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TOUGHNESS: TORSION

Toughness is a term applied to more than one property of wood.
Thus wood that is difficult to split is said to be tough. Again,
a tough wood is one that will not rupture until it has deformed
considerably under loads at or near its maximum strength, or one
which still hangs together after it has been ruptured and may be
bent back and forth without breaking apart. Toughness includes
flexibility and is the reverse of brittleness, in that tough
woods break gradually and give warning of failure. Tough woods
offer great resistance to impact and will permit rougher
treatment in manipulations attending manufacture and use.
Toughness is dependent upon the strength, cohesion, quality,
length, and arrangement of fibre, and the pliability of the
wood. Coniferous woods as a rule are not as tough as hardwoods,
of which hickory and elm are the best examples.

The torsion or twisting test is useful in determining the
toughness of wood. If the ends of a shaft are turned in opposite
directions, or one end is turned and the other is fixed, all of
the fibres except those at the axis tend to assume the form of
helices. (See Fig. 19.) The strain produced by torsion or
twisting is essentially shear transverse and parallel to the
fibres, combined with longitudinal tension and transverse
compression. Within the elastic limit the strains increase
directly as the distance from the axis of the specimen. The
outer elements are subjected to tensile stresses, and as they
become twisted tend to compress those near the axis. The
elongated elements also contract laterally. Cross sections which
were originally plane become warped. With increasing strain the
lateral adhesion of the outer fibres is destroyed, allowing them
to slide past each other, and reducing greatly their power of
resistance. In this way the strains on the fibres nearer the
axis are progressively increased until finally all of the
elements are sheared apart. It is only in the toughest materials
that the full effect of this action can be observed. (See Fig.
20.) Brittle woods snap off suddenly with only a small amount of
torsion, and their fracture is irregular and oblique to the axis
of the piece instead of frayed out and more nearly perpendicular
to the axis as is the case with tough woods.

[Illustration: FIG. 19.--Torsion of a shaft.]

[Illustration: FIG.