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Article

Variations in Physical and Mechanical Properties Between Clear and Knotty Wood of Chinese Fir

by
Yingchao Ruan
1,2,
Zongming He
1,2,
Shaohui Fan
3,
Zhiyun Chen
4,
Ming Li
1,2,
Xiangqing Ma
1,2 and
Shuaichao Sun
1,2,*
1
College of Forestry, Fujian Agriculture and Forestry University, Fuzhou 350002, China
2
Chinese Fir Engineering Research Center of National Forestry and Grassland Administration, Fuzhou 350002, China
3
International Center for Bamboo and Rattan, Beijing 100102, China
4
Shaowu Weimin State-Owned Forest Farm, Shaowu 354006, China
*
Author to whom correspondence should be addressed.
Forests 2024, 15(11), 2007; https://doi.org/10.3390/f15112007
Submission received: 11 October 2024 / Revised: 3 November 2024 / Accepted: 12 November 2024 / Published: 14 November 2024
(This article belongs to the Section Wood Science and Forest Products)
Browse Figures
Versions Notes

Abstract

:
Significant market value discrepancies exist between clear and knotty Chinese fir (Cunninghamia lanceolata) wood, distinguished not only by their aesthetic variations but also by their distinct material properties. This study aimed to explore the differences in physical and mechanical properties between clear and knotty Chinese fir wood. Nine standard trees were chosen from a 26-year-old Chinese fir plantation for the experiment. Subsequent to felling, trunk segments below 7 m in length were transported to the laboratory. For each tree, detailed preparations were made to obtain clear and knotty wood specimens, and these distinct wood specimens were subjected to thorough physical and mechanical assessments. The results revealed significant variations in properties between clear and knotty Chinese fir wood. The shrinkage and swelling coefficients of knotty wood were generally lower than those of clear wood, except for higher radial and tangential air-dry shrinkage. Specifically, the swelling ratio of knotty wood was at least 0.40% lower, and the oven-dry shrinkage was at least 0.58% lower than that of clear wood. Knotty wood exhibited higher air-dry and oven-dry densities, with its density being at least 0.15 g cm−3 higher than that of clear wood. However, its mechanical properties, including tensile strength, compression strength, impact bending strength, bending strength, and modulus of elasticity, were lower than those of clear wood. For instance, the tensile strength parallel to the grain of clear wood was 40.63 MPa higher, the modulus of elasticity was 1595 MPa higher, and the impact bending strength was 27.12 kJ m−2 greater than that of knotty wood. Although the tangential and radial surface hardness of knotty wood increased significantly compared to clear wood, the end hardness remained relatively lower. Overall, knotty Chinese fir wood displayed enhanced physical properties, whereas clear wood showcased superior mechanical properties. Careful selection between clear and knotty wood is recommended based on the specific requirements of wooden structural elements to optimize timber resource utilization.

