Effect of Bamboo Nodes on the Mechanical Properties of P. edulis (Phyllostachys edulis) Bamboo

Abstract

In the context of global sustainable development, the use of natural and renewable bamboo as a building material is of great significance for building engineering. As an important part of bamboo, there is still a lack of systematic research on the effect of bamboo nodes on the mechanical properties of bamboo. In this paper, compression parallel to the grain of the bamboo, bending, tensile parallel to the grain, shear parallel to the grain, compression perpendicular to the grain and tensile perpendicular to the grain tests of an entire bamboo culm of P. edulis bamboo were carried out, and the effects of bamboo nodes on moisture content, bearing capacity and mechanical properties were analyzed. The relationship between the mechanical properties and standardized parameters of the node and internode specimens was established. A conversion method of mechanical properties of the same test type was proposed, and a conversion method among different mechanical properties was proposed. The bamboo was graded by tensile strength parallel to the grain. The results show that bamboo nodes have an obvious effect on the moisture content. Bamboo nodes have different effects on the bearing capacity and mechanical properties of different types of specimens. The mechanical properties of P. edulis bamboo can be predicted by the relationship between the mechanical properties and the standardized parameters and the mechanical property conversion method. The strength classification of P. edulis bamboo can be predicted by standardized parameters.

1. Introduction

Currently, the main building materials in China include masonry, concrete and steel, which are nonrenewable and polluting. With the implementation of sustainable development strategies, the application of new building materials is very important. In recent years, wood and bamboo materials have become increasingly valued and favored by people. As a result of the lack of forest resources, wood has difficulty meeting the needs of development. Bamboo is rich in resources, and it has the advantages of a short growth cycle, a high ratio of strength to weight, a high ratio of rigidity to weight, good toughness and so on, which is called “plant reinforcement” and is an ideal new green building material [1,2,3,4,5].

A series of achievements have been made in the study of bamboo properties. The typical achievements include the effects of moisture content, bamboo age, height, wall thickness, diameter and density on the properties of original bamboo [6,7,8,9,10]; the properties and analysis of recombined bamboo and glued bamboo [11,12,13]; and the composite components of bamboo and other materials [14,15,16,17,18]. Currently, the body of research mainly focuses on modified bamboo and the combination of modified bamboo with other materials, but research on the performance of original bamboo is still not extensive enough. When modified bamboo is made or bonded with other materials, the adhesive will cause some negative effects on the environment. Therefore, it is of great significance to study how to utilize the structure of the original bamboo, and a clear grasp of the mechanical properties of the original bamboo is the basis of engineering applications. As an important part of bamboo, the effect of bamboo nodes on the mechanical properties of bamboo remains to be studied.

In this paper, bamboo stalks with the highest yield and the most widely used species were taken as the research object. Experiments of compression parallel to the grain, bending, tensile parallel to the grain, shear parallel to the grain, compression perpendicular to the grain and tensile perpendicular to the grain were carried out. The effect of bamboo nodes on the moisture content, bearing capacity and mechanical properties was studied, and the method of mechanical property conversion and hierarchical prediction of bamboo was proposed. The research results of this paper will greatly enrich the research results of the mechanical properties of the original bamboo and lay a theoretical foundation for the engineering applications of original bamboos.

2. Materials and Methods

2.1. Materials

The bamboo species in this study was P. edulis bamboo, which was cut during winter in Chenzhou city, Hunan Province, China. Twenty-five straight bamboos with a length of 6 m and an age of approximately 4 years were randomly cut from the bamboo forest in order to carry out the mechanical properties test.

The specimens with mechanical properties were divided into six categories: compression parallel to the grain (UC), bending (B), tensile parallel to the grain (UT), shear parallel to the grain (US), compression perpendicular to the grain (CC) and tensile perpendicular to the grain (CT). The specimens are shown in Figure 1. The ratio of the length to diameter of the UC and US specimens is 1. The B specimen size is 220 mm × 15 mm × t mm (t refers to the wall thickness); the size of the UT specimen is 330 mm × 15 mm × t mm; the CC specimen size is 15 mm × 15 mm × t mm; the CT specimen length is 100 mm. The factors of bamboo node and height were taken into full consideration, and the principle of uniform distribution along the height of bamboo stalk was followed.

