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.     

Mode I interlaminar fracture property of moso bamboo (Phyllostachys pubescens)

Bamboo is a unidirectional fiber-reinforced bio-composite. Once having cracks, the delaminating propagation is not controlled by the strength but by the interlaminar fracture toughness. In this paper, the behaviors of Mode I (crack opening mode) interlaminar fracture parallel to grain of moso bamboo (Phyllostachys pubescens) were studied. Based on energy theory, the Mode I interlaminar fracture toughness, G _IC, was measured using the double cantilever beam specimens, and the fracture surfaces were examined under scanning electron microscope. The results show that: (1) the interlaminar fracture toughness of Mode I is the basic characteristic of bamboo material. The mean value of G _IC = 358 J/m² (coefficient of variation = 16.88%) represents the resistance arresting crack propagation. No significant difference was found for G _IC among the specimens located at different heights of the bamboo. (2) Due to the low G _IC of bamboo, the crack propagation parallel to grain developed easily. The crack was a self-similar fracture without fiber-bridging. On the fracture surfaces, smooth fibers and plane ground tissue were found at the extended area of Mode I fracture along the longitudinal direction. Under scanning electron microscope, it could be seen that the crack propagation developed along the longitudinal interface between fibers or ground tissue. It indicates that the longitudinal interface strength was weak among bamboo cells.     

Measurement of mode II fracture toughness of wood by the end-notched flexure test

We examined the applicability of end-notched flexure (ENF) tests for measuring the mode II fracture toughness of wood. Western hemlock (Tsuga heterophylla Sarg.) was used for the specimens. The fracture toughness at the beginning of crack propagationG _IIc and that during crack propagationG _IIR were calculated from the loadloading point compliance and load-crack shear displacement (CSD) relations. The obtained results were compared with each other, and the validity of measurement methods were examined. The results are summarized as follows: (1) The value ofG _IIc increased with the increase in initial crack length. When measuringG _IIc by ENF tests, we should be aware of the dependence ofG _IIc on the initial crack length. (2) The value ofG _IIR initially increased with the crack length, and it reached a constant value. (3) Measurement of the CSD is recommended when obtainingG _IIR because the crack length, which has a great influence on theG _IIR calculation, is implicitly included in the CSD. (4) We found that the crack length during its propagation should be evaluated by the final crack length.     

Relationship between crack propagation and the stress intensity factor in cutting parallel to the grain of hinoki (<Emphasis Type="Italic">Chamaecyparis obtusa</Emphasis>)

The method of digital image correlation (DIC) was applied to the digital image of orthogonal cutting parallel to the grain of hinoki, and the strain distribution near the cutting edge was evaluated. The wood fracture associated with chip generation was considered as mode I fracture, and the stress intensity factor K_I for fracture mode I was calculated from the strain distribution according to the theory of linear elastic fracture mechanics for the anisotropic material. The calculated K_I increased prior to crack propagation and decreased just after the crack propagation. The change in K_I before and after crack propagation, ΔK_I, decreased in accordance with the crack propagation length, although the variance in ΔK_I should depend on the relationships between the resolution of DIC method and the dimensions of cellular structure. The calculated K_I in this study was almost on the same order as reported in the literatures. It was also revealed, for the case of chip generation Type 0 or I, the stress intensity factor for fracture mode II could be negligible due to the higher longitudinal elastic properties of wood in the tool feed direction than the one radial ones, and the mode I fracture was dominant.     

Variability of fracture toughness by the crack tip position in an annual ring of coniferous wood

The effects of the location of the crack tip in an annual ring and the direction of crack propagation on the fracture toughness of the TR crack propagation system of coniferous wood (T, direction normal to the notch plane; R, propagation direction) were analyzed by the finite element method in regard of the changes in elastic modulus and strength within an annual ring. The critical point of the fracture was defined as the state where the stress of a square element (0.125 × 0.125 mm) in contact with the crack tip equals the tensile strength. The distribution of specific gravity was measured by soft X-ray densitometry. The elastic moduli in the T and R directions were estimated by the sound velocity. The tensile strengths in the T and R directions were measured by the tensile test using small specimens of l mm span length. Regarding the variability of fracture toughness (K _IC), the experimental and calculated results had the same tendency. Therefore, it was concluded that the variability ofK _IC is caused by the (1) heterogeneity of the elastic modulus and strength within an annual ring; and (2) changes in the degree of stress concentration at the crack tips, according to the direction of crack propagation.Part of this work was presented at the 40th annual meeting of the Japan Wood Research Society, Tsukuba, April 1990 and at the 6th International Conference on Mechanical Behavior of Materials, Kyoto, July 1991     

