Tetsuya Nakao

Dynamic properties of three types of wood-based composites

This study used a vibration test method to show that grain angles of face veneer have substantial effects on sound velocities and dynamic Young’s moduli of three types of wood-based composites. The sound velocity at 0° grain angle of face veneer was the highest, and it decreased with increasing grain angle in the range of 0° to 90°. This tendency was similar to that for dynamic Young’s modulus. The relationship between the grain angle of face veneer and the sound velocity of three types of wood-based composites can be expressed in the form of Hankinson’s equation or a second-order parabolic equation. This study also showed that the application of orthotropic elasticity theory was valid for the three types of wood-based composites. The relationship between the grain angle of the face veneer and the Young’s modulus of three types of wood-based composites can be expressed in the form of the Jenkin equation, Hankinson’s equation, or a second-order parabolic equation. Rule of mixture can also be used to predict the Young’s modulus of wood-based composite from the Young’s moduli of the two elements.

Vibrational properties of wood plastic plywood

Wood plastic plywood (WPPW), composed of veneer and styrofoam, was manufactured without special adhesives such as urea–formaldehyde or phenol–formaldehyde resins, and its vibrational properties were investigated. WPPW can be produced at 1 MPa and 160°C for 9 min (three-ply) and 12 min (five-ply). The dynamic Young’s modulus reached its highest value when the styrofoam thickness was 30 mm. The sound velocity and dynamic Young’s modulus had minimum values at a grain angle of 45°. The results for dynamic Young’s moduli measured by a longitudinal vibration method and an in-plane flexural vibration method were almost the same. Dynamic shear moduli were measured by an in-plane surface wave propagation test and an in-plane flexural vibration method. From the experimental results, the dynamic shear moduli at 0° and 90° by the two methods were relatively close, although the surface wave propagation test results were higher than those from the flexural vibration method. Dynamic shear moduli at a grain angle of 45° measured by the in-plane surface wave propagation test and calculated from theory were relatively close. The surface wave propagation test results were smaller than the results calculated from theory. The shear stress distribution factors were about 1.000–1.189 for WPPW.

Relationship between piezoelectric behavior and the stress – strain curve of wood under combined compression and vibration stresses

It is generally thought that the piezoelectric effect of wood results from natural cellulose crystals in the cell wall. However, natural cellulose cannot exist as a molecule in wood. Many molecular chains of cellulose form fiber structures in bundles with hemicellulose and lignin, i.e., microfibrils. The cell wall can be considered as a frame for these microfibrils. Hence, the increase and decrease of the piezoelectric voltage during the deformation of wood originates from the dynamic deformation of the cell wall. Many studies of the piezoelectric effect in wood have examined the physical properties of small specimens of wood under a minute load, but there is very little research on deformation. Therefore, we examined test specimens subjected to combined compression and vibration stresses at a 45-degree angle to the fiber direction and load direction, to clarify the relationship between the piezoelectric voltage and deformation. This study elucidated the relationship between piezoelectric behavior and the initial shape of the stress-strain curve.

Analysis of the effects of density profile on the bending properties of particleboard using finite element method (FEM)

The bending properties of particleboards with various density profiles were analyzed by calculating their modulus of elasticity (MOE) using two-dimensional finite element method (FEM). The calculation was based on the fundamental properties of homo-profile particleboards (board with flat density profile) produced from lauan (Shorea spp.) particles and an isocyanate resin. The results are summarized as follows: 1. The calculated MOE fit very well with the experimental values, with a deviation of below 5%. 2. Increment in peak density (PD) results in a proportional increase in MOE, while core density (CD) determines the optimum slope gradient between the peak and core regions. 3. The counteractive effect arising from increment in PD and simultaneous reduction in peak width resulted in merely 9% improvement in MOE, when PD was increased from 1.0 to 1.5 g/cm 3 . 4. Increment in peak distance (Pdi) results in a proportional reduction in MOE. When Pdi was doubled from 1 mm to 2 mm, MOE was reduced by about 11%. 5. In idealized density profile models, when peak and core densities remain unchanged, the maximum peak area does not necessarily result in the highest MOE. 6. Multiple regression analysis shows that the overall MOE of particleboard depends basically on the board mean density (MD), PD and Pdi.

Mechanical behavior of the crystal lattice of natural cellulose in wood under repeated uniaxial tension stress in the fiber direction

This study investigated the relationship between the cellulose crystal lattice strain (crystalline region) and the macroscopic surface strain in specimens of Chamaecyparisobtusa wood under repeated uniaxial tension stress in the fiber direction. Changes in the strain of the crystal lattice were measured from the peak of (004) reflection using the transit X-ray method. The macroscopic surface strain of each specimen was measured with a strain gauge. In both loading and unloading, the surface strain changed linearly with changes in stress. However, crystal lattice strain was not linear but exhibited changes along a curve with changing stress. Under stressed conditions, the crystal lattice strain was always less than the surface strain, regardless of the frequency of repetition in the loading and unloading cycle. The ratio of the crystal lattice strain to the surface strain showed a negative correlation for stress in both loading and unloading. That is, the ratio decreased with increasing stress, and finally tend to converge to a specific value. The ratio (I/I0) between the diffracted intensity (I0) in the (004) plane in the unloaded condition and the diffracted intensity (I) in the (004) plane in the loaded condition tend to converge on a specific value with increasing frequency of repetition. When the substantial tension Young’s modulus of the wood in the longitudinal direction decreased, the ratio of the strain of the crystal lattice to the surface strain also decreased. Moreover, the ratio decreased with increasing microfibril angle of the specimen.

