Toughness Of Wood Research Articles

Experimental study on fracture failure characteristics evaluation of wooden pallets in humid-cold environment based on piezoelectric technology

Wooden pallets often suffer mechanical damage during transportation, stacking, forklifting, and on-site handling in cold supply chain logistics. In humid-cold environments, the fracture properties of wooden pallets change, increasing the risk of sudden fracture failure. Understanding the fracture failure characteristics of wooden pallets is essential to prevent further damage to goods. Nevertheless, evaluating and identifying these characteristics currently presents significant challenges. This study investigates the changes in modulus of elasticity (MOE) and fracture failure characteristics including modulus of rupture (MOR), brittleness and toughness of wooden pallets and impact toughness of wood material under humid-cold conditions. The piezoelectric technology was applied to evaluate and identify fracture risk of wooden pallets used in various humidity and cold conditions. Quarter-sized wooden pallets and standard wood specimens were examined by laboratory study. The experimental results showed that the MOE, MOR, toughness of wooden pallets and impact toughness of wood with higher moisture content (MC) increased with decreasing temperature, while brittleness increased as temperature decreased. These results indicate that the maximum carrying ability of wooden pallets increases in humid-cold environments while the ability to resist fracture was reduced with a higher brittle rupture risk. The toughness and its corresponding piezoelectric voltage exhibited good consistency with changes in temperature and MC. The relationship between toughness and piezoelectric voltage was established with good coefficient of determination (over 0.81). It indicates that the risk of wooden pallets could be predicted effectively using piezoelectric technology.

Green bio-derived epoxidized linseed-oil plasticizer improves the toughness, strength, and dimensional stability of furfuryl alcohol-modified wood

Furfuryl alcohol (FA) modification represents a sustainable approach to wood modification; however, it significantly reduces the toughness of the wood, limiting its practical applications. This study introduced epoxidized linseed oil (ELO) as an environmentally friendly and bio-derived plasticizer to enhance the toughness and mechanical properties of FA-modified wood. Microstructural analyses confirmed the integration of FA and ELO into the cellular structure of the wood. The chemical interactions between the epoxy groups of ELO and hydroxyl groups of FA were characterized using Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy, illustrating that a ring-opening reaction occurred that grafted the long flexible aliphatic chains of ELO onto the FA chains, thereby enhancing the chain mobility and flexibility. This interaction significantly decreased the crosslinking density, thereby achieving toughening. FA–ELO modification resulted in significantly enhanced dimensional stability and mechanical properties. Specifically, the anti-swelling efficiency exceeded 80 %, while the modulus of elasticity, modulus of rupture, and along-grain compressive strength increased by up to 35.0 %, 36.8 %, and 45.7 %, respectively. Additionally, significant enhancements in the elongation at break and impact strength were achieved, at 369.1 % and 72.4 %, respectively. The loss factor also increased. The optimal ELO content was 20 wt%. This study effectively demonstrated how ELO mitigates the brittleness of FA-modified wood and supports its application in high-value fields such as construction.

On the high fracture toughness of wood and polymer-filled wood composites – Crack deflection analysis for materials design

Cracks oriented in the toughest direction across the grain of wood (0°) tend to deflect at 90° to the precrack rather than extending in 0° direction. Fracture toughness data across the grain are therefore difficult to interpret. Crack growth mechanisms and effects from replacing wood pore space with a polymer are investigated. Crack growth is analyzed in four-point bending fracture mechanics specimens of birch and two different polymer-filled birch composites using strain-field measurements and finite element analysis (FEA). Calibrated cohesive zone models in both precrack and 90°-directions describe fracture process zone properties in orthotropic FEA-models. Conditions for 0° crack penetration versus 90° crack deflection are analyzed based on cohesive zone properties. Stable, subcritical crack deflection takes place at low load, reduces crack tip stress concentration, and contributes to high structural toughness, provided the 90° toughness is not too low. Polymer-filled neat birch composites have the best structural toughness properties in the present investigation, since 90° toughness is not compromised by any chemical treatment.

