Transverse mechanical behavior of wood under high temperature and pressurized steam.
Transverse mechanical behavior of Japanese sugi (Cryptomeria japonica D. Don) under high temperature and pressurized steam environment was investigated. Measured tensile strength was compared with published data of transverse shrinkage, compression yield, and relaxed compression yield stresses. The results showed that at 100 percent relative humidity (RH), tensile strength of the wood along both radial and tangential directions gradually decreased, and the destructive tensile strain increased with increase in temperature. The increase in the destructive strain was more significant when temperatures increased above 100[degrees]C. The relationship between tensile strength and temperature was similar to that of the maximum shrinkage stress and temperature from drying under high RH and different temperatures.
Tensile strength and relaxed compression yield stress decreased at a similar rate with increase in temperature. The relationship of radial tensile strength and temperature agreed well with that of compressive yield stress and temperature in a temperature range from 80 to 180[degrees]C at 100 percent RH. The maximum shrinkage stress values from drying were similar to those of relaxed compression yield stresses at the same temperature and RH conditions. The significant reduction of shrinkage stresses during drying under high temperatures and pressurized superheated steam was caused by stress relaxation.
It is well known that stresses develop during drying of wood due to uneven shrinkage between radial and tangential directions and moisture gradients within a given board. When the stress exceeds the inherent tensile strength, wood begins to experience cracking phenomenon. The extent of cracking depends largely on modulus and strength of the wood. The Young's modulus and strength of dry wood decrease slightly with increase in temperature. Both properties can, however, be reduced significantly when the wood is in wet condition (Gerhards 1982, Tang and Zhao 2002, Cao et al. 1998, Furuta and Hiroshi 1999, Furuta et al. 2000, Furuta 2002). Since temperature, moisture content (MC), and drying speed vary during drying process, the tensile strength of wood may also vary depending on the drying conditions within a given board. Many reports on the effects of temperature and MC on wood tensile strength at temperatures below 100[degrees]C have been published (Furuta and Hiroshi 1999, Furuta et al. 2000, Furuta 2002; Iida et al. 1984, Iida and Norimoto 1987). However, there is very limited data on transverse tensile and compressive behavior of wood under high temperatures above 100[degrees]C and pressurized steam conditions.
A specially fabricated clamping system to restrain the dimensional changes of wood specimens during drying under high temperature and pressurized steam was developed (Dwianto et al. 1999, Cheng et al. 2005). The device was successfully used in applying load to small wood specimens and monitoring wood compression yield and relaxed compression yield stresses (Ohshima 2002) and wood shrinkage stresses (Cheng et al. 2004, 2007). It was shown that the shrinkage of wood was significantly reduced under high temperature and high relative humidity (RH) conditions. It was hypothesized that the phenomenon was related to the softening of wood under high temperature and high RH. However, the extent of actual softening was not clear. The objectives of this study were 1) to analyze the direct relationship between shrinkage cracking and transverse tensile characteristics in moisture saturated wood, and 2) to compare measured tensile strength with published data on the maximum shrinkage, compression yield, and relaxed compression yield stresses from the same wood species. The previously developed clamping system, which was capable of accurately applying and measuring tensile load under high temperature and pressurized steam inside an autoclave (Dwianto et al. 1999, Cheng et al. 2004, 2005), was used in the study.
Materials and methods
The cores of 135-years old Japanese sugi (Cryptomeria japonica D. Don) were used in this study. Experimental samples of two different dimensions were prepared based on the orientation of two primary timber applications, i.e., 65 mm (radial) by 50 mm (tangential) by 10 mm (longitudinal) in flatsawn boards; and 50 mm (radial) by 65 mm (tangential) by 10 mm (longitudinal) in quartersawn boards. The mean air-dry specific gravity (SG) and growth-ring width was 0.32 (SD: 0.015) and 2.2 mm (SD: 0. l 5 mm), respectively, based on 20 randomly selected specimens for each direction. Prior to the testing, the samples were vacuum pressure impregnated with water up to a saturated MC of 120 percent, which were then kept in air-tight bags and placed in a conditioning room for more than a week. In order to accurately monitor changes in wood properties under high temperature and high pressure steam over time, it was important for the internal temperature and MC of the samples to reach equilibrium with the surroundings quickly. Consequently, the samples were prepared with the smallest dimension along the longitudinal direction.
