tensile properties of bamboo units in different sizes

Proceedings of the 55th International Convention of Society of Wood Science and Technology August 27-31, 2012 - Beijing, CHINA

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Tensile Properties Of Bamboo Units In Different Sizes Wang Ge1, Chen Hong1, Yu Zi-xuan1, Sheldon Q. Shi2,

Cheng Hai-tao1, Qiu Ya-xin1

1International Center for Bamboo and Rattan, Beijing, 100102, China

2Mechanical and Energy Engineering, University of North Texas, Denton, TX

76203-1277, USA Abstract

The objective of this study was to effect of specimen size on the mechanical properties. Both single bamboo fibers and bamboo fiber bundles isolated chemically and mechanically were tested. The tensile strength of chemically macerated single bamboo fiber was 47.58% higher than that of fibers isolated mechanically. Meanwhile, tensile strength and tensile modulus of chemically macerated fiber bundles were over 2.10 times and almost 1.43 times higher than that of the mechanically isolated ones. As both chemically macerated, fiber bundles’ tensile strength, decreased by 65.73%, tensile modulus by 12.25%, and elongation by 9.69%, comparing with single fibers. For mechanical isolation, tensile strength, tensile modulus and elongation of fiber bundles were 68.81%, 52.34% and 60.93% lower than that of single fibers, respectively. Compared with single bamboo fibers and bamboo fiber bundles, the tensile strength of bamboo strips reduced by 81.72% and 41.38%, whereas tensile modulus decreased by 57.25% and 10.30%, respectively. And mechanical properties of bamboo strips were lower than that of samples made from outer portion of the bamboo but higher than that of samples made from inner portion of the bamboo. In the tensile properties were different among single bamboo fibers, bamboo fiber bundles and bamboo strips. The smaller the specimen size, the higher the tensile properties. It was shown that the tensile strength, tensile modulus and elongation reduced increasingly, and those decreasing levels were more significant when fibers were isolated mechanically.

Keywords: single bamboo fibers, bamboo fiber bundles, bamboo strips, mechanical property, changing regularity


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Introduction Bamboo resource is abundant in the world, especially in China. With the growing market for bamboo in China, bamboo units in different sizes, such as single bamboo fibers, bamboo fiber bundles and bamboo strips, are becoming the primary feedstock in the paper making, textile, and fiber-based composite. The mechanical performance of products in such fields is highly dependent on the properties of bamboo units. To improve utilization and manufacture of bamboo materials, there is an increasing need for more detailed knowledge regarding mechanical properties and relations between bamboo units in different sizes. Jayne (1959) tested the mechanical properties of earlywood and latewood fibers from 10 gymnosperm species. It was found that fibers were generally Hookean in nature, displaying a proportional stress-strain relationship. Kersavage (1973) obtained the average tensile stress and modulus of Douglas fir fibers as 0.85 GPa - 0.91 GPa and 23.60 GPa - 24.50 GPa, respectively. Mott et al. (2002) investigated the variations in mechanical properties of individual southern pine fibers and compared engineering properties of earlywood and latewood tracheids with respect to the tree height and juvenility. Groom et al.(2002) reported the definition of juvenile, transition and mature zone as classified by fiber stiffness, strength, microfibril angle, and cross-section area. Subsequently, Burgert et al. (2005) compared the tensile properties of mechanically and chemically isolated spruce fibers by micro-tensile investigation. Recently, Yu et al. (2011) presented an improved technique for single plant fiber characterization, which is easy to conduct, and the results are more accurate. Zhai et al. (2012) investigated the mechanical properties and structures of windmill palm fiber bundles, and found that the fiber bundles from the inner layer of windmill palm showed higher tensile strength (0.11 GPa) and Young's modulus (1.25 GPa) than that from the other layers. Recently, many studies have been conducted on the mechanical properties of bamboo strips. Wang (2003) explored that the fracture mechanism of bamboo / Chinese fir composite in bending and shearing tests by observing the images obtained by SEM. For longitudinal 3-point the bending and shearing strength test, the failure form if the composite was crack at tangential direction in Chinese fir or bamboo themselves under the air-dry condition, and the glue line was not breakage. But after water soaked, the failure form of the composite was peeled out at the glue interface between Chinese fir or bamboo and adhesive. Li (2009) studied the tensile properties of bamboo using the digital speckle correlation method (DSCM) and discovered that the mechanical properties of bamboo were lower than that of bamboo fibers because of the slips occurred between fibers when factures happened. In this paper, the effect of bamboo element sizes (single bamboo fibers, bamboo fiber bundles and bamboo strips) and the isolation method (chemically and mechanically) are investigated. In two previous papers (Wang, 2011; Chen, 2011), four single plant fibers, including single