Fracture toughness of wood and transparent wood biocomposites in the toughest LT-direction

Fracture toughness and mechanisms of crack growth are characterized for transparent wood polymer biocomposites and compared to native wood, with the crack normal to the fiber direction (LT fracture plane). Side-grooved specimen geometries generated pure mode I crack growth, whereas previous investigations commonly report 90° crack path deflection. Crack growth micromechanisms were analyzed by experimental fracture tests and in-situ microscopy observations. Large damage zones around the crack tip with fiber bundle bridging and pull-out were observed in the crack wake, justifying more advanced cohesive zone modeling suitable for composite materials design. The polymer matrix resulted in much higher fracture energy of the biocomposites compared to native wood due to increased local cohesive strength. This strength increased from the polymer contribution and more homogeneous stress distribution in the wood fibers.

Application of polyvinyl alcohol (PVA) as a toughening agent in wood furfurylation

Abstract In order to explore the application of furfurylated wood as engineering material, polyvinyl alcohol (PVA) was proposed as a toughening agent to incorporate with furfuryl alcohol (FA) to form FA/PVA precursor and impregnate wood in a one-step method. After in situ polymerization upon heating, a hydrophobic network composed of polyfurfuryl alcohol (PFA) and PVA was formed within the wood scaffold. The impact toughness, as well as some water-related properties of modified wood including the contact angle, water absorption, moisture adsorption, and dimensional stability were investigated in this study. The results showed that the impact toughness of wood significantly decreased after furfurylation. The incorporation of PVA could alleviate the reduction, and this effect was more obvious for highly furfurylated wood. PVA could penetrate and bulk the wood cell wall to a certain degree, but increased concentration of PVA may negatively influence the penetration of FA in wood cell wall. PVA showed only slight influence on water-related properties of FA modified wood, and it depended on both concentrations of PVA and FA. All results indicate that incorporation of PVA in FA system is a promising approach to enhance the toughness of FA modified wood.

Effect of growth rings on fracture toughness of wood

Effect of growth rings on fracture toughness of wood

Micro-tensile behavior of Scots pine sapwood after heat treatments in superheated steam or pressurized hot water

Heat treatments reduce the strength and ductility of wood, but the extent depends on the direction of load and the treatment conditions applied. The tensile behavior of wood is very sensitive to heat treatments, but there is a lack of understanding how this is related to different heat treatment conditions. In this study, we treated homogeneous micro-veneers under different time-, temperature-, and moisture-environments and compared the effect on the tensile behavior of the treated veneers based on their chemical composition changes. The results confirmed the adverse effect of the preferential hemicellulose removal on the strength and toughness of wood. However, chemical composition changes could not fully explain the tensile behavior of dry heat-treated wood, which showed an additional loss in maximum load and work in traction at the same residual hemicellulose content compared to wet heat-treated wood. The scission of cellulose chains as well as the enhanced cross-linking of the cell wall matrix under dry heat conditions and elevated temperatures was discussed as additional factors. The enhanced cross-linking of the cell wall matrix helped in preserving the tensile properties when testing the veneers in water-saturated state, but may have also promoted the formation of cracks that propagated across the cell wall during tensile loading.Graphic abstract

Effect of temperature on the fracture toughness of wood under mode I quasi-static loading

A numerical and experimental study was performed to evaluate the critical strain energy release rate of wood under mode I loading (GIc), in the range of 30 °C and 110 °C. Pinus pinaster Ait. was employed as testing material, using the double cantilever beam (DCB) test, promoting fracture in the RL fracture system. The determination of Resistance-curves was accomplished using an equivalent crack length method, which does not require crack length measurement in the course of the experimental test. This aspect was found crucial since wood develops a non-negligible fracture process zone (FPZ) ahead of the crack tip, and the experiments required the use of a climate chamber with limited dimensions for visual access. It was observed that the critical energy release rate is affected by temperature within the referred range. Cohesive laws were also determined to replicate the experimental response as a function of the tested temperatures.