2.2. Mechanical Properties Test and Calculation

A universal testing machine (ETM504C) was used to load all types of the mechanical property specimens. The loading rate of the UC, US and UT tests was 0.01 mm/s, the loading rate of the B test was 150 N/mm² per minute, the loading rate of the CC test was 20 N/mm² per minute and the loading rate of the CT test was 0.005 mm/s. The strength and elastic modulus of the specimen are calculated as follows [19,20].

$$f_W=\frac{P_{\max }}{A}$$

(1)

$$E_W=\frac{20\mathsf{\Delta }P}{A\mathsf{\Delta }l}$$

(2)

$$MO{R}_W=\frac{150P_{\max }}{t{b}^2}$$

(3)

$$MO{E}_W=\frac{1920000\Delta P}{8{\delta }_{m}t{b}^3}$$

(4)

In the formula, f_W is the strength of UC, US, UT, CC and CT specimens under moisture content W (MPa); E_W is the compressive modulus and tensile modulus along the grain under moisture content W (MPa); MOR_W is the bending strength under moisture content W (MPa); MOE_W is the flexural elastic modulus under moisture content W (MPa); P_max is the failure load (N); A is the stressed area (mm²); t is the thickness of the specimen (mm); b is the height of the specimen (mm); ΔP is the difference between the upper and lower loads (N); ∆l is the difference of the specimen deformation values under upper and lower loads (mm); and δ_m is the deflection value of the pure bending section of the specimen under the action of ∆P (mm).

2.3. Adjustment of Mechanical Properties

After the loading failure of the specimen, a small specimen with a size of approximately 20 mm × 20 mm was immediately cut off near the failure part in order to carry out the moisture content test. The moisture content was calculated by Formula (5) [6]. Since the mechanical properties of bamboo are significantly affected by its moisture content, it is necessary to uniformly adjust the mechanical properties to the value under the standard moisture content (12%) with Formula (6) [19,20]:

$$W=\frac{m_1-m_0}{m_0}\times 100$$

(5)

$$M_{12}=K_{\mathrm{W}}{M}_{\mathrm{W}}$$

(6)

$$K_{\mathrm{W}}=\frac{1}{a+b{e}^{cw}}$$

(7)

where W is the air dry moisture content (%); m₁ and m₀ are the masses of air dry and fully dry, respectively (g); an electronic balance was used for measurement; m₀ was measured after drying in an oven; M₁₂ is the strength or elastic modulus of the specimen under the standard moisture content (12%); M_W is the strength or elastic modulus of the specimen when the moisture content is W; and K_W is the moisture content correction coefficient, which is related to specific mechanical properties and the moisture content. Parameters a, b and c refer to the standards [19,20].

3. Results and Discussion

3.1. Failure Process and Morphology Analysis

The UC node and internode specimens gradually buckled with increasing load (Figure 2a) and eventually developed longitudinal cracks and failure. The typical load-displacement curve of the UC specimens is shown in Figure 3. The curve includes the elastic section, the elastic-plastic phase and the descending section. The specimens show good ductility failure characteristics. During the loading process, specimen B bent at the loading point, and the deformation gradually increased. When the load reaches a certain value, the fibre breaks, and the specimen is destroyed (Figure 2b). Bamboo bending shows a “semibrittle” failure feature (Figure 3), and the elastic plasticity of the load-displacement curve is short. The main failure mode of the UT specimen was transverse fracture failure at the centre of the specimen (Figure 2c), and the specimen showed obvious brittle failure characteristics (Figure 3). There is no obvious deformation during the process of loading of the US specimen, and the specimen was damaged due to longitudinal dislocation in the shear plane (Figure 2d). The typical load-displacement curve of the US specimen (Figure 3) shows the characteristics of brittle failure. The failure mode of the CC specimen is mainly pressure and bending failure (Figure 2e), showing a baroclinic failure mode. The typical load-displacement curve of the specimen (Figure 3) includes the elastic stage, the elastic-plastic stage and the descending stage, showing the characteristics of ductile failure. All CT specimens had fracture failure along the opening of the specimens (Figure 2f), and the typical load-displacement curve (Figure 3) indicates that the specimens exhibit brittle failure characteristics.