1. Introduction

Wood, as a natural biological material, possesses traits such as eco-friendliness, sustainability, and recyclability, holding a crucial role in various sectors including construction, papermaking, fuel production, and wood products [1,2,3]. Wood resources can act as substitutes for energy-intensive materials like plastics and metals, aiding environmental conservation and resource preservation through their recyclable nature, thereby positively impacting the mitigation of global warming [2,4]. During the natural growth process, wood is susceptible to internal defects like knots, cracks, decay, and insect infestation [5,6]. These flaws not only compromise its structural integrity but also result in performance inconsistencies during drying, processing, and utilization, impacting its physical and mechanical properties and consequently constraining the extensive utilization of wood [5,7].
Chinese fir (Cunninghamia lanceolata) is a widely utilized, rapidly growing timber species in the subtropical regions of China, with a long history of cultivation. It is valued for its straight trunk, high productivity, superior quality, wide range of uses, hardness, durability, and resistance to cold, drought, diseases, and pests, establishing it as an important multifunctional and environmentally friendly material in various industries [8,9,10,11,12]. In China, the area and stock volume of Chinese fir rank first among plantations, making it a significant high-quality economic timber that positively contributes to forest cultivation, wood production, and ecological restoration [13,14]. However, frequent human activities over the years have led to the degradation of ecosystems, posing greater challenges to wood resources. In the context of sustainable development within the wood industry, in-depth research on the wood properties of Chinese fir is of considerable significance.
Previous studies have identified numerous unique and critical wood characteristics of Chinese fir. For example, Chinese fir is capable of surviving in various growing environments, exhibits a high growth rate, good adaptability, and also possesses excellent ecological functions, such as carbon sequestration, climate regulation, and maintaining ecological balance, among others [11,15]. You et al. [16] found variability in the physical properties of Chinese fir wood from different geographical provenances, with significant correlations between climatic factors and these physical properties. Wang et al. [17] indicated that anatomical structure parameters and the microfibril angle synergistically influence the mechanical properties of Chinese fir wood. Chu et al. [18] discovered that the mechanical and physical properties of Chinese fir wood are strongly influenced by genetic factors, and the physical and mechanical properties among different annual rings remain relatively stable.
Some research has indicated that Chinese fir has many inherent drawbacks, such as a large proportion of heartwood, low density, weak strength and stiffness, as well as significant cracking and deformation, poor dimensional stability, poor fire resistance and air permeability, etc., which seriously restrict its effective utilization [8,19,20]. The wood of softwood trees typically exhibits a relatively simple cellular structure, characterized by elongated wood fibers and thin cell walls [21]. Mansfield et al. [22] reported a strong correlation between fiber properties and basic density as well as strength characteristics. Sheng et al. [23] highlighted that the chemical composition of Chinese fir wood is closely linked to soil temperature and moisture; specifically, soil warming and precipitation exclusion inhibit lignin synthesis while increasing organic extract content. Zhan et al. [24] discovered that Chinese fir wood becomes softer after moisture absorption, with its ability to resist deformation decreased and its modulus of elasticity decreased. The research of Jiang et al. [25] showed that there was a significant correlation between the tensile strength, compressive strength, etc., of Chinese fir and its moisture content. Li et al. [26] observed that the compression wood and opposite wood of Chinese fir exhibited relatively large differences in tensile modulus and ultimate tensile stress, which were attributed to significant differences in the cellulose microfibril angles of their fibers.
The research presented in the literature data primarily focuses on the connection between internal or external experimental factors and wood properties, addressing only a limited subset of indicators while predominantly concentrating on clear wood. However, given the current context of dwindling resources, the scarcity and high cost of premium clear wood underscore the critical importance of investigating and effectively utilizing flawed wood. In contrast, there is a considerable lack of comprehensive comparative analyses between clear and knotty wood regarding their performance characteristics. Therefore, we selected 26-year-old mature Chinese fir as the experimental material to evaluate and analyze various physical and mechanical properties of Chinese fir logs, including both clear and knotty wood. This research aims to address deficiencies in the comprehensiveness of wood types, particularly concerning knotty wood, and expands the coverage of performance indicators in existing studies.

2. Materials and Methods

2.1. Study Site

The experimental site is located in the Nanji region of Shaowu Weimin State-owned Forest Farm in the northwestern part of Fujian Province of China (27°06′ N, 117°44′ E), with an altitude ranging from 550 to 687 m, and it falls within the mid-subtropical zone. This area features a warm and humid climate, abundant sunlight, and an average annual precipitation of 1712 mm, with a mean temperature of 19.4 °C. It experiences 1714.4 h of sunshine annually and a frost-free period lasting 290 days. The soil in this forested zone is characterized by mountainous red soil, with a soil layer depth exceeding 100 cm, displaying high fertility. Figure 1 shows the location of the study site.

2.2. Experimental Design

In early 1996, Chinese fir seedlings were selected to establish a plantation with a density of 1800 trees per hectare. In December 1999, at the age of 4, the initial thinning was conducted, reducing the density to 1200 trees per hectare. A randomized complete block design was employed to set up pruning experimental plots, each covering an area of 600 m2. Thirty Chinese fir trees were randomly chosen for pruning in each plot, while the remaining trees were left unpruned as the control group. Pruning intensity was categorized into 4 levels, each with 4 replicates. Branches with diameters below 6, 8, 10, or 12 cm at any position on the tree trunk were pruned. Subsequently, each pruned tree underwent annual pruning in December until the clear height of the tree trunk reached 7 m, consistent with the standard size for ordinary sawn timber logs. In 2015, the second thinning was implemented, adjusting the retention density of each plot to 900 trees per hectare.