Evaluation of end-check propagation based on mode I fracture toughness of sugi (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>)

The relation between crack propagation based on fracture mechanics and end-check propagation during drying was evaluated in this study. Corresponding to the direction of end-check propagation, the mode I fracture toughness of air-dried sugi specimens in TR, TL, and intermediate systems was examined by single-edge-notched bending tests. The occurrence and propagation of end checks on sugi (Cryptomeria japonica D. Don) blocks during drying were observed at the scale of the annual rings. It was found that the critical stress intensity factor (K _IC) decreased as the crack propagation changed from TL to TR. The value of K _IC in the TR system was significantly lower than that in the TL system. As a measure of fracture energy, the area under the load-crack opening displacement curve in the TR system was more than twice that in the TL and intermediate systems. These results indicate that cracks perpendicular to the tangential direction initiate radially with ease, and then crack arrest occurs to prevent growing. This finding provides a consistent interpretation of the end-check propagation observed during drying as follows: tiny end checks, as an analog of TR cracks, occur easily and selectively in latewood or transition wood and propagate toward the pith during drying. When there is no corresponding secondary check in the forward latewood, the checks are arrested and do not propagate further.     

Influence of span/depth ratio on the measurement of mode II fracture toughness of wood by end-notched flexure test

The influence of the span/depth ratio when measuring the mode II fracture toughness of wood by endnotched flexure (ENF) tests was examined. Western hemlock (Tsuga heterophylla Sarg.) was used for the specimens. The ENF tests were conducted by varying the span/depth ratios; and the fracture toughness at the beginning of crack propagation G_IIc was calculated by two equations that require the load-deflection compliance or Young's modulus. Additionally, the influence of the span/ depth ratio on the load-deflection compliance was analyzed by Timoshenko's bending theory in which additional deflection caused by the shearing force is taken into account. The following results were obtained: (1) When the span/depth ratio was small, the fracture toughness calculated with the data of load-deflection compliance was large. In contrast, the fracture toughness calculated with the equation containing Young's modulus tended to be constant. (2) In the small span/depth ratio range, the load-deflection compliance was estimated to be larger than that predicted by Timoshenko's bending theory. (3) To obtain the proper fracture toughness of wood with a single load-deflection relation, the span/depth ratio should be larger than that determined in several standards for the simple bending test method of wood, 12:16.     

Characterization of crack propagation in particleboard

Summary In this paper the wedge-splitting-method (Tschegg 1986) for mode I-testing was applied to specimens of particleboard. Specimen geometry and loading-device used for this method allow testing under the condition of steady state crack propagation. Therefore the full load-displacement curves can be recorded. Using the loaddisplacement-curves, different ways of evaluation have been carried out to determine relevant fracture properties like the specific fracture energy G_f and the crack resistance R_c of the investigated material for two different orientations.The authors thank Dr. D. M. Tan for conducting the FE-calculations. Financial support by the Fonds zur Förderung der wissenschaftlichen Forschung, Wien is gratefully acknowledged     

Mechanical performance of linseed oil impregnated pine as correlated to the take-up level

 The mechanical performance of pine sapwood (pinus sylvestris), impregnated with linseed oil to different take-up levels, is evaluated using several test methods. SEM is used to study morphological changes following the impregnation procedure. The reduction of mechanical properties is attributed to a) localized cell wall damage in the ray region that facilitates longitudinal inter-cell split in L-R plane (macrocrack) initiation and propagation; b) submicroscopical cracking in the S1 sublayer that reduces the resistance to Mode I and Mode II inter-cell splitting at any location where the oil front has passed. Mechanical testing shows the following effect of the impregnation on failure a) the Mode I fracture toughness G _Ic in L-T and L-R planes, determined in DCB test, is significantly lowered with no significant difference in fracture resistance reduction in between planes; b) 3-point flexural test for specimen geometry leading to cracking in R-L and T-L planes show that the flexural strength as well as flexural modulus are reduced due to impregnation; c) 3-point flexural tests on longitudinal specimens used to determine the impregnation effect on longitudinal modulus E _L and shear moduli G _LT and G _LR , reveal only minor changes. Fracture surfaces in mechanical tests are analyzed using SEM, and differences are explained by described microdamage mechanisms. Received 10 August 1999     