3.2. Location Distribution of the Bamboo Nodes

The distance between the bamboo nodes is different at different heights of the bamboo stalk, which increases gradually along the direction of bamboo growth (Figure 4). Suppose a bamboo stalk has n bamboo nodes, each node is numbered from bottom to top, and the lowest node is numbered one. The distribution of bamboo nodes of a randomly selected bamboo stalk was obtained by calculating statistics on the location of the nodes in the bamboo, as shown in Figure 5. As shown in Figure 5, the increase in bamboo node spacing gradually increases along the height direction. The fitting relation between the height of the bamboo stem and the number of the bamboo nodes is shown in Formula (8).

$$h=5.82n^2+174.19n-182.65{(\mathrm{R}}^2=0.9997)$$

(8)

3.3. Effect of Bamboo Nodes on Moisture Content

The bamboo culms were evenly divided into six segments, each with a length of 1000 mm. The average moisture content of the specimens in each section was counted in order to obtain the statistical results of the moisture content, as shown in Figure 6. According to the results in Figure 6, the average moisture content of the specimens with nodes in each section is higher than that of the specimens without nodes. The average moisture content in each section of the specimens with nodes increased by 10.04%, 2.32%, 4.78%, 9.76%, 12.67% and 8.43%, respectively, compared with that of the specimens without nodes, and the average moisture content increased by 8.83%. Bamboo nodes have an obvious effect on moisture content, and bamboo nodes can lock in water to a certain extent.

3.4. Influence of Bamboo Nodes on Bearing Capacity

The results shown in

Table 1. Bearing capacity statistics.

Specimens Type	0 < h < 1000 mm	1000 ≤ h < 2000 mm	2000 ≤ h < 3000 mm	3000 ≤ h < 4000 mm	4000 ≤ h < 5000 mm	5000 ≤ h < 6000 mm	Average
UCN	149.82	135.60	119.42	106.39	85.17	84.93	113.56
UCI	150.37	132.67	121.29	104.45	89.82	80.67	113.21
BN	0.05	0.06	0.06	0.05	0.04	0.04	0.05
BI	0.06	0.07	0.07	0.05	0.05	0.04	0.06
UTN	4.45	4.35	3.81	3.71	3.98	3.70	4.00
UTI	4.89	4.64	4.65	4.65	4.65	4.56	4.67
USN	53.89	56.53	43.71	39.83	40.70	33.49	44.69
USI	61.17	52.11	46.14	39.46	36.06	32.73	44.61
CCN	4.61	4.45	2.41	4.93	3.68	3.18	3.88
CCI	3.47	3.18	2.42	3.33	3.13	2.92	3.07
CTN	2.83	2.01	2.65	2.32	2.01	1.99	2.30
CTI	2.70	1.47	2.58	2.31	1.98	1.88	2.15

Note: “_N” stands for node specimen, “_I” stands for internode specimen and the rest is the same.

were obtained by statistical analysis of the bearing capacity of the node and internode specimens with various mechanical properties. According to Table 1, among all kinds of specimens, specimen UC has the highest bearing capacity, while specimen B has the lowest bearing capacity. The bearing capacity of the UC, CC and CT node specimens is higher than that of internode specimens. The bearing capacity of the UT and B internode specimens is higher than that of the node specimens, and the bearing capacity of the US node specimens is similar to that of the internode specimens.

3.5. Effect of the Nodes on Mechanical Properties

3.5.1. Characteristic Values of the Mechanical Properties of Entire Bamboo Stalks

The box normal curve shown in Figure 7 was obtained by statistical analysis of the mechanical properties of bamboo. After the outliers are removed, the statistical results are shown in

Table 2. Statistical results of the characteristic values of the mechanical properties.

