000 | 1.59 |
| Douglas | 8,870 | 1,579,000 | 2.79 |
| white | 7,230 | 1,326,000 | 2.21 |
| Hemlock | 6,330 | 1,025,000 | 2.19 |
| Pine, lodgepole | 6,870 | 1,142,000 | 2.31 |
| longleaf | 9,680 | 1,739,000 | 3.02 |
| red | 7,480 | 1,438,000 | 2.18 |
| sugar | 6,740 | 1,083,000 | 2.34 |
| western yellow | 7,070 | 1,115,000 | 2.51 |
| white | 6,490 | 1,156,000 | 2.06 |
| Spruce, Engelmann | 6,300 | 1,076,000 | 2.09 |
| Tamarack | 7,750 | 1,263,000 | 2.67 |
|-------------------------------------------------------|

Impact testing is difficult to conduct satisfactorily and the
data obtained are of chief value in a relative sense, that is,
for comparing the shock-resisting ability of woods of which like
specimens have been subjected to exactly identical treatment.
Yet this test is one of the most important made on wood, as it
brings out properties not evident from other tests. Defects and
brittleness are revealed by impact better than by any other kind
of test. In common practice nearly all external stresses are of
the nature of impact. In fact, no two moving bodies can come
together without impact stress. Impact is therefore the
commonest form of applied stress, although the most difficult to
measure.

_Failures in Timber Beams_

If a beam is loaded too heavily it will break or fail in some
characteristic manner. These failures may be classified
according to the way in which they develop, as tension,
compression, and horizontal shear; and according to the
appearance of the broken surface, as brash, and fibrous. A
number of forms may develop if the beam is completely ruptured.

Since the tensile strength of wood is on the average about three
times as great as the compressive strength, a beam should,
therefore, be expected to fail by the formation in the first
place of a fold on the compression side due to the crushing
action, followed by failure on the tension side. This is usually
the case in green or moist wood. 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