Initial shapes of stress-strain curve of wood specimen subjected to repeated combined compression and vibration stresses and the piezoelectric behavior

This study investigated the relationship between the initial shape of the stress (σ)-strain (ε) curve of a Chamaecyparis obtusa wood specimen subjected to repeated combined compression and vibration stresses at various angles between the fiber direction and load direction and the piezoelectric behavior. The main findings of the study are: (1) the σ-ε curve became convex initially, and then the stress was proportional to the strain. The σ-ε curve had almost the same shape during both loading and unloading. (2) The σ-piezoelectric voltage (P) curve was nonlinear, with a maximal point or cusp on the curve, which had almost the same shape during both loading and unloading, as was also observed for the σ-ε curve. (3) The plot of the first derivative of the stress [dσ/dε (= σ′)] against ε was nonlinear. The σ′-ε and P-ε curves at various angles were fairly similar. (4) The stress at the maximal point (or cusp) of the σ-P curve decreased with an increase in the angle between the fiber direction and load direction. The tendency of the stresses was very similar to that of Young’s modulus and compression strength calculated from Hook’s law and Hankinson’s law, respectively.

Effects of fiber length and orientation on elasticity of fiber-reinforced plywood

Short carbon fibers, a reinforced material in wood veneer composites, were used to investigate the effects of fiber length and orientation of fibers on the elasticity of plywood. The technical feasibility, elasticity, and strength of the reinforced plywood with short carbon fiber were evaluated. In a short fiber reinforcement system, the fiber length does not directly influence the reinforcement in Coxs theory when the fiber length exceeded a certain length. When the length of short carbon fiber is beyond 3 mm, the high reinforced result was obtained in the experiment. However, if fiber length was too long, the reinforced result was less owing to the bridge between fibers and the increase of holes. The optimum fiber length must be considered. The orientation of fibers has a strong influence on the reinforcement. Unidirectional, perpendicular, and random orientation displayed different influence on the elasticity. Experimental results were discussed with Coxs method. Reinforced plywood with short carbon fibers in random orientation has a higher shear modulus and bending strength than the controls, in addition to other mechanical properties.

The relationship between macroscopic strain and crystal lattice strain in wood under uniaxial stress in the fiber direction

Wood, a natural composite material, is composed mainly of cellulose and noncellulose matrix. The native natural cellulose that forms the skeleton comprises about half of the components (crystallinity: about 50%–60%). Although it is probable that natural cellulose greatly affects the dynamic behavior of wood, there have been few reports on the effects of the crystalline and noncrystalline regions in wood on the macroscopic and semi-microdynamic behaviors in small clear specimens. In the present study, X-ray stress measurements were performed by applying uniaxial compression stress and tensile stress to test specimens in the direction of the fibers. The relationship between the crystal lattice strain and the macroscopic strain in the test specimens (hereafter referred to as the surface strain) was investigated in detail. Meridional (004) diffraction was used to calculate the crystal lattice strain. The experiment was carried out taking into account the microfibril inclination angle (MFA, by Cave’s method) of the test piece, because it is thought that the orientation of the fibers greatly affects the dynamic behavior.

Mechanical behavior of the crystalline region of wood and the piezoelectric response of wood in tension tests

We investigated the relationship between the crystal lattice strain and the piezoelectric response in Japanese cypress (Chamaecyparis obtusa Endl.) wood fibers subjected to tension stress in the fiber direction. As a result, the piezoelectric voltage was very sensitive to the mechanical behavior (deformation) of the wood crystalline regions obtained from the x-ray stress measurement. Thus, by investigating the behavior of piezoelectric voltage, it was possible to simply estimate the mechanical behavior of the crystalline regions in the wood.

The relationship between sap flow rate and diurnal change of tangential strain on inner bark in Cryptomeria japonica saplings

The relationship between sap flow rates and diurnal fluctuation of stems was investigated in cloned 3-year-old saplings of Cryptomeria japonica D. Don grown in a phytotron with irrigation every 2 days. The improved stem heat balance method and a strain gauge were used to measure sap flow rate and diurnal fluctuation of the stem. The sap flow rate reacted to lighting conditions, increasing and decreasing immediately after lights-on and lights-off, respectively. The tangential strain on the surface of the inner bark exhibited a reaction that followed but opposed the reaction of the sap flow rate to lighting conditions. Based on the changes in sap flow rate, there seemed to be four phases in diurnal sap flow: phase A1 began with lights-on, when the sap flow rate increased, and lasted about 2 hours. In the following phase, A2, the sap flow rate remained almost constant at 1.3 g/min for about 10 h, and then declined for about 2 h as lights-off approached. In phase B, the early period of darkness, the sap flow declined quickly and then more slowly, for about 4 h, until the start of the second dark period, phase C, when the sap flow rate became almost constant at 0.05 g/min for about 6 h. The first derivative of each sap flow rate and the corresponding tangential strain were calculated, and the results indicated a negative correlation between the two variables in all periods. In particular, the relationship between the first derivative values exhibited a highly negative correlation in phases A1 and B, expressed as a primary formula. Sap flow rate was found to continue for some time after lights-off, and this compensated for reduced evaporative effects, albeit at a slow rate, over 4 h. The total amount of sap flow in the dark was only about 9% of that in the light, disregarding transpiration in the dark for simplicity. Thus, the total amount of sap flow responsible for swelling of the stem was about 9% of that consumed in transpiration during the light period.