Characterization of tangential shrinkage stresses of wood during drying under superheated steam above 100 [degrees]C.
The characteristics of tangential stresses experienced by Japanese sugi (Cryptomeriajaponica D. Don) during drying under superheated steam with high temperature and pressure were examined in this study. A load cell capable of operating under high temperature and pressure inside an autoclave was used to measure the shrinkage stresses of water-saturated specimens under various drying conditions. The results showed that significant shrinkage stresses occurred even at 100 percent relative humidity (RH), probably due to decomposition and/or modification of wood components. The relationships between tangential shrinkage stresses and moisture content change at various RHs from high-temperature saturated steam drying were significantly different from those of the radial stresses reported in an early study. The occurrence of these stresses had a strong effect on the overall stress development during wood drying under high RH conditions. The shrinkage stresses were significantly reduced by pretreatment using high-temperature saturated steam to prevent drying defects such as surface checks. The occurrence of shrinkage stresses were also effectively restrained by drying under high-temperature steam. The study showed that temperature and RH level had no direct effect on the maximum tangential shrinkage stresses for drying wood above 20 percent RH and within 160 to 180 [degrees]C.
Under high-temperature drying of wood above 100 [degrees]C water evaporates very rapidly from wood surface due to large difference between equilibrium moisture content (EMC) in the surrounding air and moisture content (MC) of wood surface. Consequently, moisture gradient within the wood becomes much greater compared to conventional wood drying. This phenomenon can result in greater stresses and deformation in wood even at high MC conditions (Cheng et al. 2005). In order to overcome these problems in wood under high-temperature drying, it is crucial to understand the characteristics of the occurrence and propagation of these stresses in relation to the rheological properties of wood under high-temperature drying.
Several studies have been reported on high-temperature wood drying technology (Rosen et al. 1983, Kawasaki 1998, Haslett et al. 1999, Yoshida et al. 2000, Yamamoto et al. 2001). Among the studies, the work by Haslett et al. (1999) on pressure drying and pressure steaming of Radiata pine showed that pressure steaming significantly reduced twist and residual stress compared to high-temperature drying and atmospheric pressure steaming. Their study also showed that mechanical properties of structural lumber dried under these conditions were not adversely affected. It is suggested that further studies are needed to determine steaming conditions (e.g., pressure and duration) on warp and twist reduction and to understand fundamental wood response under cyclic pressure and steaming processes. Thus, detailed investigation on the changes in viscoelasticity, stress occurrence, and stress relief of wood during drying is of practical importance to the development of suitable conditions of high-temperature fast-drying technology.
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A specially fabricated clamping system to restrain the dimensional changes of wood specimens during drying was developed (Cheng et al. 2005). The device was successfully used to measure wood shrinkage stresses along the radial direction under high-temperature drying (Cheng et al. 2004). Wood is weakest along the tangential direction. As a result, surface checks from wood drying are more evident on the tangential surface (Wu and Milota 1995). Understanding of tangential stress development is thus of practical significance for quality drying of wood, especially flat-sawn boards. The objective of this study was to apply the developed technique for measuring shrinkage stresses along the tangential direction; and to study the effect of temperature and relative humidity (RH) on stress development during drying under hightemperature superheated steam.
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Materials and methods Specimens
The core of a 135-year old Japanese sugi (Cryptomeriajaponica D.Don) from Nara Prefecture, Japan, was used. The ovendry density of the sample was 0.32 g/[cm.sup.3], with a mean annual ring width of 2.2 mm. The dimensions of the specimens were 65 mm by 50 mm by 10 mm along the radial, tangential, and longitudinal directions, respectively. The specimens were kept under ambient conditions, and their MC was adjusted to about 120 percent by using a pressure impregnation method. The moisture-saturated specimens were then sealed in air-tight plastic bags and kept in a conditioning chamber for a week prior to the experiment.
Experiments were conducted at four temperatures (80, 140, 160, and 180 [degrees]C) and six RH levels (0, 20, 40, 60, 80, and 100%). To study the effect of presteaming on shrinkage stress development, experiments were also made by presteaming the samples at 180 [degrees]C for 30 minutes and followed by drying under pressure at six RH levels (0, 20, 40, 60, 80, and 100%).
