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Elastic properties of compressed spruce with respect to its cross section obtained under various compression ratios.

Abstract

We examined the elastic properties of Sitka spruce (Picea sitchensis Carr.) compressed wood with respect to its cross section. Using specimens with various compression ratios, tension tests were conducted for measuring Young's modulus in the radial and tangential directions and Poisson's ratio on the cross section, whereas asymmetric four-point bending tests were conducted for measuring the shear modulus on the cross section. The results are summarized as follows: (1) Young's modulus in the radial direction decreased with increasing the compression ratio, whereas that in the tangential direction showed the converse tendency. (2) Poisson's ratio decreased with increasing the compression ratio. (3) The shear modulus was maximum at the compression ratio of 60 percent.

Recently, the technique for fabricating compressed wood has dramatically advanced, and several products using compressed wood, such as flooring and handrails, have been developed, When considering the development of these products, it is important to characterize the mechanical properties of compressed wood properly as well as those of solid wood, and several research studies have been conducted (Asaba and Nishimura 2000; Haller and Wehsener 2004; Kubojima et al. 2004a, 2004b; Ohtani et al. 2005; Tsunematsu and Yoshihara 2006; Yoshihara and Tsunematsu 2007). In these studies, the mechanical properties concerning the longitudinal axis such as Young's modulus in the longitudinal direction, and the shear moduli and Poisson's ratios on the radial and tangential sections were examined, whereas the properties about the radial and tangential axes such as Young's modulus in the radial and tangential directions and the shear modulus and Poisson's ratio on the cross section were not examined. As pressure is laterally applied in the process of fabricating compressed wood, the mechanical properties in the lateral directions are influenced by the pressure. To develop a technique for fabricating compressed wood, it is important to know the change in mechanical properties in the lateral directions. In this study, we measured Young's modulus in the radial and tangential directions, and the shear modulus and Poisson's ratio in the cross section of compressed wood with various compression ratios, and examined the influence of compression ratio.

Experimental procedure

Sitka spruce (Picea sitchensis Cart.) lumber, with a density of 0.48 g/[cm.sup.3] at 12 percent MC and eight or nine annual rings contained in the radial length of 10 mm, was used for the tests. The annual rings were flat enough to ignore their curvature. This lumber had no defects such as knots and grain distortions so that the specimens cut from it could be regarded as "small and clear." The lumber was stored for about 1 year at a constant 20[degrees]C and 65 percent relative humidity before the test, and was confirmed to be in air-dried condition. The equilibrium MC condition was approximately 12 percent. All of the specimens were prepared from this lumber.

Compressed wood was fabricated by applying a compression load in an airtight atmosphere. Plates with the thickness of 10, 15, 20, 25, and 30 mm (radial direction), width of 150 mm (tangential direction), and length of 500 mm (longitudinal direction) were cut from the lumber mentioned above, and the compressed wood was fabricated with the plates except those with the thickness of 10 mm, which were used as the control condition. When a wood plate is deformed in high-temperature steam, recovery from deformation can be effectively prevented, so the plate is often soaked in water for circulating the steam prior to the compression process (Inoue et al. 1990, 1993; Yi et al. 2004; Tsunematsu and Yoshihara 2006; Yoshihara and Tsunematsu 2007). When fabricating surface-compressed wood, several grooves were cut on the surface of the plate to circulate the steam effectively (Inoue et al. 1990). This method, however, cannot be used for fabricating wholly-compressed wood. Therefore, instead of cutting grooves, holes with the diameter of 1 mm were cut on the tangential surface of the plate at an interval of 20 mm. There was a concern that strength properties would deteriorate when the diameter of the hole was too large, so the hole diameter was determined by trial and error. In the control condition (thickness = 10 mm), plates without holes were also prepared for examining the influence of holes. After the plates were soaked in water at 20[degrees]C for 2 days, they were set in an airtight device, which was similar to those developed by Inoue et al. (Inoue et al. 1993; Yi and et al. 2004; Tsunematsu and Yoshihara 2006; Yoshihara and Tsunematsu 2007). Figure 1 shows the diagram of the airtight device used in this experiment. The compression load was applied to the tangential of the plate at the temperature of 180[degrees]C for 10 minutes, and then the steam was exhausted through the leak valve at a similar temperature with applying the same compression load. After the period for exhausting the steam, which was about 1 hour, the load was released and the plate was cooled in the room at a constant 20[degrees]C and 65 percent relative humidity until the specimen was confirmed to be in air-dried condition. The compression ratio was controlled with a spacer so that the thickness of the specimen, which corresponded to the radial direction, was 10 mm. The compression ratio Cr can be represented as follows:

Cr = [t.sub.0] - [t.sub.c]/[t.sub.0] x 100(%) [1]

where [t.sub.0] and [t.sub.c] are the lengths in the radial direction before and after the compression treatment, respectively. As described above, [t.sub.c] varied from 10, 15, 20, 25, to 30 mm, whereas [t.sub.0] was fixed as 10 mm, so the value of Cr in this experiment varied from 0 (control condition), 33, 50, 60, to 67 percent. After the procedure mentioned above, the compressed lumber was conditioned at 20[degrees]C and 65 percent relative humidity. From this lumber, cubic specimens with side length of 10 mm were cut and used for the tension and asymmetric four-point bending tests as described below.

[FIGURE 1 OMITTED]

Tension tests were conducted for obtaining Young's modulus in the radial and tangential directions and Poisson's ratio on the cross section. Figure 2(a) shows the tension test specimen. Two sticks made of birch were oppositely bonded by epoxy resin on the radial or tangential sections of the cubic specimen. To measure the strains in the radial and tangential directions during loading, biaxial strain gages (gage length = 2 mm; FCA-2- 11, Tokyo Sokki Co., Tokyo) were bonded on both cross sections by cyano-acrylate adhesive. When bonding a gage on the cross section, there is a concern that the adhesive is absorbed in the specimen. Nevertheless, it was confirmed that the strain gage was bonded tightly on the specimen without any slippage. The portions of birch were gripped and tension load was applied to the specimen at the crosshead speed of 1 mm/min. The load P and strains in the loading and transverse directions, [epsilon] and [epsilon]', respectively, were measured. Young's modulus in the radial and tangential directions, [E.sub.R] and [E.sub.T], respectively, was determined from the initial inclination of the P-[epsilon] relation, whereas Poisson's ratio on the cross section [v.sub.RT] was determined from the initial inclination of the [epsilon]-[epsilon]' relation.

[FIGURE 2 OMITTED]

Asymmetric four-point bending tests were conducted for obtaining the shear modulus on the cross section of compressed wood [G.sub.RT]. Figure 2(b) shows the testing diagram of the asymmetric four-point bending test. In the same manner as in the tension test, two sticks of birch were bonded by epoxy resin on the radial section of the cubic specimen. To measure the shear strains on the cross sections, biaxial strain gages, which were similar to those used in the tension tests, were bonded on both cross sections, but the gage axes were inclined at [+ or -]45[degrees] with respect to the length direction of the specimen. The total span, which corresponded to the distance between the outer support and outer loading point, was 150 mm. The specimen was eccentrically supported at two trisected points so that the gages were at the side surface of the midspan, and the loads were applied at the remaining two trisected points at a crosshead speed of 2 mm/min. According to elementary beam theory, the shear stress is distributed parabolically along the depth direction, and the shearing force is maximum in the middle section of the specimen by this loading method. The shear stress at the neutral axis [[tau].sub.RT], which is maximum in the depth direction, is obtained as (Yoshihara and Suzuki 2005):

[[tau].sub.RT] = 3P/4bh [2]

where P is the applied load, and b and h are the width and depth of the specimen, respectively. The shear strain [[gamma].sub.RT] is derived from the following relation:

[gamma].sub.RT] = [[epsilon].sub.+45] - [[epsilon].sub.-45] [3]

where [[epsilon].sub.+45 ]and [epsilon].sub.-45] are the normal strains in the 45[degrees] and -45[degrees] inclined directions, respectively, with respect to the length direction. The shear modulus [G.sub.RT] was determined from the linear portion of the [[tau].sub.RT]-[[gamma].sub.RT] relation as:

[G.sub.RT] = [DELTA][[tau].sub.RT]/[DELTA][[gamma].sub.RT] [4]

where [DELTA][[tau].sub.RT] and [DELTA][[gamma].sub.RT] are the stress and strain increments in the linear portion of [[tau].sub.RT]-[[gamma].sub.RT] relation.