Cutting force variability as a consequence of exchangeable cleavage fracture and compressive breakdown of wood tissue

In the present study, the conditions of chip propagation or fracture in orthogonal oblique cutting of beech wood (Fagus silvatica) in the 90°–0° direction for a type-I chip has been investigated. The force required for orthogonal wood cutting is pronouncedly variable, which is the consequence of exchangeable different ways of material breakdown. The chip formation process is discontinuous because of interrupted splitting of the material in front of the cutting tool. A 10-mm-thick specimen was cut at a rake angle of 31° and 42° with chip thicknesses ranging from 0.1 to 0.3?mm. The chip segment length increased with the chip thickness. A chip of varying length and thickness was modelled using the finite element method. For each case, the bending or compressive stress in the chip and the stress intensity factor at the crack tip was calculated. The segment length of the chip can be calculated by taking into account the condition that a crack propagates when the stress intensity factor K _I at the crack tip equals the critical stress intensity factor K _IC, and the bending or compressive stress σ _x in the chip is smaller than the strength σ _u . Good agreement between the calculated and the measured values was observed. The chip segment length can change considerably already with small changes in the bending strength and critical stress intensity factor. This large sensitivity is also confirmed by the fluctuation of the measured chip segment lengths by as much as 400%.     

Resistance curve for the mode II fracture toughness of wood obtained by the end-notched flexure test under the constant loading point displacement condition

 The measurement method of mode II fracture toughness-crack propagation length relation (i.e., the resistance curve, or R-curve) was examined by end-notched flexure tests on sitka spruce (Picea sitchensis Carr.). The tests were conducted by varying the span/depth ratios under the constant loading point displacement condition. The fracture toughness was measured from the load-crack shear displacement (CSD) and load-longitudinal strain relations. The crack length was determined by a combination of load-CSD and load-strain compliances and Williams's end correction theory, as well as the observation of crack propagation. When the specimen had an appropriate span/depth ratio, the fracture toughness and crack propagation length were measured from the load-CSD compliance and combined load-CSD and load-strain compliances, respectively, and the R-curve could be determined properly under the constant loading point displacement condition. Received: March 15, 2002 / Accepted: July 25, 2002     

Elastic parameters of poplar wood with end-cracks

? In this study, longitudinal specific modulus of elasticity along the grain (specific MOE_L = MOE_L/ρ) as well as radial and tangential shear moduli (G _LR and (G _LT ) of Populus Deltoides wood were examined in free flexural vibration in a free-free bar method, where end-cracks produced manually in LT plane along and parallel to annual rings in four different sizes.

? The effects of four different crack sizes (0, 6, 12, and 18 cm) on elastic parameters of the bars were examined for their vibration properties based on Timoshenko bar equations, in order to find a procedure to make a confident choice of a clear specimen among the cracked ones, considering three initial modes of vibration.

? Based on research findings, a significant correlation existed between radial and tangential shear moduli of the clear bars as G _LR was approximately 15 percents higher than (G _LT . After making the shortest crack sizes, however, this correlation entirely faded. Statistically for 6 cm crack, decreases in specific MOE _L for measurement on the tangential impact were not significant though they were for longer cracks.

? It was also revealed that in specimen under the study if longitudinal specific modulus of elasticity from both LR and LT flexural vibrations were almost equal and G _LR was slightly larger than (G _LT , the user could be confident enough to consider the specimen without severe longitudinal cracks.

