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Article

Bending Performance of Plantation Teakwood and Its Mechanism Based on Radial and Tangential Directions

by
Linghua Yao
1,2,
Lina Ji
1,*,
Zhangheng Wang
2 and
Junnan Liu
1
1
College of Landscape Architecture and Art, Xinyang Agriculture and Forestry University, Xinyang 464000, China
2
College of Furnishings and Art Design, Central South University of Forestry and Technology, Changsha 410004, China
*
Author to whom correspondence should be addressed.
Forests 2024, 15(12), 2203; https://doi.org/10.3390/f15122203
Submission received: 9 November 2024 / Revised: 1 December 2024 / Accepted: 9 December 2024 / Published: 14 December 2024
(This article belongs to the Section Wood Science and Forest Products)
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Abstract

:
Curvilinear components made from solid wood bending not only enhance the use value of wood but also add a unique aesthetic to solid wood products, in line with the concept of green and sustainable development. In this study, we focused on the influence of the direction of the wood growth rings on the bending performance of teakwood and its bending mechanism, and radial and tangential bending experiments were conducted on teakwood using the synergistic softening method of a triethanolamine compounding solution and steaming. The results showed that the radial bending coefficient of the teakwood after softening treatment was 1/9.26, 41.39% higher than that for chordwise bending, 1/15.8. Through a macroscopic analysis, the stress distribution of the teakwood during radial bending was found to be uniform, while during tangential bending, the stress was mainly borne by many longitudinal growth rings, which are prone to the bending “destabilization” phenomenon. From SEM and AFM observations, it was found that the earlywood zone and latewood zone on the outermost tensile surface of the tangential bending are longitudinal; the stress redistribution problem still exists in the bending process; and ray parenchyma are the most vulnerable part, gradually extending to the earlywood zone and causing more serious bending tearing. It can be seen that the bending performance of teakwood is not only affected by the softening process but also obviously constrained by the direction of the growth rings, which is of great significance for the optimization of hardwood bending processing technology and product design.

