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Effects of vascular fiber content on abrasive wear of bamboo
Wear 259 (2005) 78–83
Short communication
Effects of vascular fiber content on abrasive wear of bamboo Jin Tong ∗ , Yunhai Ma, Donghui Chen, Jiyu Sun, Luquan Ren The Key Laboratory of Terrain-Machine Bionics Engineering (Ministry of Education, China) and College of Biological and Agricultural Engineering, Jilin University (Naling Campus), 5988 Renmin Street, Changchun 130025, PR China Received 25 August 2004; received in revised form 16 March 2005; accepted 23 March 2005
Abstract Natural biomaterials have unique structures and some distinguishing properties for adapting themselves to natural surroundings. Bamboo is a natural composite reinforced with longitudinal fibers (vascular fibers). The abrasive wear property of the cross section of bamboo (Phyllostachys pubescens) stem was examined. The abrasive material used for tests was the mixture of quartz sand (96.5 wt.%) and powdered bentonite (3.5 wt.%) and contained a water content of 3 wt.%, simulating soil condition The size of the quartz sand particles was 0.104–0.214 mm, 0.214–0.420 mm and 0.420–0.840 mm, respectively. The abrasive wear tests were run on a rotary disk type of abrasive wear testing machine. The relative sliding velocity was 1.68, 2.35 and 3.02 m/s, respectively. It was concluded that the abrasive wear resistance of the cross section of the bamboo stem was a function of the vascular fiber content of bamboo, the abrasive particle size and the relative sliding velocity. The wear resistance of bamboo was increased with its vascular fiber content. A higher sliding velocity or a larger abrasive particle size resulted in a higher abraded volume of bamboo. The effects of the tensile strength and the impact strength of bamboo on its abrasive wear were discussed. It was found by scanning electron microscopy that the geometric morphology of the abraded surfaces of the sections of bamboo stem displays a not-smooth structure. This not-smooth surface morphology provides a clue to develop anti-abrasion morphological surfaces of soil-engaging components. © 2005 Published by Elsevier B.V. Keywords: Bamboo; Fiber content; Composite; Abrasive wear; Mechanical property
1. Introduction Natural biomaterials have some unique properties and functions and have attracted many researchers’ interest, including their structures, mechanical properties, physical and chemical behavior and their biomimetics [1–13]. Bamboo is a natural fiber-reinforced composite. Though various cells can be observed in bamboo, these cells can, in mechanics, be classified into two types: matrix tissue cells and sclerenchyma cells. Matrix tissue cells are leptodermous and act as the matrix of general composites. Sclerenchyma cells mainly consist of vascular boundless enveloped in the matrix tissue. A vascular bundle is made of many phloem fibers, which may be considered as fibers in general composites. A phloem fiber is composed of several layers of pillar fibers. ∗
Corresponding author. Tel.: +86 431 5095730; fax: +86 431 5095575. E-mail address: [email protected] (J. Tong).
