Yoshitaka Kubojima

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 Youngs modulus was measured by the free-free flexural vibration test, the static Youngs 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 Youngs 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 becauseI12 andI23 were reduced by the heat treatment.

Applicability of the losipescu shear test on the measurement of the shear properties of wood

We examined the applicability of the Iosipescu shear test for measuring the shear properties of wood. Quarter-sawn board of sitka spruce (Picea sitchensis Carr.) and shioji (Japanese ash,Fraxinus spaethiana Lingelsh. were used for the specimens. Iosipescu shear tests were conducted with two types of specimen whose longitudinal and radial directions coincided with the loading direction. The shear modulus, yield shear stress, and shear strength were obtained and were compared with those obtained by the torsion tests of rectangular bars. The results are summarized as follows: (1) The Iosipescu shear test is effective in measuring the shear modulus and the yield shear stress. (2) To measure the shear strength properly by the Iosipescu shear test, the configuration of specimen and the supporting condition should be examined in more in detail.

Vibrational properties of Sitka spruce heat-treated in nitrogen gas

Sitka spruce (Picea sitchensis Carr.) wood was heated for 0.5–16.Oh at temperatures of 120°–200°C in nitrogen gas or air. The values for Youngs modulus, shear modulus, and loss tangent were measured by free-free flexural vibration tests. X-ray diffractometry was carried out to estimate the crystallinity index and crystallite width. The results obtained are as follows: (1) Density decreased at higher temperatures and longer heating times. The specific Youngs modulus, specific shear modulus, crystallinity index, and crystallite width increased during the initial stage and were constant after this stage at 120°C and 160°C, whereas they increased during the initial stage and decreased later when the temperature was high. Loss tangent in the longitudinal direction increased under all conditions, whereas that in the radial direction increased at 120°C and decreased at 160°C and 200°C. (2) From the relation between Youngs modulus and moisture content, it can be safely said that Youngs modulus is increased by the crystallization and the decrement in equilibrium moisture content, and that crystallization (rather than degradation) is predominant at the initial stage of the heat treatment, whereas the latter is predominant as the heating time increases. (3) It is implied that the specific Youngs modulus, specific shear modulus, crystallinity index, and crystallite width decreased more in air than in nitrogen gas because of oxidation in air.

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 Timoshenkos 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 Timoshenkos 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 Youngs 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.

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 Timoshenkos 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 Youngs modulus and the shear modulus were calculated by Timoshenkos 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. Youngs 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.

Moisture Content of Green Wood in High Temperature Water Vapor

Summary Changes in the moisture content of green sitka spruce (Picea sitchensis Carr.) during heating in environments of over 100°C and 1 atm were measured. Temperature and weight were measured in a highly accelerated stress test (HAST) chamber. The set temperature and relative humidity ranges were 107–160°C and 75–99%, respectively. The change in the temperature of a specimen was roughly divided into 4 steps. Equilibrium moisture contents at lower temperatures and pressures were similar to previous estimates, while at higher temperatures and pressures they were higher than previous results. A diagram of temperature–relative humidity–pressure–equilibrium moisture content in the range of 105–160°C, 75–99% RH and 0.02–0.39 MPa was obtained experimentally.

Prediction of the buckling stress of intermediate wooden columns using the secant modulus

We made several buckling tests of wooden columns with intermediate slenderness ratios (λ) and examined the empirical formulas. On the basis of the examination, we formulated an equation for predicting the buckling stress (σ) of an intermediate wooden column. Sitka spruce (Picea sitchensis Carr.) and buna (Japanese beech, Fagus crenata BI.) were used for the studies. A compressive load was applied on the specimen supported with pin ends, and the buckling stress was predicted by the tangent modulus theory and two empirical equations: those of Tetmajer and Newlin-Gahagan. The predicted σ-λ relations were compared with the test results, and the applicability of these predictions were examined. Based on the comparisons, we formulated an equation that can predict the σ-λ relations of materials with various stress-strain characters in the plastic strain range.

Vibrational properties of wetwood of todomatsu (Abies sachalinensis) at high temperature

The object of this study was to understand precisely the drying characteristics of wetwood of todomatsu (Abies sachalinensis Mast.). For this purpose, the vibrational properties of wetwood of todomatsu at high temperature were compared with those of normal parts that had lower green moisture content than the wetwood. Specimens were cut respectively from the wetwood and normal parts, and matched in the radial direction. The specimens and the measuring systems were placed in an electric drying oven and free-free vibration tests were conducted in the oven under absolutely dry conditions. The wetwood and the normal parts were tested separately. The temperature was raised from room temperature to 200°C and then lowered to 50°C in steps of 25°C. The specific Young’s modulus decreased with an increase in temperature during the heating process while it increased with the decreasing temperature during the cooling process. There was no significant difference in the specific Young’s modulus between the wetwood and the normal part at all tested temperatures. The loss tangent took a minimum value at about 100°C in both the heating and cooling processes. There was no significant difference in the loss tangent between the wetwood and the normal part. Thus, the elastic and viscoelastic behaviors of the wetwood appear to be similar to those of the normal part in the temperature range of an actual kiln-drying process.

Fixed-fixed flexural vibration testing method of beams for timber guardrails

Resonance frequencies of beams with various types of end supports were examined for flexural vibration. Rectangular beams with dimensions of 300 (L) × 25 (R) × 5 or 10mm (T) were used as the test specimens. Various compressing stresses were applied to the parts around both ends of a test beam and flexural vibration tests were conducted. The measured resonance frequency started to increase from the resonance frequency of a beam with simply supported ends and was stable around the resonance frequency of a beam with fixed ends as the compressing stress increased. The stable resonance frequency was lower than the theoretical value because perfect fixation of a beam to a post was difficult. From these results, the temporal change in resonance frequency itself, rather than the stable resonance frequency, is effective to examine whether a beam has enough strength as a guardrail.

Vibrational properties of green wood in high-temperature water vapor

Abstract The object of this study was to measure basic wood properties when water in wood is lost by heating under high temperature and high humidity. Using Sitka spruce (Picea sitchensis Carr.) specimens with dimensions of 110 mm (R)×25 mm (L)×8 mm (T), temperature and weight were measured and free-free flexural vibration tests were conducted in water vapor at 115–150°C and 75–98% relative humidity. The results were compared to those at high temperatures without humidity control under atmospheric pressure. Early in the drying process, the resonance frequency and loss tangent showed a minimum and maximum, respectively. Temporal changes in the temperature, moisture content and vibrational properties were similar to those in heating without humidity control, while the specific Youngs modulus and loss tangent during the drying processes were much smaller and greater, respectively, than those in heating without humidity control. Hence, we concluded that the wood was remarkably softened by high temperature and moisture.