3.1 Appearance change
Table 1 showed the contrast of the outer surface color of the bamboo ring under different moisture contents. In CIE L*a*b* color system, L* represents the lightness, a* represents the red-green value, b* represents the yellow-blue value. With the decrease of moisture content, the L* value increased gradually at first, indicating that the bamboo ring became lighter. When the moisture content was 0% after drying, the L* value decreased slightly, indicating that the bamboo ring became darker slightly. The green-red (a*) and yellow-blue (b*) color coordinates also changed significantly with the decrease of moisture content. The positive values of Δa* indicated a tendency to turn red for the outer surface of the bamboo ring. The positive values of Δb* indicated a tendency to turn yellow for the outer surface of the bamboo ring. The color change of the outer surface of the bamboo ring was mainly due to lignin and extractives. During drying process, with the movement of water, part of the water-soluble extracts would move to the surface, causing oxidative discoloration. When drying was carried out at high temperature later, this discoloration was even more severe. At the same time, it was accompanied by complex chemical reactions such as oxidation, degradation and condensation of lignin (Wang, 2015).
Table 1 The contrast of the outer surface color of the bamboo ring under different moisture contents
As the moisture content decreased, the dry shrinkage stress gradually increased, especially at the outside bamboo wall and near the end cross sections of the bamboo ring. Figure 3 showed the variation of cracks morphology in the bamboo ring under different moisture contents. On the outside bamboo wall, cracks appeared as short vertical lines in the longitudinal direction, while as radially zigzag wavy lines in the cross section. Finally, two almost symmetrical cracks were developed into large cracks, extending radially to the inside of the bamboo wall, and longitudinally running through the entire height, even causing the entire bamboo ring to be split in half. When large cracks were formed and expanded, a reverse compressive stress would be generated on the rest of the bamboo ring in the circumferential direction, so the width of other small cracks gradually decreased and tended to close.
Figure 4 showed the changes of the wall thickness and longitudinal height of the bamboo ring under different moisture contents. It could be seen that with the decrease of moisture content, both the wall thickness and longitudinal height of the bamboo ring tended to decrease. The absolute dry shrinkage rate of the wall thickness was 8.14%, while the absolute dry shrinkage rate of the longitudinal height was 0.48%. Obviously, the dry shrinkage rate of the radial dimension was significantly larger than that of the longitudinal dimension.
3.2 Shape fitting
Figure 5 showed the shape fitting of the bamboo ring under different moisture contents. In Fig. 5a, b, there was no obvious crack, so the shape of each bamboo ring could be fitted by only one circle. In Fig. 5c, d, there were obvious cracks, so each bamboo ring was supposed to be divided into two parts according to the cracks first, then circular fitting of them were carried out separately. Table 2 showed the comparison of the circular fitting parameters of the bamboo ring under different moisture contents, including radian, radius and arc length. It could be seen that with the decrease of moisture content from 64.03–0%, the radian of the bamboo ring decreased from 6.28 rad to 4.89 rad decreasing by 22.13%; the radius of the bamboo ring first decreased slightly from 4.2 cm to 4.12 / 4.11 cm, a decrease of 2.02%, and then increased to 5.06 / 5.13 cm, an increase of 21.31%; the arc length of the bamboo ring decreased from 26.41 cm to 24.91 cm, a decrease of 5.68%. The absolute dry shrinkage rate of the bamboo ring arc length was 6.02%, which was not significantly different from the dry shrinkage rate of the wall thickness (8.14%). This was related to the fact that the internodes hardly contain any radially oriented cells that could limit the dry shrinkage.
Table 2 The comparison of the circular fitting parameters of the bamboo ring under different moisture contents
Moisture content (%)
|
Radian (rad)
|
Radius (cm)
|
Arc Length (cm)
|
64.03
|
6.28
|
4.20
|
26.41
|
19.03
|
6.28
|
4.15
|
26.04
|
11.62
|
3.14 + 3.09 = 6.23
|
4.12 / 4.11
|
12.94 + 12.70 = 25.64
|
0
|
2.48 + 2.41 = 4.89
|
5.06 / 5.13
|
12.55 + 12.36 = 24.91
|
With the decrease of moisture content, the asynchronous circular dry shrinkage occurred in the radial direction of bamboo ring. The degree of dry shrinkage on the outside bamboo wall was greater than that on the inside bamboo wall. On the one hand, the outside bamboo wall played a leading role in dry shrinkage, even driving the dry shrinkage of the inside bamboo wall (Yan et al., 2020). On the other hand, the dry shrinkage of the outside bamboo wall was limited by the inside bamboo wall and bonding strength between cells. When the moisture content was high, the dry shrinkage stress was relatively small and far less than the bonding strength between cells, the bamboo ring would not crack significantly, but circumferential shrinkage occurred, that was, the radian basically unchanged, and the arc length and radius were slightly reduced. When the moisture content was low, the dry shrinkage stress was greater than the bonding strength between cells, and the bamboo ring would tend to crack. Due to the dry shrinkage stress of the outside bamboo wall was greater than that of the inside bamboo wall, the cracks always occurred in the outside bamboo wall first, and then gradually expanded to the inside bamboo wall. The cracked bamboo ring lost the restriction of intercellular binding force, resulting in a reduction of its radian and an increase of its radius. The decrease of the arc length of the bamboo ring further explained the circumferential dry shrinkage of the bamboo ring.