1. Introduction

Teak is a type of wood recognized around the world as being precious, has a beautiful texture, has an aromatic smell, is an excellent material, and has other characteristics, so it is widely used to manufacture high-grade wood products, but its price is high and natural forest resources are in short supply [1,2]. China began to introduce and cultivate teak in plantation forests in the 1960s, and after decades of development, a large-scale plantation base has been formed, which not only significantly improves the production of teak but also effectively alleviates the pressure on natural forest resources and provides stable and high-quality raw teakwood materials for the Chinese market [3]. However, as a kind of oil-rich broadleaf hardwood [4], the traditional wood processing technology makes it difficult to achieve effective bending of teakwood, which to a certain extent restricts the scope of application of teakwood materials and the innovative development of new products.
As early as the 1830s, the German furniture manufacturer Michael Thonet adopted hydrothermal treatment technology, successfully softened beech wood, then bent it into various furniture bending components and made a series of creative bentwood furniture [5]. With the development of science and technology, researchers have explored the bending species and softening technology of solid wood in depth and achieved significant results.
At present, scholars at home and abroad mainly analyze the bending process of wood regarding its parallel grain tensile deformation, its parallel grain compression deformation, and the changes in its cellulose molecular chain to reveal its bending mechanism. For example, Li [6] analyzed the bending mechanism of wood from macroscopic and microscopic perspectives; from the macroscopic point of view, wood can achieve bending as a result of the action of tension on the convex side of the specimen, compression on the concave side, and shear on the neutral layer, while from the microscopic point of view, it is a result of the change in the molecular bond angles and bond lengths of the cellulose molecular chains on the tensile side of the wood, the relative sliding between the molecular chains, and the curling of the molecular chains on the compressive side. Luo [7] used fast-growing fir as a test material, deduced the theoretical bending properties of fir by studying the maximum stress–strain in tension and compression of the wood along the smooth grain, and carried out experimental validation and analyses. Guo et al. [8] prepared a new type of construction wood; analyzed the mechanical properties of mineralized wood in bending experiments; and used X-ray tomography analysis to reveal the damage mechanisms of native wood and mineralized wood under radial and tangential bending at the microscopic level, such as cell collapse and structural delamination. Báder et al. [9] used hydrothermal softening to compress oak wood longitudinally, analyzed the changes in the cell wall before and after treatment using micromechanical and ultrastructural characterization, and found that some microfibrils in the S1 and S2 layers of the cell wall of the compressed wood underwent corrugated folds and that their bending properties were significantly enhanced. Yao et al. [10] used the response surface method to study the synergistic treatment of teakwood with triethanolamine vacuum impregnation and steam to improve its bending properties and to obtain the ideal softening parameters but did not study the bending mechanism from the direction of the annual rings of teakwood, which is one of the key factors in bending mechanism research. Numerous studies have shown that wood’s bending properties are mainly determined by the parallel grain tensile property of the outer side and the compression property of the inner side. The wood’s cellulose has a greater influence on the parallel grain tensile ability, while its lignin and hemicellulose affect the compression property [11,12]. The lignin glass transition temperature of the softened wood is reduced, showing a viscous flow state; at the same time, lignin and hemicellulose degradation occurs to some extent, the wood cellulose part of the crystalline zone surface microfibrils becomes exposed, the degree of association between each part becomes weaker, and the cellulose molecular chain can be easily slipped and curled [13,14], thus providing the conditions for compression of the inner side of the deformation and tension of the outer side. Therefore, the plastic deformation capacity of the wood is improved.
Based on the current state of research, as noted above, in this study, based on the experimental results from the optimization of the wood-softening process from previous studies [10,15,16], teakwood treated via vacuum impregnation with triethanolamine-saturated steam and synergistic softening was used as the test object and bent radially and tangentially using a customized bending machine to obtain the minimum bending radius of the curvature and to explore the change rule of the radius of curvature under the two bending states. At the same time, the morphological characteristics of the teakwood, such as its cross-section, and the tensile and compressive surfaces, before and after bending, were observed using an electron microscope (SEM) and atomic force microscope (AFM), and the changes in cell morphology were compared to explore the bending performance of teakwood at the microscopic and ultramicroscopic levels. In addition, the changes in the mechanical curves of the teakwood in the bending process were tested using the Mechanical Experimentation Machine to analyze the load–deformation relationship in radial and tangential bending. This study is expected to not only further elucidate the bending mechanism of hardwood after softening treatment but also enrich the theoretical system of solid wood bending and provide a scientific basis for improving the bending yield of teakwood.

2. Materials and Methods

2.1. Materials and Chemicals

Plantation teakwood (Tectona grandis L.F.) was purchased from Xishuangbanna Dai Autonomous Prefecture, Yunnan Province, with an age of 15 years, a diameter of 20 cm, and no quality defects (decay, slash, knots, etc.). The wood samples were taken from the middle part of the trees, with dimensions of 500 × 40 × 20 mm3 (L × T × R), straight grain, and an air-dry density of 0.577 g/cm3. The softening solution triethanolamine (TEA, C6H15NO3), the penetrant sodium chloride (NaCl, AR), and the surfactant sodium dodecylbenzene sulfonate (SDBS, C18H29NaO3S) were purchased from the Sinopharm Chemical Reagent Group Co., Ltd., Shanghai, China.

2.2. Equipment

The following equipment was used for the experiment: Wood Vacuum Impregnation Tank (Customized, Changsha Juchuang Technology Co., Changsha, China), Wood Softening Test Tank (Customized, Changsha Juchuang Technology Co., Changsha, China) (Figure S1), Wood Bending Machine (Customized, Shanghai Tongzhou Machinery Co., Shanghai, China) (Figure S2), Universal Mechanical Tester (KHQ-002H, Jiangsu Jianhao Test Instrument Technology Co., Suzhou, China), and Constant Temperature and Humidity Chamber (GDJS-500B, Jiangsu Emerson Test Instrument Technology Co., Suzhou, China).

