Measurement of the shear modulus of wood by asymmetric four-point bending tests

We conducted asymmetric four-point bending tests of wood and obtained the shear moduli on the basis of Timoshenko's theory of bending. Akamatsu (Japanese red pine,Pinus densiflora D. Don) and shioji (Japanese ash,Fraxinus spaethiana Lingelsh.) were used for the tests. Asymmetric four-point bending tests were undertaken by varying the depth/span ratios; and Young's modulus and the shear modulus were calculated by Timoshenko's bending theory. Independent of the asymmetric bending tests, we also conducted three-point bending tests, free-freeflexural vibration tests, and numerical calculations by the finite element method. Young's and shear moduli obtained by these methods were compared with those derived from the asymmetric bending tests. Based on these comparisons, we concluded that the shear modulus can be properly obtained by the asymmetric four-point bending tests when the span is 20 times larger than the depth.     

Measurement of the shear modulus of wood by static bending tests

When measuring the shear modulus of wood by static bending tests, the basic theory is dependent on Timoshenko's bending theory. The shear modulus obtained by static bending is a much smaller value than that derived by other methods. We examined the applicability of Timoshenko's theory and propose an empirical equation that can derive the shear modulus properly. Three softwoods and three hardwoods were used for the tests. First, the Young's modulus and shear modulus were measured by free-free flexural vibration tests. Then the three-point static bending tests were undertaken, varying the depth/span ratios. Additionally, the bending tests were simulated by the finite element method (FEM). The shear moduli obtained by these methods were then compared. The deflection behaviors in static bending were not expressed by the original Timoshenko bending theory because of the stress distortion near the loading point. Based on the experimental results and numerical calculations, we modified the original Timoshenko bending equation. When using our modified equation the stress concentration must be carefully taken into account.     

Influence of loading point on the static bending test of wood

We conducted three-point bending tests by changing the condition at the loading point and then examined the influence of the loading point on the test data. Yellow poplar (Liriodendron tulipfera L.) was used for the tests. First, using loading noses with various radii, static bending tests were conducted by varying the depth/span ratios. Deflections were measured from the displacement of the cross head and at the point against the loading nose: Young's and shear moduli were obtained from the modified Timoshenko's bending equation proposed in a previous paper. Then a similar testing procedure was undertaken by inserting cushion sheets of Teflon between the specimen and the nose. After the measuring these moduli, bending strengths were measured using the loading noses and cushion sheets. The following results were obtained: (1) When the deflection was measured from the displacement of the cross head, the radius of the loading nose had an influence on the additional deflection when the depth/span ratio was high, causing the dependence of the shear modulus on the radius. In contrast, the radius had little influence on the measurement of Young's modulus. By placing cushion sheets between the nose and the specimen, the effect of the radius was moderated. When the deflection was measured at the point against the loading nose, the radius of the nose had little influence on the additional deflection; hence the loading nose had little influence when obtaining Young's and shear moduli. This tendency was commonly observed regardless of whether the cushion sheets were in place. (2) When the specimen had a high depth/span ratio, the bending strength increased with the increase in the radius of the loading nose. However, the influence of the radius was small when the specimen had a low depth/span ratio. There was no significant effect of the cushion sheets used here on the measurement of bending strength.Part of this paper was presented at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998     

Bending strength and toughness of heat-treated wood

The load-deflection curve for static bending and the force-time curve for impact bending of heat-treated wood were examined in detail. The effect of oxygen in air was also investigated. Sitka spruce (Picea sitchensis Carr.) was heated for 0.5–16.0h at a temperature of 160°C in nitrogen gas or air. The dynamic Young's modulus was measured by the free-free flexural vibration test, the static Young's modulus and work needed for rupture by the static bending test, and the absorbed energy in impact bending by the impact bending test. The results obtained were as follows: (1) The static Young's modulus increased at the initial stage of the heat treatment and decreased later. It decreased more in air than in nitrogen. (2) The bending strength increased at the initial stage of the heat treatment and decreased later. It decreased more in air than in nitrogen. (3) The work needed for rupture decreased steadily as the heating time increased. It decreased more in nitrogen than in air. It is thought that heat-treated wood was more brittle than untreated wood in the static bending test because W₁₂ was reduced by the heat treatment. This means that the main factors contributing to the reduction of the work needed for rupture were viscosity and plasticity, not elasticity. (4) The absorbed energy in impact bending increased at the initial stage of the heat treatment and decreased later. It decreased more in air than in nitrogen. It was concluded that heat-treated wood became more brittle in the impact bending test becauseI ₁₂ andI ₂₃ were reduced by the heat treatment.     