0043-1648/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.wear.2005.03.031
There are many micro-fibers in each layer of the pillar fibers. The micro-fibers are spirally arranged at a certain spiral angle, which is varied as different layers of pillar fibers. Also a vascular bundle is composed of many right-handed spiral phloem fibers at a certain spiral angle. Shen et al. [2] compared the surface properties of bamboo fibers with the linter fiber. It was demonstrated that bamboo has unique functional graded microstructure and excellent mechanical properties [3–6]. Lo et al. [7] determined the effects of the diameter and age of bamboo Bambusa pervariabilis and Phyllostachys pubescens on the compressive strength capacity of bamboo stick. The composites filled with bamboo fibers were prepared, such as bamboo fiber filled natural rubber composites [8,9], bamboo–glass fiber reinforced polypropylene hybrid composite [10] and bamboo pulp fiber reinforced cement [11]. The structural feature of bamboo gives a clue to design new composite materials. A biomimetic model of the reinforcing elements was developed based on the bamboo bast
J. Tong et al. / Wear 259 (2005) 78–83
fibers. Each reinforcing element is composed of two layers of helically wound fibers. A biomimetic model was proposed and the biomimetic composite was prepared [12,13]. Yakou and Sakamoto [14] examined the abrasive property of bamboo with carborundum paper as the counterface. They found that the abrasive wear rate of the outside surface layer was lower than that of the inner layer for bamboo specimens of normal- and parallel-oriented cellulose fibers with respect to the sliding surface. Tong el al. [15] examined the abrasive wear behavior of bamboo using quartz sand particles as abrasive material and found that the abrasive resistance of bamboo stem was affected by the relative orientation of the vascular fibers with respect to the sliding surface and by the size of abrasive particles. Specimens with the normal orientation of vascular fibers to the sliding surface gave a higher abrasive resistance than those with the parallel orientation. Tong et al. [16] also examined the dry sliding wear behavior of bamboo on a block-on-ring machine. It was demonstrated that the worn volume of bamboo was a function of the normal load, the sliding velocity and the relative orientation of bamboo fibers with respect to the sliding surface. The effects of the vascular fiber content of bamboo on its abrasive wear against free-abrasive material have been studied in this work.
2. Experimental details 2.1. Materials Three different parts of bamboo specimens with different fiber content were taken from an air-dry bamboo (Phyllostachys pubescens) stem, not including bamboo knots. The volume contents of the vascular fibers were measured using a stereoscope with a computer image manipulation system. The vascular fiber content can be given automatically by this system. The standard dimensions for the free-abrasive wear tests are 60 mm × 35 mm × 6 mm for steel materials. But, it was difficult to prepare the standard specimens of bamboo since the limitation of the size of the bamboo stem. So, the bamboo specimens of dimensions 20 mm × 8 mm × 3 mm were cut from the same bamboo stem and three bamboo blocks were mounted in each steel plates of dimensions 60 mm × 35 mm × 6 mm as shown in Fig. 1. The vascular
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Fig. 2. Schematic diagram showing vascular fiber orientation with respect to the sliding surface for free abrasive wear specimen of bamboo.
fiber orientation was normal to the sliding surface for all bamboo specimens during the abrasive wear tests as shown in Fig. 2. Three different sizes of quartz sand (0.104–0.214 mm, 0.214–0.420 mm and 0.420–0.840 mm, respectively) and bentonite (76 m) were used as abrasive, which contained quartz sand of 96.5 wt.% and bentonite of 3.5 wt.%. The water content of the abrasive material was 3 wt.%. 2.2. Abrasive tests The abrasive wear tests were run on a rotary disk type of abrasive wear testing machine as shown in Fig. 3 100 kg quartz sand was filled in the disk. The rotary disk rotated to drive the abrasive against the abrading specimen during the tests. A set of four specimens were installed on the specimen holder at 90-degree interval and their positions were changed successively to abrade them in turn every 803.4 m of sliding distance. The abrading specimen was embedded to a depth of 40 mm in abrasive. The impact angle of the tangential direction of the abrasive against the sliding surface of the testing specimen was 35◦ as shown in Fig. 4. The total sliding distance for each specimen was 25708.8 m. New abrasive material was used for each set of four specimens. The disk velocity at the position of the testing sample was considered as relative sliding velocities of abrasive material against the specimen, which were 1.68, 2.35 and 3.02 m/s, respectively. The temperature of the surrounding air was 21 ◦ C. The three compacting wheels were kept their unchanged height in order to maintain the same density of the abrasive during all tests. The abraded height of the bamboo specimens was determined through measuring the change of sections with a stereoscope and then the abraded volume was calculated. The morphologies of the abraded surfaces were examined by scanning electron microscopy. 2.3. Tensile and impact property tests
Fig. 1. Dimensions of specimens used for free-abrasive wear tests.