     

Influence of intermediate deformation rates in softwoods characterized with fracture resistance R-curves

Fracture resistance curves of Norway spruce were evaluated over a range of crosshead speeds between 0.05 and 200 mm/min using compact tension specimens. The fracture resistance curves were determined via J-integral K _R versus Δa (crack extension) using hybrid experimental–finite element method with stepwise quasi-static and transient dynamic analysis. Coupling experiments and high-speed camera with a rate of 0.5–135 frames/s capture enabled determination of crack kinetics and velocities. Digital image correlation method was employed to evaluate strain distribution maps of the fracture process zone. Deformation measurements suggested strong strain localization and significant influence of the loading rate. The most distinct strain map was evaluated for the loading rate of 200 mm/min, suggesting inertial effects. Rising fracture resistance develops for smaller crack lengths (0.15–0.20 a/w) for all loading rates, with an increase of up to 20–50% compared to initiation values. Fracture resistance R-curves were evaluated for single individual specimens as well as for averaged load–deformation responses, which hide strong localisations in single responses. Influence of deformation rate on the shape and magnitude of fracture resistance curves is significant. Inertial effects in transient dynamic response give rise to twofold higher fracture resistance of softwoods compared to quasi-static resistance.     

New splitting method for wood fracture characterization

Summary A testing procedure with a new and simple specimen shape is presented which is appropriate to characterize fracturing of inhomogeneous and complex materials like wood. With this, the fracture energy of spruce wood is determined in the TL and RL direction. The size effect, i.e. influences of specimen dimensions on K_IC and G_f (specific fracture energy) are investigated. Stress and deformation distribution in the newly developed specimens are analysed with FE methods. The measured load-displacement curves are approximated by bilinear softening diagrams and FE analysis. Based on these results, it is tried to interpret typical deviations from LEFM's behaviour by mechanisms like microcracking, crack branching or crack tip bridging.The authors thank Dr. A. Teischinger for supplying the testing material and Dipl. Ing M. Elser for preparation of the diagrams. Financial support of the Fonds zur Förderung der wissenschaftlichen Forschung, Wien is gratefully acknowledged.     

Investigation of wood properties that influence the final moisture content of air-dried sugi (Cryptomeria japonica) using principal component regression analysis

Sugi (Cryptomeria japonica D. Don) lumber is known to have a large variability in final moisture content (MC_f) and is difficult to dry. This study investigated the variability in MC_f of sugi in relation to wood properties. The wood property variables included initial moisture content (MC_i), basic density (BD), annual ring orientation (ARO), annual ring width (ARW), heartwood ratio (HR) and CIE L ^* color (L ^*). Sugi samples were cut from flat-sawn lumbers and air-dried; a principal component regression (PCR) model for predicting MC_f was developed with the wood property variables. The wood properties that contributed to the prediction of MC_f were evaluated by PCR analysis. Significant positive regression coefficients of the PCR model were observed in the MC_i, BD, ARO and HR, whereas negative ones in the L ^*. There were no significant regression coefficients in the ARW. These results suggest that the MC_i, BD, ARO and HR had a positive influence, the L ^* had a negative influence, and the ARW had little influence on the MC_f of air-dried sugi wood. This finding is in line with the general view on the drying characteristics in relation to these wood properties.     

Change in fracturing and colouring of solid spruce and ash wood after thermal modification

Effects of different thermal treatments (maximum treatment temperatures of 200, 210 and 220°C for 2.5 hours) on solid spruce (Picea abies L. Karst.) and ash (Fraxinus excelsior L.) were investigated in this study. The fracture behaviour in radial/longitudinal as well as in tangential/longitudinal and the change of the wood colour (CIEL*a*b* colour space) on all principal anatomical surfaces (cross sectional, radial and tangential) were analysed. The specific fracture energy and the maximum breaking load decreased almost significantly after all thermally treated samples in comparison to the untreated (standard dried) sample. The wood colour changed also significantly, in particular the lightness decreased with increasing intensity of thermal treatment on all investigated surfaces. A comparison of the percentage loss of the different fracture and colour values has shown a strong correlation between the maximum breaking load and the lightness after several thermal treatments, in both investigated crack propagation systems, on all measured surfaces and for both analysed species.     

基于竞争势的林分生长量分配模型研究

以湖南省攸县黄丰桥林场杉木林分2008年至2011年每木检尺调查结果为数据源,开展对开阔比数模型fo,简单竞争因子模型fH和竞争压力指数模型fCSI的研究,提出一个新的林木综合竞争因子即竞争势模型fC=1/(fo+fH+fCSI),计算结果经过残差分析方法剔除异常,以竞争势为依据将林分总体生长量分配到林木个体,构建林分生长量分配模型.为了验证模型的准确性,对比分别以fC、fH、fCSI三个模型为依据的林分生长量分配结果,计算通过模型分配到个体的生长量与个体实际生长量之间的相关指数分别为0.6,0.44,0.29.分析结果表明竞争势模型可以作为林分生长可视化模拟中林分生长量向个体分配的一个依据,为林分生长可视化模拟提供有效支持.     

