Thermo-Hydro-Mechanical Wood Behaviour and Processing” Selected Papers

Abstract: Thermo-hydro-mechanical (THM) treatment is a combined action of temperature, moisture, and mechanical force, which leads to modified wood (THMW). Various types of eco-friendly THM processes have been developed to enhance wood properties and generate new materials, such as welding, densification, molding, bending, profiling, artificial aging, panel manufacture, and surface densification. The various transformation processes in the course of THM bring about positive effects in terms of the mechanical and physical properties as well as the biological durability. To the negative effects belong the loss in strength and fracture toughness, and one of the challenges is to minimize these negative aspects. The present paper reviews the chemical transformations processes during THM treatment in a closed processing system and presents the relationship between processing parameters and THMW properties. The discussion includes the problems associated with eliminating the set recovery of densified wood by THM posttreatments and the chemical origin of the relaxation of internal stresses induced by densification. Keywords: chemical-mechanical, chemistry, densification, fixation of compression set, moisture content (MC), temperature, woo

Micromechanical approach to wood fracture by three-dimensional mixed lattice-continuum model at fiber level

To investigate the fracture behavior of wood, the porosity and heterogeneities of its microstructure should be taken into account. Considering these features of wood microstructure in a continuum-based model is still a difficult problem and the lattice model might be an alternative. In the developed mixed lattice-continuum model, the probable crack propagation volume was modeled by defining a three-dimensional lattice of different beam elements and the other regions were considered as continuum medium. Different beam elements of lattice represented the earlywood fibers, latewood fibers, ray cells and bonding medium between the fibers. The proposed model was used to investigate the mechanism of mode I fracture in a small notched wood specimen in RL orientation. The resulting pre-peak and softening curve and also the crack opening trajectory in both cross-section and longitudinal-section in model were in good agreement with the experimental observations. This model shows the importance of considering the three-dimensional and distributed propagation of microcracks and main cracks in fracture stability. It was also shown that in mode I fracture, RL orientation, the main crack propagates in the earlywood rin

Micromechanics of creep and relaxation of wood. A review COST Action E35 2004-2008: Wood machining - micromechanics and fracture

Wood, like all polymeric materials, shows viscoelastic behaviour. The time dependent behaviour of wood depends on material anisotropy, temperature, moisture and stresses. To predict the behaviour of wood, numerous mathematical models have been developed largely relying on experimental results. In this paper, time dependent viscoelastic behaviour of wood is reviewed under constant and cyclic climatic conditions, separately. More emphasis is given on results obtained in recent years on the behaviour of thin wood tissues, single fibres, thermo-viscoelasticity of wood, influence of hemicelluloses and the modelling of the effect of transient moisture at the molecular level on the mechanical respons

Experimental observations and micromechanical modeling of successive-damaging phenomenon in wood cells' tensile behavior

Single wood cells have complex tensile behavior. To gain insight into this complex functionality, the behavior of single wood tracheids was studied under controlled cyclic tensile loading. The cyclic tensile stress-strain curves show that beyond the yield point the tracheid undergoes permanent deformations and its rigidity increases. As in plasticity elastic (or visco-elastic) unloading takes place and energy is dissipated by permanent deformation. Consequently, single tracheids show a load-history dependent behavior. To understand the intervening mechanism in the process of elasto-plastic response of a wood tracheid, a micromechanical based model was developed. This model permits us to describe the influence of non-uniformity of microfibril angle (MFA) and other defects on the wood cell rigidity and to discuss different scenarios, which may occur during the tensile test. Successive damage of the hemicelluloses and lignin matrix and reduction of MFA as mainly responsible for elasto-plastic response of a wood cell were suggested. It should be noted that this paper is part of the research work which has been reported previously (Navi et al. in Wood Sci Technol 29:411-429, 1995; 36:447-462, 2002; Sedighi-Gilani et al. in Wood Sci Technol 39:419-430, 2005

Modeling crack propagation in wood and wood composites. A review COST Action E35 2004-2008: Wood machining - micromechanics and fracture

