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.  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. 17, No. 4.) It is characteristic of a brittle wood which gives way suddenly without warning, like a piece of chalk.  In this case the surface of fracture is described as brash.

Compression failure (see Fig. 17, No. 5) has few variations except that it appears at various distances from the neutral plane of the beam.  It is very common in green timbers.  The compressive stress parallel to the fibres causes them to buckle or bend as in an endwise compressive test.  This action usually begins on the top side shortly after the elastic limit is reached and extends downward, sometimes almost reaching the neutral plane before complete failure occurs.  Frequently two or more failures develop at about the same time.