3.3 Fiber volume fraction
Figure 6 showed the variation of the area value and volume fraction of different tissues in the bamboo ring under different moisture contents. With the decrease of moisture content, the area of the fiber sheath, the rest except the fiber sheath and the cross section of the bamboo ring all decreased, the volume fraction of the fiber sheath decreased, while the volume fraction of the rest except the fiber sheath increased. It was defined that the volume fraction of the fiber sheath was equal to the fiber sheath area divided by the cross-sectional area, and the volume fraction of the rest except the fiber sheath was equal to the rest area divided by the cross-sectional area. The cross-sectional area was equal to the sum of the fiber sheath area and the rest area, consisting of the area of the fibers, vessels, sieve tubes and parenchyma cells. It was obvious that the dry shrinkage rate of the fiber sheath was greater than that of the rest except the fiber sheath and the cross section. The dry shrinkage rate of the fiber-rich vascular bundles was greater than that of the parenchyma cells, which was the main reason for the dry shrinkage of the bamboo ring. The fiber volume fraction of the outside bamboo wall was larger, resulting in larger dry shrinkage.
3.4 Area of different types of fiber sheaths
According to the classification of fiber sheaths in Fig. 1, the change in the area of different types of fiber sheaths under different moisture contents were studied as shown in Fig. 7. With the decrease of moisture content, the decreasing trend of different types of fiber sheaths was similar. Because there was no essential difference in the structure of fibers in different types of fiber sheaths, which were all sclerenchyma cells with similar chemical components and structures. The only significant difference among different types of fiber sheaths was the fiber content. The dry shrinkage of different types of fiber sheaths was almost the same, but the absolute dry shrinkage varied with the fiber content.
3.5 Area of divided cross sections
Figure 8 showed the variation in area value and change rate of different cross-sectional zones divided according to vascular bundles type Ⅰ-Ⅳ under different moisture contents. With the decrease of moisture content, the reduction of the inner part of the cross-section area was larger in absolute value, but smaller in change rate. This might be because there were fewer vascular bundles but more parenchyma cells in the inside bamboo wall than the outside. The dry shrinkage of the inside bamboo wall was mainly caused by the dry shrinkage of a large number of parenchyma cells and a small number of vascular bundles, while the dry shrinkage of the outside bamboo wall was mainly caused by the dry shrinkage of a small number of parenchyma cells and a large number of vascular bundles. Besides, there was a big difference in the area value among different regions. In comparison, the type Ⅳ area was the largest, followed by the type Ⅲ area, the type Ⅱ area was the third, and the type Ⅰ area was the smallest. There was a larger area change rate of the outside bamboo wall because of its high fiber content and greater extent of dry shrinkage of fibers. There was a larger absolute value but a smaller change rate of dry shrinkage in the inside bamboo wall due to its large number of parenchyma cells.
3.6 Arrangement of vascular bundles
Figure 9 showed the changes of arrangement of vascular bundles under different moisture contents. It could be clearly observed from Fig. 9a that with the decrease of moisture content, the vascular bundles would be displaced in both radial and tangential directions. The series lines of vascular bundles were superposed based on the middle position of the bamboo wall thickness, but the dry shrinkage on the outside bamboo wall was greater actually, so the real superimposed benchmark should be shifted toward the inside bamboo wall, and the vascular bundles on the outside bamboo wall were supposed to have larger radial dry shrinkage displacements. Figure 9b showed the variation of a series of consecutive radial spacings of the bamboo ring under different moisture contents, trying to quantify the radial dry shrinkage. From the initial moisture content of 64.03% to the final moisture content of 0%, all the radial spacings decreased finally. The dry shrinkage rate for the sum of all radial spacings was 5.96%, in line with the dry shrinkage rate of the bamboo wall thickness above. As the moisture content decreased, not all spacing decreased constantly. The reduction of the different spacing did not gradually decreased from the outside to the inside of the bamboo wall. This might be due to the continuous dry shrinkage along the direction of bamboo wall thickness, and the adjacent vascular bundles had similar fiber content, resulting in their similar dry shrinkage. Large displacements of vascular bundles might happen actually due to dry shrinkage, but the displacements in terms of the adjacent vascular bundles were small. It was not advisable to quantify and compare the changes in the dry shrinkage displacement of vascular bundles with too small unit spacing. Taking the middle of the bamboo wall thickness as the benchmark, the bamboo ring was divided into two integral parts, containing 1–8 and 9–13 spacing respectively. The results showed that the radial dry shrinkage of the outside bamboo wall was 6.25%, while that of the inside bamboo wall was 5.68%. This was consistent with the statement that the dry shrinkage rate of the outside bamboo wall was greater than that of the inside bamboo wall.