Effect of Bamboo Nodes on the Mechanical Properties of Phyllostachys iridescens

Abstract

Bamboo is a significant natural resource, recognized for its rapid growth, lightweight composition, high strength, and excellent mechanical properties, making it increasingly valuable in the furniture and construction industries. A critical structural aspect of bamboo is its nodes, yet there has been limited research on their impact on bamboo’s mechanical properties. This study investigates the mechanical properties of round bamboo tubes in three different states: internodes (S1), nodes with diaphragm removed (S2), and nodes with diaphragm (S3). The results show that the mechanical properties of S1 are a compressive strength (CS) of 29.72 MPa, a shear strength parallel to grain (SS_p) of 11.82 MPa, a radial stiffness (S_r) of 155.59 MPa, an impact toughness (IT) of 20.74 kJ/m², a modulus of rupture (MOR) of 16.45 MPa, a modulus of elasticity (MOE) of 408.53 MPa, a tensile modulus of rupture parallel to grain (MOR_T) of 189.62 MPa, and a tensile modulus of elasticity parallel to grain (MOE_T) of 431.05 MPa. Compared with S1, these above parameters change by CS +11%, SS_p 6%, S_r +100%, IT −29%, MOR +5%, MOE +63%, MOR_T −29%, and MOE_T −58% in S2 and CS +10%, SS_p 28%, S_r +250%, IT −31%, MOR +28%, MOE +92%, MOR_T −25%, and MOE_T −42% in S3. It demonstrates that the bamboo diaphragm and nodes significantly influence the mechanical properties of bamboo; they have a significant positive effect on the bending properties across the transverse grain, radial ring stiffness, and shear properties along the grain, but negatively impact the tensile properties along the grain.

1. Introduction

Bamboo resources are widely distributed across the globe, primarily concentrated in the tropical and subtropical regions of the Asia–Pacific, with the global bamboo planting area reaching 22 million hectares. Bamboo’s remarkable characteristics—such as its rapid growth rate, high strength-to-weight ratio, and wide distribution—make it a versatile material extensively used in construction, home furniture, and daily necessities [1]. With appropriate modification treatments, the service life of bamboo products can exceed 30 years, making it an effective carbon sequestration material and a renewable resource with significant potential.

For many Asian and African countries, bamboo is a vital natural resource with substantial economic value. In Asia, it is extensively utilized in industries such as construction, furniture, and papermaking, contributing to economic development. In Africa, bamboo, as an emerging resource, is driving green economic growth, poverty alleviation, and environmental protection, demonstrating its significant development potential.

China is rich in diverse bamboo resources, boasting the largest bamboo forest area in the world [2]. The output value of the bamboo industry continues to grow, supported by numerous processing enterprises, and its products are widely employed in construction, furniture, papermaking, and other sectors. This has led to the formation of a complete industrial chain, making significant contributions to economic development.

Bamboo culms exhibit excellent bending toughness and serve as the primary material for bamboo processing and utilization. The bamboo culm is characterized by a thin, hollow wall, with its length segmented by nodes that appear every few centimeters to several tens of centimeters [3]. The cross-sectional area of the culm gradually decreases from the base upwards. This unique structure has evolved to help bamboo adapt to its growth environment, providing effective protection against external forces like wind and snow. The structural characteristics of these nodes vary across different bamboo species, with their form and frequency depending on the species and the height of the culm [4].

Inside the bamboo, there is a bamboo diaphragm corresponding to the position of the bamboo node. The presence of bamboo nodes significantly impacts both the processing efficiency and the product quality of bamboo [5]. For instance, bamboo nodes enhance the gluing properties of bamboo laminated timber but they reduce its flexural strength. During the bamboo flattening process, nodes can have a detrimental effect, exacerbating cracking during both the flattening and the dividing stages. Additionally, the importance of bamboo nodes in the intermediate growth and the material transport of bamboo should not be overlooked. The rapid growth of bamboo relies on the internode meristem, while the lateral transport of water and nutrients occurs within the bamboo nodes [6]. Consequently, bamboo nodes play a crucial role in maintaining the overall structural integrity of bamboo culm’s hollow structure.

The differences in the microstructure of bamboo nodes and internodes are believed to influence the processing performance of bamboo. The vascular bundles and the parenchyma cells in the internode are mainly parallel to the bamboo’s height in bamboo internode, while they are disordered in the node [7,8]. The effect of bamboo nodes on bamboo function and processing is influenced by the bamboo’s microstructure, which differs between the nodes and internodes. In the internodes, the vascular bundles and parenchyma cells are primarily aligned parallel to the height of the bamboo, whereas they are more disordered within the nodes [9]. Additionally, the bamboo diaphragm inside the nodes forms a partition that effectively prevents splitting, damage, and deformation of the round bamboo [10].

