In this work, we used tensile tests to analyze the tangential failure forms of raw bamboo and determine a relationship between tangential tensile strength, elastic modulus, position, density, and moisture content. We found that the tangential mechanical properties of the culm wall were mainly dependent on the mechanical properties of the basic structure of the thin wall. Formulas for calculating the tangential tensile strength of moso bamboo and adjusting the moisture content were also determined. The tangential tensile strength and the tangential tensile modulus of elasticity (TTMOE) followed: outer > middle > inner, and diaphragm > bamboo node > culm wall. Below the fiber saturation point, the tangential tensile strength and TTMOE values of the bamboo gradually decreased with increasing moisture content. When the moisture content was 15%, the tangential tensile strengths of the inner, middle, outer, culm wall, bamboo node, and diaphragm samples of the five-year-old moso bamboo were 3.17, 3.29, 3.31, 3.24, 3.67, and 8.85 MPa, respectively. Furthermore, their TTMOE values were 215.09, 227.98, 238.45, 224.04, 267.21, and 559.27 MPa, respectively. Hence, this study provides a theoretical basis for future research on bamboo cracking.
Large-diameter moso bamboo (
Bamboo loses water due to the equilibrium moisture content of the external environment and consequently, shrinks and deforms; therefore, cracks will easily occur in raw bamboo when the tensile stresses on its inner and outer surfaces exceed the tangential tensile strength. Hence, the tangential tensile strength and the TTMOE play vital roles in moso bamboo cracking.
Extensive research has been conducted to analyze the longitudinal tensile strength, compressive strength, flexural strength, stiffness, and elastic modulus of bamboo along the fiber direction. However, the radial and tangential mechanical properties of bamboo have rarely been explored [
Bamboo exhibits better mechanical properties than ordinary wood and has excellent compression and bending resistance [
Mo et al. [
The GB/T15780-1995 (Testing methods for the physical and mechanical properties of bamboo) and JG/T199-2007 (Testing methods for the physical and mechanical properties of bamboo used in buildings) standards do not have regulations for assessing the tangential tensile strength of raw bamboo. According to these two standards, the length of a tensile strength test piece must be at least 280 mm along the grain direction, and for the GB/T14017-2009 standard (Method of testing in tensile strength perpendicular to grains of wood), the length of the horizontal grain must be 150 mm. However, moso bamboo does not typically have a sufficient tangential length of 150 mm; thus, the tangential tensile strength of moso bamboo is rarely studied.
Zeng et al. [
In this work, we studied the tangential failure forms of raw bamboo and established a relationship between its tangential tensile strength, elastic modulus, location, density, and moisture content through experimentation.
Moso bamboo was collected from bamboo forests in Liyang City and Changzhou City in Jiangsu Province (east longitude = 119.43° and north latitude = 31.18°). The distance between the bamboo joints at 1.3 m height was 228.57 ± 21.81 mm, with an average outer diameter of 89.32 ± 9.19 mm, and the average wall thickness was 8.95 ± 0.74 mm.
According to Guan et al. [
According to the GB/T15780-1995 standard, 100 sets of specimens were obtained for each group, where the average moisture content was 17.35 ± 5.17%, the volume shrinkage coefficient was 0.84 ± 0.46%, the air-dry density at 12% moisture content was 0.81 ± 0.07 g/cm3, and the total dry density was 0.76 ± 0.08 g/cm3.
A schematic depicting the intersected test specimen is shown in
The inner, middle, and outer specimens were collected from the inner, middle, and outer 1/3 sections of the culm wall in the thickness direction, respectively. To reduce the additional bending moment caused by the tangential curved surface of the moso bamboo, we used the inner, middle, and diaphragm specimens without a curved surface. The culm wall, bamboo node, and outer specimens also had a curved surface in the tangential direction. Because the additional bending moment decreases the actual detected values, the effect of the additional bending moment was ignored. The cross-sections and dimensions of the inner, middle, outer, culm wall, bamboo node, and diaphragm specimens are shown in
The moso bamboo was first cut into 25-mm-long bamboo rings using a sliding table saw. The bamboo rings were then cut into 25 mm × 31 mm bamboo blocks using a micro band saw machine, and the obtained bamboo blocks were finally cut into the inner, the middle, and the outer specimens with a small sliding table saw. The specimens were then polished to corresponding thicknesses with a sanding machine and milled to the sizes shown in
The two opposite sides of the effective cross-sections of each test piece were flat and parallel to each other. The test pieces also had no defects and were numbered randomly after processing. The allowable deviations for specimen length, width, and effective width were ± 1.0 mm, ± 0.5 mm, and ± 0.2 mm, respectively.