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bamboo fibers, single Chinese fir fibers, single kenaf fibers and single ramie fibers, were tested. The single bamboo fibers shown higher tensile stress, 1.78 GPa, other three tensile stress were 1.26 GPa, 1.02 GPa, 1.00 GPa, respectively. Therefore Cizhu bamboo were chosen as material in our research.

Methods Samples Preparation Materials were from the 1-yr-old Cizhu bamboo (Neosinocalamus affinis) grown in Chengdu, Sichuan province, China, with an initial moisture content of 8-12%. Single bamboo fibers were isolated both chemically and mechanically. The bamboo samples were cut into strips (20 mm longitudinally and 2×2 mm in cross-section), and immersed in the chemical solution at a temperature of 60℃ for 42 h to separate. The other bamboo fibers

were isolated mechanically using tweezers from 90-um-thick tangential slices after being softened with about 40 h of hot water treatment. Bamboo fiber bundles: Bamboo fiber bundles were also isolated chemically and mechanically. Bamboo strips (15x 4×2 cm (length x width x thickness) were immersed in alkaline solution for being soften, then were isolated using a comb. The mechanically isolated bamboo fiber bundles were obtained using tweezers from the strips being fibrillated 6 times in the instrument designed at International Center for Bamboo and Rattan. In our previous studies, it was found the tensile strength of bamboo strips fibrillated 4-8 times were stable keeping between 0.09 GPa and 0.10 GPa. Although the tensile strength was 47.6% lower than that of bamboo strips without fibrillation treatment, the bamboo strips fibrillated 4-8 times was well distributed and suited to be used as primary units in the process of bamboo based composites. The strips being fibrillated 6 times were chosen for this study. The bamboo strips were selected between 4-6 m from the bottom of the trunk. Tensile samples, including bamboo strip, outside of bamboo strip and inside of bamboo strip, were cut into a dog born shape in accordance with requirements described in GB/T 15780-1995 as shown in Figures 1 and 2 using a laser cutting machine (CMA-6040, Guangdong, China).

Figure 1. Bamboo strip sample Figure 2. Bamboo samplemaking

Inside Bamboo strip

Outside


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Tensile properties testing Testing of single bamboo fibers: Tensile strength testing of the single bamboo fibers was conducted following the testing procedure described by Cao (2010). In brief, the fibers were first glued on an organic, channeled glass with one droplet of glue in each end, and then placed in an oven at 60℃ for 24 h followed by 22℃ for at least 24 h. The tensile testing of the

single fibers was conducted with newly designed instrument (SF-Microtester I) located at the International Center of Bamboo and Ratten, Beijing, China. A constant strain rate of 48 um/min was set at 25 ℃ and 20% RH. Fibers were removed from the tensile apparatus

immediately upon failure and saved for subsequent cross-sectional area measurement with a confocal laser scanning microscope (CLSM; Zeiss, LSM 510 Meta, Germany) for tensile modulus and strength calculations, Figure 4 (a). Ten samples were tested for chemically and mechanically isolated single bamboo fibers. Testing of bamboo fiber bundles: Bamboo fiber bundles were glued onto an organic, channeled glass designed to carry the fiber bundles and to be easily fixed with one droplet of glue in each end (Fig. 3). The free length of the fiber bundles were about 10 mm. Before the testing, the samples should be kept at the room temperature for at least 3 h for glue droplet solidifying. The tensile testing of single fibers was conducted at a high-resolution commercial mechanical tester (Microtester 5848, Instron, USA) with a constant strain rate of 48 um/min in the room environment (25 ℃ and 20% RH). To calcul ate tensile stress and modulus of

