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Glossary of Terms

Checks: Longitudinal separation of the fibers in wood that do not go through the whole cross section. Checks result from tension stresses during the drying process.

Compressive strength parallel to grain: Maximum stress sustained by a compression parallel-to-grain specimen having a ratio of length to least dimension of less than 11.

Compressive stress perpendicular to grain: Reported as stress at proportional limit. There is no clearly defined ultimate stress for this property.

Density: Weight per unit volume. Density of wood is influenced by rate of growth, percentage of late wood and in individual pieces, the proportion of the heartwood.

Dimensional Stability: A term that describes whether a section of wood will resist changes in volume with variation in moisture content (other term: movement in performance).

Figure: The pattern produced in a wood surface by annual growth rings, rays, knots, deviations from regular grain, such as interlocked and wavy, and irregular coloration.

Grain: The direction, size arrangement, appearance, or quality of the fibers in sawn wood. Straight grain is used to describe lumber where the fibers and other longitudinal elements run parallel to the axis of the piece.

Gum Pocket: An excessive local accumulation of resin or gum in the wood.

Hardness: Generally defined as resistance to indentation using a modified Janka hardness test, measured by the load required to embed a 11.28 mm (0.444-in.) ball to one-half its diameter. Values presented are the average of radial and tangential penetrations.

Hardwood: A description applied to woods from deciduous broad-leafed trees (Angiosperms). The term has no reference to the actual hardness of the wood.

Heartwood: The inner layers of wood in growing trees that have ceased to contain living cells. Heartwood is generally darker than sapwood, but the two are not always clearly differentiated.

Impact bending: In the impact bending test, a hammer of given weight is dropped upon a beam from successively increased heights until rupture occurs or the beam deflects 152 mm (6 in.) or more. The height of the maximum drop, or the drop that causes failure, is a comparative value that represents the ability of wood to absorb shocks that cause stress beyond the proportional limit.

Modulus of Elasticity: An imaginary stress necessary to stretch a piece of material to twice its length or compress it to half its length. Values for the individual species are given in megapascals (MPa-equivalent to N/mm2), and are based on testing small clear pieces of dry wood.

Modulus of Rupture: Reflects the maximum load-carrying capacity of a member in bending, and is proportional to maximum moment borne by the specimen. Modulus of rupture is an accepted criterion of strength, although it is not a true stress because the formula by which it is computed is valid only to the elastic limit.

Moisture Content (M.C.): The weight of water contained in wood expressed as a percentage of the weight of the oven dry wood.

Pith Flecks: Pith-like irregular discolored streaks of tissue in wood, due to insect attack on the growing tree.

Sapwood: The outer zone of wood in a tree, next to the bark. Sapwood is generally lighter than heartwood.

Shear strength parallel to grain: Ability to resist internal slipping of one part upon another along the grain. Values presented are average strength in radial and tangential shear planes.

Shrinkage: The contraction of wood fibers caused by drying below the fiber saturation point (usually around 25 to 27% M.C.). Values are expressed as a percentage of the dimension of the wood when green.

Specific Gravity: The relative weight of a substance compared with that of an equal volume of water. The S.G. of wood is usually based on the green volume and oven dry weight.

Split: Separation of the fibers in a piece of wood form face to face (other term: end-split).

Stain: Materials used to impart color to wood.

Tensile strength perpendicular to grain: Resistance of wood to forces acting across the grain that tend to split a member. Values presented are the average of radial and tangential observations.

Texture: Determined by relative size and distribution of the wood elements. Described as coarse (large elements), fine (small elements) or even (uniform size of elements).

Warp: Distortion in lumber causing departure from its original plane, usually developed during drying. Warp includes cup, bow, crook and twist.

Weight: The weight of dry wood depends upon the cellular space, the proportion of wood substance to air space.

Work to maximum load in bending: Ability to absorb shock with some permanent deformation and more or less injury to a specimen. Work to maximum load is a measure of the combined strength and toughness of wood under bending stresses.

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