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.