In dry material the first visible failure is not infrequently on the lower or tension side, and various attempts have been made to explain why such is the case.[15] [Footnote 15: See Proc.Int.Assn.for Testing Materials, 1912, XXIII_{2}, pp.

12-13.] Within the elastic limit the elongations and shortenings 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.