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The effects of microwave and steam treatment were analyzed relative to the immediate (thermo-hygro-plasticity) and post-assessed (permanent changes) properties of wood. The study was conducted using European beech (Fagus sylvatica L.) standard and 1.5 times up-scaled (only for microwave-heated and reference samples) bending specimens tested in a static three-point loading mode. The specimens were plasticized by heat and moisture (1) separately and (2) simultaneously by heating moist specimens using (i) various microwave regimes in continuous mode, and (ii) heated saturated steam in discontinuous mode. Oven-dried specimens tested at 20 degrees C served as references. The thermo-hygro-plasticity was studied immediately after treatment, whereas the permanent changes were assessed after oven-drying of plasticized specimens to 0% moisture content. Permanent structural changes were analyzed using scanning electron microscopy. Microwave treatment increased the plasticity of wood (decreasing the modulus of elasticityby 70%) comparably to steam treatment, when the output moisture content was 30% or higher. A similar degree of plasticity was found in up-scaled specimens heated by microwaves. Further analyses confirmed that microwave treatment did not cause any permanent damage to wood structure or reduce mechanical performance. The results showed that microwave treatment is an efficient alternative to steaming when plasticizing moist wood.

Thermal modification of wood is an environment-friendly method, which, among others, enhances wood water related properties, whereas the use of hot oil as a heating medium generally leads to better performance. In this study, European beech wood was hemp oil-impregnated, thermally modified at 200 °C and both ways of treatment were combined. Analysis of the treated wood included the determination of the change in important wood characteristics—colour, hygroscopicity, dimensional stability, bending properties, and decay resistance against white rot. The results indicate that heat treatment combined with oil has influenced the properties of beech wood more than heat treatment itself. In certain cases, the high effectiveness is only relative with regard to mass increase caused by high oil retention.

One-sided surface charring of beech wood (Fagus sylvatica L.) was analyzed. Specimens were charred on one surface using contact heating system with a hot plate, at a temperature of 220 °C and atmospheric pressure for 15 or 40 min. Surface charring was applied on the radial or tangential surface. Spectrophotometric chemical analysis was carried out to determine the total amount of soluble carbohydrates and phenolic compounds beneath the surface in order to evaluate the depth (within 1, 2, 3 and 4 mm) of the degradation affected by the charring. Specimens of dimensions 14 × 14 × 14 mm were used to determine the moisture behavior of charred specimens. The equilibrium moisture content (EMC) of specimens was calculated at a temperature of 20 °C and 65% relative humidity. Water absorption (WA) was recorded during the water immersion for 240 h at various intervals. Finally, the three-point bending tests were undertaken and modulus of rupture (MOR) and modulus of elasticity were calculated. Degradation of the main wood compounds due to chemical changes occurred during the surface charring is closely associated with a mass loss (3.5–5.5%) and further related to the severity of process and orientation of specimens to heat flow. Application of surface charring resulted in the significant reduction in EMC (~ 20%) as a result of a decrease in hydroxyl groups, an increase in cellulose crystallinity as well as further cross-linking of lignin. Furthermore, the WA of one-sided surface-charred wood considerably decreased (15%) when compared to that of the control specimens. A significant increase in soluble carbohydrates as well as phenolic compounds was found, and it can be stated that one-sided charring affected wood properties to depth 2–3 mm from the surface. Significant differences (14–24%) of bending strength MOR results are most likely due to improved sorption behavior of wood. The results show that beech wood charring improved moisture-related characteristics and consequently led to better mechanical behavior, but more studies are needed to exploit the potential of surface charring method for future use.

This work employs the acoustic emission (AE) method for material state monitoring. AE presents a non-destructive evaluation technique, which could be used for detection of microstructural changes in composite material. Work describes the process of acquisition of AE in tensile loading of carbon composite materials. In course of tensile stress, the composite was monitored with optical method, applying principles of digital image correction (DIC). Optical stereovision method enables calculation of field shift and field of proportional deformation at composite surface. The objective is analysis of damage in carbon composite materials and employ the methodology of AE signal processing for facilitation of early damage diagnostics and prediction of structural failure. For this purpose, the experimental setup was designed to obtain results from 50 nominally identical composite samples in tensile loading test. Force load applied on samples was synchronically recorded along with AE and image data. Experimental data were subsequently analysed in a way enabling the description of typical phenomena in course of every measurement. Results show that observation of AE sources could be employed in facilitation of early damage diagnostics and establishment of failure prognosis. It is about internal changes in composite material. info:eu-repo/semantics/openAccess

The hardwood is a natural composite, which finds applications in many fields such as an important building material. Attention was paid to the accurate description of the European beech wood under high strain rates in order to constitute a reliable input for the finite element method. It was done on the basis of split Hopkinson tensile bar tests, taking also into consideration the previously reported pressure bar tests to capture the tensile–compressive failure asymmetry, which was incorporated through the stress triaxiality. Strains measured on the steel bars for specimens oriented in all principal loading directions were used to calibrate the fully orthotropic behaviour of elasticity, plasticity and fracture. The concept of continuum damage mechanics was applied to the damage accumulation in order to simulate the crack propagation within explicit finite elements realistically.

