For. Stud. China, 2012, 14(2): 107–111 DOI 10.1007/s11632-012-0201-7
RESEARCH ARTICLE
Viscoelastic properties of wood from Chinese-fir and poplar plantations ZHAO You-ke1*, Ikuho IIDA2, FENG Shang-huan1, LU Jian-xiong1 1 2
Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, P. R. China Laboratory of Wood Technology, Kyoto Prefectural University, Kyoto 606-8522, Japan
© Beijing Forestry University and Springer-Verlag Berlin Heidelberg 2012
Abstract Elastic and strength properties (proportional-limit stress (σprop), Young’s modulus (E), breaking stress (σmax) in static bending parallel to grain in a longitudinal direction), as well as stress relaxation in air-dried condition and water-saturated conditions at seven different constant temperatures and increasing and decreasing temperatures were investigated for wood from Chinese-fir and poplar plantations. The results show that hygrothermal conditions considerably affect these mechanical properties. The higher the moisture content (MC) or temperature, the lower the strength of wood. Further investigation of the effects of constant temperature on stress relaxation indicates that high temperature specimens have low relaxation moduli and high fluidity. In the case of increasing temperature the range of the modulus of relaxation is larger than in the case of a reduction in temperature, while the residual moduli do not show large differences. This is because the modulus at high temperatures decreases more than that at low temperatures. The fluidity of specimens in a state of water desorption increases slowly at the beginning, increases quickly until the MC reaches an equilibrium moisture content (EMC) and then becomes stable, which is quite different from that in a water-saturated state. Fluidity in a desorption state is much higher than in a water-saturated state. This is probably due to the fact that the former is in an unstable state which can be interpreted as a state with internal strain and has therefore a greater potential to release strain. Key words Chinese fir, poplar, visco-elasticity, mechanical properties
1 Introduction Chinese fir and poplar are two of the most important timber species in China due to their fast growth. As it is, no other species could replace these species in our domestic timber resource supply. During the processes of manufacturing wood products, temperature and moisture content (MC) are two of the most important factors that impact and affect wood processes, such as bending and drying. These two factors are subject to visco-elastic properties of wood. In wood bending processes, the visco-elastic properties of wood will directly affect rates of the finished products, while the stress relaxation characteristics after bending will have a direct effect on shape in the bending of wood products. During the process of wood drying, changes in moisture content and temperature will affect the fluidity of wood, as well as the development of stress, checks and deformation. Understanding the viscoelastic properties of wood from Chinese-fir and poplar plantations will no doubt assist in improved utilization of these two important species in China in the form of Author for correspondence. E-mail:
[email protected]
*
higher value-added products, especially given the current situation in which more and more plantations of these two species are established in the country. However, the visco-elastic properties of wood from Chinese-fir and poplar plantations have not been studied systematically. With this study, we intend to explore the effects of temperature and MC on the elastic and strength properties and stress relaxation characteristics of wood from these two species.
2 Materials and methods Chinese-fir (Cunninghamia lanceolata) and poplar (Populus euramericana cv. ‘San Martina I-72/85’) specimens were successively cut from the same pieces of wood in order to minimize individual variation. Two longitudinal-direction close-contact couples were marked, one for the stress-strain test, the other for the stress relaxation test whose deformation is determined for 80% by the elastic deformation of the stress-strain test. The dimensions of the specimens were 11 cm in
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a radial direction (R), 1.5 cm in the tangential direction (T) and 0.4 cm in the longitudinal direction (L). In order to minimize the stress variation in individual specimens, these specimens were oven-dried and then saturated with water under vacuum. The elastic and strength properties, as well as the stress relaxation test were conducted on a testing machine with the load cell and specimens immersed in a temperature-program-controlled water bath or in a conditioning room. The detailed testing method and equipment are similar to those of a previous study (Iida et al., 2002a). Seven constant temperatures (20, 30, 40, 50, 60, 70 and 80°C) and a range of two variable temperatures (20 to 80°C and 80 to 20°C in two hours at speeds changing at a constant rate) were chosen for the tests. We opted for two levels of a constant MC (watersaturated and air-dried specimens) and a variable MC (from water saturated MC to air-dried MC) for the study of the effects of MC on stress relaxation. The real-time changing MC of the specimens for stress relaxation was determined by other specimens in close contact in the same conditioning room by recording the change in weight from the water-saturated condition to the air-dried condition during the entire course of the stress relaxation test.