2.3. Bending Properties and Mechanical Test Methods

2.3.1. Preparation of Bending Specimens

The specimens were prepared following the radial and tangential grain directions of the teakwood. The radial direction of the wood is through the longitudinal section perpendicular to the growth rings of the pith; the growth rings parallel to each other, the color of the sapwood and heartwood, the direction of the rays, and the direction of the grain can be seen through the radial section. The tangential direction of the wood is along the longitudinal direction of the wood; the growth rings are parallel to each other in this section, and the growth rings are a parabolic shape seen through the tangential section.
In the experiment, teakwood bending can be categorized into two types according to the relationship between the bending surface of the wood and the growth rings: one in which the bending surface of the teakwood is parallel to the direction of the growth rings, i.e., radial bending (Figure 1a), and the other in which the bending surface of the teakwood is perpendicular to the direction of the growth rings, i.e., tangential bending (Figure 1b). The radius of bending curvature and the bending quality of teakwood during bending are strongly influenced by the annual layer and wood ray.

2.3.2. Evaluation of Bending Performance Test

Based on a previous study, we made teakwood bending specimens by adopting the best softening process parameters (the softening temperature was 125 °C, softening time was 175 min, and softening solution concentration was 15%) and by the bending method of applying a steel strip on the convex side of the softened wood [10]. Teakwood bending specimens were determined as shown in Figure 2, and the radius of curvature and bending coefficient of the specimens were calculated according to Equations (1) and (2), respectively [17].
R m i n = a 2 / 8 b   +   b / 2
K b = h / R m i n
where a is the inner chord length of the specimen (mm); b is the perpendicular distance from the center of the specimen to the chord length (mm); and h is the thickness of the specimen (mm).

2.3.3. Bending Mechanics Test Evaluation

The teakwood bending mechanical test was carried out by the bending machine in cooperation with the universal mechanical testing machine; the maximum load force of the mechanical testing machine was set 10 × 103 N, and the test speed was 100 mm/min.

2.4. Microscopic Morphology Analysis

2.4.1. SEM Morphology Analysis

Small samples were taken from the maximum bend in the middle of the teakwood bending specimen, with a size of 10 × 10 × 5 mm, and the micromorphological changes of the cell wall in the two bending states were observed by using an ambient scanning electron microscope (SEM) (Sigma 300, Zeiss, Berlin, Germany). Before testing, platinum was sprayed on the sample surface to improve the conductivity of the material, and observations were made in low and high magnification modes. The instrument was tested with a resolution of 1.0 nm@15 kV and an accelerating voltage of 10 kV.

2.4.2. AFM Morphology Analysis

The weak-phase structure of the teakwood was studied by atomic force microscopy (AFM) when tiny cracks sprouted in the cross-section of the cell wall between the tensile side and the neutral layer after bending to analyze the change rule of the cellular morphology of the teakwood when it reached the minimum bending radius. The specimens were embedded with LR White resin, and then an ultrathin slicer (Leica UC7, Leica, Berlin, Germany) was used to make the cut surface smooth and flat with a surface roughness of 1 μm. Then, the interface was scanned morphologically by AFM in 200 μm steps. The instrument was used in knockout mode with a Bruker Multimode 8, and the RTESP-300 cantilever beam needle was selected with a cantilever beam length of 125 μm and a resonance frequency of 300 kHz.

3. Results and Discussion

3.1. Analysis of the Change Rule of Bending Radius of Curvature

3.1.1. Radial Bending

Table 1 and Figure 3 show the radius of bending curvature, standard deviation, coefficient of variation, and bending coefficient of the specimens before and after the teakwood impregnation treatment. After the saturated steam softening, the radial bending coefficient of the teakwood was 1/11.62, the bending qualification rate was low, and the fracture of teakwood bending mainly occurred in the middle region of the tensile side (Figure 4). The reason for this phenomenon includes two main aspects: one is because during the bending of wood, the compressive deformation ability of the concave side is greater than the tensile deformation ability of the convex side, and the middle region of the tensile side of the convex side is subjected to the maximum tensile stress and tensile deformation, so the tensile side of the specimen will be damaged when it is increased to the limiting value [18]; the other is that though steam, as a plasticizer, can penetrate the amorphous area of cellulose, hemicellulose, and lignin, it is unable to moisten and expand the crystalline area of the cellulose of the wood, so that the relative sliding ability of the cellulose molecular chain is limited, which further reduces the plastic tensile deformation of the wood [19,20]. After the softening solution impregnation and saturated steam co-treatment, the bending properties of the teakwood were significantly improved compared with the unimpregnated wood, the bending coefficient was increased to 1/9.26, which was 20.31% higher than that of the untreated wood, and the qualification rate reached 80%, while the coefficient of variation of the bending radius of curvature was not much different from that of the unimpregnated wood.
The reason for this is mainly related to the penetration of the softening solution, the glass transition temperature of the wood, the chemical composition changing, and the dissolution of extractives [21,22,23,24,25]. During teakwood softening, the reaction of the triethanolamine compound with the main chemical composition of the wood causes internal swelling [10]. At the same time, the alkaline nature of the softening solution makes it easy to separate the linked ester bonds between hemicellulose and lignin, reducing the structural cohesion of teakwood itself [26,27,28]. In addition, under high temperature and humidity conditions, hemicellulose and lignin are degraded to different degrees, some microfibrils in the crystalline area of cellulose are exposed [29], and the degree of adhesion between them is reduced, so the softening bending properties of teakwood are significantly improved.

