Analysis of mechanical behaviour of wood species by speckle interferometry

Abstract

The understanding of the mechanical behaviour of wood is a very important task in modern structural engineering. This is due to the increasing role played by wood structures in modern architecture as well as to its use in the restoration of ancient buildings. However, solid mechanics of wood is very difficult since it is a complex, polymer-based, cellular, inhomogeneous material. Moreover, at the cellular scale wood is not a continuum due to the presence of vascular vacuums and it can be regarded as an anisotropic material. Conversely, at a macroscopic scale when the analysis is carried out on a structural member which is large in comparison to the size of cell, wood is usually considered as a continuous homogeneous orthotropic material and its mechanical behaviour is usually approached in cylindrical or rectangular coordinates. The above considerations emphasize the importance of experimental techniques able to investigate the behaviour of wood either at fibre or at macroscopic scale. Strain measurement at the fibre level has not yet been accomplished due to the loading effect on the measured quantity. Aim of the paper is to apply Electronic Speckle-Pattern Interferometry handled by phase-stepping technique to the experimental study of wood mechanical behaviour. The selected wood species are Red Spruce, Pitch Pine, Swedish Pine, Larch, belonging to the conifers, usually classified as softwoods, and Cherry-wood, Baky, Teak, Iroko, Doussiè, Bahia belonging to the latifoliae, usually classified as hardwoods. In the paper the mechanical behaviour due to bending is investigated by adopting a four-point bending test. The obtained results are compared with those for an isotropic material (Plexiglas) in order to show the differences. The results confirm the complexity of the mechanics of wood and suggest further investigations in order to gain a deeper insight in of developing a correct approach from a structural point of view.