The tensile strength and impact strength along the longitudinal direction of the bamboo stem were tested in order to discuss the relationship of the abrasive wear property with the mechanical properties. The tensile specimens of dimensions 160 mm × 10 mm × 10 mm and the impact specimens of
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Fig. 3. A rotary disk type of abrasive wear testing machine. (a) A photograph of the main part; (b) the schematic diagram; 1–3, compacting wheels; 4–7, test specimens; 8, subsoiler; 9, rotary disk.
Fig. 4. Schematic diagram showing the impact angle (35◦ ) of abrasive material against specimen surface.
dimensions 250 mm × 20 mm × 10 mm were prepared. The tensile tests were run on a universal electron tensile testing machine, Shanghai Hualong Test Instrument Co. Ltd., model WDW and the impact tests were run on an impact testing machine, Wuzhong Material Testing Techincal Co., model JB-5.
ume of the bamboo specimen with the vascular fiber content against the abrasive material of dimension 0.420–0.840 mm at the sliding velocity 2.35 m/s. The abraded volume of the bamboo specimens decreased with the fiber content. It can be found that the abraded volume of the specimens with the fiber content 28.9 and 34.4 vol.% were 67.9 and 52.4% of the abraded volume of specimen with fiber content 23.1%, respectively. It can be seen from Figs. 6 and 7 that the abraded volume of bamboo specimens decreased with the fiber content at three sliding velocities, 1.68, 2.35 and 3.02 m/s and under the three dimensions of abrasive particles, 0.104–0.214, 0.214–0.420 and 0.420–0.840 mm and the abraded volume increased as the sliding velocity was increased or as the abrasive particle size was increased.
3. Experimental results 3.1. Abrasive wear The experimental results of abrasive wear showed that the vascular fiber content had a significant effect on the abrasive wear as shown in Figs. 5–7. Fig. 5 illustrates the abrasive volFig. 6. Effects of the sliding velocity on the abraded volume of bamboo specimens against the abrasive particles of dimension 0.420–0.840 mm at the varied fiber contents.
Fig. 5. Effect of the vascular fiber contents on the abraded volume of bamboo specimens against the abrasive particles of dimension 0.420–0.840 mm at the sliding velocity 2.35 m/s.
Fig. 7. Effects of the abrasive particle size on the abraded volume of bamboo specimens at the sliding velocity 2.35 m/s at varied fiber contents.
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Fig. 8. SEM micrographs of the surfaces of bamboo specimen with vascular fiber content 34.4 vol.% abraded by abrasive particles of 0.104–0.214 mm at the sliding velocity 3.02 m/s (arrows mean the sliding direction of the abrasive particles).
3.2. Abraded surface morphologies Fig. 8 illustrates the morphologies of the abraded surfaces of bamboo specimen with a vascular fiber content of 34.4 vol.% against the abrasive particles of 0.104–0.214 mm at the sliding velocity 3.02 m/s. As seen in Fig. 8a, a certain depth of the matrix tissue was removed to make the vascular bundle fibers protrude on the matrix, which suggested that the vascular fibers had higher abrasive wear resistance than the matrix tissue. The morphology of the abraded surface illustrated in Fig. 8a is a kind not-smooth morphological surface formed during the abrasive wear. The protruding fiber ends on the sliding surfaces can reduce the further wear of the matrix. So, this time, the abrasive wear procedure was controlled by the wear rate of the vascular fiber ends. This not-smooth surface is a better abrasive wear resistant surface because the vascular fibers had higher anti-wear resistance than the matrix tissue. Fig. 8b shows the details of the abraded surface of a protruding fiber end and near it.