2.3. Specimen Processing

To investigate the disparities in characteristics between clear and knotty timber, in July 2022, three average standard Chinese fir trees were chosen from each of the 10 cm and 12 cm pruning intensities with more prominent and larger knots, as well as the control group (CK), totaling nine experimental trees. Following tree felling, the lower segment of each tree trunk below 7 m was sectioned, marked, and subsequently transported back to the laboratory. In compliance with the International Organization for Standardization (ISO) 3129:2019 [27] guidelines, clear wood specimens were meticulously prepared from wood sections measuring 1.3 to 3.3 m, while knotty wood specimens were crafted utilizing the same technique on the identical tree, with efforts made to preserve knots and other flaws as thoroughly as possible.
A section of wood measuring 1.3 to 3.3 m in height was cut into two 1 m long sections, which were subsequently sawed along their lengths into square strips with end dimensions of 25 × 25 mm (yielding 15 to 20 square strips per section) and 55 × 55 mm (yielding 3 to 5 square strips per section), respectively. When selecting the squares, sapwood should be prioritized over heartwood, particularly when sapwood is available. All squares must be free of bark. The sawn strips should be placed in a cool location, spread out to dry, and marked accordingly to indicate their position in relation to the tree height. Subsequently, smaller wood specimens were prepared according to the requirements of specific wood property tests. The detailed process for preparing wood specimens is illustrated in Figure 2.
The characteristics of knots in the batten were measured. In a wood segment of height 1.3 to 2.3 m from a single tree, the proportion of the knot area within the unit area was approximately 5.88%. For small-sized specimens, the percentage of the knot area varied from 4.46 to 31.47%. The total number of knots observed on the surface of the batten was 98. When examined on a smaller scale, each specimen contained 1 to 3 knots. Additionally, we recorded the size of the knot area on these smaller specimens, which ranged from 8.22 to 89.52 mm2.
In this research on the physical and mechanical properties of wood, the basic information such as the specific testing items, the main testing equipment and their models, the dimensions of the wood specimens, and the number of wood specimens required for each treatment group are detailed in Table 1.

2.4. Testing of Wood Properties

The physical properties of wood specimens were tested following the guidelines outlined in ISO standards 13061-2:2014 [28], 13061-13:2016 [29], 13061-14:2016 [30], 13061-15:2017 [31], and 13061-16:2017 [32]. The measured indicators include air-dry and oven-dry densities, volumetric dry shrinkage coefficient, and radial, tangential, and volumetric shrinkage (both air-dry and oven-dry) and swelling (air-dry and water-absorbent), with all relevant values determined at a moisture content of 12%.
The determination of shrinkage in the radial, tangential, or volumetric directions, whether oven-dry or air-dry, was based on the ratio of changes in dimensions or volume from a water-saturated state to either oven-dry or air-dry conditions, relative to the dimensions in the water-saturated state. Conversely, for measuring swelling in the radial, tangential, or volumetric directions, whether air-dry or water-absorbent, the calculations relied on the ratio of changes in dimensions or volume from oven-dry to either air-dry or water-saturated conditions, relative to the dimensions in the oven-dry state.
The wood specimens’ mechanical properties were evaluated in accordance with ISO standards 13061-3:2014 [33], 13061-4:2014 [34], 13061-6:2014 [35], 13061-10:2017 [36], 13061-12:2017 [37], and 13061-17:2017 [38]. The parameters measured include tensile strength parallel to grain, ultimate stress in compression parallel to grain, bending strength, modulus of elasticity in bending, impact bending strength, and static hardness (end, tangential, and radial sections), with all pertinent indicators determined at a moisture content of 12%.
The determination of impact bending strength was based on the load applied at the center of the specimen until flexural failure occurred. The modulus of elasticity in bending was calculated from the relationship between load and deformation during the bending of wood under stress, with the loading rate set at 1–3 mm min−1. The lower and upper load limits for vertical deformation were established at 300 N and 700 N, respectively. The assessment of bending strength involved applying a uniform load at the midpoint of the test span until failure, with the maximum load recorded during testing serving as the determining factor; the loading rate was set at 5–10 mm min−1. The tensile strength and compressive strength parallel to the grain were measured by applying tensile or compressive forces in the grain direction at a uniform rate until failure occurred, within a timeframe of 0.5 to 5 min. Hardness measurements were conducted by pressing a hemispherical steel indenter with a radius of 5.64 mm into the wood at a uniform speed of 3–6 mm min−1, with the resistance encountered upon penetration to a depth of 5.64 mm recorded as the hardness value.