Mechanical Performance Index	Quantity	Mean	Standard Deviation	Coefficient of Variation
UCSN	74	59.8 MPa	4.13 MPa	0.069
UCSI	231	57.2 MPa	4.68 MPa	0.082
UCEN	74	14.5 GPa	1.17 GPa	0.080
UCEI	231	13.6 GPa	1.18 GPa	0.087
MORN	75	130 MPa	6.65 MPa	0.046
MORI	80	133 MPa	7.19 MPa	0.054
MOEN	75	17.4 GPa	0.804 GPa	0.046
MOEI	80	17.7 GPa	1.37 GPa	0.077
UTSN	167	140 MPa	12.3 MPa	0.088
UTSI	147	149 MPa 9.40926	9.41 MPa	0.063
UTEN	167	16.5 GPa	1.15 GPa	0.070
UTEI	158	16.3 GPa	1.18 GPa	0.072
USSN	61	15.9 MPa	1.62 MPa	0.109
USSI	144	15.9 MPa	1.1 MPa	0.069
CCSN	77	37.3 MPa	4.65 MPa	0.125
CCSI	100	27.9 MPa	1.37 MPa	0.049
CTSN	47	6.33 MPa	0.489 MPa	0.077
CTSI	73	3.77 MPa	0.517 MPa	0.137

, where “N” represents the node specimen and “I” represents the internode specimen. According to Figure 7 and Table 2, the bamboo is anisotropic with respect to the mechanical properties. UCS and MOR are high, while USS is low. The tensile and compressive strengths parallel to the grain (UCS and UTS) are significantly higher than the compressive strength perpendicular to the grain (CCS). The tensile strength parallel to the grain (UTS) is slightly higher than the bending strength (MOR). The tensile strengths parallel to the grain (UTS) and bending strength (MOR) are significantly higher than the compressive strength parallel to the grain (UCS), and the compressive strength perpendicular to the grain (CCS) is significantly higher than the tensile strength perpendicular to the grain (CTS).

Bamboo nodes have different effects on different mechanical properties. The UCS, UCE, UTE, CCS and CTS of the node specimens are higher than those of the internode specimens. The MOR, MOE and UTS of the internode specimens are higher than those of the node specimens. The USSs of the node specimen and the internode specimen are similar. The relative difference in mechanical properties between nodes in the transverse striation direction is more obvious.

3.5.2. Relationship between Bamboo Mechanical Properties and Standardized Parameters Considering the Effect of Bamboo Nodes

Based on the least square method, the mechanical properties of bamboo are fitted with the standardized parameter h/D_B (where D_B is the diameter of the second bamboo node at the bottom), and the fitting curve, as shown in Figure 8, and the fitting relation, as shown in

Table 3. Relationship between the physical and mechanical properties and h/D_B.

The Fitting Parameters	Relations	R2
UCSN	UCSN = 0.203 h/DB + 54.331	0.678
UCSI	UCSI = 0.186 h/DB + 50.665	0.661
UCEN	UCEN = 0.058 h/DB + 12.796	0.546
UCEI	UCEI = 0.062 h/DB+ 11.554	0.595
MORN	MORN = 0.277 h/DB + 124.2	0.541
MORI	MORI = 0.302 h/DB + 125.48	0.585
MOEN	MOEN = 0.03 h/DB + 16.7	0.523
MOEI	MOEI = 0.063 h/DB + 16.24	0.517
UTSN	USSN = 0.451 h/DB + 123.7	0.574
UTSI	USSI = 0.666 h/DB + 132.18	0.675
UTEN	UTSN = 0.041 h/DB + 14.98	0.299
UTEI	UTSI = 0.048 h/DB + 14.841	0.445
USSN	UTEN = 0.061 h/DB + 13.86	0.694
USSI	UTEI = 0.055 h/DB + 14.185	0.640
CCSN	CCSN = −0.33 h/DB + 43.74	0.591
CCSI	CCSI = 0.0335 h/DB + 26.423	0.353
CTSN	CTSN = −0.04 h/DB + 7.62	0.228
CTSI	CTSI = −0.017 h/DB + 4.236	0.107