bamboo fiber bundles, the cell wall cross section-sections were determined. The tested fibers were stained with a dilute concentration of acridine orange, attached to a glass slide and covered with a number 1 cover slip. The images of cell wall cross section were obtained by CLSM and the areas were calculated by the software in the CLSM (Fig. 4 (b). Six samples were tested.

Figure 3. Typical images pf bamboo bundle samples


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(a) cross section of single bamboo fiber (b) cross section of bamboo fiber bundle Figure 4. Typical CLSM images of area of cross section

Testing of bamboo strip: The testing of bamboo strip was performed by a high-resolution commercial mechanical tester (Microtester 5848, Instron, USA) with the strain rate at 2 mm/min at a ambient environment of 25℃ and 55% RH. Ten samples were tested for each types.

Results and Discussion

Tensile Properties of Single Bamboo Fibers Table 1 Tensile properties of single bamboo fibers isolated by different methods (TS - Tensile Strength, TM - Tensile Modulus, E - Elongation) Single Bamboo Fibers T S/GPa(CV) TM/GPa(CV) E/%(CV) Chemically Isolated 1.77(0.15) 26.85(0.06) 2.89(0.16) Mechanically Isolated 0.93 (0.19) 34.62(0.17) 4.30(0.17) Tensile properties of single bamboo fibers isolated chemically and mechanically are shown in Table 1. Isolation methods affected the mechanical properties of single bamboo fibers significantly. Tensile strength of chemically isolated fibers was higher than that of mechanically isolated fibers, whereas the modulus and elongation were lower. An analysis of the chemical component of the fibers was conducted by FT-Raman spectra to explain why the tensile strength of fibers isolated chemically was higher while the modulus and elongation was lower comparing with the fibers isolated mechanically. In Fig. 5, the most intense band in the fingerprint region at 1600 cm-1 for mechanically isolated fibers, attributing to aryl ring stretching and aryl ring symmetric vibration, meant the existence of lignin. Big differences were observed in the fingerprint regions, with the absence of the peak at 1600 cm-1 for chemically isolated fibers. Therefore, we can conclude that lignin degradation happened in the chemically isolated fibers. Zhang (2011) found that the tensile strength of single Chinese fir fibers increased, while the modulus and elongation decreased when the lignin content was reduced.


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BFMBFC

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900 1000 1100 1200 1300 1400 1500 1600 Raman shift (cm-1)

Figure 5. FT-Raman spectra of single bamboo fibers isolated chemically and mechanically in the fingerprint area. (BFC- bamboo fiber isolated chemically, BFM- bamboo fiber isolated mechanically) Table 2 Assignment of bands in the FT-Raman spectrum of cell wall polymers(Agarwal, 1997)

Wavenumber(cm-1) Band assignment Lignin 1600 Aryl ring stretching,symmetric

Cellulose 895 HCC and HCO bending at C6 1377 HCC,HCO and HOC bending 1456 HCH and HOC bending

Carbohydrates 1095 CC and CO stretching The cell wall structure model established by Boyd (1982)allowed for an essentially lamellar distribution of cellulose, but a non-lamellar distribution of lignin. Neighboring cellulose fibrils linked to each other and formed the disc-shaped openings filled up with matrix made up of hemicellulose and lignin. With the degradation of cellulose in the cell wall, cellulose fibrils aggregated more and connected each other more in longitudinal direction, which increased fracture resistant. Meanwhile, Chen (2011) also found that the cross section area of mechanically isolated fibers was smaller comparing with those isolated chemically. On the other hand, no fracture is observed (Fig. 6(a) on the surface of fiber isolated chemically, while predominant plane fractures were (Fig. 6 (b) produced when the fiber was peeled with fine tweezers, which may be another reason for the lower tensile strength of mechanically isolated fibers.