2.5. Data Statistics and Analysis Methods

SPSS 27 was utilized to calculate the mean and standard error of the data, and one-way ANOVA along with multiple comparisons using the LSD method was performed to assess the significance of the wood performance indicators. The results are presented graphically using Origin 2021.

3. Results

3.1. Comparison of Physical Properties Between Clear and Knotty Wood

Figure 3 illustrates the variance analysis of the physical properties between clear and knotty wood. Significant differences were observed in radial, tangential, and volumetric air-dry and water-absorbent swellings, as well as in radial, tangential, and volumetric oven-dry shrinkage. Moreover, notable distinctions were found in volumetric air-dry shrinkage, volumetric dry shrinkage coefficient, and air-dry and oven-dry densities.
As depicted in Figure 3A, the radial, tangential, and volumetric air-dry and water-absorbent swellings of the knotty wood were lower than those of the clear wood by 0.41%, 1.36%, and 1.41%, and by 0.63%, 1.90%, and 2.49%, respectively. The differences in radial air-dry and water-absorbent swellings between clear and knotty wood were minimal, less than 0.63%. Conversely, considerable differences were identified in the tangential and volumetric air-dry and water-absorbent swellings between clear and knotty wood, notably in the tangential and volumetric water-absorbent swellings, surpassing 1.9%. The most substantial differences in air-dry and water-absorbent swellings between clear and knotty wood were in the volumetric direction, while the smallest discrepancies were in the radial direction, with the volumetric water-absorbent swelling notably high and the radial air-dry swelling significantly low.
Figure 3B shows that knotty wood exhibited lower radial, tangential, and volumetric oven-dry shrinkage compared to clear wood by 0.58%, 1.68%, and 1.80%, respectively. In contrast, the radial and tangential air-dry shrinkage of knotty wood was higher by 0.33% and 0.78%, respectively, while the volumetric air-dry shrinkage was lower by 0.76%. The differences in radial oven-dry shrinkage and radial, tangential, and volumetric air-dry shrinkage between clear and knotty wood were relatively minor, all below 0.79%. Notably, the volumetric shrinkage values were the highest, and the radial shrinkage values were the lowest for both oven-dry and air-dry conditions, with the volumetric oven-dry shrinkage notably high and the radial air-dry shrinkage exceptionally low.
In Figure 3C, both clear and knotty wood showed volumetric dry shrinkage coefficients below 0.44%, with the knotty wood’s coefficient 0.11% lower than that of clear wood. Figure 3D illustrates the densities of clear wood under air-dry and oven-dry conditions, which were relatively low (both under 0.33 g cm−3), while knotty wood densities surpassed those of clear wood by 0.17 g cm−3 and 0.16 g cm−3, respectively. The discrepancy in air-dry and oven-dry densities of clear wood was negligible (0.02 g cm−3), and, similarly, the variance in densities of knotty wood was also modest (0.04 g cm−3).

3.2. Comparison of Mechanical Properties Between Clear and Knotty Wood

Analysis of variance of the mechanical properties between clear and knotty wood revealed significant differences in tensile strength parallel to the grain, ultimate stress in compression parallel to the grain, modulus of elasticity in bending, bending strength, impact bending strength, and static hardness of the tangential and radial sections. However, there was no notable difference in the static hardness of the end section (Figure 4).
In Figure 4A, the tensile strength parallel to the grain of both clear and knotty wood was below 55 MPa, showing a substantial difference between the two, with the knotty wood’s tensile strength being 40.63 MPa lower than that of the clear wood. Figure 4B shows that the impact bending strength of both clear and knotty wood was under 42 kJ m−2, demonstrating a significant gap, with the knotty wood’s impact bending strength 27.12 kJ m−2 lower than that of the clear wood.
Figure 4C shows that the modulus of elasticity in bending exceeded 5256 MPa for both clear and knotty wood, with a considerable difference between the two. The knotty wood exhibited a modulus of 1595 MPa lower than the clear wood. Figure 4D indicates that the bending strength of both clear and knotty wood was below 50 MPa, with the knotty wood’s bending strength 6.18 MPa lower than that of the clear wood. Analysis of Figure 4E reveals that the ultimate stress in compression parallel to the grain of both clear and knotty wood was below 34 MPa, with the knotty wood exhibiting a 5.71 MPa lower ultimate stress than the clear wood.
Figure 4F indicates that the tangential and radial sections of the knotty wood had static hardness levels higher by 1812 N and 1181 N, respectively, compared to the clear wood, while the end section’s static hardness was lower by 39.33 N. Notably, the static hardness levels of the end, tangential, and radial sections of the knotty wood, as well as the end section of the clear wood, were notably high, exceeding 2285 N. On the contrary, the clear wood displayed lower static hardness in the tangential and radial sections (below 1255 N), with a minimal difference of only 149.78 N between them. The variance in static hardness between the clear wood’s end section and the knotty wood’s end and radial sections ranged modestly from 39.33 N to 155 N.
In comparing the mechanical properties using consistent units (MPa or N), both clear and knotty wood displayed a significantly higher modulus of elasticity in bending compared to the tensile strength parallel to grain, ultimate stress in compression parallel to grain, and bending strength (Figure 4). Among the observed properties, clear wood demonstrated the lowest ultimate stress in compression parallel to grain, whereas knotty wood exhibited the lowest tensile strength parallel to grain. Additionally, clear wood showed the highest static hardness in the end section, with a relatively minor discrepancy between the tangential and radial sections. Contrarily, knotty wood showcased the highest tangential static hardness, while the difference in static hardness between the end and radial sections was also relatively slight.

