When a short column is compressed until it breaks, the manner of failure depends partly upon the anatomical structure and partly upon the degree of humidity of the wood. The fibres (tracheids in conifers) act as hollow tubes bound closely together, and in giving way they either (1) buckle, or (2) bend.[5]
The first is typical of any dry thin-walled cells, as is usually the case in seasoned white pine and spruce, and in the early wood of hard pines, hemlock, and other species with decided contrast between the two portions of the growth ring. As a rule buckling of a tracheid begins at the bordered pits which form places of least resistance in the walls. In hardwoods such as oak, chestnut, ash, etc., buckling occurs only in the thinnest-walled elements, such as the vessels, and not in the true fibres.
According to Jaccard[6] the folding of the cells is accompanied by characteristic alterations of their walls which seem to split them into extremely thin layers. When greatly magnified, these layers appear in longitudinal sections as delicate threads without any definite arrangements, while on cross section they appear as numerous concentric strata. This may be explained on the ground that the growth of a fibre is by successive layers which, under the influence of compression, are sheared apart. This is particularly the case with thick-walled cells such as are found in late wood.
| TABLE VI | |||
|---|---|---|---|
| RESULTS OF ENDWISE COMPRESSION TESTS ON SMALL CLEAR PIECES OF 40 WOODS IN GREEN CONDITION | |||
| (Forest Service Cir. 213) | |||
| COMMON NAME OF SPECIES | Fibre stress at elastic limit | Crushing strength | Modulus of elasticity |
| Lbs. per sq. inch | Lbs. per sq. inch | Lbs. per sq. inch | |
| Hardwoods | |||
| Ash, white | 3,510 | 4,220 | 1,531,000 |
| Basswood | 780 | 1,820 | 1,016,000 |
| Beech | 2,770 | 3,480 | 1,412,000 |
| Birch, yellow | 2,570 | 3,400 | 1,915,000 |
| Elm, slippery | 3,410 | 3,990 | 1,453,000 |
| Hackberry | 2,730 | 3,310 | 1,068,000 |
| Hickory, big shellbark | 3,570 | 4,520 | 1,658,000 |
| bitternut | 4,330 | 4,570 | 1,616,000 |
| mockernut | 3,990 | 4,320 | 1,359,000 |
| nutmeg | 3,620 | 3,980 | 1,411,000 |
| pignut | 3,520 | 4,820 | 1,980,000 |
| shagbark | 3,730 | 4,600 | 1,943,000 |
| water | 3,240 | 4,660 | 1,926,000 |
| Locust, honey | 4,300 | 4,970 | 1,536,000 |
| Maple, sugar | 3,040 | 3,670 | 1,463,000 |
| Oak, post | 2,780 | 3,330 | 1,062,000 |
| red | 2,290 | 3,210 | 1,295,000 |
| swamp white | 3,470 | 4,360 | 1,489,000 |
| white | 2,400 | 3,520 | 946,000 |
| yellow | 2,870 | 3,700 | 1,465,000 |
| Osage orange | 3,980 | 5,810 | 1,331,000 |
| Sycamore | 2,320 | 2,790 | 1,073,000 |
| Tupelo | 2,280 | 3,550 | 1,280,000 |
| Conifers | |||
| Arborvitæ | 1,420 | 1,990 | 754,000 |
| Cedar, incense | 2,710 | 3,030 | 868,000 |
| Cypress, bald | 3,560 | 3,960 | 1,738,000 |
| Fir, alpine | 1,660 | 2,060 | 882,000 |
| amabilis | 2,763 | 3,040 | 1,579,000 |
| Douglas | 2,390 | 2,920 | 1,440,000 |
| white | 2,610 | 2,800 | 1,332,000 |
| Hemlock | 2,110 | 2,750 | 1,054,000 |
| Pine, lodgepole | 2,290 | 2,530 | 1,219,000 |
| longleaf | 3,420 | 4,280 | 1,890,000 |
| red | 2,470 | 3,080 | 1,646,000 |
| sugar | 2,340 | 2,600 | 1,029,000 |
| western yellow | 2,100 | 2,420 | 1,271,000 |
| white | 2,370 | 2,720 | 1,318,000 |
| Redwood | 3,420 | 3,820 | 1,175,000 |
| Spruce, Engelmann | 1,880 | 2,170 | 1,021,000 |
| Tamarack | 3,010 | 3,480 | 1,596,000 |
The second case, where the fibres bend with more or less regular curves instead of buckling, is characteristic of any green or wet wood, and in dry woods where the fibres are thick-walled. In woods in which the fibre walls show all gradations of thickness—in other words, where the transition from the thin-walled cells of the early wood to the thick-walled cells of the late wood is gradual—the two kinds of failure, namely, buckling and bending, grade into each other. In woods with very decided contrast between early and late wood the two forms are usually distinct. Except in the case of complete failure the cavity of the deformed cells remains open, and in hardwoods this is true not only of the wood fibres but also of the tube-like vessels. In many cases longitudinal splits occur which isolate bundles of elements by greater or less intervals. The splitting occurs by a tearing of the fibres or rays and not by the separation of the rays from the adjacent elements.
Figure 8
Failures of short columns of green spruce.
