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For series B and C, the ends of the specimens were waterproofed by paraffin. Wood samples were obtained from sugi Cryptomeria japonica lumber and materials used in series A were cut from the same lumber.

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The displacement applied to series A and B was controlled by the rotation angle of the nut according to the pitch of the bolt. For PE and FE specimens, the initial stress level was set sufficiently below the resistance value of sugi in compression perpendicular to the grain to evaluate the behavior within the elastic region. The bolts and the lag screw were tightened manually with a wrench, in which case the tightening period was not constant.

Considering the stress relaxation that occurs during tightening, it is necessary to keep the torque imposition time constant [ 18 ]. Since the relaxation test periods are sufficiently long with respect to the torque imposition time, and it is difficult to keep the tightening time constant in actual construction conditions, the imposition time was not considered in this study.

The clamp force was monitored by strain gages attached to the bolt, and the influence of temperature changes was corrected with dummy gages.

The axial force of the lag screw was monitored by a load cell. The specimens were placed in an indoor environment where the temperature and humidity were not constantly controlled, but the room was heated occasionally during the day during winter months. However, P2 maintained a higher stress than other specimens Fig. RH relative humidity, T temperature. Since the speed of data acquisition while tightening was relatively slow and the tightening speed was also indefinite due to manual fastening, the maximum clamp force could not be recorded.

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Although P0. Thus, a high initial stress is considered to maintain a high clamp force. Many previous studies have indicated that wood can be treated as a linear viscoelastic material within certain limits of stress, but at higher levels it became nonlinear [ 11 , 12 ]. On the other hand, Manrique [ 17 ] reported that stress relaxation of wood is not a linear stress function at any stress level.

Kuwamura [ 18 ] suggested that the judgment of linearity of wood is unclear in previously conducted research due to differences in the imposed time needed to reach the target load or strain, merging creep strain at supports, and individual difference between specimens.

He carried out the experimental tests excluding the above pitfalls and concluded that the viscoelastic behavior of wood is nonlinear even at a low stress level. However, in the case of restressing wood in its plastic region, the behavior was reported to be close to linear [ 19 ]. It is important to note that Fig. From Fig. Therefore, the effect of imposition time can be neglected over the long term, and the behavior of the wood appeared to be relatively close to linear viscoelastic, regardless of embedded displacement in the plastic region.

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Comparison of the stress relaxation curves in the different initial stress levels and ambient air condition during the tests. The stress ratio of PE virtually disappeared i. Thus, it is difficult to conclude that a clamp force can be maintained over a long period of time. In addition, it is difficult to maintain a clamp force over the long term under full compression in the elastic region because the stress of FE decreased to zero at the early stage. Materials P1.

Stress relaxation behavior of wood in the plastic region under indoor conditions

In addition, the time histories of the stresses were substantially similar, as shown in Fig. For specimens P1. Accordingly, the influence of the thickness of the specimen on the stress ratio is small in this test condition. The entire period of stress ratios obtained from P2, LP2.

All the specimens showed a tendency to asymptotically approach some value under the influence of humidity fluctuation after the initial rapid stress relaxation. Comparison of the stress ratio curves and ambient air condition during the tests. Using a lag screw in LP2. In addition, short-term fluctuations due to changes in humidity were considerably smaller compared with other specimens, despite their lack of moisture-proof treatment. These behaviors may be attributed to the uniform shear stress distribution in the axial direction of the threaded portion [ 20 ], unlike the bolted joints where the strain concentrates around the washer [ 16 ].

With the larger embedded displacement of P2 compared to that of P1.

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Since the influence of the embedded displacement was considered to be small in the plastic region as described in the previous section, the stress ratio may be affected by the extra end distance of the specimen. Although the tightening date for P1. Although creep deformation, which is viscoelastic behavior in wood, is often evaluated by the power law [ 22 ], Eq.

Ohashi et al.

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In this study, the stress relaxation is estimated by Eq. The predicted curves obtained with Eq. Both predictive curves were consistent with the observed experimental trends. Stress-relaxation curves and estimated curves by Eq. In particular, the predicted stress ratio of LP2. This fluctuation is related to the humidity fluctuation and depends on the season at the beginning of the measurement. For the P2 specimen, which had the longest measurement, the overall predicted trend decrease, and there a tendency to converge had not yet been observed.

The plastic embedded specimens of series B displayed low values at an early stage. The time axis is based on the test start date of P2. The impact of air pollution on anatomical structure of wood of Scots pine Pinus silvestris L. Acta Agraria et Silvestria Series Silvestris 25 — Comparative research on the impact of industrial pollution based on the properties of Scots pine Pinus sylvestris L.

Acta Agraria et Silvestria Series Silvestris 34 — Paschalis P. Staniszewski P. Changes in some indicators of properties of pine wood originated from industrially polluted regions. Sylwan 8 35— Schweingruber F. Wood structure and environment. Springer Berlin 87— Long- and short-term incremental response of Pinus sylvestris L. Dendrochronologia 36 1— Variations of tree ring width and chemical composition of wood of pine growing in the area nearby chemical factories.

Geochronometria 44 1 — Sheppard L. Causal mechanisms by which sulphate nitrate and acidity influence forest hardiness in red spruce: review and hypothesis New Phytologist 1 69— Stravinskiene V. Bartkevicius E. Plausinyte E. Dendrochronological research of Scots pine Pinus sylvestris L. Dendrochronologia 31 3 — Tulik M.

Kozakiewicz P. Some physical and mechanical properties of pine wood Pinus sylvestris L. Watmough S. Monitoring historical changes in soil and atmospheric trace metal levels by dendrochemical analysis.

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Environmental Pollution 3 — Wertz B. Dendrochronological evaluation of the impact of industrial imissions on main coniferous species in the Kielce Upland. Sylwan 5 — Productivity of Scots pine stands Pinus sylvestris L. Prace IBL Ser. CiteScore 0. Home About us Subjects Contacts. Show all.

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Table of contents 11 chapters Table of contents 11 chapters Introduction Pages Conclusions Pages Show next xx. Recommended for you. PAGE 1.