Norway spruce (Picea abies L. Karst.) and European beech (Fagus sylvatica L.) samples were loaded in shear mode aimed at testing their elastic material characteristics applicable in finite element analysis (FEA). More precisely, experimental and numerical analyses of uniaxial tensile test parallel to grain in longitudinal-radial (LR) or longitudinal-tangential (LT) shear of plane are described. The elastic material models in the FEA are based on own experimental data and those of the literature. The verification of material characteristics was performed by 3D numerical models with the same parameters as for the experimental tests. The fully orthotropic elastic material model was applied in the uniaxial tensile tests. The digital image correlation (DIC) method served for verification of the numerical models with proposed elastic material characteristics. Good correlation was found between numerically predicted and experimentally measured data. The minor differences between the two data sets could be mainly attributed to certain natural wood characteristics, which were neglected in the proposed models, i.e. especially variation of earlywood and latewood density. The proposed elastic material models offer general data sets for the evaluation of mechanical response of timber structures and especially in timber connexions.

Ammonia treatment was used in order to change the colour of wood and also to plasticize the wood before bending or compressing. The mechanism and the process of gaseous ammonia treatment impact on wood are not very well known. The goal of this study was to quantify the effect of gaseous ammonia on mechanical properties, i.e., modulus of elasticity (MOE), modulus of rupture (MOR) and deflection at maximal loading force (yFmax). When wood was treated by water and ammonia vapour together, significant changes in plasticity of wood were observed. The samples which were firstly exposed to water vapour and then to ammonia vapour exhibit the lowest values of MOE and MOR and an enhanced flexibility of the material. The results show that ammonia treatment increases wood plasticity. This process can be used for manufacturing of bent furniture.

The hardwood, as a natural composite, may be subjected to the dynamic loading in many applications, such as the high-speed disintegration or structural collapses. The correct material description is needed for finite element modelling that can take place in the design stages of wood manufacturing processes or building the wooden structures. The split Hopkinson pressure (Kolsky) bar test provides the data for high strain rates. The experiments were carried out on the European beech wood in all the principal loading directions. The tests were repeated computationally in order to obtain the material constants of the complex material model, which covered the anisotropy of elasticity, plasticity as well as fracture behaviour. Moreover, the failure was considered non-symmetric with respect to the tension and compression. This was achieved by incorporating the stress triaxiality dependency into the failure model. The crack initiation and propagation were realized through the element deletion technique within LS-DYNA code. The real behaviour was captured quite well including the pulses in the incident and transmission bars.

The reinforcement of timber and concrete load bearing beams is increasingly utilized. The most promising effect can be achieved by CFRP (carbon fibre reinforced polymer). However, the difference in mechanical properties of the materials used may be so vast that the two components slip from each other at high stresses. This study deals with the delamination of CFRP from a beam. Design of corrugated reinforced surface is presented. It is based on area enlargement, which leads to a higher applied force needed for CFRP delamination during 4-point bending. Moreover, the corrugated shape comes to synergy with the timber beam which deforms during the flexural load and cyclic humidity. Corrugated CFRP is able to adapt to transverse dimensional changes and vice-versa increases the bending stiffness in the longitudinal direction. All in all, the corrugated shape of CFRP constrains the delamination and seems to be applicable for long-term reinforcement.

The goal of the study is to investigate the non-standard deformation behaviour of wood loaded by compression parallel to the grain. This is represented as a negative increment of strain in the range of plastic deformations when the load continues to increase. The objectives of this study are to point out this problem and to provide its description based on the deformation fields that have been analysed using three approaches: a) full-field optical technique based on digital image correlation (DIC); b) “clip on” extensometer and its virtual analogy, and c) crosshead displacement method. Further, the negative strain phenomenon was studied depending on the sample length. The samples were made from the European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L. Karst.). Based on the strain analysis, it can be concluded that the deformation field consists of three sub-regions exhibiting different stiffness values (three-spring model). The failure of less stiff zones near the compression plates during the “non-standard” compression behaviour causes almost zero compression deformation of the stiffer middle zone or even leads to its expansion. The three-zone heterogeneity of deformation field induces a deviation of the displacement and strain measured by the proposed approaches. This phenomenon substantially influences the resulting longitudinal Young’s modulus and, therefore, should be of concern when measuring wood in such mode.