at 20°C, while the strain in Chinese-fir under similar conditions is almost the same, suggesting that poplar is more readily affected by temperature than Chinesefir. Table 1 shows that the stress of the proportional limit, the breaking stress and the modulus of elasticity in our static bending tests decreased clearly in poplar with the increase in temperature, while Chinese-fir did not show such a clear pattern. Although the specimens were cut in succession from the same pieces of wood in order to minimize the variation in mechanical properties between individual specimens, there was still variation among them. This is probably a factor that can be attributed to the uncertain impact of temperature on the mechanical properties of Chinese-fir. From Fig. 1, it can be seen that the strain of Chinesefir in the saturated condition at 80°C does not differ much from that at 20°C, while large differences exist in poplar. These large differences do not readily decrease by individual variation in mechanical properties, while small differences are probably more readily diminished by individual variation. This is one more reason for a not-very-clear temperature impact pattern in Chinese-fir. 3.2 Stress relaxation
3 Results and discussion 3.1 Stress and strain Temperature and MC showed a marked effect on the stress of the proportional limit (σ prop), the bending strength (σmax), as well as the modulus of elasticity in bending (MOE), in both Chinese-fir and poplar (Fig. 1), suggesting that both properties have a very obvious effect on the strength of wood. The higher the MC or temperature, the lower the strength of wood. In addition, the strain of poplar in the water-saturated condition at 80°C increased considerably compared with that in the air-dried and water-saturated conditions
The modulus of relaxation of poplar at different temperatures in all relaxation periods decreased clearly with the increase in temperature. During the entire relaxation period, the modulus of relaxation at higher temperatures was lower than that at lower temperatures. In the case of Chinese-fir, no clear pattern emerged, i.e., not every modulus of relaxation of the specimens tested at lower temperatures showed a higher value than those at higher temperatures (Fig. 2). The concept of the fluidity of wood was adopted from the study by Kudo (2003), who defined it as changes in the relative modulus of relaxation (1 − Et / E0), where Et is the modulus of relaxation at time t and
Fig. 1 Stress-strain curves of specimens in air-dried and water-saturated (wet) conditions, at temperatures of 20°C and 80°C
ZHAO You-ke et al.: Viscoelastic properties of wood from Chinese-fir and poplar plantation
E0 the modulus before relaxation. Further analysis of fluidity showed clearly that the fluidity of Chinesefir increased with the increase in temperature (Fig. 3), indicating that stress relaxes more in high temperature than it does in low temperature. This result confirms that the uncertain pattern of Chinese-fir (Fig. 2) is attributed to variation in large specimens which can be eliminated by fluidity. Although the specimens were cut successively, and the specimens were also ovendried and then saturated with water under vacuum to minimize stress variation, the effect of variation of individual specimens on visco-elastic properties must be taken into account. The fluidity of poplar shows a similar result except for the unexpected and unexplained low results of fluidity at 80°C compared with
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that at 60°C and 70°C. Future studies should focus on this problem. Figure 4 shows stress relaxation curves during elevation and reduction in temperature, showing similar results with those of the study by Iida et al. (2002a). Temperature remained constant for the first 30 min and then increased or decreased evenly from 30 to 150 min and finally remained constant from 150 to 180 min. In the case of increased temperature, the modulus of relaxation decreased markedly. As a result, large differences in the modulus showed up after relaxation. In the case of a decrease in temperature, the modulus remained almost constant and therefore, the tests showed only small differences in relaxation over the same time period (180 min) at the same temperature
Table 1 Changes in elastic and strength properties in static bending parallel to grain in longitudinal direction by temperatures increasing from 20°C to 80°C in a water-saturated condition Specimens Temperature (°C) σprop (MPa) σmax (MPa) MOE (MPa)
Chinese-fir
Poplar
20
1.28
1.00*
2.88
1.00*
180.51
1.00*
30
1.35
1.05
*
3.15
1.09
*
209.25
1.16*
40
1.24
0.97*
3.11
1.08*
214.30
1.19*
50
0.83
0.65
*
2.85
0.99
*
190.94
1.06*
60
0.91
0.71*
3.12
1.08*
266.86
1.48*
70
0.73
0.57
*
2.78
0.97
*
220.26
1.22*
80
0.60
0.47*
2.03
0.70*
160.90
0.89*
20
1.30
1.00*
4.43
1.00*
395.87
1.00*
30
1.24
0.95
*
3.82
0.86
*
227.91
0.58*
40
0.96
0.74*
3.27
0.74*
211.34
0.53*
50
0.78
0.60
*
2.63
0.59
*
171.25
0.43*
60
0.62
0.48*
2.32
0.52*
141.98
0.36*
70
0.44
0.34
1.97
0.44
141.31
0.36*
110.55
0.28*
*
*
80 0.39 0.30* 1.61 0.36* Note: shows the ratio of the specific values at different temperatures divided by that at 20°C. *
