Mechanical Behaviour of Alternatively Laminated LVL Composed of Rubberwood veneer and Falcata veneer

Transcription

1 Mechanical Behaviour of Alternatively Laminated LVL Composed of Rubberwood veneer and Falcata veneer Koji Murata Assistant Professor Graduate School of Agriculture, Kyoto University Kyoto, Japan Sayaka Nakao Graduate Student Graduate School of Agriculture, Kyoto University Kyoto, Japan Kuniharu Yokoo Managing Director Uni-Wood Corporation Osaka, Japan Summary A new type of laminated veneer lumber (LVL) was developed by the alternate piling of the rotary lathe veneer of rubberwood and falcata. This mixed-species LVL is of moderate hardness, and like solid falcata timber, it does not display a tendency to warp. The rubberwood LVL warped almost eight times more than the falcata LVL. The alternately laminated LVL made of rubberwood and falcata veneer warped to the same extent as the falcata LVL. The soft layer of the falcata veneer probably mitigates the warping of the hard layer of the rubberwood veneer. Further, the mechanical properties of the LVL were also tested. The shear strength and transverse compression strength of the mixed-species LVL were similar to those of the falcata LVL, but its bending strength was identical to that of the rubberwood LVL. The soft layer of the falcata veneer appears to increase the amount of energy required to rupture the mixed-species LVL. 1. Introduction It has become important to utilize fast-growing plantation trees in order to prevent the exhaustion of the natural forest resource. Fast-growing trees like eucalyptus and poplar have been planted all over the world to be cut and utilized deliberately. However, they are considered low value trees because their applications are mostly limited to producing pulp and fuel. We hope to develop higher value applications of fast-growing trees such as using them as building materials to increase their contribution to the local economy of the plantation area. The shape of most fast-growing trees is not suitable for their use as building materials and thus their low value timber needs to be technically processed to yield higher value materials. One popular technique is to process reconstruction materials into engineered wood products (EWP) such as OSB and Glulam. The higher the strength of EWP, the larger its element such as lamina, veneer is. However, the larger element tends to be affected from the defects of the tree, that is, low density and knots will degrade the quality of EWP. Many researchers have engineered two different species of fast-growing trees into composites for overcoming the defects of these trees, for example, composites of eucalyptus and poplar [1], beech and eucalyptus [2], poplar and beech [3] and Japanese sugi and Douglas-fir [4-6]. This composite lumber is generally a gradient material whose outer layer elements have a higher density in order to acquire higher strength. One of the commonly planted fast-growing plantation trees is falcata (Paraserianthes falcataria) in Southeast Asia. It has been planted in a wide area and is expected to have many applications

2 because of its white colour, lightness, softness and ease of processing. However, it is not used as a building material because of its lack of bending strength and nail withdrawal resistance. On the other hand, para rubber tree (Hevea brasiliensis) was planted in Southeast Asia for the production of raw rubber and cut every 25 years for renewal. The rubberwood of the trunk is white and has a hard surface. Though once plagued by the problem of blue stains because of fungi, rubberwood has now, with the development of a chemical treatment process to remove the stains, become valuable in the furniture industry. However, rubberwood is not yet used as a building material because the trunk of the para rubber tree curves at times, and the rubberwood tends to twist and warp due to moisture absorption. In order to use these two species as building materials, a mixed-species composite lumber that has moderate density and strength has been developed. Further, in order to improve falcata s nail withdrawal resistance and rubberwood s tendency to twist and warp, falcata veneer and rubberwood veneer are alternately laminated [7-9]. Here, it is important to note that the thin hard veneer, which tends to twist and warp, is placed between the thin soft veneer in contrast to the heavy outer layer and light inner layer of conventional mixed-species composite lumbers. In this report, we discuss the various mechanical properties of alternately laminated lumber made from falcata veneer and rubberwood veneer. 2. Experimental 2.1 Materials Alternately laminated lumber composed of falcata veneer and rubberwood veneer (mixed-species LVL) are prepared for the following experiments (Table 1). Rotary lathe veneer (thickness: approximately 2.5 mm) of falcata and rubberwood with scarf jointing are laminated alternately and glued with phenol resin for making the mixed-species LVL. Moreover, LVL containing only falcata and LVL containing only rubberwood made from the same veneer and resin are prepared for comparison. The prepared LVL is 13 ply, and the cross section is 30 mm (thickness) 40 mm (width). The materials are sufficiently seasoned in a climate chamber (20 C, 60% RH) and cut into specimens for the various mechanical tests. The moisture content is approximately 11%. 2.2 Warp due to moisture absorption LVL specimens (length: 350 mm) are set in a conditioning chamber (20 C) with a water tray (Fig. 1); the warps because of moisture absorption are measured. Changes in deflection are measured for a month using a deflection sensor (span: 300 mm) with a dial gauge set at its centre (Fig. 2). Five specimens are prepared for each type of LVL. Table 1 Prepared materials for mechanical experiments. Type Density (kg/m 3 ) Composition Symbol Rubberwood LVL 680 RRRRRRRRRRRRR R Falcata LVL 400 FFFFFFFFFFFFF F Mixed-species LVL(surface: falcata) 540 RFRFRFRFRFRFR RFR Mixed-species LVL(surface: rubberwood) 540 FRFRFRFRFRFRF FRF Fig. 1 Conditioning chamber for absorbing moisture. Fig. 2 Setup for measuring deflection due to moisture absorption.

