WHAT IS TOUGH WOOD? 49 



FLEXIBILITY. 



Pine is brittle, hickory is flexible; the former breaks, the latter 

 bends. Being the opposite of stiffness, want of stiffness would seem to 

 indicate flexibility. This, however, is only partly true ; hickory and 

 ash are stiff and yet among the most flexible of woods. Their small 

 dimensions cause shavings and thin strands of most woods to appear 

 pliable. For this reason the pliable, twisted wicker willow is not a fair 

 measure of the flexibility of the wood of this species. Generally hard 

 woods are more flexible than conifers, wood of the butt surpassing 

 in this respect that of the main part of the stem, the latter being 

 usually superior to that of the limbs. Moisture softens wood and 

 thereby increases its flexibility. Knots and crossgrain diminish flexi- 

 bility, but the irregular structure of elm, ash, etc. (particularly the 

 arrangement of bodies of extremely firm fibers, like so many strands, 

 among the softer tissue, as well as the interlacement of fibers, due to 

 post-cambial growth), favorably influences the flexibility of these woods. 



TOUGHNESS. 



So far the load by which the exhibition of the various kinds of 

 strength in compression, tension, cross bending, etc., was produced has 

 always been assumed as applied slowly and gradually. When a wagon 

 goes lumbering along a cobble pavement the load on the spokes is not 

 thus applied. Every stone deals the wheel a blow, and a mile's journey 

 means many thousand blows to every wheel rim and spoke. In chop- 

 ping, the ax handle is jarred and a handle made of pine wood, which 

 shears easily along the fiber, would soon be shattered to pieces. Loads 

 thus applied are "shocks," and resistance to this form of loading 

 requires a combination of various kinds of strength possessed only by 

 "tough" woods. Toughness is a familiar word to woodworkers, and 

 yet is rarely defined. Tough wood must be both strong and pliable. 

 Thus a willow is not tough when dry; it is weak and brittle, and re- 

 quires, notwithstanding its small lateral dimensions, to be moistened 

 and twisted or sheared into still smaller strands so that its fibers are sub- 

 jected almost exclusively to tension, if great deflection and great 

 strength are to be combined (handles of wicker baskets). Hickory is 

 both strong and pliable; in the dimensions of a willow twig it can be 

 used almost like a rope. The term "tough," therefore, is properly 

 applied to woods like hickory and elm and improperly to willow. 



Judging from the behavior of elm and hickory, wood may be pro 

 nounced "tough" if it offers great resistance to — 



(1) Longitudinal shearing over 1,000 pounds per square inch, 



(2) Tension over 16,000 pounds per square inch, 



and permits, when tested dry, of an aggregate distortion in compres- 

 sion and tension amounting to not less than 3 per cent. 



3521— No. 10 i 



