Failure of wood in tension parallel to the grain occurs sometimes in flexure, especially with dry material. The tension portion of the fracture is nearly the same as though the piece were pulled in two lengthwise. The fibre walls are torn across obliquely and usually in a spiral direction. There is practically no pulling apart of the fibres, that is, no separation of the fibres along their walls, regardless of their thickness. The nature of tension failure is apparently not affected by the moisture condition of the specimen, at least not so much so as the other strength values.[3]
[Footnote 3: See Brush, Warren D.: A microscopic study of the mechanical failure of wood. Vol. II, Rev. F.S. Investigations, Washington, D.C., 1912, p. 35.]
Tension at right angles to the grain is closely related to cleavability. When wood fails in this manner the thin fibre walls are torn in two lengthwise while the thick-walled fibres are usually pulled apart along the primary wall.
|--------------------------------------------| | TABLE III | |--------------------------------------------| | TENSILE STRENGTH AT RIGHT ANGLES TO THE | | GRAIN OF SMALL CLEAR PIECES OF 25 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 | | | | | | | | Ash, white | 645 | 671 | | Basswood | 226 | 303 | | Beech | 633 | 969 | | Birch, yellow | 446 | 526 | | Elm, slippery | 765 | 832 | | Hackberry | 661 | 786 | | Locust, honey | 1,133 | 1,445 | | Maple, sugar | 610 | 864 | | Oak, post | 714 | 924 | | red | 639 | 874 | | swamp white | 757 | 909 | | white | 622 | 749 | | yellow | 728 | 929 | | Sycamore | 540 | 781 | | Tupelo | 472 | 796 | | | | | | Conifers | | | | | | | | Arborvitæ | 241 | 235 | | Cypress, bald | 242 | 251 | | Fir, white | 213 | 304 | | Hemlock | 271 | 323 | | Pine, longleaf | 240 | 298 | | red | 179 | 205 | | sugar | 239 | 304 | | western yellow | 230 | 252 | | white | 225 | 285 | | Tamarack | 236 | 274 | |--------------------------------------------|