4. Discussion

The density of wood is influenced by several factors, including its cellular structure, chemical composition, and growth environment [17]. Clear wood exhibits a relatively regular cell structure, while knotty wood demonstrates structural specialization. Knotty wood features a complex architecture with irregularly arranged cells and often contains localized accumulations of lignin and cellulose. In some coniferous species, the cells surrounding knots increase the thickness of the secondary wall to enhance support, thereby resulting in greater wood density. Additionally, knots frequently contain higher concentrations of resins, gums, and other substances that fill cell gaps or adhere to cell walls, further elevating the mass per unit volume and contributing to the relatively high density of knotty wood [7]. Density directly impacts the quality and strength of wood. Denser wood is harder, more resistant to decay, and better able to withstand wind [39].
In this study, the air-dry and oven-dry densities of clear wood were significantly lower than those of knotty wood. This difference can be attributed to the higher lignin and cellulose content as well as the lignification degree of knotty wood, which are natural high-molecular compounds in wood pivotal for wood cell wall formation, imparting beneficial mechanical properties and stability [40,41]. Furthermore, the static hardness of the tangential and radial sections in knotty wood was notably higher than that of clear wood, indicating a higher presence of polymeric compounds in knotty wood, enhancing its hardness and durability. The distinct structure of knots, differing from the surrounding clear wood, alters the organization of wood components such as conduits and fibers, leading to irregular texture and increased susceptibility to drying or water absorption, potentially resulting in cracking, warping, reduced dimensional stability, and compromised wood aesthetics [42]. This study found that the volumetric dry shrinkage coefficient, as well as various swelling and shrinkage (excluding radial and tangential air-dry shrinkage), of knotty wood were smaller than those of clear wood. This comparison illustrates that knotty wood experienced less deformation and exhibited superior dimensional stability. This improved stability may be attributed to the more complex fibrous tissues present in knotty wood, resulting in variations in fiber arrangement and orientation that lead to reduced wood deformation. Additionally, the lower moisture content conductivity in knotty wood facilitated a more uniform absorption or release of moisture, thereby reducing overall deformation. In our study, we found that swelling and dry shrinkage rates varied in different directions, a phenomenon attributed to the complexity and anisotropy of the wood’s internal structure. This anisotropy is manifested in the varying shapes and arrangements of cells, as well as in the composition and structure of the cell walls across different directions [39].
Wood is a highly heterogeneous material characterized by a complex structure, with internal knots serving as significant factors that influence its mechanical properties [43]. The presence of knots alters the deformation capacity of wood, leading to stress concentration when subjected to compression loads, which in turn diminishes the wood’s strength [44]. Additionally, knots contribute to a reduction in local density and create an uneven structural distribution within the wood, causing variances in tissue density and microstructure around these regions. This unevenness affects strain distribution under load, resulting in localized increases in strain, thereby reducing the flexural modulus of elasticity and overall strength. Furthermore, the size, shape, and positioning of knots vary in degree, and each has a distinct impact on the mechanical properties of wood [45]. Notably, wood knots can also enhance the density and brittleness of wood, further influencing its mechanical characteristics.