, are obtained. As observed in Figure 8, UCS_N, UCS_I, UCE_N, UCE_I, MOR_N, MOR_I, MOE_N, MOE_I, UTS_N, UTS_I, USS_N, USS_I and CCS_I increase with the increase in the standardization parameter h/D_B. CCS_N, CTS_N and CTS_I decrease with increasing h/D_B. According to the fitting relation in Table 3, the mechanical properties of bamboo can be predicted by the standardized parameter h/D_B.

The variation in the mechanical properties of bamboo with the standardized parameter h/D_B reflects the variation in the mechanical properties along the height of the bamboo stalk to a certain extent. When the bamboo is stressed along the grain direction, the vascular bundle plays the role of bearing the load, and the basic organization plays the role of connecting and transferring the load [21]. The vascular bundle density increases with increasing bamboo stalk height, so the mechanical properties along the grain direction increase with increasing standardized parameter h/D_B. The primary tissue plays a major role in the transverse tensile direction of bamboo, and the proportion of the primary tissue decreases with increasing bamboo stem height, so the CTS decreases with increasing standardized parameter h/D_B. The vascular bundle plays a major role in the compression of the stria. Before the test, the protruding parts of the node specimens containing vascular bundles were disjointed and polished, and the proportion of vascular bundles worn off increased with increasing bamboo stalk height, so the CCS_N showed an opposite trend with increasing standardized parameter h/D_B.

3.5.3. Relationship between the Node and Internode Mechanical Properties

Based on the linear relationship between the mechanical properties of the node and internode specimens and the standardized parameter h/D_B, the relationship between the mechanical properties of the node and internode specimens at the same position in the bamboo was derived, as shown in

Table 4. The relationship between the mechanical properties of bamboo nodes and internodes.

Mechanical Performance Index	Node-Internode	Internode-Node
UCS	UCSN = 1.09UCSI − 0.965	UCSI = 0.916UCSN + 0.884
UCE	UCEN = 0.945UCEI + 1.987	UCEI = 1.07UCEN − 2.12
MOR	MORN = 0.917UCEI + 9.11	MORI = 1.09UCEN − 9.93
MOE	MOEN = 0.476UCEI + 8.97	MOEI = 2.1UCEN − 18.83
UTS	UTSN = 0.677UTSI + 34.2	UTSI = 1.47UTSN − 50.5
UTE	UTEN = 0.854UTEI + 2.30	UTEI = 1.17UTEN − 2.70
USS	USSN = 1.11USSI − 1.87	USSI = 0.9USSN + 1.69
CCS	CCSN = −9.85CCSI + 304	CCSI = −0.1CCSN + 30.86
CTS	CTSN = 2.35CTSI − 2.35	CTSI = 0.CTSN + 0.99

.

3.5.4. Mutual Relationship of Bamboo Mechanical Properties Considering the Effect of Bamboo Nodes

It is impossible to measure multiple mechanical properties of a single specimen at the same time, and the relationship between mechanical properties plays a vital role in the establishment of a bamboo mechanical property evaluation system. The establishment of the relationship between mechanical properties can greatly reduce material consumption and test costs.

It is generally believed that the relationship between mechanical properties is based on the same level and that the value of different mechanical properties with the same failure probability has a one-to-one relationship. From a mechanics performance value of 99 percentile values (percentile choose 0.01 + 0.01n, n = 0~98 and n is rounded; schematic diagram as shown in Figure 9) to establish the relationship between quantile values of each mechanical properties, the relationship between mechanical properties was fitted according to the percentile data. The relationship between mechanical properties form the basis of the bamboo performance evaluation system. The establishment of a relationship between the two mechanical properties can dramatically simplify the quality control process of mechanical stress classification and reduce material consumption in the performance test, especially for materials with a large performance variation coefficient [22]. The relationship between UCS_N fitting based on the above method and the mechanical properties of the other node specimens is shown in Figure 10. Due to space constraints, the fitting curves of the other mechanical properties are not shown.

