are
equal, and the neutral plane lies in the middle of the beam.
(See TRANSVERSE OR BENDING STRENGTH: BEAMS, above.) Later the
top layer of fibres on the upper or compression side fail, and
on the load increasing, the next layer of fibres fail, and so
on, even though this failure may not be visible. As a result the
shortenings on the upper side of the beam become considerably
greater than the elongations on the lower side. The neutral
plane must be presumed to sink gradually toward the tension
side, and when the stresses on the outer fibres at the bottom
have become sufficiently great, the fibres are pulled in two,
the tension area being much smaller than the compression area.
The rupture is often irregular, as in direct tension tests.
Failure may occur partially in single bundles of fibres some
time before the final failure takes place. One reason why the
failure of a dry beam is different from one that is moist, is
that drying increases the stiffness of the fibres so that they
offer more resistance to crushing, while it has much less effect
upon the tensile strength.

There is considerable variation in tension failures depending
upon the toughness or the brittleness of the wood, the
arrangement of the grain, defects, etc., making further
classification desirable. The four most common forms are:

(1)~Simple tension,~ in which there is a direct pulling in two
of the wood on the under side of the beam due to a tensile
stress parallel to the grain, (See Fig. 17, No. 1.) This is
common in straight-grained beams, particularly when the wood is
seasoned.

[Illustration: FIG. 17.--Characteristic failures of simple
beams.]

(2)~Cross-grained tension,~ in which the fracture is caused by a
tensile force acting oblique to the grain. (See Fig. 17, No. 2.)
This is a common form of failure where the beam has diagonal,
spiral or other form of cross grain on its lower side. Since the
tensile strength of wood across the grain is only a small
fraction of that with the grain it is easy to see why a
cross-grained timber would fail in this manner.

(3)~Splintering tension,~ in which the failure consists of a
considerable number of slight tension failures, producing a
ragged or splintery break on the under surface of the beam. (See
Fig. 17, No. 3.) This is common in tough woods. In this case the
surface of fracture is fibrous.

(4)~Brittle tension,~ in which the beam fails by a clean break
extending entirely through it. (See Fig