Fig. 2 Stress relaxation curves at different temperatures: poplar showing a clear pattern of decreases in the modulus with increasing temperature, while no clear pattern in Chinese-fir
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Fig. 3 Fluidity of Chinese-fir and poplar at different temperatures
difference (60°C). However, the moduli of residual relaxation were similar. The higher range of relaxation during temperature elevation shows up because at the beginning of relaxation, lower temperatures are associated with little variation in fluidity (Fig. 3), while the modulus at this low temperature is large (Fig. 2). With the increase in temperature, the modulus decreased (Fig. 2) and fluidity increased (Fig. 3) as did relaxation. In the case of temperature reduction, at the beginning of the relaxation, the temperature was high, which reduces the modulus to a much greater extent compared with the modulus at low temperature. During the relaxation period, with the decrease in temperature, the fluidity also decreased. This turned out to be a similar residual modulus regardless of whether temperature increases or decreases. In the case of desorption of water-saturated specimens in the conditioning room (T: 25°C, RH: 65%), the fluidity curves (Fig. 5) show up quite different from those in a water-saturated condition at constant temperatures (Fig. 3). The fluidity of the latter increased quickly at the beginning of relaxation and increased more slowly afterwards (Fig. 3). The fluidity of the former increased slowly at the beginning of relaxation, but after the MC reached about 90%, the fluidity increased quickly until the MC reached about 45%, after which the fluidity increased slowly (Fig. 5). The fluidity of specimens in the state of airconditioned desorption has a much higher fluidity than water-saturated specimens at similar temperatures (in this case, 20°C water-saturated specimens and 25°C in the air-conditioned desorption state). According to the study by Ishimaru et al. (2001a), the desorption specimens are in an unstable state, showing “lower elasticity and strength and higher fluidity than wood in a true equilibrium state” (Tokumoto, 1994; Ishimaru et al., 2001b; Iida et al., 2002b). In addition, the unstable
state of the wood can be interpreted as a state with internal strain (Iida et al., 2002b) and has therefore a greater potential to release the stress, resulting in a higher fluidity in a water desorption state than that in a water-saturated state. At the beginning of the desorption state, the fluidity increased very slowly which is probably due to the fact that too much water in wood cell structures deters stress relaxation and more energy is required to release that water from the wood. When the MC is between about 90% to 45%, the wood has an appropriate amount of water to soften the lignin and hemicellulose and allows enough water molecules to lubricate the microfibril and facilitate the movement of microfibrils. That is probably why fluidity increases quickly when the MC is about 90%. Fluidity became almost constant after the MC decreased, in this case to about 45% (Fig. 5). On the one hand, this is attributed to the stress relaxation which has a tendency to decrease the modulus; on the other hand, the modulus increases with a decrease in MC when the MC is below 30%.
Fig. 4 Stress relaxation curves with changes in temperature. The temperature remained constant from 0−30 min and from 150−180 min.
ZHAO You-ke et al.: Viscoelastic properties of wood from Chinese-fir and poplar plantation
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state. The fluidity of specimens in the desorption state is much higher than in water-saturated specimens. This is probably due to the fact that the former is in an unstable state which can be interpreted as a state with internal strain and therefore has a greater potential to release strain.
Acknowledgement
Fig. 5 Fluidity curves as a function of decreasing MC. The specimens were tested in a conditioning room from a watersaturated condition to an air-dried condition.
4 Conclusions Hygrothermal conditions considerably affect the mechanical properties of wood from Chinese-fir and poplar plantations. The higher the MC or the temperature, the lower the strength of wood. Further investigation into the effect of a constant temperature on stress relaxation indicates that the specimens at high temperatures have a low modulus of relaxation and high fluidity. In the case of increasing temperatures the range of the modulus of relaxation is larger than in the case of decreasing temperatures, while the residual modulus shows no large differences. This is because this modulus at high temperatures declines more than that at low temperatures. The fluidity of specimens in a state of water desorption increases slowly at the beginning and then increases quickly until the MC reaches the EMC (equilibrium moisture content) and then stabilizes, which is quite different from the water-saturated
This work was funded by the Special Funded Project for Basic Scientific Research of the Nationallevel Research Institute for Public Welfare (No. CAFINT2007C03).
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