Wood Definitions


Modulus of rupture


Modulus of rupture is the maximum load carrying capacity of a member. It is generally used in tests of bending strength to quantify the stress required to cause failure. It is reported in units of psi.


Modulus of elasticity

Modulus of elasticity or Young's modulus is the ratio of stress to strain. Within the elastic range below the proportional limit, this ratio is a constant for a given piece of wood, making it useful in static bending tests for determining the relative stiffness of a board. The modulus of elasticity is normally measured in pounds per square inch (psi) and is abbreviated as MOE or E. Values for E relating to wood properties are commonly in terms of million psi; for simplicity, a board with a modulus of elasticity of 2,100,000 psi. (2.1 x 106) may be reported as 2.1E.


Internal Bond Strength


Internal bond strength is the strength of the bond between the wood particles in the panel.


Moisture Content

Moisture content for a given sample of wood is defined as the weight of water in wood expressed as a percentage of the weight of wood fibrous material (which is considered to be the oven dry weight of the sample).


Density

Density is the weight or mass of a unit volume of wood, and specific gravity the ratio of the density of wood to that of water. Tensile


Tensile


Tensile stress elongates or expands an object. Measurements of tensile stress perpendicular to the grain are useful for quantifying resistance to splitting. Examples of such stress include splitting firewood, driving nails, and forcing cupped boards to be flat. Wood is relatively weak in tension perpendicular to the grain but it is very strong in tension parallel to the grain (visualize a board being pulled from both ends). Due to difficulties in testing and the limited use for such data, tension parallel to the grain has not been extensively measured and/or reported to date. Tensile stress is measured in psi.


Compression


Compression stress shortens or compresses the material. For the woodworker, the primary types of compression to consider are parallel to the grain and perpendicular to the grain. Compression parallel to the rain shortens the fibers in the wood lengthwise. An example would be chair or table legs which are primarily subjected to downward, rather than lateral pressure. Wood is very strong in compression parallel to the grain and this is seldom a limiting factor in furniture design. It is considerably weaker in compression perpendicular to the grain. An example of this type of compression would be the pressure that chair legs exert on a wooden floor. If the applied pressure (weight) exceeds the fiber stress at proportional limit for the wood, permanent indentations will result in the floor. Compression stress is measured in psi.