After processing, the bamboo test pieces were placed in a cool and ventilated place without stacking so that they could freely release water.
The test pieces were then placed in an HWS-250 constant temperature and humidity box before the experiment, and their moisture content values were adjusted to 5%, 10%, 15%, and 20% to obtain the four different sample sets.
In this experiment, we used the following equipment: a D-54518 Niersbach sliding table saw, an MBS240/E mini band saw machine, a No. 27070 mini sliding table saw, an MF70 mini bench drilling and milling machine (PROXXON Co., Ltd., Germany), a LUXTER-MM491G sand tray machine (Jinhua Maituo Power Tools Co., Ltd., China), an HWS-250 constant temperature and humidity box (Shanghai Jinghong Experimental Equipment Co., Ltd., China), a BS423S electronic balance (Shanghai Meiyingpu Instrument Manufacturing Co., Ltd., China), a WDW-200 electronic universal testing machine (Changchun Xinte Testing Machine Co., Ltd., China), and 150 T vernier calipers (Shanghai Menet Industrial Co., Ltd., China).
The bamboo fibers were naturally dried and finely ground after steam explosion, and their sieving value was determined with a Bauer-McNett fiber sieving instrument according to the TAPPI T233 method.
The bamboo specimens were tested at 20 ± 2°C with 65 ± 5% relative humidity. Each specimen was clamped with a clamping device and placed vertically in the tensile testing machine, and the stress (
The tangential tensile strengths (ftw) of the specimens were calculated by
Linear numerical fitting was performed on automatically recorded stress and strain values to fit a straight line σ = Eε + a (a is an arbitrary constant and E is the TTMOE), on the basis of a significance level of
To analyze the rules of the test data, we obtained the standard deviations for each data group and checked whether the number of test pieces in each group met the requirements. In this study, we calculated the relevant statistics in accordance with JG/T199-2007.
The failure modes of each specimen are shown in
As shown in
As shown in
According to Chen et al. [
We found that the tangential fracture of the culm wall mainly occurred due to matrix failure, which was accompanied by a small amount of interfacial dissociation (
The formulas for calculating stresses at any point in a cross-section under the actions of a crack as follows:
where σ
As shown in
The tangential tensile strengths of the inner, the middle, outer, culm wall, bamboo node, and diaphragm specimens were measured with moisture content values of 5%, 10%, 15%, and 20%. We used eight test pieces for each group, according to article 6.5.3 of the JG/T199-2007 standard.
The tangential tensile strengths and related statistical data of the moso bamboo with moisture content values of 5%, 10%, 15%, and 20% are presented in
Location | Average value (MPa) | Standard deviation (MPa) | Mean standard deviation (MPa) | Accuracy index (%) | Minimum number of test pieces (piece) |
---|---|---|---|---|---|
Inner | 4.42 | 0.99 | 0.35 | 15.84 | 6 |
Middle | 4.57 | 0.93 | 0.33 | 14.39 | 7 |
Outer | 4.63 | 0.95 | 0.34 | 14.51 | 7 |
Culm wall | 4.51 | 0.87 | 0.31 | 13.64 | 8 |
Bamboo node | 4.88 | 1.22 | 0.43 | 17.68 | 5 |
The diaphragm | 10.63 | 2.14 | 0.76 | 14.24 | 8 |
Location | Average value (MPa) | Standard deviation (MPa) | Mean standard deviation (MPa) | Accuracy index (%) | Minimum number of test pieces (piece) |
---|---|---|---|---|---|
Inner | 3.82 | 0.85 | 0.30 | 15.73 | 6 |
Middle | 3.91 | 0.78 | 0.28 | 14.11 | 8 |
Outer | 3.95 | 0.84 | 0.30 | 15.04 | 7 |
Culm wall | 3.86 | 0.80 | 0.28 | 14.66 | 7 |
Bamboo node | 4.32 | 0.95 | 0.34 | 15.55 | 6 |
The diaphragm | 9.04 | 1.80 | 0.64 | 14.11 | 8 |
Location | Average value (MPa) | Standard deviation (MPa) | Mean standard deviation (MPa) | Accuracy index (%) | Minimum number of test pieces (piece) |