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Measurement of shrinkage stresses
A special clamping device was used to restrain the shrinkage movement of the specimen during drying by keeping the dimensions of the specimen constant (Cheng et al. 2005). Figure 1 shows a schematic diagram of the apparatus for measuring shrinkage stress when the shrinkage deformation is restrained during drying under superheated steam. The upper chuck is integrated with the framework of the apparatus, and the restrained shrinkage force is transferred to the load cell through the lower chuck. A load cell, which is both pressure and heat resistant, was used to measure the continuous change in the tangential tensile stresses (i.e., shrinkage stresses) within the specimen from the moisture-saturated conditions to the EMC or until checks occurred under high-temperature superheated steam. The measurement of the shrinkage stresses was initiated after the specimen was placed in the autoclave for 3 minutes in order to allow the specimen to reach an equilibrium condition (Dwianto et al. 1999, Higashihara et al. 2000, Ohshima et al. 2001). Five replicates were used for each test condition.
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Measurement of MC
The MC of the specimens was calculated based on the ovendry weight. Under each specific temperature and RH level, the wet weight of each specimen was determined immediately upon removal from the autoclave at a fixed interval of time. The samples were finally ovendried at 104 [degrees]C to determine their ovendry weight. Five samples were used for this measurement, and the result was averaged.
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Results and discussion
Figure 2 shows the trend of variation in the shrinkage stresses at 80 [degrees]C and varying RHs as a function of drying time. As shown, the shrinkage stresses increased as the drying proceeded at 80 [degrees]C, and the rate of stress increment increased significantly with reduction in RH. The maximum stresses reached a similar value under all RHs. Thus, the maximum stress was not directly affected by RH used. Figure 3 shows the relationship between tangential shrinkage stresses and sample MC at various RH levels and 80 [degrees]C. At 0 percent RH level, stresses developed at wood MC level as high as 80 percent, and increased rapidly as MC was further reduced. Increase in RH levels delayed early stress development and significant stresses occurred only at wood MC less than 40 percent for drying at RH levels greater than 20 percent. Compared to the results reported by Cheng et al. (2005), the rate of change in tangential shrinkage stress was in a similar trend as this along the radial direction for drying at 80 [degrees]C.
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Figure 4 shows the variation of shrinkage stresses during drying under various RH levels at 140, 160, 170, and 180 [degrees]C, respectively. It can be seen in Figure 4 that irrespective of temperature, the maximum shrinkage stresses gradually reduced as the RH increased from 0 percent to 100 percent. At 0 percent RH, wood specimen experienced an uneven rate of drying as the water evaporated rapidly from the surface under high temperature, hence creating the greatest shrinkage stresses. However, at 80 percent RH, the duration taken by the specimen to reach the EMC increased, with a corresponding increase in the time taken for the specimen to reach checking failure. Figure 4(D) shows that at 180 [degrees]C, wood specimens experienced the same variation in shrinkages stresses at 80 percent and 100 percent RH in the first 25 minutes However, the shrinkage stresses at 80 percent RH began to surpass those at 100 percent RH after that. This shows that the shrinkage stresses caused by drying actually occurred only after the first 25 minutes, and the difference between these two curves indicates the true shrinkage stress resulted from drying, which is in fact relatively small. Besides, Figure 4(D) also shows a different trend of shrinkage stress variation before and after the first 10 minutes Samples dried at 20 percent, 40 percent, and 60 percent RH experienced a point of deflection at 1, 4, and 7 minutes, respectively. The stresses within the above durations might be caused by the decomposition or modification of wood components. The changes in stress level and trend, which occurred with subsequent drying, represent the shrinkage of cell wall materials due to loss of moisture.
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The trend of the stress development from 20 percent to 80 percent RH can be divided into three stages. An initial or primary stage was caused by decomposition or modification of wood components (for example, the first 7 minutes for 60 percent RH and 25 minutes for 80 percent RH). The initial stage was followed by a period of continuous increase due to wood shrinkage during drying. The greatest stress occurred at the final stage of drying, where the specimen experienced clearly visible cracks. This is the unique trend of variation in the shrinkages stresses during drying under high temperature superheated steam. The trend can be typically represented by the curve of 180 [degrees]C and 80 percent RH. An important observation here is that all the test specimens reached similar MCs of below 5 percent within the duration of measurement. At RHs above 20 percent, the specimens showed basically similar levels of the maxirnum stress when they experienced obvious failure, regardless of the RH levels.
Figure 5 shows a comparison of the tangential stresses at 100 percent RH and various temperatures as a function of time. It can be seen that large shrinkage stresses occurred even at 100 percent RH. The occurrence of such stresses may have an effect on shrinkage stresses during drying under high RH. The results clearly indicate the occurrence of shrinkage stresses, which were not due to drying process, especially at above 160 [degrees]C. As discussed in earlier studies (Cheng et al. 2005, Higashihara et al. 2004), these stresses may be attributed to the changes in the main components of the wood materials.
