it lies flat on the platform of the machine 

 and pressing the piece of metal against the block of wood by means 

 of the crosshead of the machine. This test is carried only slightly 

 beyond the elastic limit of the wood under test, as loading beyond 

 that point has at present no significance. The action is similar to 

 that of a rail on a tie. 



In making a shearing test the block is clamped firmly in a frame 

 with the lip projecting. The frame is placed on the platform of the 

 machine and the lip sheared off by means of a sliding plate applied 

 against the upper surface of the lip and parallel to the gram. 



Table 1 1 shows the oven-dry weight of tanbark oak to be about 

 43.2 pounds per cubic foot. Air-dry tanbark oak, containing 15 per 

 cent moisture, weighs about 50 pounds per cubic foot, or 4,160 

 pounds per 1,000 board feet. This weight is about- the same as that 

 of white oak and is somewhat higher than that of red oak. 



The average bending strength (modulus of rupture) of green tan- 

 bark oak is 10,707 pounds per square inch, and the average crushing 

 strength, 4,845 pounds per square inch. The results of similar tests 

 on several kinds of hickory by the Forest Service, including pignut, 

 shagbark, mockernut, big shellbark, nutmeg, and water hickory 

 show a bending strength of from 9,200 pounds per square inch for 

 green nutmeg hickory to 11,450 pounds per square inch for green 

 pignut hickory. The average oven-dry weight of pignut hickory is 

 about 51 pounds per cubic foot. 



Such tests as have been made on eastern white and red oaks indi- 

 cate that tanbark oak in bending and crushing strength ranks about 

 the same as white oak and is somewhat superior to red oak. 



