This property of wood is very important in certain uses such as firewood, fence rails, billets, and squares. Resistance to splitting or low cleavability is desirable where wood must hold nails or screws, as in box-making. Wood usually splits more readily along the radius than parallel to the growth rings though exceptions occur, as in the case of cross grain.
Splitting involves transverse tension, but only a portion of the fibres are under stress at a time. A wood of little stiffness and strong cohesion across the grain is difficult to split, while one with great stiffness, such as longleaf pine, is easily split. The form of the grain and the presence of knots greatly affect this quality.
| TABLE XIII | ||
|---|---|---|
| CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF 32 WOODS IN GREEN CONDITION | ||
| (Forest Service Cir. 213) | ||
| COMMON NAME OF SPECIES | When surface of failure is radial | When surface of failure is tangential |
| Lbs. per sq. inch | Lbs. per sq. inch | |
| Hardwoods | ||
| Ash, black | 275 | 260 |
| white | 333 | 346 |
| Bashwood | 130 | 168 |
| Beech | 339 | 527 |
| Birch, yellow | 294 | 287 |
| Elm, slippery | 401 | 424 |
| white | 210 | 270 |
| Hackberr | 422 | 436 |
| Locust, honey | 552 | 610 |
| Maple, red | 297 | 330 |
| sugar | 376 | 513 |
| Oak, post | 354 | 487 |
| red | 380 | 470 |
| swamp white | 428 | 536 |
| white | 382 | 457 |
| yellow | 379 | 470 |
| Sycamore | 265 | 425 |
| Tupelo | 277 | 380 |
| Conifers | ||
| Arborvitæ | 148 | 139 |
| Cypress, bald | 167 | 154 |
| Fir, alpine | 130 | 133 |
| Douglas | 139 | 127 |
| white | 145 | 187 |
| Hemlock | 168 | 151 |
| Pine, lodgepole | 142 | 140 |
| longleaf | 187 | 180 |
| red | 161 | 154 |
| sugar | 168 | 189 |
| western yellow | 162 | 187 |
| white | 144 | 160 |
| Spruce, Engelmann | 110 | 135 |
| Tamarack | 167 | 159 |
PART II
FACTORS AFFECTING THE MECHANICAL PROPERTIES OF WOOD
INTRODUCTION
Wood is an organic product—a structure of infinite variation of detail and design.[17] It is on this account that no two woods are alike—in reality no two specimens from the same log are identical. There are certain properties that characterize each species, but they are subject to considerable variation. Oak, for example, is considered hard, heavy, and strong, but some pieces, even of the same species of oak, are much harder, heavier, and stronger than others. With hickory are associated the properties of great strength, toughness, and resilience, but some pieces are comparatively weak and brash and ill-suited for the exacting demands for which good hickory is peculiarly adapted.