Whenever forces act upon a body in such a way that one portion tends to slide upon another adjacent to it the action is called a ~shear~.[8] In wood this shearing action may be (1) ~along the grain~, or (2) ~across the grain~. A tenon breaking out its mortise is a familiar example of shear along the grain, while the shoving off of the tenon itself would be shear across the grain. The use of wood for pins or tree-nails involves resistance to shear across the grain. Another common instance of the latter is where the steel edge of the eye of an axe or hammer tends to cut off the handle. In Fig. 10 the action of the wooden strut tends to shear off along the grain the portion _AB_ of the wooden tie rod, and it is essential that the length of this portion be great enough to guard against it. Fig. 11 shows characteristic failures in shear along the grain.
[Footnote 8: Shear should not be confused with ordinary cutting or incision.]
[Illustration: FIG. 10.--Example of shear along the grain.]
[Illustration: FIG. 11.--Failures of test specimens in shear along the grain. In the block at the left the surface of failure is radial; in the one at the right, tangential]
|---------------------------------------------| | TABLE VII | |---------------------------------------------| | SHEARING STRENGTH ALONG THE GRAIN OF SMALL | | CLEAR PIECES OF 41 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, black | 876 | 832 | | white | 1,360 | 1,312 | | Basswood | 560 | 617 | | Beech | 1,154 | 1,375 | | Birch, yellow | 1,103 | 1,188 | | Elm, slippery | 1,197 | 1,174 | | white | 778 | 872 | | Hackberry | 1,095 | 1,161 | | Hickory, | | | | big shellbark | 1,134 | 1,191 | | bitternut | 1,134 | 1,348 | | mockernut | 1,251 | 1,313 | | nutmeg | 1,010 | 1,053 | | pignut | 1,334 | 1,457 | | shagbark | 1,230 | 1,297 | | water | 1,390 | 1,490 | | Locust, honey | 1,885 | 2,096 | | Maple, red | 1,130 | 1,330 | | sugar | 1,193 | 1,455 | | Oak, post | 1,196 | 1,402 | | red | 1,132 | 1,195 | | swamp white | 1,198 | 1,394 | | white | 1,096 | 1,292 | | yellow | 1,162 | 1,196 | | Sycamore | 900 | 1,102 | | Tupelo | 978 | 1,084 | | | | | | Conifers | | | | | | | | Arborvitæ | 617 | 614 | | Cedar, incense | 613 | 662 | | Cypress, bald | 836 | 800 | | Fir, alpine | 573 | 654 | | amabilis | 517 | 639 | | Douglas | 853 | 858 | | white | 742 | 723 | | Hemlock | 790 | 813 | | Pine, lodgepole | 672 | 747 | | longleaf | 1,060 | 953 | | red | 812 | 741 | | sugar | 702 | 714 | | western yellow | 686 | 706 | | white | 649 | 639 | | Spruce, Engelmann | 607 | 624 | | Tamarack | 883 | 843 | |---------------------------------------------|