4. Discussion 4.1. Mechanical properties of bamboo stem and their effects on abrasive wear Fig. 9 illustrates the tensile strength and elastic modulus of the bamboo stem. Obviously, the tensile strength and elastic modulus of the bamboo stem increased with the vascular fiber content. The tensile strength and the elastic modulus of the bamboo stem were approximately proportional to its vascular fiber content. The tensile strength of the bamboo with the vascular fiber content of 28.9 and 34.4 vol.% was higher by 74.5 and 153.3% than that with the vascular fiber content of 23.1 vol.%, respectively. The elastic modulus of the bamboo with the vascular fiber content of 28.9 and 34.4 vol.% was higher by 24.2% and by 88.7% than that with the vascular fiber content of 23.1 vol.%, respectively. Zeng et al. [17] also demonstrated that an increase in the vascular fiber content
Fig. 9. Relationship of: (a) tensile strength and (b) elastic modulus of the bamboo stem with its vascular fiber content.
can results in increase of the elastic modulus and longitudinal tensile strength of bamboo. Table 1 lists the experimental results of the impact strength. It is shown that the impact strength of the bamboo stem increased with its vascular fiber content, similar as the tensile strength and elastic modulus. The effects of the vascular fiber content on the tensile strength and elastic modulus just corresponded with their effects on the abrasive wear resistance. That was, an increase Table 1 Impact strength of bamboo stem Fiber content (vol.%)
Impact strength (kJ/m2 )
23.1 28.9 34.4
72.8 90.7 117.5
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Fig. 10. The relationship of the abraded volume of bamboo stem with the tensile strength for the wear tests against abrasive particles of dimension 0.420–0.840 mm at the sliding velocity 2.35 m/s.
of the mechanical properties of the bamboo stem resulted in an improvement of its abrasive wear property, as shown. Fig. 10 illustrates the relationship of the abraded volume of bamboo stem with the tensile strength. It can be found that the tensile strength of the bamboo stem was approximately proportional to its vascular fiber content as shown in Fig. 9a and the abraded volume of bamboo stem was approximately inversely proportional to its tensile strength as shown in Fig. 10. Lo et al. [7] demonstrated that the compressive strength of bamboo Bambusa pervariabilis and Phyllostachys pubescens increased with their vascular fiber content. The increase of both tensile strength and compressive strength is in favor of the abrasive wear resistance of materials. Amada and Untao [8] found that the fracture toughness K1C of the bamboo stem is proportional to the volume fraction of vascular fibers. Yakou and Sakamoto [15] measured the hardness of Phyllostachys pubescens and considered that reason for the higher abrasive wear resistance of vascular fibers than the matrix tissue is the higher hardness of the former than that of the latter. A better mechanical properties of the bamboo with higher vascular content will result in a higher abrasive wear resistance. 4.2. Effects of not-smooth morphology The abrasive particles are in freely flowing state in the sliding contact system of a solid surface with the free abrasive material in this work. The motion of one abrasive particle at the sliding contact is restricted by other particles nearby and the contacting solid surface. The abrasive wear action of the abrasive particles to the solid surface is dependent upon the motion state of the abrasive particles, including the motion of the bulk abrasive material and the motion of each particle. As far as a solid surface is concerned, there are two factors influencing its abrasive wear, the property of solid material and the geometrical morphology of the solid surface. The material factor mainly includes the mechanical properties, such as tensile strength, impact strength, hardness, and/or fracture