3 2.3 Shear strength tests For testing the shear strength, we performed the block shear and short-span 3-point bending tests. Block shear specimens are based on the JIS standard for a small and clear wood specimen (Fig. 3), in which the species of veneer on the tip of the notch of a mixed-species LVL is equal to that on the surface. On the short-span bending test, the span is 300 mm (Fig. 4). Strength tests are executed with a universal material testing machine (Instron 8502, 250 kn); the crosshead speed is 0.5 mm/min for block shear and 2 mm/min for short-span bending. We prepared six block shear specimens and twelve short-span bending specimens for each type of LVL. RL-specimen TL-specimen Fig. 3 Block shear specimen. Fig. 4 Short-span bending specimen. 2.4 Transverse compression test Specimens (length: 30 mm) for transverse compression are cut from common materials and compressed in a direction that is parallel to the laminating direction with a compression attachment (Φ = 80 mm). The compression test is executed by a universal material testing machine (Shimadzu, 5 kn), and the crosshead speed for measuring the Young s modulus and proportional limit stress is 0.5 mm/min. Sixteen specimens of each type were considered. In addition, the deformation on the cross section of a specimen is photographed using a digital camera, and the strain distribution is analyzed using a digital image correlation technique. 2.5 Static 3-point bending test Specimens (length: 480 mm) for static 3-point bending test are cut from common materials. Twenty four specimens of each type are considered. The flat-wise bending test with a 420-mm span is executed by using a universal material testing machine (Shimadzu, 5 kn), and the crosshead speed for measuring the Young s modulus (MOE), modulus of rupture (MOR), proportional limit and work to failure is 3 mm/min. The bending strength of LVL is affected by the joints of veneer. In this test, the scarf jointing of the surface layer on tension side is not set just under the loading head. Therefore, the bending strength will not be affected by the joints and depend only on the structure, e.g. alternate lamination. 3. Results and Discussion 3.1 Warp due to moisture absorption Changes in moisture content and deflection because of moisture absorption are shown in Fig. 5, and the deflection of each type of LVL because of a moisture content of approximately 16% is listed in Table 2, where deflection indicates the average absolute displacement at the centre of the specimen, and the moisture content is calculated with an oven-dry weight measured after the examination.