Predictive simulation of crack growth and fracture of wood and wood composites has always been difficult, and it has been limited by the availability of appropriate fracture models. As summarized herein, progress has been made on several fronts. First, a variety of fictitious crack model refinements have been made, along with the corresponding effects on bulk load-deformation response and R-curve behavior. Second, progress has been made on several different discrete element approaches that can explicitly represent material heterogeneity and variability. While progress has been substantial, a universal fracture law for wood remains elusiv

Fracture behaviour of wood and its composites. A review COST Action E35 2004-2008: Wood machining - micromechanics and fracture

Fracturing of wood and its composites is a process influenced by many parameters, on the one hand coming from the structure and properties of wood itself, and on the other from influences from outside, such as loading mode, velocity of deformation, moisture, temperature, etc. Both types of parameters may be investigated experimentally at different levels of magnification, which allows a better understanding of the mechanisms of fracturing. Fracture mechanical methods serve to quantify the fracture process of wood and wood composites with different deformation and fracturing features. Since wood machining is mainly dominated by the fracture properties of wood, knowledge of the different relevant mechanisms is essential. Parameters that influence the fracture process, such as wood density, orientation, loading mode, strain rate and moisture are discussed in the light of results obtained during recent years. Based on this, refined modelling of the different processes becomes possibl

Microfibril angle non-uniformities within normal and compression wood tracheids

The pattern and extent of variation of microfibril angle (MFA) in normal and compression tracheids of softwood were investigated by using confocal laser scanning microscopy technique. All measurements support the idea that the orientation of microfibrils in single wood tracheids is not uniform. MFA of the radial wall of earlywood tracheids was highly non-uniform and had an approximately circular form of arrangement around the bordered pits (inside the border). Between the bordered pits the measured MFAs were less than the other parts of the tracheid. In the latewood tracheids MFA was less variable. The average orientation of simple pits in the crossfield region was consistent with the mean MFA of the tracheids; however some of the measurements showed a highly variable arrangement in the areas between the simple pits. In many cases the local measured MFAs of compression wood tracheids agreed with the orientation of natural helical cavities of compression wood. Comparing the measured results in different growth rings showed that MFAs in juvenile wood are generally larger than in perfect woo

Within-Fiber Nonuniformities of Microfibril Angle

The pattern and extent of variation of microfibril angle of macerated spruce fibers were investigated by confocal laser scanning microscopy. All measurements supported the idea that the orientation of the microfibrils is not uniform along the radial wall of earlywood fibers. Microfibrils had an approximately circular form of arrangement around the bordered pits (inside the border). Between the bordered pits, lower microfibril angles were measured than in the other parts of the fiber. This phenomenon was interpreted by assuming the existence of crossed microfibrils in these zones. Variation of microfibril angle in earlywood fibers was observed only in the vicinity of the bordered pits, not in the nonpitted zones and tangential walls. Within the latewood fibers, microfibril angle was approximately uniform, even close to the pitted areas. The average orientation of simple pits in the crossfield region was consistent with the mean microfibril angle of the fibers; however, some of the measurements showed a highly variable arrangement in the areas between the simple pits

Influence of the moisture content on the fracture characteristics of welded wood joint. Part 1: Mode I fracture

Friction welding is a joining technique for wood materials. The positive aspects of this technique are the speed of processing and the absence of chemical or mechanical agents, but the welded joints are not water resistant. To understand better the effect of moisture on the fracture behavior of welded joints, their fracture characteristics have been investigated. The double cantilever beam specimens were tested, which permit to compute the mode I energy release rate of a welded joint. The results confirm the negative effect of moisture on the fracture properties of the joint. The data concerning the maximal tensile strength of the joining material were collected by uniaxial tests and implemented in a finite element model to establish a cohesive law, which describes the behavior of welded pieces in terms of moisture conten

Influence of the moisture content on the fracture characteristics of welded wood joint. Part 2: Mode II fracture

As a second part of this series, the present study also addresses the water resistance of joints obtained by friction welding. Here, the mode II fracture is in focus, that is, 4-points end-notched flexure specimens (4-ENF) were investigated with various moisture contents (MCs). The critical energy release rate was decreasing at higher MCs. The maximal shear strength of the joining material, as determined by torsion tests, was also affected by high MCs. The experimental data were implemented in a finite element model (FEM) based on the cohesive law to simulate the behavior of welded connection in 4-ENF tests. The FEM results describe well the experimental load-displacement curve