toughness. The effects of the geometrical morphology means, in this work, mainly the effects of the not-smooth geometrical morphology of the abraded surfaces of the bamboo stem on the abrasive wear. The friction force acting on a smooth solid surface by the abrasive particles can be considered uniform in distribution and the abrasive wear of the solid surface is also uniform in distribution. When a solid surface is a not-smooth surface with an arrangement of not-smooth geometrical construction units, the streaming direction and contacting state of the free abrasive material are changed near the construction units, not like smooth surface. The total friction force acting on this not-smooth surface consists of two parts: the friction force acting on the flat parts of the solid surface and on the geometrical not-smooth construction units. The abraded surface of bamboo stem is a not-smooth one. The fiber ends protruding on the matrix surface are the not-smooth construction units. The not-smooth units would guide abrasive material stream and would make more abrasive particles roll near the protruding fiber ends to reduce the damaging action on the vascular fibers and matrix tissue. The size of the fiber spacing along the sliding direction was 0.5–1.0 mm or more. The size of the abrasive particles of 0.104–0.214 mm and 0.214–0.420 mm was less than that of the fiber spacing. The size of the abrasive particles of 0.420–0.840 mm was near to that of the fiber spacing. So, it was found that the not-smooth surface morphology appeared for all abrasive particles in this work. It can be estimated that the not-smooth surface morphology can be easily formed if the spacing of the construction units is more than that of the abrasive particles. 4.3. Effects of experimental conditions on the abrasive wear The not-smooth morphology appearing on the abraded surface against the bigger size was not obvious as compared to that on the abraded surface under smaller size. The abraded surfaces against the larger abrasive particles possessed a morphology displaying serious damage, comparing the morphology shown in Fig. 11 with that shown in Fig. 8a. The force acting on the sliding surface of a unit area by the abrasive particles was mainly resulted from the centrifugal force of the abrasive particles for the tests running on the rotary disk type of abrasive wear testing machine. The centrifugal force was related to the density and the motion velocity of the abrasive material. The centrifugal force was increased as the sliding velocity because density of the abrasive material was a constant during tests in this work. When the sliding velocity was a constant, the force acting on the abrading surface of a unit area can be considered a constant. The stress acting on the abrading surface consisted of the two components: shearing stress and compressive stress. The average shearing stress on an abrading surface of unit apparent contact area was the sum of the shearing force generated by all contacting abrasive particles on this unit area. Likewise, the average compressive stress on an abrading surface of unit apparent
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impact strength. A higher mechanical property results in an increase of the abrasive wear resistance.
Acknowledgements This project was supported by National Science Fund for Distinguished Young Scholars of China (grant no. 50025516) and by National Natural Science Foundation of China (grant no. 50275037).
References
Fig. 11. SEM micrograph of the surface of bamboo specimen with vascular fiber content: 28.89 vol.% abraded by abrasive particles of 0.420–0.840 mm at the sliding velocity 1.68 m/s (arrow means the sliding direction of the abrasive particles).
contact area was the sum of the normal force generated by all contacting abrasive particles on this unit area. There were more abrasive particles contacting the abrading surface on a unit area for the smaller size of abrasive particles than those for the larger size of abrasive particles. Therefore, the real shearing stress and real normal load acting on the abrading surface by a larger abrasive particle was higher than those by a smaller abrasive particle. As a result, the larger abrasive particles tended to quicken the rupture procedure of the abrading surface layer under an identical sliding velocity due to the higher real shearing stress and normal stress. So, the larger abrasive particles were resulted in a larger abraded volume. The higher centrifugal force of the abrasive material appeared when the sliding velocity was higher. This would be resulted in a higher abraded volume of the bamboo stem at a higher sliding velocity.
