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Elastic deformation of a material occurs when the magnitude of loads applied to a sample are small enough that when released the sample returns to its original state (Hibbeler, 2000). Here we need to define some particular terminology and assumptions. The proportionality limit is the point at which the relationship between stress and strain stops being linear, although not necessarily elastic. The end of the elastic state is characterised by the yield point (elastic limit), after the yield point plastic (irreversible) deformation occurs, although this deformation does not necessarily result in a loss of stiffness (Reiterer et al., 1999).  It was assumed that the proportionality limit and the yield point are equal and theterms the terms  yield point, proportionality limit and failure point are used interchangeably to indicate what is strictly the proportionality limit. There is argument for and against the assumption that wood is a linear elastic material in literature (Mackenzie-Helnwein et al., 2005a). Within this work models are restricted to the limit of proportionality in order to retain simplicity.