Mechanical properties of wood
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Mechanical properties of wood
- 2. DEFINITION The mechanical properties of wood are its fitness and ability to resist applied or external forces The mechanical properties of wood considered are (1) stiffness and elasticity, (2) tensile strength, (3) compressive or crushing strength, (4) shearing strength, (5) transverse or bending strength, (6) toughness, (7) hardness, (8) cleavability, (9) resilience.
- 3. STIFFNESS The property by means of which a body acted upon by external forces tends to retain its natural size and shape, or resists deformation. Thus a material that is difficult to bend or otherwise deform is stiff; one that is easily bent or otherwise deformed is flexible. Flexibility is not the exact counterpart of stiffness, as it also involves toughness and pliability.
- 4. TENSILE STRENGTH The tensile strength of wood parallel to the grain depends upon the strength of the fibers and is affected not only by the nature and dimensions of the wood elements but also by their arrangement.
- 5. COMPRESSIVE OR CRUSHING STRENGTH Is very closely related to hardness and transverse shear. There are two ways in which wood is subjected to stress of this kind, namely, (1) with the load acting over the entire area of the specimen, and (2) with a load concentrated over a portion of the area.
- 6. SHEARING STRENGTH 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. In wood this shearing action may be (1) along the grain, or (2) across the grain.
- 7. TRANSVERSE OR BENDING STRENGTH: BEAMS When external forces acting in the same plane are applied at right angles to the axis of a bar so as to cause it to bend, they occasion a shortening of the longitudinal fibers on the concave side and an elongation of those on the convex side.
- 8. FAILURES IN BEAMS (1) Simple tension, in which there is a direct pulling in two of the wood on the underside of the beam due to a tensile stress parallel to the grain (2) Cross-grained tension, in which the fracture is caused by a tensile force acting oblique to the grain. This is a common form of failure where the beam has diagonal, spiral or other form of cross grain on its lower side. (3) Splintering tension, in which the failure consists of a considerable number of slight tension failures, producing a ragged or splintery break on the under surface of the beam. This is common in tough woods.
- 9. (4) Brittle tension, in which the beam fails by a clean break extending entirely through it. It is characteristic of a brittle wood which gives way suddenly without warning, like a piece of chalk. (5) Compression failure has few variations except that it appears at various distances from the neutral plane of the beam. It is very common in green timbers. The compressive stress parallel to the fibers causes them to buckle or bend as in an endwise compressive test. (6) Horizontal shear failure, in which the upper and lower portions of the beam slide along each other for a portion of their length either at one or at both ends is fairly common in air-dry material and in green material when the ratio of the height of the beam to the span is relatively large.
- 11. TOUGHNESS: TORSION Wood that is difficult to split is said to be tough Toughness includes flexibility and is the reverse of brittleness, in that tough woods break gradually and give warning of failure. Toughness is dependent upon the strength, cohesion, quality, length, and arrangement of fiber, and the pliability of the wood
- 12. HARDNESS The term hardness is used in two senses, namely: (1) resistance to indentation, and (2) resistance to abrasion or scratching In the latter sense hardness combined with toughness is a measure of the wearing ability of wood and is an important consideration in the use of wood for floors, paving blocks, bearings, and rollers.
- 13. CLEAVABILITY Cleavability is the term used to denote the facility with which wood is split. A splitting stress is one in which the forces act normally like a wedge.
- 14. RESILIENCE Is the amount of work done upon a body in deforming it. Within the elastic limit it is also a measure of the potential energy stored in the material and represents the amount of work the material would do upon being released from a state of stress