5. Conclusions For the bamboo stem specimens with a normal orientation of the vascular fibers with respect to the sliding surface, the vascular fiber of bamboo content had a significant effect on the abrasive wear. The abrasive wear resistance increased with the fiber content. An anti-wear not-smooth morphological surface was produced during the abrasive wear procedure. The effects of the vascular fiber content on the abrasive wear resistance corresponded right to the fiber content effects on some mechanical properties, such as tensile strength and
[1] J. Tong, B.Z. Moayad, L. Ren, B. Chen, Biomimetics in soft terrain machines: a review, Int. Agric. Eng. J. 13 (2004) 71–86. [2] Q. Shen, D.S. Liu, Y. Gao, Y. Chen, Surface properties of bamboo fiber and a comparison with cotton linter fibers, Colloids Surf. B 35 (2004) 193–195. [3] F. Nogata, H. Takahashi, Intelligent functionally graded material: bamboo, Compos. Eng. 5 (1995) 743–751. [4] S. Amada, Y. Ichikawa, T. Munekata, Y. Nagase, H. Shimizu, Fiber texture and mechanical graded structure of bamboo, Compos. Part B 28 (1997) 13–20. [5] S.C. Lakkad, J.M. Patel, Mechanical properties of bamboo, a natural composite, Fibre Sci. Technol. 14 (1981) 319–322. [6] K.F. Chung, W.K. Yu, Mechanical properties of structural bamboo for bamboo scaffoldings, Eng. Struct. 24 (2002) 429–442. [7] T.Y. Lo, H.Z. Cui, H.C. Leung, The effect of fiber density on strength capacity of bamboo, Mater. Lett. 58 (2004) 2595–2598. [8] H. Ismail, S. Shuhelmy, M.R. Edyham, The effects of a silane coupling agent on curing characteristics and mechanical properties of bamboo fibre filled natural rubber composites, Eur. Polym. J. 38 (2002) 39–47. [9] H. Ismail, M.R. Edyham, B. Rirjosentono, Bamboo fiber filled natural rubber composites: the effects of filler loading and bonding agent, Polym. Test. 21 (2002) 139–144. [10] M.M. Thwe, K. Liao, Environmental effects on bamboo–glass/polypropylene hybrid composites, J. Mater. Sci. 38 (2003) 363–376. [11] R.S.P. Coutts, Y. Ni, Autoclaved bamboo pulp fibre reinforced cement, Cement Concrete Compos. 17 (1995) 99–106. [12] S.H. Li, Q.Y. Zeng, Y.L. Xiao, S.Y. Fu, B.L. Zhou, Biomimicry of bamboo bast fiber with engineering composite materials, Mater. Sci. Eng. C 3 (1995) 125–130. [13] B.L. Zhou, Some progress in the biomimetic study of compositematerials, Mater. Chem. Phys. 45 (1996) 114–119. [14] T. Yakou, S. Sakamoto, Abrasive properties of bamboo, Jpn. J. Tribol. 38 (1993) 491–497. [15] J. Tong, L. Ren, J. Li, B. Chen, Abrasive wear behaviour of bamboo, Tribol. Int. 28 (1995) 323–327. [16] J. Tong, R.D. Arnell, L. Ren, Dry sliding wear behaviour of bamboo, Wear 221 (1998) 37–46. [17] Q.Y. Zeng, S.H. Li, B.L. Zhou, The characteristics of biomaterials and biomimetics of composite materials, Acta Mater. Compos. Sin. 10 (1) (1993) 1–7.
Short communication
Effects of vascular fiber content on abrasive wear of bamboo Jin Tong ∗ , Yunhai Ma, Donghui Chen, Jiyu Sun, Luquan Ren The Key Laboratory of Terrain-Machine Bionics Engineering (Ministry of Education, China) and College of Biological and Agricultural Engineering, Jilin University (Naling Campus), 5988 Renmin Street, Changchun 130025, PR China Received 25 August 2004; received in revised form 16 March 2005; accepted 23 March 2005
Abstract Natural biomaterials have unique structures and some distinguishing properties for adapting themselves to natural surroundings. Bamboo is a natural composite reinforced with longitudinal fibers (vascular fibers). The abrasive wear property of the cross section of bamboo (Phyllostachys pubescens) stem was examined. The abrasive material used for tests was the mixture of quartz sand (96.5 wt.%) and powdered bentonite (3.5 wt.%) and contained a water content of 3 wt.