Baño et al. [46] demonstrated that knots and grain deviation significantly affect the bending load capacity of timber beams. Similarly, Wright et al. [47] found that both the modulus of elasticity and the modulus of rupture in loblolly pine (Pinus taeda) timber decline as the percentage of knots increases, indicating a negative correlation with knottiness. Generally, the strength of wood is greater in the down-grain direction than in the cross-grain direction due to the continuous and close arrangement of wood fibers in the down-grain direction, which facilitates efficient stress transfer [39]. The conformational tensile strength is particularly crucial for components subjected to tensile forces, such as tie rods [48]. Cherry et al. [7] established that knots adversely impact tensile strength, reducing not only tensile and flexural strength but also the flexural modulus. They also noted variability in the wood’s hardness in different directions, corroborating our findings. However, tensile strength is also influenced by factors such as fiber length, diameter, and surface treatment [49]. Rocha et al. [44] reported a strong correlation between knot size in Eucalyptus wood and both the modulus of elasticity and compressive strength; larger knots were associated with greater reductions in these mechanical properties. Additionally, the stiffness of Eucalyptus wood is more sensitive to the presence of knots than its strength. In our study, we determined that the down-grain compressive strength of 26-year-old fir wood, which contains no knots, is approximately 33 MPa, aligning closely with the findings of Wang et al. [17] who reported a down-grain compressive strength of 31 to 34 MPa in 20-year-old fir asexual lines. This suggests that down-grain compressive strength may be less influenced by age and genetic factors. Furthermore, the impact toughness of wood correlates with its density and fiber length [39]. Impact toughness is a critical consideration in applications that may experience impact or dynamic loading, such as sports equipment, tool handles, and earthquake-prone infrastructures. Wood with higher impact toughness is better equipped to withstand sudden impacts, thereby reducing the risk of fracture [48]. Our study indicates that the low impact toughness observed in knotty timber may result from high fiber content, which leads to fiber agglomeration and resultant stress concentrations [50].
In this research, the knotty wood exhibited significantly lower values in ultimate stress in compression parallel to grain, tensile strength parallel to grain, bending strength, modulus of elasticity in bending, and impact bending strength compared to clear wood, indicating a notable disadvantage in mechanical properties of the knotty specimens. This discrepancy can be attributed to the more disordered fiber structure in knotty wood, weaker fiber connections, and the stress concentration induced by knots, all contributing to the deterioration of wood mechanical properties. Moreover, the presence of natural defects and cracks in knotty wood might diminish its load-bearing capacity, making it more prone to damage from external forces. The surrounding complex growth environment of knots could have also led to inferior inherent wood quality.
Overall, there is a marked distinction in the properties of knotty and clear wood. Knotty wood, with its superior physical characteristics, provides significant advantages in specific applications. For example, in the production of staircase handrails for interior decoration, its excellent dimensional stability, wear resistance, and low deformation enhance the quality and longevity of the final product [51]. Conversely, the mechanical properties of clear wood present a distinct advantage, making it the preferred choice for load-bearing elements, such as beams and columns in buildings, as well as for the manufacture of high-quality furniture [52]. This preference ensures the safety, reliability, and durability of timber structures over time.