---|---|---|---|---|---|
Inner | 3.17 | 0.61 | 0.22 | 13.61 | 8 |
Middle | 3.29 | 0.68 | 0.24 | 14.61 | 7 |
Outer | 3.31 | 0.82 | 0.29 | 17.52 | 5 |
Culm wall | 3.24 | 0.69 | 0.24 | 15.06 | 7 |
Bamboo node | 3.67 | 0.71 | 0.25 | 13.68 | 8 |
The diaphragm | 8.85 | 1.79 | 0.63 | 14.34 | 7 |
Location | Average value (MPa) | Standard deviation (MPa) | Mean standard deviation (MPa) | Accuracy index (%) | Minimum number of test pieces (piece) |
---|---|---|---|---|---|
Inner | 2.62 | 0.62 | 0.22 | 16.73 | 5 |
Middle | 2.68 | 0.59 | 0.21 | 15.57 | 6 |
Outer | 2.73 | 0.71 | 0.25 | 18.39 | 5 |
Culm wall | 2.65 | 0.56 | 0.20 | 14.97 | 7 |
Bamboo node | 2.70 | 0.61 | 0.22 | 16.00 | 6 |
The diaphragm | 6.76 | 1.37 | 0.48 | 14.29 | 8 |
Of note, the outer section of bamboo had the highest tangential tensile strength followed by the middle and the inner sections; however, the difference was not significant. The tangential tensile strength of the culm wall was similar to the middle section. This occurred because more thin-walled basic tissues and fewer vascular bundles were present in the inner bamboo section (tangential tensile strength damage mainly occurs in thin-walled basic tissues).
Moreover, the diaphragm had the highest tensile strength followed by the bamboo node and the internode culm wall. The vascular bundles in the bamboo nodes had different degrees of bending, bifurcation or merging, and inward winding, and the horizontal vascular bundles in the diaphragm were arranged irregularly in a network. The tangential failure of the diaphragm and bamboo node led to fiber damage, and the diaphragm broke relatively more fibers.
With 15% moisture content, the tangential tensile strength values of the inner, middle outer, culm wall, bamboo node, and diaphragm specimens of the five-year-old moso bamboo were 3.17, 3.29, 3.31, 3.24, 3.67, and 8.85 MPa, respectively.
We found that below the fiber saturation point, the tangential tensile strength of each moso bamboo section gradually decreased as the moisture content increased. This mainly occurred because bamboo is composed of lignin, cellulose, hemicellulose, and lignin, which acts as the glue. Lignin is affected by humidity, and the bonding performance will decrease as wettability increases. In addition, the flexibility of hemicellulose will increase after water exposure.
The TTMOE values of the inner, middle, outer, culm wall, bamboo node, and diaphragm specimens were measured with moisture content values of 5%, 10%, 15%, and 20%. Eight test pieces were used for each group, according to Article 6.5.3 of the JG/T199-2007 standard.
The TTMOE values and related statistical data of the moso bamboo with moisture content values of 5%, 10%, 15%, and 20% are presented in
Location | Average value (MPa) | Standard deviation (MPa) | Mean standard deviation (MPa) | Accuracy index (%) | Minimum number of test pieces (piece) |
---|---|---|---|---|---|
Inner | 301.52 | 60.33 | 21.33 | 14.15 | 8 |
Middle | 312.83 | 67.08 | 23.72 | 15.16 | 7 |
Outer | 318.47 | 72.24 | 25.54 | 16.04 | 6 |
Culm wall | 306.45 | 60.17 | 21.27 | 13.88 | 8 |
Bamboo node | 324.47 | 78.45 | 27.74 | 17.10 | 5 |
The diaphragm | 636.32 | 124.22 | 43.92 | 13.80 | 8 |
Location | Average value (MPa) | Standard deviation (MPa) | Mean standard deviation (MPa) | Accuracy index (%) | Minimum number of test pieces (Piece) |
---|---|---|---|---|---|
Inner | 265.17 | 57.51 | 20.33 | 15.34 | 7 |
Middle | 277.30 | 62.07 | 21.95 | 15.83 | 6 |
Outer | 281.21 | 63.14 | 22.32 | 15.88 | 6 |
Culm wall | 270.34 | 55.62 | 19.66 | 14.55 | 7 |
Bamboo node | 292.46 | 58.83 | 20.80 | 14.22 | 8 |
The diaphragm | 584.78 | 119.08 | 42.10 | 14.40 | 7 |
Location | Average value (MPa) | Standard deviation (MPa) | Mean standard deviation (MPa) | Accuracy index (%) | Minimum number of test pieces (Piece) |