4 Fig. 5 Changes in deflection due to moisture absorption. Table 2 explains that the deflection of the F- type LVL is much less than that of the R-type LVL and equal to that of the mixed-species LVLs (FRF-type and RFR-type). It is commonly believed that LVL does not warp much upon moisture absorption because of its symmetrical lamination. However, in reality, LVL warps to a certain extent because the unevenness of density or the irregularity of the fibre direction within the veneer creates an asymmetrical lamination structure [10]. It was reported that rubberwood frequently includes a considerable amount of tension wood [11]. Thus, it is supposed that the large deflection of the R-type LVL depends on the unevenness of material properties generated by the tension wood component. Table 2 Deflection of specimens with a moisture content of approximately 16%. Type Number of Moisture content Absolute deflection Days specimens (%) (mm) R (±0.055) F (±0.015) RFR (±0.025) FRF (±0.019) 3.2 Shear strength The shear strength (τ RL and τ TL ) in the block shear test is obtained using Eq. (1), where τ RL indicates the maximum shear stress on the area parallel to the laminated veneer, and τ TL indicates the maximum sheer stress on the area perpendicular to the veneer. The shear strength (τ RL ) in the shortspan 3-point bend test is calculated using Eq. (2). = τ = P max τ RL TL (1) Ablock 3 P = max τ RL (2) 4 Abend Where A block is 900 mm 3, and A bend is 1200 mm 3. The shear strengths obtained from the block shear test are shown in Figs. 6 and 7. When the shear area is perpendicular to the laminated veneer, that is, TL-specimen, the shear strength (τ TL ) of mixed-species (RFR-type and FRF-type) LVLs is between those of the falcata (F-type) LVL and rubberwood (R-type) LVL. However, when the shear area is parallel to the laminated veneer, i.e. RL-specimen, the shear strength (τ RL ) of mixed-species LVLs is equal to that of the F-type LVL. The species of veneer on the tip of the notch is the same as that of the surface veneer of the block shear specimen. The shear crack in the RFR-type LVL does not begin on the tip of the notch, and the specimens rupture in the falcata veneer adjacent to the rubberwood veneer on the tip (Fig. 10). This tendency results in a large variation in the shear strength of the RFR-type LVL (Fig. 6). Moreover, RL-specimens of R-type mostly rupture near adhesive layer and the fracture plane is on the tip of notch or on the next layer (Fig. 11). This also results in a large variation in the shear strength of the R-type LVL (Fig. 6). The results obtained from the short-span 3-point bending test are shown in Fig. 8. The shear cracks usually appear on the ends of the short-span bend specimen, and some R-type specimens break along the edge near the supporting block. The shear strengths (τ RL ) of the RFR-type and FRF-type LVLs are equal to that of

5 the F-type LVL in the short-span bending test. We think that this is because the mixed-species LVL breaks in the falcata layer. Figure 9 shows the comparison between the shear strength (τ RL ) calculated by using the block shear test and that calculated by using the short-span bending test. The shear strengths in the short-span bending test are smaller than that in the block shear test. A tension stress transverse to the fibre is also applied to the end of the short-span bend specimen. The complex stress with shear and tension probably weakens the shear strength in the short-span bend test. Fig. 6 Shear strength τ RL (Block shear). Fig. 7 Shear strength τ TL (Block shear). Fig. 8 Shear strength τ RL (Short-span bend). Fig. 9 Comparison of shear strengths obtained from two shear strength tests. Fig. 10 Fracture figures on RL-specimen of RFR-type. Fig. 11 Fracture figures on RL-specimen of R-type.

6 3.3 Transverse compression The elastic modulus and proportional limit stress in a transverse compression test are shown in Figs. 12 and 13. The R-type LVL is excluded because we cannot calculate the proportional limit stress until the applied load equals the maximum load of the testing machine. The elastic modulus of the mixed-species LVLs RFR-type and FRF-type is larger than that of the F-type LVL; however, the proportional limit stress of the former is equal to that of the latter. The failure probably occurs in the falcata veneer layer, and we suppose that the restriction effect of the rubberwood veneer intensifies the compression strength of the falcata veneer. Figure 12 indicates the no restriction effect of the hard layer. The strain distribution measured by using the digital image correlation method is shown in Fig. 14. Compression failure generally occurs along the edge of the specimen; however, Fig. 14 shows that a large strain, which indicates a local compression failure, is found in the inner layer of the specimen in this experiment. It can be considered that the lathe checks in the rotary lathe veneer expand to the local failure. We suppose that the lathe checks in the rotary lathe veneer probably release the restriction of the hard layer. Fig. 12 Elastic modulus of LVL in transverse compression test. Fig. 13 Proportional limit of LVL in transverse compression test. Fig. 14 Strain distributions on the cross section of mixed-species LVL (FRF-type); left: observed image, centre: normal strain and right: shear strain. 3.4 Static 3-point bending strength The mechanical properties obtained from the static 3-point bending test are shown in Figs The numerical values of all mechanical properties of the RFR-type LVL lie between those of the F- type (falcata) LVL and the R-type (rubberwood) LVL. Moreover, the numerical values of the modulus of elasticity (MOE) and the proportional limit of stress of the FRF-type LVL are between those of the F-type LVL and the R-type LVL (Figs. 15 and 16). However, Fig. 17 shows that the

