From clear soft wood to reaction wood :

From clear soft wood to reaction wood :
modeling of the influences of the microfibril angle and specific gravity on mechanical properties, elasticity and shrinkage

Daniel Guitard1and Christophe Gachet1

1 TREFLE UMR 8508, University Bordeaux 1, France

Abstract

A predictive mechanical modelling, based on a micro-macro approach, integrating four successive levels (macromolecules - cell wall - ligneous tissues - annual ring) and using 29 parameters of the structure and the ultra structure, allows to calculate three elastic modulus ER, ET and EL of clear wood. A particular set of those parameters characterises the standard softwood.(see SWW column A on table 1).

The purpose of this paper is to illustrate the utilisation of this mechanical model to analyse the influence of structural and ultra structural changes in reaction wood, according to bibliography, and to discuss the consequences on the macroscopic elastic properties of compression wood.

1. INTRODUCTION

It is now well known that the specific gravity, the moisture content and eventually the temperature are physical parameters governing the mechanical properties of woods, in particular the elastic anisotropy. Predictive models were proposed [1] for clear softwoods to characterise the three young modulus ER , ET and EL, respectively, along the radial, tangential and longitudinal anatomical directions as a linear functions of the specific gravity r.

That is to say that the observed variability of elastic properties of different normal woods is well explained by those parameters.

The paradox is that the variability of elastic properties inside a given tree can be significantly larger and the specific gravity, the moisture content and the temperature are no more pertinent parameters. This variability, inside the tree, is governed by more complex anatomical parameters that characterise the different woods, juvenile wood, adult normal wood, compression wood,…

The purpose of this paper is to illustrate and analyse the influence of small realistic variations of structural and ultra structural parameters on the macroscopic elastic anisotropy of softwood, to characterise more specifically the transition from normal wood to reaction wood.

2. Principle of the mechanical modelling and properties of the Standard Softwood (SSW)

A predictive mechanical modelling, based on a micro-macro approach, integrating four successive levels and using 29 parameters of the structure and the ultrastructure, allows to calculate three elastic modulus ER , ET and EL of a given clear wood [2]. The modelling presented previously [2], [3] should not be discussed here.

The two first steps of the mechanical modelling (here combined in a single step [1]) give an estimation of the longitudinal ELp (1) and transverse ETp (2) Young modulus of the double cell wall belonging to two adjacent wood cells, which is assimilated to an isotropic matrix of amorphous hemicellulose and lignin reinforced by a three dimensional skeleton of crystalline cellulose microfibrils. Note that the model considers mainly the S2 sub layer, influences of S1 and S3 are neglected .

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