Predicting layer cracks in cross-laminated timber with evaluations of strategies for suppressing them

Cross Laminated Timber (CLT) is made by laminating three or more timber layers with grain directions in alternate layers at right angles to each other. This structure, with its numerous wood bonds glued in cross-grained directions, develops residual stresses whenever panels are exposed to variations in temperature or moisture content. This paper analyzes how these residual stresses combined with mechanical stresses can cause CLT cracks by deriving an expression for energy released when a new crack forms within one layer. By using finite fracture mechanics concepts, where a crack forms when energy released by that crack exceeds the transverse toughness of wood, this equation can predict layer cracking under many conditions. Two specific examples given are cracking due to residual shrinkage alone and cracking due to uniaxial tension. Finite fracture mechanics and energy release calculations provide useful tools for designing durable CLT products and structures. Various options for improving CLT are considered along with needs for more analysis and experiments. Two simple changes to CLT that could greatly enhance durability are to use thinner layers and to sufficiently dry the timber before panel fabrication.

Simultaneously improving the toughness and stiffness of wood flour/polypropylene composites using elastomer A669/talcum blends

It is important for wood plastic composites to possess satisfactory toughness and stiffness in practical applications, such as construction formworks. In this study, an elastomer‐grafted polyolefin (A669) was introduced to improve the toughness of wood flour/polypropylene (WF/PP) composites, with talcum powder used as the reinforced phase to maintain the flexural strength and stiffness of the WF/PP composites. The impact strength and bending properties of the composites were tested. Differential scanning calorimetry, dynamic mechanical analysis, and scanning electron microscopy were used to analyze this composite system. Test results show that the combination of A669 and talcum powder can provide the WF/PP composites with both good toughness and flexural properties. The optimum additions of A669 and talcum powder were 6% and 9%, respectively. Talcum powder plays an important role in the heterogeneous nucleation, which is helpful for increasing crystallinity and improving flexural properties. However, this function could be offset by the addition of A669. The WF/PP composites with elastomer A669 and talcum powder present more viscosity characteristics and rougher fracture surfaces. In addition, the WF/PP composites containing elastomer A669 and talcum powder have better processability. POLYM. COMPOS., 40:1335–1341, 2019. © 2018 Society of Plastics Engineers

低輻射加熱を受けた木材の燃焼性能および強度性能

The purpose of this study was to obtain the fundamental values which were required to establish the decisions for the fire resistance verification method in case of localized fire scenario. The species of wood samples tested in this study was limited to Sugi (Cryptomeria japonica) controlled at the moisture content of less than 15%. In Chapter 1, the present circumstances of the fire resistance verification method since 2000 when Building Standard Law of Japan had been revised was explained. And the definitions for fire resistive buildings and what kind of values needed for this method were put in order. Then previous studies regarding the ignition, burning and the charring behavior are reviewed and the content of experiments carried out in this study was introduced In Chapter 2, the ignitability tests and cone calorimeter tests (CCM) at the incident heat fluxes of less than 20 kW/m2 from 20 to 30 minutes were conducted. The lowest radiant heat flux to cause ignition within 30 minutes was found to be from 13 to 15 kW/m2 because neither glowing nor flaming was observed at the incident heat fluxes of less than 13 kW/m2. And the linear function, which could lead to the time to ignition (tig), based on the date by plotting 1/√tig against incident heat fluxes was proposed. On the other hand, the temperature, which had been quoted as one of the determinants for ignition typically, was not suitable to the criterion to ignite because even if the temperature reached to 260°C or more on the surface of the test samples, ignition didn't occurred or delayed because of the lack of the amount of enough vaporized fuel to cause ignition below 20 kW/m2. Then even if ignition occurred, each test sample was extinguished automatically at the incident heat fluxes of 15~20 kW/m2. And it was found that the char depth could be obtained by the linear regression that were made through the char depth to the product of the given incident heat fluxes by the difference between the experimental period and the time to ignition. But the darkening was observed on the surface of the samples even if not ignited, and the temperature at the depth of 5mm from the bottom of the char layer was also measured more than 180°C. These phenomena could influence to the strength and the toughness of the wood. Therefore, in Chapter 3, the static bending test and impact test of small pieces of wood exposed to heating at 120°C, 150°C or 180°C for 20 minutes and stored later in the desiccator at 20°C for several days were performed. And the date from the experiments was compared with the date from the test samples not to be exposed to heating in order to investigate the possibility of deterioration of the structural performance of wood by heating. And by the comparison using statistics, it was not seemed that the strength and the toughness of wood would fall. In addition, the effect of the thermal degradation characteristics of wood on the duration of load was considered by the previous study and the date from the tests conducted in this report. And the heating at below 150°C was unlikely to degrade the structural performance of wood. Consequently, we propose the equation for the time to ignition, the equation for the char depth and the effective section size of the beams to check the structural performance as the decision for the fire resistance verification method in case of localized fire scenario for 20 minutes.

