Resistance may be measured in several ways. Another approach is to measure the deformation or strain that results from a given level of stress before the point of total failure. Strength of wood is often thought of in terms of bending strength. This is certainly a useful yardstick of strength but is by no means the only one. A number of other strength criteria are described below.
Stress is the amount of force for a given unit of area.
Strain is defined as unit deformation or movement per unit of original length. It is typically expressed in inches per inch. Elasticity 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. 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.