Permanent set is due to the plasticity of the material. A perfectly plastic substance would have no elasticity and the smallest forces would cause a set. Lead and moist clay are nearly plastic and wood possesses this property to a greater or less extent. The plasticity of wood is increased by wetting, heating, and especially by steaming and boiling. Were it not for this property it would be impossible to dry wood without destroying completely its cohesion, due to the irregularity of shrinkage.
A substance that can undergo little change in shape without breaking or rupturing is brittle. Chalk and glass are common examples of brittle materials. Sometimes the word brash is used to describe this condition in wood. A brittle wood breaks suddenly with a clean instead of a splintery fracture and without warning. Such woods are unfitted to resist shock or sudden application of load.
The measure of the stiffness of wood is termed the modulus of elasticity (or coefficient of elasticity). It is the ratio of stress per unit of area to the deformation per unit of length.
| ( | unit stress | ) | ||
| E | = | ------------- | ||
| unit strain |
It is a number indicative of stiffness, not of strength, and only applies to conditions within the elastic limit. It is nearly the same whether derived from compression tests or from tension tests.
A large modulus indicates a stiff material. Thus in green wood tested in static bending it varies from 643,000 pounds per square inch for arborvitæ to 1,662,000 pounds for longleaf pine, and 1,769,000 pounds for pignut hickory. ([See Table IX].) The values derived from tests of small beams of dry material are much greater, approaching 3,000,000 for some of our woods. These values are small when compared with steel which has a modulus of elasticity of about 30,000,000 pounds per square inch. ([See Table I].)
| TABLE I | |||||
|---|---|---|---|---|---|
| COMPARATIVE STRENGTH OF IRON, STEEL, AND WOOD | |||||
| MATERIAL | Sp. gr.,dry | Modulus of elasticity in bending | Tensile strength | Crushing strength | Modulus of rupture |
| Lbs. per sq. in. | Lbs. per sq. in. | Lbs. per sq. in. | Lbs. per sq. in. | ||
| Cast iron, cold blast (Hodgkinson) | 7.1 | 17,270,000 | 16,700 | 106,000 | 38,500 |
| Bessenger steel, high grade (Fairbain). | 7.8 | 29,215,000 | 88,400 | 225,600 | |
| Longleaf pine, 3.5% moisture (U.S.) | .63 | 2,800,000 | 13,000 | 21,000 | |
| Redspruce, 3.5% moisture (U.S.) | .41 | 1,800,000 | 8,800 | 14,500 | |
| Pignut hickory, 3.5% moisture (U.S.) | .86 | 2,370,000 | 11,130 | 24,000 | |
| NOTE.—Great variation may be found in different samples of metals as well as of wood. The examples given represent reasonable values. | |||||
TENSILE STRENGTH
Tension results when a pulling force is applied to opposite ends of a body. This external pull is communicated to the interior, so that any portion of the material exerts a pull or tensile force upon the remainder, the ability to do so depending upon the property of cohesion. The result is an elongation or stretching of the material in the direction of the applied force. The action is the opposite of compression.