Given the significance of the bamboo nodes, numerous studies have examined their influence on the properties of bamboo-based panels. For instance, Widjaja et al. explored the relationship between bamboo fibers and mechanical properties, finding that flexural strength, compressive strength, and tensile strength are closely related to the bamboo fiber [11]. Murphy et al. reported that the distinctive secondary wall structure of bamboo fibers, characterized by alternating width and thickening, significantly influences bamboo’s mechanical properties [12]. Garcia et al. assessed the transverse elastic modulus, shear modulus, and Poisson’s ratio of Guadua bamboo through flexural testing of ring specimens, providing data directly applicable to the numerical simulation of this bamboo species [13]. Despite these advancements, relatively few studies have examined the impact of bamboo nodes on these properties.

Currently, most studies about the influence of bamboo nodes on the mechanical properties of bamboo primarily use a bamboo strip as the test unit. However, the effects of the bamboo node and the diaphragm on mechanical properties of round bamboo are often not considered. This paper investigates the effects of the bamboo node and the diaphragm on the mechanical properties of bamboo, providing a scientific and effective basis for its practical application. These insights can enhance the utilization rate of bamboo resources and promote the healthy and sustainable development of China’s bamboo industry.

2. Materials and Methods

2.1. Materials

Phyllostachys iridescens, commonly known as red bamboo, was selected as the research subject due to its frequent use in furniture manufacturing. The bamboo specimens were sourced from Anji, Zhejiang Province, and were between 4 and 6 years old. The bamboo sections selected for sample preparation were from 0.5 m to 2.3 m above the ground. They had an average diameter of 40.0 ± 1.0 mm, with a thickness of the bamboo wall of 4.0 ± 0.5 mm. The moisture content of the red bamboo was 12.0 ± 1.5%. Three types of bamboo were investigated in this study: bamboo internodes (S1), bamboo tubes with nodes but without diaphragms (S2), and bamboo tubes with nodes and diaphragms kept (S3) (Figure 1a).

2.2. Test Methods

The mechanical properties evaluated in this study include modulus of rupture (MOR), modulus of elasticity (MOE), impact toughness (IT), radial stiffness (Sr), shear strength parallel to grain (SSp), compressive strength parallel to grain (CS), tensile modulus of rupture parallel to grain (MOR_T), and tensile elastic modulus parallel to grain (MOE_T). Detailed sample information for each parameter is presented in Figure 1b. Six parallel samples were prepared for each test, and the arithmetic mean values were calculated, with the standard deviations represented as error bars.

The parameters MOR, MOE, SR, SSP, CS, MOR_T, and MOE_T were tested according to the LY/T2564-2015 standard, “Test Method for Physical and Mechanical Properties of Round Bamboo” [14]; an MMW-50 mechanical testing machine (Jinan Nair Testing Machine Co., Ltd., Jinan, China) was employed for testing these mechanical properties. IT was assessed following the GB/T 1927.17-2021 standard, “Test methods for physical and mechanical properties of small clear wood specimens-Part 17: Determination of impact bending strength” [15]; a JBS-300S pendulum impact testing machine (Beijing Time High Technology Ltd., Bejing, China) was used for the IT tests.

3. Results

3.1. Compressive Strength Parallel to Grain

At the beginning of the testing, the bamboo did not exhibit significant changes under loading. However, as the load gradually increased, slight bulging occurred in the bamboo tube near the loading end. In specimen S1, cracks developed along the longitudinal direction of the outer surface near the bulged area, extending towards the inner surface. For S2 and S3, the stiffening effect of the bamboo node reduced the extent of bulging compared to S1. Nevertheless, cracks still formed along the bamboo wall, extending from the wall to the center of the bamboo diaphragm in S3 (Figure 2a).

Figure 2b presents the load-displacement curves, which demonstrate similar behavior across all specimens. Initially, the load-displacement relationship exhibits a concave shape. Subsequently, the load increases linearly with the displacement, indicating the elastic deformation phase. When the load exceeds the proportional limit, the slope of the curve decreases. Upon reaching the maximum load, the load gradually declines, reflecting a reduction in bearing capacity. As the load continues to increase, the specimen eventually loses its bearing capacity, leading to complete failure. Notably, the slopes of the curves differ among the groups, with S1 showing the steepest slope, followed by S2 and then S3.

