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Materials testing laboratories determine wood strength and stiffness in a standard static-bending test where the test specimen's resistance to slowly applied loads is measured. The standard tests are generally conducted at 12% moisture content and 20¡ã C temperature. The ends of the test specimen are supported on rollers, usually with growth rings horizontal, and a load is applied at the beam centre so that a constant rate of deflection is maintained until the piece fractures. Instruments measure and plots the load (stress) and the deflection (strain) at intervals, as shown in the diagram. The vertical axis reflects the increasing stress and the horizontal axis the increasing strain.
The first part of the curve is a straight line where the deflection is directly proportional to the load and where, once the load is removed, the beam will return to its original state; i.e. it retains its elasticity. With increasing load, a limit point of proportionality is reached after which the increase in amount of deflection is greater than (i.e. no longer proportional with) the increase in load; but elasticity is still retained until an elastic limit is reached. If stress is further increased, the material loses elasticity and becomes plastic (i.e when the load is removed the deformation caused by deflection will be more or less permanent). At the point of maximum load, ultimate load or ultimate strength, the material begins to yield and will fracture unless load is substantially reduced.
stress_strain.gif
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