| | | | | | Arborvitae | 5,290 | 778,000 | 2.04 | | Cypress, bald | 8,290 | 1,431,000 | 2.71 | | Fir, alpine | 5,280 | 980,000 | 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 | |-------------------------------------------------------| re>

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.]