Experimental set-up and methods
Tensile stresses at temperature above 100[degrees]C were applied and measured within a steel autoclave equipped with a specially designed hydraulic experimental system (Hisa Co. Ltd., Japan). The apparatus consisted of upper and lower platens, and the lower platen was moved upward or downward using a hydraulic system (PQCS2R, Taiyo, Japan). The degree of movement was measured by a displacement gauge to an accuracy of 0.001 mm. A pressure- and heat-resistant load cell was installed in the autoclave (CLP-5KNHS by Tokyo Sokki Kenkyujo Co. Ltd., Japan), as shown in Figure 1. Tensile stresses at temperatures below 100[degrees]C were applied and measured using a similar set-up of load cell and a constant temperature water bath on a universal testing machine (TOM5000X by Shinkoh Com. Ind. Co. Ltd., Japan)
Matched wood specimens with initial saturated MC of 120 percent were placed in different constant temperature water baths from 20 to 100[degrees]C for 10 minutes, or in 120 to 180[degrees]C saturated steams for 3 minutes The exposure allowed samples to reach complete equilibrium with the corresponding surrounding conditions. The samples were then secured onto the tensile fixture and tested until failure occurred under each specific condition. Five replicates were tested along each of the radial and tangential directions, and means were calculated. The gauge length of the tensile specimen was 33 mm and the loading speed was 2 mm/min. To study the effect of presteaming on the tensile behavior, matched radial and tangential specimens were exposed to saturated steam at 160 and 180[degrees]C for periods of 3, 30, and 60 minutes and then tested.
[FIGURE 1 OMITTED]
Results and discussion
Tensile stress and strain relationship
Figure 2 shows typical tensile stress and strain curves from both radial and tangential directions at various temperatures. Linear stress and strain relationship was observed during the initial stage of loading, especially at low temperatures and/or along the radial direction. Temperature increase reduced the slope (i.e., tensile modulus) and extent of the linear portion of the curve. Thus, wood became less stiff and had more plastic flow at higher temperatures along both radial and tangential directions.
Tensile strength and destructive strain
Figure 3(a) shows the variation of tensile strength of wet samples with temperature ranging from 20 to 180[degrees]C. With increasing temperature, the samples experienced a significant reduction in tensile strength along both radial and tangential directions because of wood softening. However, the maximum tensile strain increased significantly with increase in temperature up to 120[degrees]C (Fig. 3(b)). The strain remained much the same (radial direction) and slightly decreased (tangential direction) at temperatures above 120[degrees]C. It is expected that wood with MC below the fiber saturated point experiences a similar phenomenon. The fact that significant increase in tensile strain occurred due to the combined effects of stress, moisture, and heat may be used to explain the effectiveness of high temperature and high pressurized steam in preventing wood cracking during drying process.
It can be seen from Figures 2 and 3 that both tensile strength and maximum strain in the radial direction were higher than those along the tangential direction at the same temperature. An abrupt failure of the tensile specimens was observed when failing in the radial direction at below temperatures below 100[degrees]C. However, at higher temperatures, tensile stresses did not experience any sudden drop (i.e., gradual failure), even when the sample was completely failed. Observation of the fractured surface from failed radial specimens above 140[degrees]C revealed that the wood fibers were pulled apart one by one, creating a fibrous failure pattern, which prevented sudden reduction in tensile stress. For the tangential direction, the maximum stress dropped dramatically once the sample failed, and the fractured surface was smooth.
[FIGURE 2 OMITTED]
The effects of prestreaming treatment time on the radial and tangential tensile strengths of wood are shown in Figure 4, which depicts the relationship between tensile strength and steam treatment time at 160 and 180[degrees]C. Extension of steam treatment duration did not have a strong effect on the tensile strength, and only a slight decrease in tensile strength was observed. Both radial and tangential tensile strength at 160[degrees]C experienced a greater reduction with treatment time than at 180[degrees]C. This could be due to the rapid achievement of equilibrium condition at 180[degrees]C and softening within a short period of time, or an initiation of change in the cell wall compositions. At higher temperatures, radial tensile strength underwent a great reduction after the 30 minutes treatment, but remained almost constant up to the 60 minutes treatment level. At 180[degrees]C, the tangential tensile strength did not experience any noticeable change over treatment periods of 3, 30, and 60 minutes
Comparison with shrinkage stress from drying
Tensile strength of wood under high temperature and high humidity conditions was closely related to the values of shrinkage stress at cracking during drying (i.e., the maximum shrinkage stress) reported by Cheng et al. (2004, 2007). Figure 5 shows tensile strength and maximum shrinkage stress at
60 percent and 80 percent RH and different temperatures. Both curves show a similar trend of stress reduction at higher temperatures. If the net shrinkage stress during drying was calculated by subtracting the maximum shrinkage stress at 100 percent RH and corresponding temperatures, then the reduction in the maximum drying stress was even more apparent. During early stages of drying, the wood surface layer experiences high tensile stress (i.e., shrinkage stress) due to shrinkage deformation. An extended period of stressed condition causes some stress relaxation. Hence, the maximum shrinkage stress when cracks occur tends to be smaller than the tensile strength under similar environmental conditions. Since this difference is closely related to the relaxation of tensile stress during the drying process, it is, therefore, necessary to examine the relaxation of tensile stress during drying under the same temperature and RH.
[FIGURE 3 OMITTED]
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[FIGURE 5 OMITTED]
Comparison with compression stresses
A comparison was also made among tensile strength, compression yield stress, and relaxed compression yield stress from the same wood species measured at the same environment conditions (Ohshima 2002). Figure 6 shows radial relaxed compression yield stress (1 hour relaxation after 50 percent compression ratio) and tensile strength with temperature in wet samples at 100 percent RH. Both responses had a similar decrease with increase in temperature from 20 to 140[degrees]C. At temperatures above 140[degrees]C, the stress relaxation value dropped drastically with increase in temperature, showing an evident deviation from the tensile stress curve. Such phenomenon could be due to a significant change in the chemical composition of wood above 140[degrees]C, which led to a substantial reduction in the compression yield stress caused by stress relaxation.