3.1.2. Tangential Bending

The test results of the teakwood were analyzed for tangential bending based on ensuring the qualification rate. Table 2 and Figure 5 show the radius of bending curvature, standard deviation, coefficient of variation, and bending coefficient of the specimens before and after the impregnation treatment. After the steam softening, the tangential bending radius of the curvature of the teakwood was 371.7 mm, and the bending coefficient was 1/18.58, with a larger bending radius of curvature and a smaller bending coefficient. Compared with radial bending, tangential bending is more prone to bending fracture (Figure 6), which is mainly due to the perpendicular relationship between the bending surface of the wood and the annual ring layer; in addition, the bending surface is divided into many columns by layer in the longitudinal direction, which in turn causes the layer to produce a mismatch under the action of bending stress, and the wood is prone to cracking at the junction of the earlywood and the latewood [30,31]. Although the bending properties of teakwood have been improved to some extent after synergistic treatment with a softening solution and steam, the bending radius of curvature is still large, which makes it difficult to meet the production needs of bentwood furniture and wood products.

3.1.3. Analysis of Rule Differences Between Radial and Tangential Bending Modes

According to the test results of radial and tangential bending, it can be seen that the radial bending performance of the teakwood is better than that for chordwise bending, which is closely related to the variation in earlywood and latewood, the direction of the annual ring line, and wood rays and other organizational structures [32,33,34].
In radial bending, the direction of the bending load is perpendicular to the growth rings of the wood. The outer tensile surface of teakwood is latewood, and the inner compression surface is earlywood. As the lumen of the outer latewood is small and thick-walled, the tensile bearing ability is greater than that of the earlywood, while the lumen of the earlywood on the inner side of teakwood is large and thin-walled, which provides favorable conditions for the parallel tensile compression of teakwood. In addition, the bending stress is uniformly born by the stacked layer of growth rings, such as “layers” of paper; therefore, the bending process is more stable in the radial bending. However, in tangential bending, the direction of the bending load is parallel to the growth rings of teakwood, and tensile stress and compressive stress are born by the longitudinal growth of the rings; the earlywood and latewood are alternatively distributed, the wood rays and wood fibers are arranged at a certain angle, and thus there is a “destabilization” phenomenon in the bending [35,36,37], which causes bending tearing phenomena. At the same time, it was also found that in the tangential bending of teakwood, the outermost “jagged” tensile fracture is located in the wood rays, indicating that the teakwood is likely to crack in the position of the wood rays in the bending.

3.2. Bending Load–Deformation Relationship Analysis

3.2.1. Radial Bending

The load–deformation relationship of the teakwood obtained under the softening solution impregnation–steam synergistic softening conditions was tested by a concave–convex mold bending machine and mechanical machine to explore the mechanical change rule of teakwood in radial bending and tangential bending. As shown in Figure 7, in the initial elastic phase of teakwood bending, we can see that a linear relationship between load and deformation, which is known as elastic strain in the mechanics of materials, following Hooke’s law [38]. At this stage, due to the greater thickness of the specimen, the required load is greater, and the deformation is less. As the load increases, the teakwood load–deformation relationship then shows a flatter, i.e., elastic–plastic deformation stage. The elastic–plastic deformation stage is caused by the curling and sliding of the molecular chains of fibers, and the bending deformation of teakwood is somewhat reversible [39,40]. With further increases in the load and the bending deformation amplitude, it is at the plastic deformation stage. As the degree of bending increases, a greater load force is required to produce a certain deformation, and this stage is close to the bending failure limit value within the permissible deformation range of teakwood.
Figure 7 shows the load–deformation relationship for the radial bending of the unimpregnated and impregnated teakwood. From Figure 7, it can be seen that for the same degree of deformation, the unimpregnated wood requires a larger load force in bending, and shows even a small curve with a steep rise and fall (marked by the arrows). In addition, its load–deformation relationship curve is not smooth enough, which indicates that the unimpregnated wood has a localized micro-fracture in the bending. While the impregnated material requires less load in the bending, the load–deformation relationship curve is smoother, and the bending performance is more satisfactory, which is mainly because the softening solution can not only enter the amorphous area of teakwood but also penetrate the crystalline area to improve the softening bending property of the teakwood.

