Is Wood a Material? Taking the Size Effect Seriously

Abstract

This review critically examines the various ways in which the mechanical properties of wood have been understood. Despite the immense global importance of wood in construction, most understanding of its elastic and inelastic properties is based on models developed for other materials. Such models neglect wood’s cellular and fibrous nature. This review thus questions how well models that were originally developed for homogeneous and effectively continuous materials can describe wood’s mechanical properties. For example, the elastic moduli of wood have been found by many authors to depend on the size of the test specimen. Such observations are incompatible with classical elasticity theory. There is also much uncertainty about how well elastic moduli can be defined for wood. An analysis of different models for size effects of various inelastic properties of wood shows that these models only approximate the observed behaviour, and do not predict or explain the scatter in the results. A more complete understanding of wood’s mechanical properties must take account of it being in some sense intermediate between a material and a structure.

Highly Mechanical Strength, Flexible and Stretchable Wood-Based Elastomers without Chemical Cross-Linking

“…Research on the structural control of wood to prepare elastic materials is limited. If the elastic properties of wood can be accurately regulated, it can be used as a high-performance green material in various fields including machinery, automobiles, sports and leisure, aerospace, home packaging, and other applications [8][9][10].…”

Differential Energy Criterion of Brittle Fracture as a Criterion for Wood’s Transition to the Plastic Deformation Stage

“…To date, many strength and fracture criteria have been developed for homogeneous isotropic materials and, to a lesser extent, for anisotropic materials [1,2]. Studies show that many criteria have limited application, and some of them do not fully correspond to real materials at the micro-, meso-and (or) macro levels [3,4]. At the same time, the best results in this area are based on energy concepts [3].…”

“…One of these criteria is based on the concept of an equivalent material, i.e., on the transformation of a physically nonlinear material into an equivalent linearly elastic material [6]; however, the prediction accuracy decreases for materials with defects [6,7]. Therefore, the development of a fracture criterion that takes into account the influence of all material defects requires further research, which is especially important for wood since the physical and mechanical properties of this material are very variable [4].…”

“…A number of other destruction criteria are known, which are based on the analysis of the conversion of input energy into the potential energy of elastic deformation, as well as into the energy of micro-, meso-and macro-destruction, acoustic emission [12][13][14], heat transfer [15] and other physical effects [16][17][18][19][20][21][22][23]. In any case, it is important to know the load-displacement (stress-strain) relationship since energy is determined by integrating the above dependence [24], and the graphical representation of these dependencies in the form of load-displacement (stress-strain) curves improves the understanding of the behavior of materials under load [4,25].…”

“…Load-displacement curves for uniaxial compression of wood [4,26,27] and materials such as sandstone and concrete [18,28] have certain similarities. The similarity of loaddisplacement curves for various indicates that some methods of analyzing rock behavior under load can be adapted to analyze wood failure, subject to certain limitations associated with different material properties [4,22,29]; for example, the acoustic emission analysis mentioned above is an effective support tool for experimental studies of both rock and wood failure [10,11,[29][30][31]. The generalization of experimental data has formed the basis of a number of defining models of the mechanical behavior of wood.…”