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(a) Fibers isolated chemically (b) Fibers isolated mechanically Figure 6. ESEM images of single bamboo fibers isolated differently（Chen, 2011） Tensile Properties of Bamboo Fiber Bundles It is seen from Table 3 that tensile strength, modulus and elongation of chemically isolated bamboo fiber bundles were all higher than that of mechanically isolated bamboo fiber bundles. Table 3 Tensile properties of bamboo bundles made by different methods Bamboo Fiber Bundles TS/Gpa（CV） TM/GPa（CV） E/%（CV） Chemically Isolated 0.61（0.37） 23.56（0.35） 2.61（0.32） Mechanically Isolated 0.29（0.71） 16.50（0.42） 1.68（0.31） For the chemical isolation process, the first alkali treatment lead to a large removal of lignin (Xu, 2006). This process, removed part of lignin-rich middle lamella, which reduced the damage caused by the comb. In contrast, bamboo fiber bundles isolated mechanically (fiber bundles were peeled with tweezers from fibrillated bamboo strips) was damaged more directly in middle lamella and single bamboo fibers in the fiber bundles. In addition, the failures happened near the glue droplet of the mechanically isolated bamboo fiber bundles were much more comparable to the fiber bundles isolated mechanically. That also revealed that the mechanical isolation caused more damage because of the stress concentration. Tensile Properties of Bamboo Strip As shown in Table 4, the tensile strength and modulus along the grain direction of samples made from outer portion of bamboo were nearly double compared to those made from the inner portion of bamboo, while the tensile strength and modulus of bamboo strips were higher than that of samples made from outer portion of the bamboo but lower than that of samples made from inner portion of the bamboo.


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Table 4 Tensile properties of bamboo strips (BS — bamboo strips, IBS —inside of bamboo strips , OBS — outside of bamboo strips) Samples TS/GPa(CV) TM/GPa(CV) E/%(CV) BS 0.17 (0.34) 14.80 (0.12) -- OBS 0.20 (0.21) 18.40 (0.17) -- IBS 0.11 (0.27) 10.60 (0.07) -- The structure of bamboo is mainly responsible for such results. Bamboo can be considered as a natural composite. The cells in bamboo can be classified into two types: 1)the cell in basic tissues for transmitting load which has high tensile strength, low modulus and low density; 2) vascular bundles structurally consisting of fibers, lignified vessels and etc. The fibers in vascular bundles, with high tensile strength, modulus and density, are the main component responsible to the mechanical properties of bamboo (Zhao, 2002). Vascular bundles were surrounded in the basic tissues and decreased from inner to outer portions of the bamboo, which meant the fibers in outer portion of bamboo was more comparable to those from the inner portion of bamboo as shown in Figure 7 (Jiang, 2002). The amount of vascular bundles determined the mechanical properties of bamboo, more vascular bundles higher tensile strength and modulus (Amada, 1997).

Figure 7. Distribution of vascular bundles in the cross-section of bamboo strip During the test, shear failure happened first at one end of most samples, followed by the tensile failure. Most of the specimens that in shear failures were those made from the outer portion of the bamboo. Most of the specimens that failed in tension were those made of the inner portion of bamboo. Generally speaking, that strength of the specimen that in shear failure is higher than that failed in tension.


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Relationship of Tensile Properties Between Bamboo Units in Different Sizes

