Properties Of Wood. Continued

Description

This section is from the "Educational Woodworking For Home And School" book, by Joseph C.Park . Also see Amazon: Educational Woodworking For Home And School.

Properties Of Wood. Continued

E. Cross-Breaking Or Bending Strength

Laws

1. The strength of a timber varies approximately with the squares of the thickness and decreases directly with increasing length.

1 Less than.

2. Seasoning increases, moisture decreases, the strength.

3. Knots and cross grain weaken the holding power of timber.

The cross-breaking strength, or "modulus of rupture" is obtained by means of the formula: -

Strength of extreme fiber = 3Wl/2bd2 .

The following table gives the strength of well-seasoned, select pieces, load acting in the middle of the block, which is supported at the ends: -

Strength In Cross-Breaking Of Well-Seasoned, Select Pieces

	Strength of lite extreme fiber f= 3Wl/2bd2 per square inch	Approximate weight which breaks a stick -
	Strength of lite extreme fiber f= 3Wl/2bd2 per square inch	1 by 1 inch and 12 inches long	2 by 2 inches and 10 feet long
(1) Robinia (locust), hard maple, hickory, oak, birch, best ash and elm, long-leaf, shortleaf, and Cuban pines, tamarack....	Pounds	Pounds	Pounds
	13,000	720	570
(2) Soft maple, cherry, ash, elm, walnut, inferior oak, and birch, best poplar, Norway, loblolly and pitch pines, black and white spruce, hemlock, and good cedar......	10,000	550	440
(3) Tulip, bass wood, sycamore, butternut, poplars, white and other soft pines, firs, and cedars......	6,500	350	280

F. Tension And Compression

Tension

The amount of force required to cause the fibers to pull apart.

Compression

The approximate weight required to crush a given block of wood endwise.

Affected by the position of the fibers, as crossgrained, oblique position of grain, and straightgrained.

The following table gives the ratio of strength in tension and compression: -

Ratio Of Strength In Tension And Compression, Showing The Difference Between Rigid Conifers And Tough Hard Woods

	Ratio: Tensile strength R= -----------compressive strength	A stick 1 square inch in cross section. Weight required to -
	Ratio: Tensile strength R= -----------compressive strength	Pull apart	Crush endwise
		Pounds	Pounds
Hickory..............	3.7	32,000	8,500
Elm................	3.8	29,000	7,500
Larch...............	2.3	19,400	8,600
Longleaf pine...........	2.2	17,300	7,400

Strength In Compression Of Common American Woods In Well-Seasoned Select Pieces

[Approximate weight per square inch of cross section requisite to crush a piece of wood endwiss.l

		Pounds
(1)	Black locust, yellow and cherry birch, hard maple, best hickory, longleaf and Cuban pines, and tamarack.....	9,000 +
(2)	Common hickory, oak, birch, soft maple, walnut, good elm, best ash, short leaf and loblolly pines, western hemlock and douglas fir.............	7,000 +
(3)	Ash, sycamore, beech, inferior oak, Pacific white cedar, canoe cedar, Lawson's cypress, common red cedar, cypress, Norway and superior spruces, and fir............	6,000 +

		Pounds
(4)	Tulip, basswood, butternut, chestnut, good poplar, white and other common soft pines, hemlock, spruce, and fir........	5,000 +
(5)	Soft poplar, white cedar, and some Western soft pines, and firs...........	4,000 +

G. Shearing

The breaking of wood along the fibers, as the shearing of a mortise in a mortise-and-tenon joint. Green wood shears more easily than dry wood.

H. Cleavability

That property by virtue of which the fibers can be split, as with an ax in splitting stove-wood. Cleavage along the radius is easier than along the tangent of the block, also easier in green wood than in dry wood.

I. Flexibility

That property of wood by virtue of which it can be bent, as hickory - hard woods more flexible than soft woods - moisture increases flexibility - knots and cross grain diminish flexibility.