5. Conclusions

This study systematically analyzed the differences in the physical and mechanical properties between clear and knotty wood derived from 26-year-old Chinese fir. The results indicated that knotty wood had distinct advantages in terms of dry shrinkage, swelling, density, and hardness, whereas clear wood exhibited superior mechanical strength in several areas. These findings offer a scientific basis for the rational selection of timber for various applications, with knotty wood being advantageous in contexts emphasizing physical properties, while clear wood is more suitable for applications requiring enhanced mechanical properties.

Author Contributions

Conceptualization, Y.R., Z.H., X.M. and S.S.; data collection, Y.R., Z.H. and Z.C.; methodology and software, Y.R., Z.H. and S.F.; drafting of the manuscript, Y.R. and S.S.; visualization, Y.R. and M.L.; revisions and suggestions, S.S., Z.H., X.M., M.L. and S.F.; funding acquisition, S.S., Z.H. and X.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key Research and Development Program of China (grant number 2021YFD2201302).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of Shaowu city showing the study site. (A) Standing trees of Chinese fir. (B) Logs of Chinese fir.
Figure 1. Location of Shaowu city showing the study site. (A) Standing trees of Chinese fir. (B) Logs of Chinese fir.
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Figure 2. Schematic diagram of the manufacturing process of wood specimens.
Figure 2. Schematic diagram of the manufacturing process of wood specimens.
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Figure 3. Comparison of physical properties between clear and knotty wood of Chinese fir. (A) Swelling ratio in different directions, (B) shrinkage ratio in different directions, (C) volumetric dry shrinkage coefficient, (D) air-dry and oven-dry density. The presence of different lowercase letters (a or b) among the parameters indicates a significant difference; conversely, identical lowercase letters (a or b) suggest no significant difference.
Figure 3. Comparison of physical properties between clear and knotty wood of Chinese fir. (A) Swelling ratio in different directions, (B) shrinkage ratio in different directions, (C) volumetric dry shrinkage coefficient, (D) air-dry and oven-dry density. The presence of different lowercase letters (a or b) among the parameters indicates a significant difference; conversely, identical lowercase letters (a or b) suggest no significant difference.
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Figure 4. Comparison of mechanical properties between clear and knotty wood of Chinese fir. (A) Tensile strength parallel to the grain, (B) impact bending strength, (C) modulus of elasticity in bending, (D) bending strength, (E) ultimate stress in compression parallel to the grain, (F) static hardness in different directions. The presence of different lowercase letters (a or b) among the parameters indicates a significant difference; conversely, identical lowercase letters (a or b) suggest no significant difference.
Figure 4. Comparison of mechanical properties between clear and knotty wood of Chinese fir. (A) Tensile strength parallel to the grain, (B) impact bending strength, (C) modulus of elasticity in bending, (D) bending strength, (E) ultimate stress in compression parallel to the grain, (F) static hardness in different directions. The presence of different lowercase letters (a or b) among the parameters indicates a significant difference; conversely, identical lowercase letters (a or b) suggest no significant difference.
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Table 1. List of basic information for wood property testing.
Table 1. List of basic information for wood property testing.
Property Testing ItemMain Testing Equipment (Model and Factory Address )Dimensions of Specimen/mmNumber of Specimen in Each Group/Piece
DensityElectronic balance (QUINTIX513-1CN, Shanghai, China)20 × 20 × 2030
ShrinkageElectric—heating blast hot-air drying oven (DHG-9023A, Shanghai, China), Digital micrometer20 × 20 × 2030
SwellingElectric—heating blast hot-air drying oven (DHG-9023A, Shanghai, China), Digital micrometer20 × 20 × 2030
Ultimate stress in compression parallel to grainMaterial tester (AGS-X 20KN, Suzhou, China)20 × 20 × 3030
Tensile strength parallel to the grainMaterial tester (AGS-X 20KN, Suzhou, China)20 × 20 × 37034
Bending strengthMaterial tester (AGS-X 20KN, Suzhou, China)20 × 20 × 30030
Modulus of elasticity in bendingMaterial tester (AGS-X 20KN, Suzhou, China)20 × 20 × 30030
Static hardnessMaterial tester (AGS-X 20KN, Suzhou, China)50 × 50 × 7030
Impact bending strengthImpact tester (KRJB-100, Jinan, China)20 × 20 × 30030
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MDPI and ACS Style

Ruan, Y.; He, Z.; Fan, S.; Chen, Z.; Li, M.; Ma, X.; Sun, S. Variations in Physical and Mechanical Properties Between Clear and Knotty Wood of Chinese Fir. Forests 2024, 15, 2007. https://doi.org/10.3390/f15112007

AMA Style

Ruan Y, He Z, Fan S, Chen Z, Li M, Ma X, Sun S. Variations in Physical and Mechanical Properties Between Clear and Knotty Wood of Chinese Fir. Forests. 2024; 15(11):2007. https://doi.org/10.3390/f15112007

Chicago/Turabian Style

Ruan, Yingchao, Zongming He, Shaohui Fan, Zhiyun Chen, Ming Li, Xiangqing Ma, and Shuaichao Sun. 2024. "Variations in Physical and Mechanical Properties Between Clear and Knotty Wood of Chinese Fir" Forests 15, no. 11: 2007. https://doi.org/10.3390/f15112007

APA Style

Ruan, Y., He, Z., Fan, S., Chen, Z., Li, M., Ma, X., & Sun, S. (2024). Variations in Physical and Mechanical Properties Between Clear and Knotty Wood of Chinese Fir. Forests, 15(11), 2007. https://doi.org/10.3390/f15112007

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