Estimative of Wooden Toughness by the Apparent Density and Bending Strength

The wooden toughness is a mechanical property of interest in the project in the case of the design of structures and structural elements subjected to impact loads, however, not an integral part of the mechanical properties commonly investigated, such as the modulus of elasticity and strength modulus in tensile, in bending and others. In order to corroborate with the most knowledge about the toughness of wood, this study aimed to investigate the possibility of estimating the toughness by the apparent density and strength in bending, using linear, quadratic and cubic regression models. Were used 15 species of wood, equally distributed in the five strength classes defined by the Brazilian standard ABNT NBR 7190:1997, enabling greater coverage of the results to estimate the toughness of wood for various species. The results of the regression models showed to be representative all the functions evaluated, presenting the quadratic fits in function of the apparent density the best results.

Study on wood fracture parallel to the grains based on fractal geometry

The fracture toughness (KIC) parallel to the grains of five kinds of wood was tested by compact tension specimen and the profile contour analysis method was employed to measure fractal dimensions Ds of their fracture surfaces. The results show that fracture toughness parallel to the grains of various woods is different because of their textural diversity and such differences are also shown on the morphology of fracture surfaces. Furthermore, the fractal dimension Ds and fracture toughness \({K_{IC}^{TL} }\) parallel to the grains have evident direct proportional relation, and this helps to reveal the inherent relationship between fracture toughness of wood and its microstructure.

The effect of elevated temperature exposure on the fracture toughness of solid wood and structural wood composites

Fracture toughness of wood and wood composites has traditionally been characterized by a stress intensity factor, an initiation strain energy release rate (G init) or a total energy to fracture (G f). These parameters provide incomplete fracture characterization for these materials because the toughness changes as the crack propagates. Thus, for materials such as wood, oriented strand board (OSB), plywood and laminated veneer lumber (LVL), it is essential to characterize the fracture properties during crack propagation by measuring a full crack resistant or R curve. This study used energy methods during crack propagation to measure full R curves and then compared the fracture properties of wood and various wood-based composites such as, OSB, LVL and plywood. The effect of exposure to elevated temperature on fracture properties of these materials was also studied. The steady-state energy release rate (G SS) of wood was lower than that of wood composites such as LVL, plywood and OSB. The resin in wood composites provides them with a higher fracture toughness compared to solid lumber. Depending upon the internal structure of the material, the mode of failure also varied. With exposure to elevated temperatures, G SS for all materials decreased while the failure mode remained the same. The scatter associated with conventional bond strength tests, such as internal bond and bond classification tests, renders any statistical comparison using those tests difficult. In contrast, fracture tests with R curve analysis may provide an improved tool for characterization of bond quality in wood composites.