The relationship between the mechanical properties was derived by the above method, and the relationship between the mechanical properties and the standardized parameter h/D_B was derived by using the linear relationship between mechanical properties and the standardized parameter h/DB. Formula (9) is used as the conversion formula between mechanical properties. α and β are defined as the conversion parameters of the bamboo’s mechanical properties by considering the effect of bamboo nodes, as shown in

Table 5. Mechanical property conversion parameters of node specimens.

Parameters		P2	UCSN2	UCEN2	MORN2	MOEN2	UTSN2	UTEN2	USSN2	CCSN2	CTSN2
	P1
α	UCSN1		1	0.313	1.388	0.16	2.337	0.280	0.258	1.137	−0.339
	UCEN1		3.128	1	4.4	05	7.421	0.889	0.817	3.604	−1.084
	MORN1		0.709	0.225	1	0.114	1.673	0.201	0.186	0.813	−0.245
	MOEN1		6.116	1.913	8.538	1	14.296	1.717	1.587	6.957	−2.056
	UTSN1		0.42	0.134	0.589	0.067	1	0.12	0.109	0.485	−0.147
	UTEN1		3.516	1.115	4.93	0.562	8.329	1	0.917	4.041	−1.223
	USSN1		3.787	1.198	5.324	0.608	8.921	1.073	1	4.316	−1.303
	CCSN1		0.861	0.273	1.205	0.138	2.04	0.244	0.223	1	−0.298
	CTSN1		−2.879	−0.889	−4203	−0.471	−6.954	−0.797	−0.752	−3.227	1
β	UCSN1		0	−3.615	48.665	7.962	0.155	−0.439	0.377	−30.961	26.627
	UCEN1		12.601	0	65.281	9.975	27.964	2.909	3.489	−17.363	22.712
	MORN1		−33.614	−14.519	0	2.505	−80.342	−10.135	−8.611	−70.082	38.551
	MOEN1		−47.358	−18.427	−17.913	0	−110.53	−13.744	−11.986	−84.857	42.386
	UTSN1		0.967	−3.588	49.289	8.117	0	−0.39	0.508	−30.819	26.872
	UTEN1		2.39	−3.12	51.162	8.336	3.938	0	0.834	−28.96	26.156
	USSN1		−0.094	−3.862	47.467	7.899	−1.16	−0.647	0	−31.264	26.967
	CCSN1		27.936	4.988	87.063	12.424	64.394	7.3	7.567	0	17.406
	CTSN1		78.111	20.733	157.22	20.516	181.86	21.39	20.594	57.533	0

and

Table 6. Mechanical property conversion parameters of internode specimens.

Parameters		P2	UCSN2	UCEN2	MORN2	MOEN2	UTSN2	UTEN2	USSN2	CCSN2	CTSN2
	P1
α	UCSN1		1	0.334	18.526	0.328	3.057	0.301	0.296	0.326	−0.200
	UCEN1		2.971	1	5.504	0.98	9.11	0.895	0.881	0.97	−0.598
	MORN1		0.531	0.177	1	0.175	1.624	0.16	0.157	0.175	−0.105
	MOEN1		3.002	1.008	5.58	1	9.248	0.905	0.888	0.987	−0.600
	UTSN1		0.325	0.109	0.602	0.108	1	0.098	0.096	0.106	−0.065
	UTEN1		3.31	1.108	6.151	1.089	10.137	1	0.983	1.08	−0.658
	USSN1		3.349	1.121	6.196	1.098	10.246	1.01	1	1.09	−0.671
	CCSN1		3.026	1.013	5.653	1.002	9.284	0.911	0.895	1	−0.605
	CTSN1		−4.954	−1.658	−9.337	−1.649	−15.268	−1.496	−1.457	−1.631	1
β	UCSN1		0	−5.688	26.965	−1.175	−27.072	−1.09	−1.276	8.906	15.297
	UCEN1		17.28	0	58.987	4.441	25.368	4.095	3.835	14.529	11.892
	MORN1		−13.334	−10.143	0	−5.636	−68.108	−5.17	−5.2	4.362	17.853
	MOEN1		4.345	−4.315	34.669	0	−15.088	0.189	0.042	10.188	14.432
	UTSN1		9.224	−2.654	43.938	1.728	0	1.652	1.444	11.856	13.449
	UTEN1		3.867	−4.415	33.807	0.064	−15.607	0	−0.172	10.15	14.478
	USSN1		4.707	−4.129	35.815	0.405	−12.836	0.285	0	10.484	14.386
	CCSN1		−26.196	−14.475	−22.879	−9.993	−108.09	−8.995	−8.978	0	20.549
	CTSN1		76.365	19.841	169.08	23.985	206.81	21.903	21.310	33.895	0

