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.
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. Fiber stress at proportional limit represents the maximum stress a board can be subjected to without exceeding the elastic range of the wood. Permanent set will result if an applied stress exceeds the proportional limit. This property is typically reported in units of psi.