%, simulating soil condition The size of the quartz sand particles was 0.104–0.214 mm, 0.214–0.420 mm and 0.420–0.840 mm, respectively. The abrasive wear tests were run on a rotary disk type of abrasive wear testing machine. The relative sliding velocity was 1.68, 2.35 and 3.02 m/s, respectively. It was concluded that the abrasive wear resistance of the cross section of the bamboo stem was a function of the vascular fiber content of bamboo, the abrasive particle size and the relative sliding velocity. The wear resistance of bamboo was increased with its vascular fiber content. A higher sliding velocity or a larger abrasive particle size resulted in a higher abraded volume of bamboo. The effects of the tensile strength and the impact strength of bamboo on its abrasive wear were discussed. It was found by scanning electron microscopy that the geometric morphology of the abraded surfaces of the sections of bamboo stem displays a not-smooth structure. This not-smooth surface morphology provides a clue to develop anti-abrasion morphological surfaces of soil-engaging components. © 2005 Published by Elsevier B.V. Keywords: Bamboo; Fiber content; Composite; Abrasive wear; Mechanical property
1. Introduction Natural biomaterials have some unique properties and functions and have attracted many researchers’ interest, including their structures, mechanical properties, physical and chemical behavior and their biomimetics [1–13]. Bamboo is a natural fiber-reinforced composite. Though various cells can be observed in bamboo, these cells can, in mechanics, be classified into two types: matrix tissue cells and sclerenchyma cells. Matrix tissue cells are leptodermous and act as the matrix of general composites. Sclerenchyma cells mainly consist of vascular boundless enveloped in the matrix tissue. A vascular bundle is made of many phloem fibers, which may be considered as fibers in general composites. A phloem fiber is composed of several layers of pillar fibers. ∗
Corresponding author. Tel.: +86 431 5095730; fax: +86 431 5095575. E-mail address: [email protected] (J. Tong).
0043-1648/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.wear.2005.03.031
There are many micro-fibers in each layer of the pillar fibers. The micro-fibers are spirally arranged at a certain spiral angle, which is varied as different layers of pillar fibers. Also a vascular bundle is composed of many right-handed spiral phloem fibers at a certain spiral angle. Shen et al. [2] compared the surface properties of bamboo fibers with the linter fiber. It was demonstrated that bamboo has unique functional graded microstructure and excellent mechanical properties [3–6]. Lo et al. [7] determined the effects of the diameter and age of bamboo Bambusa pervariabilis and Phyllostachys pubescens on the compressive strength capacity of bamboo stick. The composites filled with bamboo fibers were prepared, such as bamboo fiber filled natural rubber composites [8,9], bamboo–glass fiber reinforced polypropylene hybrid composite [10] and bamboo pulp fiber reinforced cement [11]. The structural feature of bamboo gives a clue to design new composite materials. A biomimetic model of the reinforcing elements was developed based on the bamboo bast
J. Tong et al. / Wear 259 (2005) 78–83
fibers. Each reinforcing element is composed of two layers of helically wound fibers. A biomimetic model was proposed and the biomimetic composite was prepared [12,13]. Yakou and Sakamoto [14] examined the abrasive property of bamboo with carborundum paper as the counterface. They found that the abrasive wear rate of the outside surface layer was lower than that of the inner layer for bamboo specimens of normal- and parallel-oriented cellulose fibers with respect to the sliding surface. Tong el al. [15] examined the abrasive wear behavior of bamboo using quartz sand particles as abrasive material and found that the abrasive resistance of bamboo stem was affected by the relative orientation of the vascular fibers with respect to the sliding surface and by the size of abrasive particles. Specimens with the normal orientation of vascular fibers to the sliding surface gave a higher abrasive resistance than those with the parallel orientation. Tong et al. [16] also examined the dry sliding wear behavior of bamboo on a block-on-ring machine. It was demonstrated that the worn volume of bamboo was a function of the normal load, the sliding velocity and the relative orientation of bamboo fibers with respect to the sliding surface. The effects of the vascular fiber content of bamboo on its abrasive wear against free-abrasive material have been studied in this work.