Results and discussion

Figure 3 shows the dependence of Young's modulus in the radial and tangential directions, [E.sub.T] and [E.sub.R], respectively, and Poisson's ratio and shear modulus on the cross section, [v.sub.RT] and [G.sub.RT], respectively, on the compression ratio Cr. The value of [E.sub.R] decreases with the increase of compression ratio, whereas that of [E.sub.T] increases with the increase of compression ratio. As shown in the previous papers (Tsunematsu and Yoshihara 2006; Yoshihara and Tsunematsu 2007), the fractures along the fiber direction were distributed around the midthickness of radial plane, so the decrease of [E.sub.R] is thought to be because of the fractures. In the tangential plane, however, significant fractures were not found, so the increase of [E.sub.T] is thought to be because of the densification. Poisson's ratio [v.sub.RT] decreases with the increase of compression ratio. The decrease of Poisson's ratio is thought to be because of the reason similar to the decrease of [E.sub.R]. It was thought that the shear modulus [G.sub.RT] would also decrease because of these fractures. Nevertheless, the value of [G.sub.RT] shows the maximum when the compression ratio reaches 60 percent. This trend cannot be described by the induction of fractures. Although further studies should be conducted to clarify this trend, it is thought that the high densification and fracture induction influenced the trend of shear modulus with compression ratio in a complex manner. These results indicate that the compression processing has a marked influence on the elastic properties about the cross section of wood, and that the change in these properties should be taken into account when fabricating compressed wood.

[FIGURE 3 OMITTED]

Conclusion

We conducted tension and asymmetric four-point bending tests of compressed wood of Sitka spruce (Picea sitchensis Carr.) with various compression ratios, and obtained Young's modulus in the radial and tangential directions, and shear modulus and Poisson's ratio on the cross section corresponding to the compression ratios. The results are summarized as follows:

1. Young's modulus in the radial direction decreased with increasing the compression ratio, whereas that in the tangential direction showed the converse tendency.

2. Poisson's ratio on the cross section decreased with increasing the compression ratio.

3. The shear modulus on the cross section was maximum at the compression ratio of 60 percent.

Literature cited

Asaba, M. and H. Nishimura. 2000. Effect of manufacturing conditions on bending strength of compressed wood. Trans. Jpn. Soe. Mech. Eng., A 67:83-88.

Haller, P. and J. Wehsener. 2004. Mechanical properties of densified spruce. Holz als Roh- und Werkstoff 62:452-454.

Inoue, M., M. Norimoto, Y. Otsuka, and T. Yamada. 1990. Surface compression of coniferous wood lumber 1. A new technique to compress the surface layer. Mokuzai Gakkaishi 36:969-975.

--, --, M. Tanahashi, and R.M. Rowell. 1993. Steam or heat fixation of compressed wood. Wood and Fiber Sci. 25: 224-235.

Kubojima, Y., T. Ohtani, and H. Yoshihara. 2004a. Effect of shear deflection on bending properties of compressed wood. Wood and Fiber Sci. 36:210-215.

--, --, and --. 2004b. Effect of shear deflection on vibration properties of compressed wood. Wood Sci. and Tech. 38:237-244.

Ohtani, T., Y. Kubojima, and K. Matsushita. 2005. Failure morphology and effect of compression volume on tensile strength of compressed wood. Mokuzai Gakkaishi 51 : 189-195.

Tsunematsu, S. and H. Yoshihara. 2006. Influence of the compression ratio on the elastic properties of compressed wood. Mokuzai Kogyo 61:146-152.

Yi, X., N. Kinoshita, M. Yoshinobu, and H. Guan. 2004. Sugi rotary veneers compressed by hygro-thermal treatment with an airtight device using moisture in the veneers. J. Soc. Mater. Sci. Jpn. 53:686-691.

Yoshihara, H. and A. Suzuki. 2005. Shear stress/shear strain relation of wood obtained by asymmetric four-point bending test of side-tapered specimen. J. Test. Eval. 33:55-60.

-- and S. Tsunematsu. 2007. Bending and shear properties of compressed Sitka spruce. Wood Sci. and Tech. 41: 117-131.

Hiroshi Yoshihara * Shogo Tsunematsu

The authors are, respectively, Associate Professor and Graduate Student, Dept. of Natural Products Resource Engineering, Shimane Univ., Matsue, Shimane, Japan (yosihara@riko.shimane-u.ac.jp; tnmt@hotaru.yoitoko.jp). This research was supported in part by a Grant-in-Aid for Scientific Research (No. 15580147) from the Japan Soc. for the Promotion of Sci. This paper was received for publication in June 2006. Article No. 10210.

* Forest Products Society Member.
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Author:Yoshihara, Hiroshi; Tsunematsu, Shogo
Publication:Forest Products Journal
Geographic Code:1USA
Date:Apr 1, 2007
Words:2303
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