Prediction of the load carrying capacity of bolted timber joints

Failure of bolted timber joints is analyzed experimentally and numerically. In this study, the prediction of the load-carrying capacity of dowel-type joints with one dowel under static loading is based on the analysis of fracture in wood contrarily to most engineering methods that are based on the yield theory. Mechanical joints consist of glued laminated spruce members and steel dowels. In the different analyzed tests, the bolt loads the wood parallel or perpendicular to the grain. The wood member thickness is chosen sufficiently thin to avoid the fastener from presenting plastic hinges. The influences of different structural parameters such as the dowel diameter, the edge- and end-distances are investigated. The fracture propagation analysis is carried out with the Finite Element (FE) method in the framework of Linear Elastic Fracture Mechanics (LEFM). The only identified parameter is the critical energy release rate in mode I (G_Ic). The comparison between experimental and numerical results shows that the fracture must be considered for a correct prediction of the ultimate load and that LEFM can help to improve design codes. Received 11 August 1997     

Influence of woody and herbaceous vegetation control on leaf gas exchange,water status,and nutrient relations of eastern white pine (Pinus strobus L.) seedlings planted in a central Ontario clearcut

The influence of herbaceous and woody vegetation control, either singly or in combination, on leaf gas exchange, water status, and nutrient relations of planted eastern white pine (Pinus strobus L.) seedlings was examined in a central Ontario clearcut over four consecutive growing seasons (GSs). Net carbon assimilation (A_n), leaf conductance to water vapour (G_wv), water use efficiency (WUE), and midday leaf water potential (ψ_m) were measured periodically during the second to fourth GSs of vegetation control treatments, while leaf nutrient relations were examined in GS five. Leaf A_n and G_wv were reduced (p ≤ 0.05) in the presence of herbaceous vegetation in GS two, by both herbaceous and woody vegetation in GS three, and only by woody vegetation (largely trembling aspen (Populus tremuloides Michx.)) in GS four. Leaf WUE was increased (p ≤ 0.05) in all three GSs in which herbaceous vegetation control was applied and where woody vegetation provided partial shading of planted white pine. Leaf water status was comparatively less responsive to vegetation control treatments, but leaf ψ_m was increased (p ≤ 0.05) in the presence of woody vegetation in GSs two and four, likely due to shading and reduced atmospheric evaporative demand of the white pine seedling environment. Within a given GS, the effects of vegetation control on A_n, G_wv, and ψ_m were strongly linked to treatment-induced changes in total vegetative cover, and light and soil moisture availability. Seedling height, diameter, and volume growth rates were positively correlated with A_n and WUE in GSs two and three, but less so in GS four. Vector analysis suggested that herbaceous competition induced foliar N, P, and K deficiencies in five-year-old white pine seedlings while competition from aspen resulted in foliar Ca deficiency.     

Surface characterization of a series of cured polyurethane adhesives by inverse gas chromatography and adhesion strength testing of plywood

Inverse gas chromatography (IGC) was used to determine the surface properties of a series of polyurethane adhesives, A ₁, A ₂, A ₃ and A ₄ cured with water. The weight percentages of isocyanate group (NCO) in polyurethane adhesives were 5.3, 7.0, 13.0 and 19.6%, respectively. Four n-alkanes, C₆, C₇, C₈ and C₉ were chosen as non-polar probes to characterize the dispersive component of surface free energy, $ \gamma_{s}^{d} . $ Acetone, tetrahydrofuran and ethyl acetate were chosen as polar probes to detect the Lewis acid–base parameters, K _a and K _b. The trend of $ \gamma_{s}^{d} $ and K _a and K _b of the series of cured adhesives was also shown in this paper. The cured adhesives were all amphoteric, but predominantly Lewis basic. The adhesion strength of the poplar plywood bonded with polyurethane adhesives was tested. The results showed that the adhesion strength of plywood increased with increasing the NCO content of the adhesives and K _b/K _a of the cured adhesives.