The compressive strengths of S1, S2, and S3 were 29.72 MPa, 32.98 MPa, and 32.79 MPa respectively. Compared to S1, S2 and S3 increased by approximately 11% and 10%, respectively, although these differences were not statistically significant (Figure 2c). While the bamboo node enhances the compressive strength, the bamboo diaphragm had no significant influence. This observation is consistent with the findings of Hao [16] and Lin [17]. This suggests that the bamboo node, rather than the diaphragm, had a slight stiffening effect on the compressive strength. The modest positive effect may be attributed to the disordered arrangement of the cells, which contributes to a more stable structure compared to the single-direction arrangement in S1.

3.2. Shear Strength Parallel to the Grain

In shear resistance testing, no significant phenomena are observed initially. However, as the load increases, a sudden loud noise occurs, indicating that the sample has reached its failure load. After being removed from the test equipment, the specimen exhibits a crack along the grain at the shear location, with the bamboo pieces on either side of the cushion block misaligned. Additionally, the bamboo diaphragm in S3 shows localized tearing (Figure 3a).

Figure 3a shows the loading curves from the shear resistance tests, which exhibit similar trends between these three samples. Initially, the increase is gradual with a concave pattern. As the load increases, the load-displacement relationship becomes linear. The slope then decreases, indicating the transition to the elastoplastic stage. Upon reaching the ultimate load, the curve drops sharply, followed by subsequent stepped drops. This behavior indicates that the shear region does not fail entirely at once, allowing the specimen to sustain some load even after initial localized failure. Notably, the curves exhibit a sequential decrease in magnitude, with S3 showing the highest values, followed by S2 and S1.

Figure 3b presents the value of the shear strength parallel to the grain. The shear strength was 11.82 MPa for S1, 12.48 MPa for S2, and 15.15 MPa for S3. Compared to S1, the shear strength of S2 increased by approximately 6%, while S3 showed an increase of about 28%. These results indicate that the bamboo diaphragm enhances the shear strength of bamboo, whereas the bamboo node has minimal impact on its shear strength. This result aligns with the findings of Shao [18].

The single arrangement of fibers and parenchyma cells along the longitudinal direction made bamboo easy to crack; the positive effect of the bamboo node and the diaphragm on shear strength might be caused by the arrangement direction change in the bamboo node and the diaphragm. When S1 is subjected to shear force, the bamboo wall would be easy to crack due to the single arrangement of fibers and parenchyma cells [19]; the specimen loses its bearing capacity once the bamboo wall cracks. In contrast, for specimens S2 and S3, failure under shear force requires not only cracking the bamboo wall at the shear site, but also breaking the connection between the bamboo node, the diaphragm, and the bamboo wall [7]. This additional resistance provided by the bamboo diaphragm enhances the shear strength of the bamboo along the grain.

3.3. Radial Stiffness

During radial stiffness testing, the crack initiation and propagation patterns were similar across all groups, with initial cracks typically developing near the mid-height of the bamboo specimens (Figure 4a). Cracking began on the bamboo outer surface and progressively extended towards the bamboo inner surface as the load increased, eventually penetrating the entire bamboo wall and resulting in specimen failure. The highest number of cracks was observed in group S1. Specimens in group S3 exhibited significantly less deformation along the loading direction compared to the other two groups. Additionally, a vertical crack appeared at the center of the bamboo diaphragm, perpendicular to the direction of the loading plate, gradually extending outward until the specimen failed.

In the load-displacement curves, the change trends were similar across all groups, but the slopes differed, with S3 showing the steepest slope, followed by S2 and then S1. The curves initially displayed a concave shape during the early loading stage, transitioning into a linear increase with displacement, which indicates elastic deformation where deformation is proportional to the load and is reversible. As the slope gradually decreased, the specimens entered the elastoplastic stage with an accelerated rate of displacement. Upon reaching the maximum load, the curves sharply declined; however, the specimens did not immediately lose their bearing capacity. The curves then underwent several cycles of rising and falling before the specimens ultimately failed (Figure 4b).

The radial stiffness is shown in Figure 2c. The average radial stiffness was 155.59 MPa for S1, 306.87 MPa for S2, and 542.75 MPa for S3. Compared to S1, the radial stiffness of S2 increased by approximately 100%, and the S3 showed an increase of about 250% (Figure 4c). These results indicate that both the bamboo node and the diaphragm significantly influence radial stiffness, positively enhancing it, which aligns with the findings of Yu [20].