---|---|---|---|---|---|
Inner | 215.09 | 41.36 | 14.62 | 13.60 | 8 |
Middle | 227.98 | 46.47 | 16.43 | 14.41 | 7 |
Outer | 238.45 | 50.90 | 18.00 | 15.09 | 7 |
Culm wall | 224.04 | 43.03 | 15.21 | 13.58 | 8 |
Bamboo node | 267.21 | 58.28 | 20.61 | 15.42 | 6 |
The diaphragm | 559.27 | 107.52 | 38.01 | 13.59 | 8 |
Location | Average value (MPa) | Standard deviation (MPa) | Mean standard deviation (MPa) | Accuracy index (%) | Minimum number of test pieces (piece) |
---|---|---|---|---|---|
Inner | 190.26 | 44.09 | 15.59 | 16.39 | 6 |
Middle | 204.87 | 39.15 | 13.84 | 13.51 | 8 |
Outer | 210.71 | 45.33 | 16.03 | 15.21 | 7 |
Culm wall | 196.42 | 38.45 | 13.59 | 13.84 | 8 |
Bamboo node | 225.75 | 53.22 | 18.82 | 16.67 | 6 |
The diaphragm | 524.44 | 99.75 | 35.27 | 13.45 | 8 |
We found that the outer bamboo specimens had the largest TTMOE values, followed by the middle and inner specimens. Moreover, the diaphragm specimen had the highest TTMOE value followed by the bamboo node and the internode culm wall specimens. Below the fiber saturation point, the TTMOE values of each moso bamboo section gradually decreased as the moisture content increased.
With 15% moisture content, the TTMOE values of the inner, middle, outer, culm wall, bamboo node, and diaphragm five-year-old moso bamboo specimens were 215.09, 227.98, 238.45, 224.04, 267.21, and 559.27 MPa, respectively.
A moisture content value of 15% was selected as the research basis. The tangential tensile strength of the five-year-old bamboo wall was calculated, and the tangential tensile strength (σ) and air-dry density (ρ) of each specimen were determined. We also used 100 test specimens, according to the GB/T15780-1995 standard.
The tangential mechanical properties of the bamboo wall are mainly dependent on the mechanical properties of the basic structure of the thin wall. Shao et al. [
As shown in
As clearly shown in
When 0.8 ≤
When the moisture content values were 5%, 10%, 15%, and 20%, the tangential tensile strength values of the bamboo wall were 4.51, 3.86, 3.24, and 2.65 MPa, respectively (
The calculations fitted in
The four tangential tensile strength groups with moisture content values of 5%, 10%, 15%, and 20% were divided by the tangential tensile strength, resulting in a moisture content value of 12%. These four sets of data were linearly fitted to obtain a relationship curve between the tangential tensile strength ratio and the moisture content, as shown in
In this study, we conducted tangential tensile strength and TTMOE tests on five-year-old moso bamboo, and observed that the tangential mechanical properties of the culm walls were mainly dependent on the mechanical properties of the thin-walled basic structure.
Below the fiber saturation point, the tangential tensile strength and TTMOE values of all bamboo sections would gradually decrease with increasing moisture content. When the moisture content was 15%, the tangential tensile strength values of the inner, middle, outer, culm wall, bamboo node, and diaphragm five-year-old moso bamboo samples were 3.17, 3.29, 3.31, 3.24, 3.67, and 8.85 MPa, respectively, and their respective TTMOE values were 215.09, 227.98, 238.45, 224.04, 267.21, and 559.27 MPa.
Under the condition of 0.8 ≤ρ/ρ0 ≤ 1.2, the formula for calculating the tangential tensile strength of bamboo with a moisture content of ω could be expressed by
Thus, the moisture content adjustment formula
In this study, moso bamboo samples were collected from Liyang City and Changzhou City of Jiangsu Province. In future research, moso bamboo samples will be collected from different regions, and the influence coefficient of moso bamboo performance in different regions will be used to adjust the formulas obtained through this analysis.
All authors contributed equally to this work.