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Figure 8. Regularity of tensile properties of bamboo units in different sizes (SFC — single fibers isolated chemically, SFM — single fibers isolated mechanically, BBC — bamboo bundles isolated chemically, BBM — bamboo bundles isolated mechanically, BS — bamboo strips, IBS —inside of bamboo strips, OBS — outside of bamboo strips) The average tensile strength, modulus and elongation of single bamboo fibers isolated chemically were 1.78 GPa, 23.56 GPa, and 2.89%, respectively. For the fibers isolated chemically, the tensile strength of the bamboo fiber bundle , decreased by 65.73% compared to the bamboo single fibers, the modulus by 12.25% , and the elongation by 9.69%. For the mechanically isolated single bamboo fibers, an average tensile strength of 0.93 GPa, an average modulus of 34.62 GPa and an average elongation of 4.3% were obtained as shown in Figure 8. For both chemically macerated bamboo fiber bundles and single bamboo fibers, tensile strength, modulus and elongation of bamboo fiber bundles were obtained as 68.81%, 52.34% and 60.93%, respectively, which were lower than that of single bamboo fibers. It was attributed to most failures happened in the tissues between the single fibers. This phenomenon was also illustrated by the shapes of failures shown in Figure 9, Shao (2009) studied the behaviors of Mode I (crack opening mode) interlaminar fracture parallel to grain of moso bamboo and observed the crack propagation developed along the longitudinal interface between fibers or ground tissue indicating that the longitudinal interface strength was weak among bamboo cells.

Figure 9. Typical ESEM images of fracture modes of bamboo fiber bundles

As shown in Figure 10, some bamboo fiber bundles contain parenchyma with lower tensile


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strength in comparison with the single bamboo fibers (Shao, 2009). As the existence of weak interface among the parenchymas, the crack propagation easily happened resulting in ultimate failure (Tian, 2012).

Figure 10. Typical ESEM images of parenchyma cell in the bamboo bundles

The isolation method also affected the mechanical properties of the fiber. For both single bamboo fibers and bamboo fiber bundles, the chemical treatment caused less reduction of the properties compared to the mechanical one. As shown in Figure 8, the tensile strength of bamboo strips reduced by 81.72% compared to the single fiber, and 41.38% compared to the fiber bundle, whereas the modulus decreased by 57.25% copared to the single fiber, and 10.30% compared to the fiber bundle. And percent loss of tensile strength was more than that of modulus. Bamboo is made up of vascular bundles with basic tissue in between, where the interface is weak (Shao, 2007). Crack propagation first happened in basic tissue area when the stress was just enough to cause a crack. With adding more stress, slips would happen on the interface between bamboo fiber bundles and basic tissue, then cracks would appear on the other weak interface. In the end, the bamboo fiber bundles were pulled out slowly from basic tissue (Tian, 2012). Compared with single bamboo fibers and bamboo fiber bundles, there are many multi-scale weak interfaces in bamboo that lead to a huge decrease of mechanical properties.

Conclusions

The mechanical properties of bamboo units in different sizes (single bamboo fiber, bamboo fiber bundles and bamboo strips) were characterized. The fibers obtained from two isolation methods were compared. For the single bamboo fibers, the tensile strength of chemically isolated samples is higher than that of mechanically isolated ones, whereas the modulus and elongation are lower. For the bamboo fiber bundles, the tensile strength, modulus and elongation of chemically treated samples are higher comparing with the mechanically isolated


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ones. Relationships of the mechanical properties of bamboo units as a function of bamboo sizes were established. For the bamboo strips, the tensile strength and modulus were lower than that of samples made from the outer portion of than bamboo but higher than inner portion of the bamboo. For the chemical isolated fiber, the tensile strength of the bamboo fiber bundle was decreased by 65.73% compared to that of the single fiber, modulus by 12.25%, and elongation by 9.69%. For both isolated mechanically single bamboo fibers and bamboo bundles, the tensile strength, modulus and elongation of bamboo fiber bundles were 68.81%, 52.34% and 60.93% lower than that of single bamboo fibers, respectively. In addition, the mechanical properties of bamboo fiber bundles with chemical treatment decrease more comparing to fiber bundles isolated mechanically. Compared with the single bamboo fibers and bamboo fiber bundles, the tensile strength of bamboo strips reduced by 81.72% and 41.38%, respectively, whereas the modulus was reduced by 57.25% and 10.30%, respectively.

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Acknowledgment

This work is funded by the key technology of making large span bamboo engineering structural component (201204701). Also, Dr. Lin kindly assisted with preparing the manuscript. The constructive comments from the anonymous reviewers are greatly appreciated.

tensile properties of bamboo units in different sizes

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