The positive effects of the bamboo node and the diaphragm are likely due to their unique microstructure. In the bamboo internode, fiber bundles and parenchyma cells are predominantly arranged longitudinally, while in the bamboo node and diaphragm, the arrangement is more chaotic and disordered [21]. The fiber strength in the transverse direction is considerably lower than in the longitudinal direction [22]. During the radial stiffness test, fibers and parenchyma cells are subjected to vertical loading in S1 and partial horizontal loading in S2 and S3, effectively utilizing the high longitudinal strength of the fibers. Additionally, the bamboo diaphragm exerts a tightening effect on the bamboo tube, limiting lateral deformation and enhancing the bamboo’s radial stiffness [23].

3.4. Impact Toughness

Figure 5 presents the results of the impact toughness test. The average impact toughness of S1 was 20.74 kJ/m², S2 was 26.80 kJ/m², and S3 was 14.34 kJ/m². Compared to S1, the impact toughness of S2 increased by approximately 29%, whereas that of S3 decreased by about 31%. These results indicate that the bamboo node enhances the impact toughness of red bamboo, while the bamboo diaphragm reduces it.

Compared to S1, the IT in S2 increased, likely due to changes in the microstructure of the bamboo node. In bamboo internodes, fibers and parenchyma cells are arranged longitudinally, while in bamboo nodes, the cells display an irregular arrangement. In S2, the cells from the internode and the node formed a cross structure [24], which is more effective at withstanding impact loading than a single-direction arrangement [25,26], leading to the increased IT observed in S2.

However, the reduced IT in S3 can be attributed to the brittleness resulting from the high lignin content in the bamboo diaphragm. Bamboo primarily consists of three chemical components: cellulose, hemicellulose, and lignin. With higher lignin content, the bamboo exhibited increased brittleness [27]. The bamboo diaphragm has a higher lignin content compared to the bamboo internode [28], resulting in greater brittleness. Consequently, this increased brittleness of the bamboo diaphragm negatively affects its performance in impact toughness tests.

3.5. Bending Properties

During the initial stages of the bending tests, the bamboo specimens showed no significant changes. However, as the load increased, longitudinal cracks developed at the loading points across all groups. Notably, only the S1 specimens exhibited bending deformation. In S1, cracks formed along the grain direction at mid-height, initially appearing on the outer surface and then propagating towards the inner surface. After bending failure, S1 displayed penetrating cracks along the bamboo’s length and through its wall, whereas no penetrating cracks were observed in S2 and S3 (Figure 6a).

Figure 6b shows the load-displacement curves for the bending test, which exhibit a similar trend across the three groups. In the initial loading stage, the load-displacement curves increase linearly. As the load increases, the deflection rate accelerates, causing the curves to decelerate. The deceleration is significantly more pronounced in S1 compared to S2 and S3, with S1 showing noticeable bending deformation. Upon reaching the ultimate load, the specimens fail instantaneously, leading to a sharp drop in the load-displacement curve accompanied by significant fluctuations near the ultimate load.

The MOR values for S1, S2, and S3 were 16.45 MPa, 17.23 MPa, and 21.05 MPa, respectively, which is much lower than for bamboo strips, whose MOR values are about 150 MPa both in the bamboo node and internode species [29]. Compared to S1, S2 exhibited a 5% increase, while S3 showed an approximate 28% increase, which also shows a slightly positive effect of the bamboo node on MOR in bamboo strips [29]. Similarly, the MOE values for S1, S2, and S3 were 408.53 MPa, 667.48 MPa, and 783 MPa, respectively. Compared to S1, the MOE of S2 increased by 63% and S3 increased by 92% (Figure 6c,d). The results indicated that the presence of bamboo nodes had a slight positive effect on MOR but had a significant positive effect on MOE. In contrast, bamboo diaphragms had a notable positive effect on both MOR and MOE. These findings are consistent with the studies of Shao [18] and Yang [30].

The effects of bamboo nodes and diaphragms on the modulus of rupture (MOR) and the modulus of elasticity (MOE) are associated with structural changes introduced by these features within the bamboo tube. Bamboo’s hollow tubular structure consists of cells arranged in a single longitudinal pattern at the internodes, with weak transverse connections between fiber cells [31], making the internode section susceptible to failure under bending stress. In contrast, the cell arrangement at the bamboo nodes and diaphragms is modified into a cross-linked structure that provides enhanced support for the bamboo tube. The bamboo diaphragm, in particular, reinforces the hollow tube, increasing its rigidity [32] and improving the MOE, especially in sections S2 and S3.