「２タイプの実験法によるき裂発生評価手法の一提案 構造用集成材木材を対象としたモードI破壊靱性性能の評価方法に関する考察 その３」に対する討論

Experimental achievements on the fracture toughness of wood constituting glulam in the above paper are discussed about the following items: 1) The approach based on fracture mechanics for predicting the fracture strength of wood and wood joint has some serious defects. 2) The description of ductile-like fracture seems inadequate and the J-integral at the maximum load point is doubtful as the fracture toughness for crack initiation, because the tensile softening in the load-displacement curve of wood in 3PB and CT tests is caused by multiple steps of intermittent cracking.

Toughness of pine wood chemically modified with acetic anhydride

This communication addressed the effect of chemical modification with acetic anhydride on toughness of pine wood, a dynamic mechanical property. It was found that acetylation significantly affected the toughness of wood.

Variance of the Mechanics Properties of Populus Ussuriensis Kom Lumber Compressed by Rollers

The mechanics properties of P. ussuriensis Kom lumber compressed by rollers decreases a little, the loss of which in tangential compression is more than that in radial one. The loss of mechanics strength increases with the rise of the compression rate. The Compressive strength parallel to the grain of wood is from 2.391% to－4.589%, the Shearing strength parallel to the grain is from－2.071% to－14.213%, the Tensile strength parallel to the grain is from 2.246% to－11.535%, the Tensile strength perpendicular to the grain is from－0.217% to－16.877%, the Cleavage strength of wood is from 1.785% to－16.235%, the Bending strength of wood is from 4.417% to－27.793%, the Modulus of elasticity in static bending is from 1.045% to－19.455%, and the Toughness of wood is from 3.413% to－28.929%.

THE INFLUENCE OF WIDTH, CRACK LENGTH AND STRENGTH-GRADE TO MODE I FRACTURE TOUGHNESS ON CT TESTS

Splitting of wood loaded perpendicular to the grain is a critical issu in timber engineering, and the splitting and failure of wood at its connection are studied in some articles. But to research the characterization and to evaluate that fracture is very difficult. Now we have no standard method to determine the fracture toughness of wood for timber engineering. So we need some simple or useful methods. On the past study, we conducted CT tests with some woods for structural glulam which width is 47mm. However, we didn't under stand the fracture features which size is large, and the fracture toughness. Then we conducted additional CT tests, used its width and crack length as the parameter. Durning the tests, we checked behaviour of crack on the surfaces as the same in the previours study. As result we understood that fracture of wood starts at maximum load or after that, and the fracture feature differs when we differ the size of specimens. In other words, in the case the width is under 94mm and the dimensionless-crack-length in over 0.5, the fracture feature is in ductile and we can obtain J1C in stable. Next we conducted CT tests with different strength-graded wood which width is 47mm. As the results, the J1C doesn't almost differ as the grading it different.

Mechanical behavior of heat-treated spruce (Picea abies) wood at constant moisture content and ambient humidity

The mechanical behavior of heat-treated spruce wood was investigated in relation to the mass loss that occurs during thermal treatment. At constant wood moisture content, the strength, failure strain and toughness of wood were reduced by the heat-bath treatment, decreasing with increasing mass loss. The stiffness was unaffected up to a mass loss of about 3%, and thereafter it decreased. The mechanical properties, however, are not only dependent on the mass loss but also on the relative humidity in the heating atmosphere. As a function of mass loss, the inelastic ductility and the inelastic toughness were the lowest when wood was heated in a dry climate, as compared to a moist climate.

Influence of the Measurement Methods on the Mode I Fracture Toughness of Wood

In this work, we examined the validity of four methods for measuring the mode I fracture toughness of wood; double cantilever beam (DCB), compact tension (CT), single edge notched tension (SENT), and single edge notched bending (SENB) tests, which have been frequently conducted. In the fracture tests, the crack length was variously changed, and the validity of each method was evaluated by the dependence of fracture toughness on the crack length. From the comparisons of fracture toughnesses obtained by the different four methods, the DCB test was more recommended than any other methods because the fracture toughness could be appropriately obtained in the wide range of crack length.