3.2.2. Tangential Bending

Figure 8 shows the load–deformation relationship for the tangential bending of the unimpregnated and impregnated materials under steaming. The relationship is consistent with the trend of radial bending, but under the same radius of curvature, the tangential bending requires a greater load, which is mainly due to the perpendicular relationship between the bending surface and the annual layer of the teakwood and the greater strength of the latewood.

3.3. Micromorphological Analysis

Small samples intercepts on the bending specimen were scanned by SEM and AFM to observe the morphology changes of the wood cell walls. By observing the morphology images, the changes in the microscopic characteristics of teakwood bending can be analyzed, so that the relationship between the microstructure and the macroscopic bending properties of teakwood bending can be deeply studied.
Figure 9 shows the SEM images of the teakwood during the stretching and compression of the wood cell walls before and after radial bending. The radial section images of the material show that the conduits and pit of teakwood were rounded, the ray parenchyma was mottled, and their cellular organization was more obvious and interwoven with the wood fibers (Figure 9a–c). After radial bending, the teakwood showed a greater degree of tensile deformation on the tensile surface of the outer side of the radial section (Figure 9d,e), while on the compression surface in the middle of the inner side of the bending surface, the cell wall showed multilayered folds (Figure 9f,g), and uniformly dispersed fine folds could also be seen in the macroscopic features with the naked eye. Meanwhile, the innermost tangential section (concave surface) of teakwood undergoes the greatest compressive deformation, as seen through Figure 9h,i, where compressive misalignment occurs at the junction of the ducts and the ray parenchyma, mainly to meet the demands of the bending deformation of teakwood.
The teakwood internal softening degree plays an important role in bending performance and quality. If the internal tissue of the teakwood is not softened sufficiently, it will lead to larger folds at the junction of the radial and tangential section on the compression side, and may even produce damage; if the softening temperature conditions are too high, although the degree of softening of the wood will be increased, the tensile strength of the teakwood fibers will be reduced, and there will be premature tensile breakages at the junction of the radial and tangential section on the tension side, resulting in bending failure [41,42].
Figure 10 shows the SEM images of the teakwood during the stretching and compression of cell walls in the tangential section of the teakwood after tangential bending. It can be seen from the images that the ray parenchyma of the tangential section of the bending specimen sprouted smaller cracks at the junction with the wood fiber. The wood rays are all composed of thin-walled cells with lower tensile strength and poorer mechanical properties, which are prone to fracture from the ray parenchyma during bending and stretching (Figure 10a,b), thus affecting the bending properties of the wood [43,44]. In the tangential section on the compression side, there are more earlywood zones, which make the inner side have a relatively strong compression deformation ability along the grain (Figure 10c,d).
Figure 11 shows the AFM images of the outermost tensile and neutral layers of the cross-section of the teakwood after radial bending. Figure 11a shows the microscopic morphology of the cell wall cross-section of the outermost stretching surface, which shows that the cell wall and lumen of the wood have undergone a large tensile deformation, while the secondary wall layer of the wood cell wall shows tiny cracks, and the S1 and S2 layers show a certain state of debonding. This is mainly because, under a bending load, the outermost tensile side of the bending specimen of teakwood undergoes the maximum parallel-to-grain tensile stress, which tends to crack the S2 layer of the secondary wall, and the transverse shear stress tends to debond the S1 and S2 layers [18,45]. The specimen slices were taken at the secondary outer stretching side (at 2 mm), and the cross-sectional micromorphology is shown in Figure 11b, where the cell wall fibrous tissues show cracks, but the extent of the cracks was small. Figure 11c shows the cell wall microscopic morphology near the neutral layer, and the wall structure and fiber tissue structure of the cell were relatively intact. Sample slices were taken at the center layer, and its cross-section micromorphology is shown in Figure 11d. The neutral layer in the bending process is able to withstand a certain amount of transverse compressive stress, so that the cell wall produces compressive deformation but does not withstand tensile stress and compressive stress, so the wall structure of the cell wall is more intact.
Figure 12 shows the AFM images of the outer tensile and neutral layers of the wood cross-section of the teakwood after tangential bending. Figure 12a shows the location where cracks were produced in the outer tensile surface of the teakwood bending, i.e., at the ray parenchyma, from which it can be seen that the ray parenchyma has been damaged because tangential bending is susceptible to bending damage under the same bending radius of curvature as that of radial bending, and this is also in agreement with the previous characterization of the SEM morphology. Figure 12b shows the cell wall micromorphology near the ray parenchyma on the outer side of the teakwood bending, from which it can be seen that after the specimen cracked from the ray parenchyma, the wall structure of the cell wall of the wood was torn under the combined effect of the longitudinal tensile and transverse shear coupling stresses, which led to the expansion of the wood’s cracks parallel to the grain of wood. Sections were taken from the outer tensile side (2 mm) of the bending specimen, and the secondary wall of teakwood was still severely damaged, with debonding at the interface of the S1 and S2 layers, as shown in Figure 12c. Figure 12d shows the cell wall micromorphology of the neutral layer of the teakwood after tangential bending, which shows a large deformation of the cell wall and folds in the wall layer, which is related to the uneven stress generated by transverse compression [46,47].

