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

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

A micromechanical approach to the behaviour of single wood fibers and wood fracture at cellular level

Mechanical and fracture behaviors of wood are defined by the morphology and mechanical properties of wood fibers and their bonding medium. Parallel orientation of wood fibers makes them the most influential microstructural elements from the mechanical point of view. On the other hand, in wood fracture, the difference between the properties of fiber and bonding medium (which make weak cleavage plates) plays a more important role. Experiments show that the mechanical behavior of a single wood fiber under axial tension is complex, although the cause of this complexity has still not been clearly understood. In this thesis, in order to explain the mechanism underlying the mechanical behavior of wood fibers and the fracture of wood specimens at fiber level, a micromechanical approach has been used. Confocal laser scanning microscopy was used to investigate the pattern of the distribution of microfibrils in different wood fibers. It was shown that the microfibril angle within a single fiber is non-uniform and this non-uniformity in radial walls of earlywood fibers, which contain the bordered pits, is higher than tangential walls of earlywood fibers and also higher than in latewood fibers. Tensile and cyclic tensile tests on single spruce fibers were carried out and their non-linear and force-history dependent behaviors under axial tension were shown. It was found that the fiber behavior is affected by the range of microfibril angle non-uniformities and other defects. After a certain force limit, wood fiber undergoes irreversible strains and the elastic limit of the fiber increases in the tensile loading. To explain these results, a model based on the assumption of helical and non-uniform distribution of cellulose microfibrils in the fiber and damage of the hemicellulose and lignin matrix after yielding, was proposed. The model indicated that multi-damage and evolution of microfibrils in the damages segments are the main governing mechanisms of the tensile behavior of wood fiber. Difficulties of considering the porous and heterogeneous microstructure of wood in a continuum-based fracture model, led us to develop a mixed lattice-continuum model. The three-dimensional geometry of lattice, composed of different beam elements which represent the bonding medium and alternation of earlywood and latewood fibers, enabled us to detect the propagation of cracks in both cross sections and longitudinal sections at the fiber level. Model showed that in Mode I fracture, parallel to the fibers, the location of the developed crack and the resulting stress-strain curves have a good agreement with the experimental evidence

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

Hygric properties of Norway spruce and sycamore after incubation with two white rot fungi

In this study, changes in the hygroscopic properties of two main wood species for violin making, Norway spruce and sycamore, after treatment with Physisporinus vitreus and Xylaria longipes were investigated. Swelling and moisture sorption capacity of wood at the growth ring scale were visually and quantitatively assessed by thermal neutron radiography analysis. It was demonstrated that the fungal treatment improved the dimensional stability of both Norway spruce and sycamore, but also increased their moisture adsorption capacity. Dynamic vapor sorption tests and measurements of the changes in dimensions of the specimens in the laboratory were in good agreement with the results of neutron radiography analysis. The main difference between the moisture sorption of the untreated controls and treated wood was observed at high relative humidity, e.g., above 75%. The contradictory behavior of the increased hygroscopicity and reduced swelling was explained by selective degradation of the chemical components and condensation of the moisture content gained in the capillary voids that developed in the cell walls during fungal decay

Dynamics of microcrack propagation in hardwood during heat treatment investigated by synchrotron-based X-ray tomographic microscopy

The process of crack propagation in wood during pyrolysis is strongly linked to heterogeneities in its hierarchical porous structure. Fundamental understanding of this process is necessary for the analysis of the behavior of wood structural elements during fire exposure. Synchrotron-based X-ray tomographic microscopy combined with a recently developed laser-based furnace at the TOMCAT beamline of the Swiss Light Source provides a unique opportunity to study the heat-induced propagation of microcracks in hardwood in situ with high spatial and temporal resolutions. In this study, attention was focused on the 3D microstructure of beech and the interconnectivity between morphology and cracking patterns. It is shown that thermal cracks initiate mainly along the ray cells in hardwood and in the junction of seasonal growth layers. There is a clear indication of increased total porosity of the wood due to charrin

A hygrothermo-mechanical model for wood: part A. Poroelastic formulation and validation with neutron imaging: COST Action FP0904 2010-2014: Thermo-hydro-mechanical wood behavior and processing

The correct prediction of the behavior of wood components undergoing environmental loading or industrial process requires that the hygrothermal and mechanical (HTM) behavior of wood is considered in a coupled manner. A fully coupled poromechanical approach is proposed and validated with neutron imaging measurements of moist wood specimens exposed to high temperature. This paper demonstrates that a coupled HTM approach adequately captures the variations of temperature, moisture content, and dimensions that result in a moist wood sample exposed to one-side heating

Biomechanical evaluation of porous biodegradable scaffolds for revision knee arthroplasty

Tibial bone defect is a critical problem for revision knee arthroplasty. Instead of using metallic spacer or cement, biodegradable scaffolds could be an alternative solution. A numerical model of a revision knee arthroplasty was thus developed to estimate the mechanical resistance of the scaffold in this demanding situation. The tibia, scaffold, and prosthesis were represented by simplified parameterised geometries. The maximal gait cycle force was applied asymmetrically to simulate a critical loading. Several parameters were analysed: 1) inter-individual variability, 2) cortical bone stiffness, 3) cortical bone thickness, 4) prosthesis fixation quality, and 5) scaffold thickness. The calculated scaffold strain was compared to its experimental ultimate strain. Among the tested parameters, failure was only predicted with scaffold thickness below 5 mm. This study suggests that biodegradable bone scaffolds could be used to fill bone defects in revision knee arthroplasty, but scaffold size seems to be the limiting factor