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     

Bending and shear properties of compressed Sitka spruce

We examined the bending and shear properties of compressed wood using small and clear specimens of Sitka spruce (Picea sitchensis Carr.). For measuring the bending properties, three-point bending tests were conducted under the span/depth ratio of 14, which is standardized in the American Society for Testing and Materials [ASTM D143-94 (2005a)] and Japanese Industrial Standards [JIS Z2101-94 (1994)]. In the bending test, the load, deflection at the midspan, and strain at the bottom of the midspan were simultaneously measured, and Young’s modulus and bending strength were obtained by elementary beam theory. For obtaining the shear modulus and shear strength, asymmetric four-point bending tests were conducted using the specimens with rectangular and side-grooved cross sections, respectively, and the influence of the compression ratio on the shear properties was examined. The results are summarized as follows: (1) Young’s modulus increased with increasing compression ratio when it was determined by the load–strain relation. Nevertheless, this tendency was rather obscured when Young’s modulus was determined by the load–deflection relation. Hence, it is preferable that Young’s modulus is measured from the load–strain relation. (2) The shear modulus in the longitudinal–tangential plane was maximum at the compression ratio of 50%, whereas that in the longitudinal–radial plane was minimum at the compression ratio of 50%. (3) The influence of the compression on the bending and shear strength ratio was not significant.     

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.     

Measurement of bending properties of wood by compression bending tests

We measured Young's modulus, proportional limit stress, and bending strength by the compression bending test and examined the applicability of the testing method by comparing it with conventional bending test methods. Long columns of todomatsu (Japanese fir,Abies sachalinensis Fr. Schmidt) with various length/thickness ratios were the specimens. A compressive load was axially applied to the specimen supported with pin ends. Young's modulus, the proportional limit stress, and the bending strength were obtained from the load-loading point displacement and load-strains at the outer surfaces until the occurrence of bending failure. Four-point bending tests were also conducted, and the bending properties obtained were compared with the corresponding properties obtained by the compression bending tests. Based on the experimental results, we believe that when the stress-strain relation is measured by the load-loading point displacement relation using specimens whose length/thickness ratio is large enough, the bending properties can be obtained properly using the compression bending test.     

Cell wall thickness and tangential Young's modulus in coniferous early wood

To investigate the effect of wall thickening around cell corners on the tangential Young's modulus of coniferous early wood, tapered beam cell models in which the variation of the cell wall thickness in the axial direction was taken into account were constructed for seven species. Their tangential Young's moduli were compared with the experimental results. The calculated Young's moduli of tapered beam cell models were larger than those of the models composed of the cell walls with uniform thickness, although both models showed almost the same density. For some species the calculated Young's moduli of the models in which the cell wall thickness increased curvilinearly in the axial direction were much closer to the experimental values. The reduction of the radial cell wall deflection due to the increase of the stiffness around cell corners was considered to increase the tangential Young's modulus of a wood cell.This report was presented at the 49th annual meeting of the Japan Wood Research Society, Tokyo, April 1999     

Twelve Elastic Constants of Betula platyphylla Suk.

