ed with weak walls that are readily compressed. This
explains why on the radial surface of the test blocks the plane
of rupture passes in a direction nearly following a medullary
ray, whereas on the tangential surface the direction of the
plane of rupture is oblique--but with an obliquity varying with
the species and determined by the pitch of the spirals along
which the medullary rays are distributed in the stem." See
Jaccard, _op. cit._, pp. 57 _et seq._]

SHEARING STRENGTH

Whenever forces act upon a body in such a way that one portion
tends to slide upon another adjacent to it the action is called
a ~shear~.[8] In wood this shearing action may be (1) ~along the
grain~, or (2) ~across the grain~. A tenon breaking out its
mortise is a familiar example of shear along the grain, while
the shoving off of the tenon itself would be shear across the
grain. The use of wood for pins or tree-nails involves
resistance to shear across the grain. Another common instance of
the latter is where the steel edge of the eye of an axe or
hammer tends to cut off the handle. In Fig. 10 the action of the
wooden strut tends to shear off along the grain the portion _AB_
of the wooden tie rod, and it is essential that the length of
this portion be great enough to guard against it. Fig. 11 shows
characteristic failures in shear along the grain.

[Footnote 8: Shear should not be confused with ordinary cutting
or incision.]

[Illustration: FIG. 10.--Example of shear along the grain.]

[Illustration: FIG. 11.--Failures of test specimens in shear
along the grain. In the block at the left the surface of failure
is radial; in the one at the right, tangential]

|---------------------------------------------|
| TABLE VII |
|---------------------------------------------|
| SHEARING STRENGTH ALONG THE GRAIN OF SMALL |
| CLEAR PIECES OF 41 WOODS IN GREEN CONDITION |
| (Forest Service Cir. 213) |
|---------------------------------------------|
| | When | When |
| COMMON NAME | surface of | surface of |
| OF SPECIES | failure is | failure is |
| | radial | tangential |
|-------------------+------------+------------|
| | Lbs. per | Lbs. per |
| | sq. inch | sq. inch |
| | | |
| Hardwoods | | |
|