:

$$P_2=\alpha P_1+\beta$$

(9)

where P₁ and P₂ are mechanical performance indices.

3.6. Classification of Bamboo Considering the Effect of Bamboo Nodes

The standard deviation can reflect the degree of discreteness among the data. In order to classify the data according to the standard deviation, it is necessary to ensure that the distribution of the data is normal. A Q–Q diagram is mainly used to judge whether a series of values conform to the normal distribution. The abscissa is the theoretical value, and the ordinate is the actual value. If most of the points are on the y = x line, then the data conform to a normal distribution. Figure 11 shows the Q–Q diagram of UTS_N and UTS_I. According to the Q–Q diagram, the distribution of UTS_N and UTS_I conforms to a normal distribution. The UTSN and UTS_I were graded by 1/4 standard deviation in order to obtain the results shown in

Table 7. Bamboo tensile strength grading table along the grain.

		IV	III	II	I
UTSN	Value (MPa)	UTSN ≤ 131	131 < UTSN ≤140	140 < UTSN ≤148	UTSN < 148
	h/DB	h/DB ≤ 16	16 < h/DB ≤36	36 < h/DB ≤54	h/DB < 54
UTSI	Value (MPa)	UTSI ≤ 137	137 < UTSI ≤149	149 < UTSI ≤159	UTSI < 159
	h/DB	h/DB ≤ 7	7 < h/DB ≤25	25 < h/DB ≤40	h/DB < 40

. According to Table 7, the standardized parameter h/DB can be used for hierarchical evaluation of bamboo.

4. Conclusions

(1)

The bamboo nodes have a significant effect on the moisture content, and their influences on the bearing capacity are as follows: the bearing capacity of the UC, CC and CT node specimens are higher than that of the internode specimen; the bearing capacity of the UT and B internode specimens are higher than that of the node specimens; and the bearing capacity of the US node and internode specimens are similar.

(2)

Bamboo joints have different effects on the mechanical properties. The UCS, UCE, UTE, CCS and CTS of the node specimens are higher than those of the internode specimens. The MOR, MOE and UTS of the internode specimens are higher than those of the node specimen. The USS of the node specimen and the internode specimen are similar. The relative difference in mechanical properties between nodes in the transverse striation direction is obvious.

(3)

The linear fitting relationship was obtained by fitting the mechanical properties of bamboo with the standardized parameter h/D_B. The fitting results show that UCS_N, UCS_I, UCE_N, UCE_I, MOR_N, MOR_I, MOE_N, MOE_I, UTS_N, UTS_I, USS_N, USS_I and CCS_I increase with increasing standardized parameter h/D_B. CCS_N, CTS_N and CTS_I decrease with increasing h/D_B. The standardized parameter h/D_B can be used to predict the mechanical properties of bamboo by fitting the relationship. Based on the relationship between the mechanical properties of bamboo and h/D_B, the relationship between the performance values of the joints and the specimens with the same mechanical properties was deduced.

(4)

The relationship between the mechanical properties was established by fitting 99 quartiles of each mechanical property value.

(5)

According to the standard deviation, UTS_N and UTS_I were divided into four grades, which could be predicted by the standardized parameter h/D_B.