4. Conclusions

In this study, softening solution impregnated–steam synergistically softened teakwood was taken as the test object, and radial and tangential bending were carried out on it by using a customized bending machine to explore the variation rule of the radius of curvature under the two bending states and deeply reveal the mechanical behaviors and deformation mechanisms of teakwood during the bending process through the analysis of the bending load–deformation relationship, SEM, AFM, and other technological means. The results show that the bending stress of teakwood in radial bending is uniformly taken by the layers of stacked growth rings, like layesrs of stacked paper, showing strong bending stability, while the tensile and compressive stresses in tangential bending are born by many longitudinal growth rings, in which the earlywood and latewood distribute alternately, which will cause the phenomenon of “instability” and then cause bending cracks. The microscopic morphology of the radial and tangential bending of teakwood was analyzed in detail by using microscopic characterization techniques, which further elucidated the intrinsic relationship between its microstructural changes and bending properties. Based on this, it was found that radial bending could significantly improve the bending performance of teakwood based on the synergistic softening of softening solution impregnation–steaming, which provides an important theoretical and practical basis for the application of teakwood in the field of bentwood furniture and product design.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f15122203/s1, Figure S1: Wood vacuum impregnation tank and softening test tank; Figure S2: Wood bending equipment.

Author Contributions

Conceptualization, L.Y.; methodology, L.Y. and L.J.; software, Z.W.; validation, J.L.; formal analysis, L.J.; investigation, L.Y.; resources, Z.W.; data curation, L.Y.; writing—original draft preparation, L.Y. and L.J.; writing—review and editing, L.Y. and J.L.; visualization, L.Y.; supervision, L.J.; project administration, L.Y.; funding acquisition, L.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the China National Key Research and Development Program (NO. 2017YFD0601104) and the Youth Fund Program of Xinyang Agriculture and Forestry University (NO. QN2021053).