[FIGURE 6 OMITTED]
Figure 6 also shows a comparison of tensile strength and compression yield stress values at the 50 percent compression ratio as a function of temperature at 100 percent RH. Tensile strength and compression yield stress values had almost the same degree of decrease with increase in temperature above 40[degrees]C. Assuming that a similar relationship between these two stresses also exists under other RH conditions, the values of tensile strength above 40[degrees]C under different RH conditions may be replaced with those of compression yield stress. If the values of compression yield stress and tensile strength are almost the same, it is anticipated that the values of initial compressive stress relaxation (based on compression yield stress) should be very similar to those of the initial relaxed tensile stress (based on the tensile strength). Since compression yield stress is measured over momentary deformation, whereas shrinkage stress can only be obtained from shrinkage deformation over an extended period of drying, the values of shrinkage stress and tensile relaxation are tentatively substituted with those of compression yield and relaxation stresses, respectively, in order to examine the tensile stress relaxation under different temperature and RH conditions.
Variation of the maximum drying shrinkage stress, compression yield stress at the 50 percent compression ratio, and relaxed compression yield stress as a function of temperature at different RH levels is shown in Figure 7. The results indicated that, except for 0 percent RH, the maximum drying shrinkage stress and compression stress relaxation measured under the same temperature and RH conditions were similar, confirming the assumption that a lower shrinkage stress during high temperature and high humidity superheated steam drying was due to stress relaxation. Figure 7 also shows the compression yield stress was higher than the shrinkage stress under similar RH. Both had similar decrease with increase in temperature from 0 to 40 percent RH. However, the shrinkage stress experienced much greater reduction than compression yield stress above 60 percent RH. Hence the reduction in shrinkage stress was greater than the stress reduction incurred during wood softening.
[FIGURE 7 OMITTED]
The effects of temperature on the mechanical properties of moisture saturated wood were investigated in this work. In reality, the influence of moisture and heat on wood with equilibrium and non-equilibrium moisture conditions is expected to be different. Under the equilibrium condition, the bonding between different wood components becomes weaker due to adsorbed moisture, creating Brownian movement (i.e., transition from glass to rubberized condition) and leading to wood plasticization. Under non-equilibrium conditions, moisture is lost from the wood cell wall (drying process) causing the hydrogen bonding between wood elements to be broken, and other structural instability that accounts for wood plasticization. It is, therefore, important to investigate the changes in wood mechanical properties, not only under moisture equilibrium condition but also during superheated steam-drying process (non-equilibrium condition).
Transverse mechanical properties of moisture saturated wood under high temperature and high pressurized steam at below and above 100[degrees]C were studied in relation to crack formation in wood from drying. At 100 percent RH, tensile strength of wood decreased gradually, while the destructive tensile strain increased when the temperature increased. The trend of the destructive tensile strain increase with temperature became more significant above 100[degrees]C. At 100 percent RH, tensile strength and relaxed compression yield stress of wetwood dropped at a similar rate with increasing temperature. Tensile strength showed similar variation with temperature as the maximum shrinkage stress
under high RH conditions, both undergoing a greater degree of reduction at higher temperatures. The results clearly presented the close relationship between the shrinkage stress caused by shrinkage strain during drying, and the stress relaxation occurring over the drying duration. At 100 percent RH, radial tensile strength and compression yield stress of wood showed good agreement when the temperatures varied from 80[degrees]C to 180[degrees]C. At the corresponding temperature and RH levels, the value of the maximum drying shrinkage stress was similar to that of the relaxed compression yield stress. It is concluded that significant reduction in shrinkage stress during high temperature and high humidity superheated steam drying was caused by stress relaxation.
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Qinglin Wu *
The authors are, respectively, Professor, Key Lab. of Bio-based Material Sci. and Technology (Northeast Forestry Univ.), Ministry of Education, Harbin, China (firstname.lastname@example.org); Associate Professor, Research Inst. for Sustainable Humanosphere, Kyoto Univ., Kyoto, Japan (email@example.com); Professor, School of Renewable Natural Resources, Louisiana State Univ. Agri. Center, Baton Rouge, Louisiana (firstname.lastname@example.org); and Professor, Key Lab. of Bio-based Material Sci. and Technology (Northeast Forestry Univ.), Ministry of Education, Harbin, China (email@example.com). This work was financially supported, in-part, by the National Natural Sci. Foundation of China (Grant No. 30471354), and by Scientific Research from the Ministry of Culture, Sports, Sci. and Technology of Japan (Grant No. 14560135).This paper was received for publication in January 2008. Article No. 10449.
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|Author:||Cheng, Wanli; Morooka, Toshiro; Wu, Qinglin; Liu, Yixing|
|Publication:||Forest Products Journal|
|Date:||Dec 1, 2008|
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