Wood elastic constants are needed to describe the elastic behaviors of wood and be taken as an important design parameter for wood-based composite materials and structural materials. This paper clarified the relationships between compliance coefficients and engineering elastic constants combined with orthotropic properties of wood, and twelve elastic constants of Betula platyphylla Suk. were measured by electrical strain gauges. Spreading the adhesive quantity cannot be excessive or too little when the strain flakes were glued. If excessive, the glue layer was too thick which would influence the strain flakes?performance, and if too little, glues plastered were not firm, which could not accurately transmit the strain. Wood as an orthotropic material, its modulus of elasticity and poissons ratios are related by two formulas:μij /Ei =μji /Ej and μij < (Ei /Ej)1/2. The results showed that the elastic constants of Betula platyphylla Suk. measured by electrical strain gauges were accurate and reliable. The results of shear elastic modulus GTL and GLR show a high linear regression correlation coefficient (> 0.95) between the reciprocal of elastic modulus MOE-1 and the square of the ratio of depth to length (h/l)2, which indicate that shear modulus values measured were reliable by three point bending experiment.     

Shear strengths of wood measured by various short beam shear test methods

We conducted three types of short beam shear tests of western hemlock (Tsuga heterophylla Sarg.) under various span/depth ratios, and examined whether the maximum shear stress was used as the shear strength. The following results were obtained. (1) In the short beam shear tests under the three-point loading method, it was difficult to have the specimen failing by horizontal shear. We thought that this method should not be recommended for determining the shear strength of wood. (2) In the short beam shear tests under the asymmetric four-point loading of the specimen with a rectangular cross-section, the failure caused by horizontal shear occurred under some span/depth ratio range. Nevertheless, this range was dependent on the specimen geometry and was quite restricted. We therefore think that this method should not be recommended for determining the shear strength of wood. (3) In the short beam shear tests under the asymmetric four-point loading of the I-shaped specimen, failure caused by horizontal shear occurred under the span/depth ratio range wider than that applicable for the asymmetric four-point loading of the specimen with a rectangular cross-section. The maximum shear stress was stable in a certain span/depth ratio range and the value of the maximum shear stress is effective as a parameter for comparing the shearing strength of materials with each other.     

Determination of Young's modulus for spruce, fir and isotropic materials by the resonance flexure method with comparisons to static flexure and other dynamic methods

Summary Dynamic methods provide rapid and accurate means to determine Young's modulus, i.e. the modulus of elasticity, of wood. For dry, clear specimens of épicéa commun (Norway spruce, picea excelsa) and sapin pictiné (silver fir, abies amabilis) we present a comparison of results from tests by a resonance flexure method with results obtained from four-point static flexure tests. For a wide range of specimen size the resonance flexure method provides a simpler, more rapidly performed alternative to the classical static flexure method, giving Young's modulus values which are for the spruce and fir specimens of this study, nearly identical to those calculated from the static flexure tests. Results are also presented which show that a resonance longitudinal method yields higher values of Young's modulus and an ultrasonic method yields still higher values. We provide also a comparison of the four test methods applied to isotropic materials.The authors wish to thank Pierre Michel and André Perrin for preparing the test specimens and components of the test apparatus for this study     

不同制备工艺对木纤维/聚乳酸复合材料性能影响

采用双螺旋挤出成型、模压成型和注塑成型3种不同成型工艺制备木纤维/聚乳酸复合材料,通过对比不同制备方法对复合材料密度、静曲强度、弯曲模量、拉伸强度和冲击韧性的影响可知:使用挤出成型方法制备的木纤维/聚乳酸复合材料的密度最大,各项物理力学性能也显著高于使用注塑法和模压成型制备的复合材料试件。     

Plywood properties influenced by the glue line

Summary The effect of the number and the location of glue lines in the cross section on the bending properties has been studied on plywood bonded with tannin-formaldehyde adhesive. The thickness of glue line has also been investigated. By comparing the theoretical equations with the empirical ones, we can get the thickness of the glue line of wattle tannin adhesive plywood to be 0.15 mm. As for Young's modulus in bending of the glue line in the plywood, it is calculated to be 219 800 kg/cm² and is larger than that of the glue line of phenolic resin bonded plywood. In relation to the veneer, Young's modulus of the veneer under the condition of plywood assembly seems to be slightly larger than that of the veneer under the free condition.The author wishes to express his gratefulness to CSIRO of Australia for a Fellowship during the tenure of which this research was carried out. The direction and cooperation of Mr. K. F. Plomley, Dr. P. Grossman and Mr. P. Collins, CSIRO of Australia are gratefully acknowledged     

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     

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.     