7 modulus of rupture (MOR) of the FRF-type LVL is equal to that of the R-type LVL, and Fig. 18 shows that work to failure for the FRF-type LVL is the largest. In order to discuss this tendency of the FRF-type LVL, the MOR data in this experiment are analyzed with the cumulative distribution function for the 3-parameter Weibull distribution shown in Eq. (3). m x γ F = 1 exp (3) η Where m is the shape parameter; η, the scale parameter and γ, the location parameter. Table 3 shows the Weibull parameters obtained by the least squares approximation method with nonlinear parameters by using the solver function of Microsoft Excel. The shape and scale parameters of the RFR-type LVL are approximately equal to those of the R-type LVL. This explains that the rupture of the RFR-type and R-type LVLs occurs with the same fracture mechanism. Their MOR probably depends on the strength of the surface rubberwood veneer. However, the shape and scale parameters of the FRF-type LVL are different from those of the other types. The MOR of the FRFtype LVL probably depends on neither the strength of the surface falcata veneer nor that of the inner rubberwood veneer. We suppose that the MOR of the FRF-type LVL depends on the property of the inner rubberwood veneer reinforced by the surface falcata veneer. Fig. 15 Modulus of elasticity (MOE) in flatwise 3-point bending test. Fig. 16 Proportional limit stress in flat-wise 3- point bending test. Fig. 17 Modulus of rapture (MOR) in flatwise 3-point bending test. Fig. 18 Work to failure in flat-wise 3-point bending test.

8 Table 3 Weibull parameters of MOR in flat-wise static 3-point bending test. Type Shape parameter Scale parameter Location m η parameter γ Composition R RRR RRR F FFF FFF RFR RFR RFR FRF FRF FRF References [1] Castro G. and Paganini F., Mixed glued laminated timber of poplar and Eucalyptus grandis clones, Holz als Roh und Werkstoff, Vol. 61, No. 4, 2003, pp [2] Aydın İ., Çolak S., Çolakoğlu G. and Salih E., A comparative study on some physical and mechanical properties of Laminated Veneer Lumber (LVL) produced from Beech ( Fagus orientalis Lipsky) and Eucalyptus ( Eucalyptus camaldulensis Dehn.) veneers, Holz als Rohund Werkstoff, Vol. 62, No. 3, 2004, pp [3] Burdurlu E., Kilic M., Ilce A.C. and Uzunkavak O., The effects of ply organization and loading direction on bending strength and modulus of elasticity in laminated veneer lumber (LVL) obtained from beech (Fagus orientalis L.) and lombardy poplar (Populus nigra L.), Construction and Building Materials, Vol. 21, No. 8, 2007, pp [4] Goto T., Ikebuti T., Furuno T. and Nakayama S., Investigation on the Applicability of Konara and Sugi Woods from Shimane Prefecture to Species-mixed Laminated Veneer Lumbers (LVLs) : Fundamental strength properties of lumbers, veneers, and species-mixed LVLs, Wood industry, Vol. 59, No. 2, 2004, pp (in Japanese). [5] Hayashi T. and Miyatake A., Strength properties of sugi composite-glulam beams I, Mokuzai Gakkaishi, Vol. 37, No. 3, 1991, pp (in Japanese). [6] Hayashi T., Karube M., Harada K., Mori T., Ohno T., Komatsu K. and Iijima Y., Shear tests of timber joints composed of sugi composite glulam beams using newly developed steel connectors, Journal of Wood Science, Vol. 48, No. 6, 2002, pp [7] Yokoo K., Masuda M. and Murata K., Laminated composite wooden material and method of manufacturing material, Japanese Patent Number , [8] Murata K., Masuda M. and Yokoo K., Mitigation of bowing of LVL by alternately laminating rubberwood and falcata veneer, and observation of its swelling behaviour, Mokuzai Gakkaishi, Vol. 50, No. 5, 2004, pp (in Japanese). [9] Murata K. and Nakao S., Transverse compression behaviour of softwood and alternately laminated lumber of rubberwood veneer and falcata veneer, Journal of the Society of Materials Science, Japan, Vol. 56, No. 4, 2007, pp (in Japanese). [10] Constant T., Badia M.A. and Mothe F., Dimensional stability of Douglas fir and mixed beech poplar plywood: experimental measurements and simulations, Wood Science and Technology, Vol. 37, No. 1, 2003, pp [11] Mukogawa Y, Nobuchi T. and Hamami S.M., Tension wood anatomy in artificially induced leaning stems of some tropical trees, Forest research, Kyoto, No. 75, 2003, pp