Data Availability Statement

The datasets generated or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagram of radial bending (a) and tangential bending (b).
Figure 1. Schematic diagram of radial bending (a) and tangential bending (b).
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Figure 2. Schematic diagram for the determination of the radius of curvature of the specimen.
Figure 2. Schematic diagram for the determination of the radius of curvature of the specimen.
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Figure 3. Bending radius of curvature and bending coefficient of teakwood before and after impregnation.
Figure 3. Bending radius of curvature and bending coefficient of teakwood before and after impregnation.
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Figure 4. Location of damage in radial bending of teakwood (indicated by an orange arrow).
Figure 4. Location of damage in radial bending of teakwood (indicated by an orange arrow).
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Figure 5. Bending radius of curvature and bending coefficient of teakwood before and after impregnation.
Figure 5. Bending radius of curvature and bending coefficient of teakwood before and after impregnation.
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Figure 6. Location of damage in tangential bending of teakwood (indicated by an orange arrow).
Figure 6. Location of damage in tangential bending of teakwood (indicated by an orange arrow).
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Figure 7. Load–deformation relationship for radial bending of teakwood.
Figure 7. Load–deformation relationship for radial bending of teakwood.
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Figure 8. Load–deformation relationship for tangential bending of teakwood.
Figure 8. Load–deformation relationship for tangential bending of teakwood.
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Figure 9. SEM images of the teakwood before and after radial bending: (a–c) radial section images of the material; (d,e) radial section images of the stretched side after bending; (f,g) radial section images of the compressed side after bending; (h,i) tangential section images of the compressed side after bending.
Figure 9. SEM images of the teakwood before and after radial bending: (a–c) radial section images of the material; (d,e) radial section images of the stretched side after bending; (f,g) radial section images of the compressed side after bending; (h,i) tangential section images of the compressed side after bending.
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Figure 10. SEM images of the teakwood after tangential bending: (a,b) tangential section images of the stretched side after bending; (c,d) tangential section images of the compressed side after bending.
Figure 10. SEM images of the teakwood after tangential bending: (a,b) tangential section images of the stretched side after bending; (c,d) tangential section images of the compressed side after bending.
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Figure 11. AFM amplitude images of radial bending cell walls of the teakwood: (a,b) image of the outer stretching layer after bending; (c,d) image of the neutral layer after bending.
Figure 11. AFM amplitude images of radial bending cell walls of the teakwood: (a,b) image of the outer stretching layer after bending; (c,d) image of the neutral layer after bending.
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Figure 12. AFM amplitude images of tangential bending cell walls of the teakwood: (a–c) image of the outer stretching layer after bending; (d) image of the neutral layer after bending.
Figure 12. AFM amplitude images of tangential bending cell walls of the teakwood: (a–c) image of the outer stretching layer after bending; (d) image of the neutral layer after bending.
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Table 1. Radial bending properties of teakwood specimens before and after impregnation.
Table 1. Radial bending properties of teakwood specimens before and after impregnation.
ParametersThe Mean Value of the Radius of Curvature (mm)The Standard Deviation of the Radius of Curvature (mm)The Variation Coefficient of the Radius of Curvature
(%)		Bending Coefficient (Kb)
Specimens
Unimpregnated232.416.317.011/11.62
Impregnated (15%)185.213.127.091/9.26
Table 2. Tangential bending properties of teakwood specimens before and after impregnation.
Table 2. Tangential bending properties of teakwood specimens before and after impregnation.
ParametersThe Mean Value of the Radius of
Curvature (mm)		The Standard Deviation of the Radius of Curvature (mm)The Variation Coefficient of the Radius of Curvature
(%)		Bending
Coefficient (Kb)
Specimens
Unimpregnated371.718.154.991/18.58
Impregnated (15%)316.014.574.611/15.80
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Yao, L.; Ji, L.; Wang, Z.; Liu, J. Bending Performance of Plantation Teakwood and Its Mechanism Based on Radial and Tangential Directions. Forests 2024, 15, 2203. https://doi.org/10.3390/f15122203

AMA Style

Yao L, Ji L, Wang Z, Liu J. Bending Performance of Plantation Teakwood and Its Mechanism Based on Radial and Tangential Directions. Forests. 2024; 15(12):2203. https://doi.org/10.3390/f15122203

Chicago/Turabian Style

Yao, Linghua, Lina Ji, Zhangheng Wang, and Junnan Liu. 2024. "Bending Performance of Plantation Teakwood and Its Mechanism Based on Radial and Tangential Directions" Forests 15, no. 12: 2203. https://doi.org/10.3390/f15122203

APA Style

Yao, L., Ji, L., Wang, Z., & Liu, J. (2024). Bending Performance of Plantation Teakwood and Its Mechanism Based on Radial and Tangential Directions. Forests, 15(12), 2203. https://doi.org/10.3390/f15122203

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