3.6. Tensile Properties Parallel to Grain

The tensile failure fracture of S1 occurred on the inner surface of the bamboo, resulting from the microstructural variation of the bamboo along the radial direction. The inner side is primarily composed of parenchyma cells, while the outer side is dominated by fiber cells. Since the tensile strength of parenchyma cells is significantly lower than that of fiber cells [33,34], the inner side is more susceptible to damage under tensile stress. In S2 and S3, the tensile fractures are concentrated at the bamboo nodes. Additionally, compared to S1, the fiber tear length at the tensile fractures of S2 and S3 is shorter (Figure 7a). This is attributed to the high presence of transversely arranged cells at the bamboo nodes [18], which have weak transverse bonding strength, making them more prone to tearing under tensile load.

The load-displacement curves of the three specimens exhibit a similar trend. As the displacement increases, the load generally rises linearly, with the slope gradually increasing, and no distinct yield point was observed. Upon reaching the maximum load, the curve declines sharply, leading to a complete loss of bearing capacity, which indicates total failure of the specimen (Figure 7b). It is evident that the steepness for S3 is higher than that of S2, and S2 is higher than S1, suggesting that the brittleness of the bamboo node surpasses that of the bamboo internode, and the bamboo diaphragm strengthened its brittleness. The failure loads of S2 and S3 are comparable and both higher than those of S1, indicating that the bamboo nodes enhance tensile performance, while the bamboo diaphragm does not exhibit a significant positive effect.

The MOR_T values for S1, S2, and S3 were 189.62 MPa, 135.16 MPa, and 78.76 MPa, respectively. Compared to S1, S2 exhibited a 29% decrease, while S3 showed an approximate 58% increase (Figure 7c). Similarly, the MOE_T values for S1, S2, and S3 were 431.05 MPa, 321.02 MPa, and 249.13 MPa, respectively. Compared to S1, the MOE_T of S2 decreased by 25% and S3 decreased by 42% (Figure 7d). The results indicated that both the bamboo node and the diaphragm had a significantly negative effect on MOR_T and MOE_T. The results are consistent with Qi’s study in bamboo fiber-reinforced composite [35] and Wang’s study in laminated bamboo lumber [36].

The negative impact of bamboo nodes and diaphragms on the modulus of rupture (MOR_T) and modulus of elasticity (MOE_T) in tension is closely related to the structural characteristics of the bamboo node section. In the bamboo internode, fiber cells and parenchyma cells are arranged longitudinally; whereas, in the bamboo node section, these cells exhibit a cross or disordered arrangement [21]. The polymer chains in fiber cells and parenchyma cells are primarily oriented along the longitudinal direction, providing the highest strength along this axis due to molecular alignment [37], resulting in the superior tensile properties of the bamboo internodes compared to the bamboo nodes and the diaphragms. The bamboo diaphragm exhibits the lowest MOR_T and MOE_T in tension, likely due to the presence of weak-link areas that enlarge the tensile zone but do not positively contribute to its tensile performance.

4. Conclusions

The bamboo internode exhibits the highest tensile strength and modulus. Compared to bamboo internode, both the bamboo node and the diaphragm negatively impact tensile properties, reducing the tensile strength by 29% and 58% and the tensile modulus by 25% and 42%, respectively. While the bamboo node significantly enhances impact toughness by 29%, the bamboo diaphragm reduces it by 31%.

Compared to the bamboo internode, the bamboo node positively affects bending strength, bending modulus, radial stiffness, and shear strength, and these properties of the bamboo node can be further enhanced by the bamboo diaphragm. Among these parameters, radial stiffness showed the most significant increase, rising by 100% in the bamboo node and 250% in the bamboo diaphragm, respectively.

The bamboo node also contributes to an 11% increase in compressive strength; however, the bamboo diaphragm does not provide additional reinforcement, and it shows minimal changes compared to the bamboo node specimens.

Mechanical properties are critical reference parameters for the application of bamboo in construction. However, the influence of the bamboo’s inherent characteristics on its mechanical properties remains relatively understudied, with limited theoretical foundations and incomplete testing methodologies available. This gap has led to a lack of comprehensive guidance for practical engineering applications, and relevant standards and norms are still underdeveloped. Although bamboo is increasingly recognized as an innovative material with broad applications in daily life, further research is needed to fully explore its potential in engineering contexts.