![]() The higher the E value, the stiffer the wood and the lower the deformation under a given load. Stiffness may be quantified using the modulus of elasticity, E. It is reported in units of inches or centimeters. The height of the drop that causes failure provides a comparative measure of how well the wood absorbs shock. Impact bending involves dropping a hammer of a given weight upon a board from successively greater heights until complete rupture occurs. Maximum crushing strength is the maximum stress sustained by a board when pressure is applied parallel to the grain. This property is typically reported in units of psi. Permanent set will result if an applied stress exceeds the proportional limit. It is reported in units of psi.įiber stress at proportional limit represents the maximum stress a board can be subjected to without exceeding the elastic range of the wood. It is generally used in tests of bending strength to quantify the stress required to cause failure. Modulus of rupture is the maximum load carrying capacity of a member. 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. The modulus of elasticity is normally measured in pounds per square inch (psi) and is abbreviated as MOE or E. 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. Modulus of elasticity or Young's modulus is the ratio of stress to strain. ![]() Beyond the proportional limit, each increment of stress will cause increasingly larger increments of strain (as failure is approached) and removal of the stress will only result in a partial recovery of the strain. Below this point, each increment of stress will produce a proportional increment of strain (the stress/strain ratio is constant) and the wood will return to its original position once the stress is removed. Example: if the 10-inch long block of wood in the stress example above was compressed by 0.002 inches, the strain would be 0.002 inches/10 inches = 0.0002 inches per inch.Įlasticity is a property of wood in which strains or deformations are recoverable after an applied stress is removed, up to a certain level of stress known as the proportional limit. It is typically expressed in inches per inch. Strain is defined as unit deformation or movement per unit of original length. Example: if a 1000 pound load was applied on the edge of a block of wood measuring 2-inches by 2-inches in cross-section by 10 inches in length, the applied stress would be 1000 pounds divided by 4 square inches = 250 lb./sq. It is typically measured in pounds per square inch (psi). Stress is the amount of force for a given unit of area. A number of other strength criteria are described below. This is certainly a useful yardstick of strength but is by no means the only one. Strength of wood is often thought of in terms of bending strength. Another approach is to measure the deformation or strain that results from a given level of stress before the point of total failure. One is the maximum stress that the material can endure before "failure" occurs. Resistance may be measured in several ways. Strength may be defined as the ability to resist applied stress: the greater the resistance, the stronger the material. Shear Parallel to Grain, Max Shear Strength Parallel to Grain, Max Crushing StrengthĬompress. Impact Bending, Height of Drop Causing FailureĬompress. Tree SpeciesĪverage Specific Gravity, Oven Dry Sample ![]() Note that due to inadequacies of samples, these values may not necessarily represent average characteristics. The table below provides laboratory values for several properties of wood that are associated with wood strength.
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