FGRG: Fibreglass reinforced gluelam — A new composite

Beams of a composite made from wood and fibreglass layers glued with a phenolic adhesive were tested in three point bending. Experimental results indicate an increase in mechanical properties in comparison with equivalent beams without fibreglass. An interesting modification is obtained in the mode of fracture. Values of the modulus of elasticity for the adhesive, fibreglass and wood were obtained in separate loading tests and used to derive the modulus of the composite using a general relationship for layered systems. Good agreement was found between theoretical and experimental values of modulus.     

A comparison of static bending,compression and tension parallel to grain and toughness properties of compression wood and normal wood of a giant sequoia

Summary Compression wood (CW) of the giant sequoia studied had higher values than normal wood (NW) in crushing strength and ultimate stress in tension parallel to grain, in toughness, in modulus of rupture, and in work to maximum load and total work in static bending. In the green condition CW had higher values than NW in stress at the proportional limit and work to the proportional limit, and about the same modulus of elasticity in static bending. In the dry condition CW was about equivalent to NW in work to the proportional limit, but was slightly weaker in stress at proportional limit and modulus of elasticity in static bending. The compression wood of this giant sequoia, even though formed when the tree was suppressed and having relatively narrow rings, can therefore be said to be essentially equivalent to normal wood so far as the mechanical properties tested in this study are concerned.Given at FPRS meeting in Dallas, Texas, June 1972     

Twelve elastic constants of Betula platyphylla Suk

Wood elastic constants are needed to describe the elastic behaviors of wood and be taken as an important design parameter for wood-based composite materials and structural materials. This paper clarified the relationships between compliance coefficients and engineering elastic constants combined with orthotropic properties of wood, and twelve elastic constants of Betula platyphylla Suk. were measured by electrical strain gauges. Spreading the adhesive quantity cannot be excessive or too little when the strain flakes were glued. If excessive, the glue layer was too thick which would influence the strain flakes’ performance, and if too little, glues plastered were not firm, which could not accurately transmit the strain. Wood as an orthotropic material, its modulus of elasticity and poisson’s ratios are related by two formulas: μ ij/Ei=μji/Ej and μ ij<(Ei/Ej)1/2. The results showed that the elastic constants of Betula platyphylla Suk. measured by electrical strain gauges were accurate and reliable. The results of shear elastic modulus G _TL and G _LR show a high linear regression correlation coefficient (>0.95) between the reciprocal of elastic modulus MOE ⁻¹ and the square of the ratio of depth to length (h/l)², which indicate that shear modulus values measured were reliable by three point bending experiment.     

聚氨酯预聚体改性聚乳酸/木粉复合材料的制备及表征

通过共混挤出法制备聚氨酯预聚体(PUP)改性的聚乳酸/木粉(PLA/WF)复合材料,并对复合材料进行力学性能测试、动态热机械分析、接触角测量以及断面扫描电镜分析。力学性能分析表明:当PUP用量(以PLA和WF的质量计)为20%时,复合材料断裂弯曲应变和冲击强度分别为5.78%和18.3 k J/m2,较未改性的复合材料分别提高了209%和123%,PUP显示出较好的增韧效果。动态热机械分析表明:随着PUP用量的增加,复合材料中PUP相和PLA相的玻璃化转变温度均有所下降,并且储能模量显著降低,材料韧性得到改善。PUP的加入可显著提高复合材料对水的接触角,材料疏水性能得到改善。当PUP用量为25%时,接触角达83.7°,较未增韧复合材料接触角(66.6°)提高25.7%。拉伸断面的扫描电镜分析表明:添加PUP的复合材料断面有更多的木粉被拉出且空穴变多,断面更为不平整,呈现韧性断裂的特征。