Gaseous ammonia treatment in combination with densification of wood has been known for several decades, but these days there is no industrial production of materials modified in this way; also, little has been published in this area of wood science. In this study, selected physical and mechanical properties, i.e. density profile, bending strength, hardness and moisture absorption were investigated for Lignamon (1), which was obtained from the Czech industrial production. Selected properties were also investigated using steam-densified beech (2) and native beech (3) and compared with each other. Densitometry of Lignamon showed a large variability in the density profile compared to that of only densified beech. It is affected by the degree of densification, temperature and moisture gradients, and their relationship to the glass transition of the wood cell wall. Modulus of elasticity, hardness, moisture exclusion and anti-swelling efficiency of Lignamon are enhanced compared to densified beech. The enhanced dimensional stability and lower hygroscopicity of Lignamon are probably caused by heat treatment during the process. Further investigation will be carried out with self-produced Lignamon samples.

An alternative assessment procedure for the mechanical performance of thermally modified timber is presented. Non-defect beech wood (Fagus sylvatica L.) specimens were thermally modified at 180°C and 200°C and subjected to static three-point bending load. Based on the full-field strain data acquired during the tests, the neutral axis (NA) at a 50 % level of maximal loading force was identified and localized within the specimen height by means of image processing methods. The NA in specimens mildly thermally modified moved toward the tensile zone when compared to the control specimens, whereas the position of NA within the specimens severely thermally modified was comparable with the NA position in control specimens. The proposed procedure proved to be applicable for the assessment of the mechanical performance; however, in the present study, the NA movements caused by thermal modification were not found to be statistically significant.

A new sandwich composite structure was prepared that utilizes classic wood-based composites as the core and face materials. Particleboards were used as faces, which covered a plywood-made iso-grid core. A new type of core-face fixation was suggested and assessed. The sandwich panels can be regarded as lightweight, as their density was below 400 kg/m3. Digital image correlation (DIC) was used to determine Poisson’s ratios and obtain additional insight into the deformation behavior of the sandwich panel. DIC was also employed to assess the core-face bonding, which was based on imprinted grooves on one side of the particleboard face. The results include strength in edgewise and flatwise compression and flexural properties. The latter were determined through three-point bending tests. Comparable strength properties were found relative to the literature, which means that this new type of sandwich panel demonstrates a competitive property profile. It was concluded that the developed sandwich panel is versatile, and the hollow spaces in the core can be filled with insulation materials such as fibers or foams. Surfaces can also be covered with some overlay, delivering improved bending performance.

Wood exhibits very different behavior and properties at different scales. One important scale is the cell wall (CW) that is commonly tested by nanoindentation. Common nanoindentation provides important insight into the material but has limitations because it does not apply uniaxial stress and provides data from single spots. Therefore, the aim was to examine beech CW using two state-of-the-art techniques: micropillar compression (MCo) and nanoindentation mapping (NIP). The mean strength of the beech CW was found to be about 276 MPa and the mean yield stress was 183 MPa. These values were higher than those in most cited literature, which was attributed to the fact that libriform fibers from beech late wood were measured. Mean E obtained from MCo was about 7.95 GPa, which was lower than the values obtained on a macrolevel and about 61% of the value obtained from NIP. NIP also showed that E of the CW around the middle lamella (ML) was about 64% of the value at the location attributed to the S2 layer. Lower E from MCo may be caused by sinking of the micropillar into the wood structure under the load. Failure of the micropillars showed gradual collapse into themselves, with debonding at the S3 layer or the MLs.

[EN] The fracture properties of thermally modified beech (Fagus sylvatica) wood (TMW) at 180 degrees and 200 degrees C were evaluated in mode II using the three-point end-notched flexure (3ENF) scheme assisted by three-dimensional (3D) stereovision equipment for obtaining displacements and strains. The compliance-based beam method (CBBM) provided the strain energy release rates (G(II)) of TMW and cohesive laws for both native wood (W) and TMW. Based on the CBBM and equivalent crack length approach (ECLA), G(II) was obtained directly from the force-deflection data. The thermal modification (TM) process reduced the compressive strength by 4.4% and increased the compressive elastic modulus by 38.3%, whereas G(II) was reduced substantially by 40.8% and 67.9% at TM180 degrees C and TM200 degrees C, respectively. TM also increased wood brittleness that was visible on the displacement slip reduction. The resulting mean cohesive models can be used for numerical analyses. The fracture properties of TMW have to be taken into consideration for constructional wood application, when cyclic loading may lead to microcracking and material fatigue., The authors would like to thank COST Action FP1407 (Funder Id: https://dx.doi.org/10.13039/501100000921), the European Commission for funding the InnoRenew CoE project under the Horizon2020 Widespread-Teaming program (grant agreement #739574), the Republic of Slovenia for providing support from the European Regional Development Funds, and the financial support provided by the Internal Grant Agency (IGA) of the Faculty of Forestry and Wood Technology, Mendel University in Brno (LDF_PSV_2016015).