Figure 6 shows a comparison of the shrinkage stresses as a function of MC at various RH levels and 180 [degrees]C temperature. Except at 0 percent RH level, changes in RH had little effect on the relationship between shrinkage stresses and MC (i.e., all curves overlapping). Thus, these curves could be represented by a single master curve with similar maximum shrinkage stresses. This result was probably due to the formation of checks along the radial direction of the wood caused by the stress concentration during high temperature drying. The above characteristics of tangential shrinkage stresses are clearly different from the radial shrinkage stresses, which tend to decrease with increasing RHs (Cheng et al. 2004). Also, tangential shrinkage stresses were significantly higher than the radial stresses under the high temperature saturated steam drying, similar to the behavior under conventional wood drying process (Cheng et al. 2005).
Figure 7 illustrates the correlation between the maximum tangential shrinkage stresses and RH at three temperatures (i.e., 160, 170, and 180 [degrees]C). At RH below 20 percent, the maximum stresses increased drastically, whereas at above 20 percent RH levels, they remained much the same (i.e., without being affected by the surrounding temperature and RH). This result shows that at high temperatures, even when the specimens were exposed to high RH for a prolonged duration, the maximum shrinkage stresses could not be lowered to a level which could effectively release the internal stresses. Once the shrinkage stresses reached the maximum values, the specimens cracked. It is difficult to reduce the maximum stress in the tangential direction because of the existence of the nondrying related shrinkage stresses, which prohibited the reduction of the internal stresses during drying.
Since a specific level of stress is needed to initiate drying checks, the conditions of high-temperature drying with low maximum stress can be established, and the drying defects can be prevented. One possibility is by avoiding the shrinkage stresses caused by the decomposition of wood components under high temperature and high RH. In this respect, attempt was made to reduce the maximum shrinkage stresses during the drying process by introducing an 180 [degrees]C steam pretreatment. Figure 8 shows the effects of stream pretreatment on the shrinkage stresses during drying. At 180 [degrees]C and 80 percent RH, a 30-minute steam pretreatment was found to reduce the shrinkage stress to a large extent (i.e., below 0.1 MPa). Based on this, further drying experiments were conduced for other RH levels at 180 [degrees]C and results are shown in Figure 9. As shown, compared to untreated specimens [Figure 4(D)], steam pretreated specimens recorded significantly lower maximum shrinkage stresses for all RH levels. Although the specimens were restrained from changing in dimensions during drying, at 80 percent RH, no check was observed even when the wood sample MC was reduced to 13 percent.
The characteristics of tangential shrinkage stresses of wood from drying under high temperature and high pressure superheated steam were investigated. The results showed that under high temperature, high tangential stresses still occurred even at 100 percent RH. The occurrence of these stresses had a strong impact on overall shrinkage stress development, which tends to occur during the high RH drying process. These shrinkage stresses could be reduced by high-temperature, saturated-steam pretreatment to prevent subsequent drying defects.
The results also showed that at 180 [degrees]C, except for 0 percent RH, the tangential shrinkage stresses demonstrated similar correlation with MC at all other RH levels. The relationship can be represented by a single master curve with similar maximum values. At above 20 percent RH levels, the maximum shrinkage stresses, that lead to the surface checking, were not directly related to drying temperature and RH in a temperature range of 160 to 180 [degrees]C.
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Wanli Cheng Toshiro Morooka Qinglin Wu * Yixing Liu
* Forest Products Society Member.
The authors are, respectively, Associate Professor, Key Lab. of Bio-based Material Sci. and Technology (Northeast Forestry Univ.), Ministry of Education, Harbin, China (email@example.com); Associate Professor, Research Inst. for Sustainable Humanosphere, Kyoto Univ., Kyoto, Japan (firstname.lastname@example.org); Professor, School of Renewable Natural Resources, Louisiana State Univ. Agri. Center, Baton Rouge, Louisiana (QWu@agcenter.lsu.edu); 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 by the National Natural Sci. Foundation of China (Grant No. 30471354); by Scientific Research from the Ministry of Culture, Sports, Sci. and Technology of Japan (Grant No. 14560135); and by High-Speed Drying of Sugi from the Forestry and Forest Products Research Inst. (FFPRI). This paper was received for publication in June 2007. Article No. 10297.
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|Author:||Cheng, Wanli; Morooka, Toshiro; Wu, Qinglin; Liu, Yixing|
|Publication:||Forest Products Journal|
|Date:||Nov 1, 2007|
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