2. Experimental details 2.1. Materials Three different parts of bamboo specimens with different fiber content were taken from an air-dry bamboo (Phyllostachys pubescens) stem, not including bamboo knots. The volume contents of the vascular fibers were measured using a stereoscope with a computer image manipulation system. The vascular fiber content can be given automatically by this system. The standard dimensions for the free-abrasive wear tests are 60 mm × 35 mm × 6 mm for steel materials. But, it was difficult to prepare the standard specimens of bamboo since the limitation of the size of the bamboo stem. So, the bamboo specimens of dimensions 20 mm × 8 mm × 3 mm were cut from the same bamboo stem and three bamboo blocks were mounted in each steel plates of dimensions 60 mm × 35 mm × 6 mm as shown in Fig. 1. The vascular
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Fig. 2. Schematic diagram showing vascular fiber orientation with respect to the sliding surface for free abrasive wear specimen of bamboo.
fiber orientation was normal to the sliding surface for all bamboo specimens during the abrasive wear tests as shown in Fig. 2. Three different sizes of quartz sand (0.104–0.214 mm, 0.214–0.420 mm and 0.420–0.840 mm, respectively) and bentonite (76 m) were used as abrasive, which contained quartz sand of 96.5 wt.% and bentonite of 3.5 wt.%. The water content of the abrasive material was 3 wt.%. 2.2. Abrasive tests The abrasive wear tests were run on a rotary disk type of abrasive wear testing machine as shown in Fig. 3 100 kg quartz sand was filled in the disk. The rotary disk rotated to drive the abrasive against the abrading specimen during the tests. A set of four specimens were installed on the specimen holder at 90-degree interval and their positions were changed successively to abrade them in turn every 803.4 m of sliding distance. The abrading specimen was embedded to a depth of 40 mm in abrasive. The impact angle of the tangential direction of the abrasive against the sliding surface of the testing specimen was 35◦ as shown in Fig. 4. The total sliding distance for each specimen was 25708.8 m. New abrasive material was used for each set of four specimens. The disk velocity at the position of the testing sample was considered as relative sliding velocities of abrasive material against the specimen, which were 1.68, 2.35 and 3.02 m/s, respectively. The temperature of the surrounding air was 21 ◦ C. The three compacting wheels were kept their unchanged height in order to maintain the same density of the abrasive during all tests. The abraded height of the bamboo specimens was determined through measuring the change of sections with a stereoscope and then the abraded volume was calculated. The morphologies of the abraded surfaces were examined by scanning electron microscopy. 2.3. Tensile and impact property tests
Fig. 1. Dimensions of specimens used for free-abrasive wear tests.
The tensile strength and impact strength along the longitudinal direction of the bamboo stem were tested in order to discuss the relationship of the abrasive wear property with the mechanical properties. The tensile specimens of dimensions 160 mm × 10 mm × 10 mm and the impact specimens of
80
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Fig. 3. A rotary disk type of abrasive wear testing machine. (a) A photograph of the main part; (b) the schematic diagram; 1–3, compacting wheels; 4–7, test specimens; 8, subsoiler; 9, rotary disk.
Fig. 4. Schematic diagram showing the impact angle (35◦ ) of abrasive material against specimen surface.
dimensions 250 mm × 20 mm × 10 mm were prepared. The tensile tests were run on a universal electron tensile testing machine, Shanghai Hualong Test Instrument Co. Ltd., model WDW and the impact tests were run on an impact testing machine, Wuzhong Material Testing Techincal Co., model JB-5.
ume of the bamboo specimen with the vascular fiber content against the abrasive material of dimension 0.420–0.840 mm at the sliding velocity 2.35 m/s. The abraded volume of the bamboo specimens decreased with the fiber content. It can be found that the abraded volume of the specimens with the fiber content 28.9 and 34.4 vol.% were 67.9 and 52.4% of the abraded volume of specimen with fiber content 23.1%, respectively. It can be seen from Figs. 6 and 7 that the abraded volume of bamboo specimens decreased with the fiber content at three sliding velocities, 1.68, 2.35 and 3.02 m/s and under the three dimensions of abrasive particles, 0.104–0.214, 0.214–0.420 and 0.420–0.840 mm and the abraded volume increased as the sliding velocity was increased or as the abrasive particle size was increased.
3. Experimental results 3.1. Abrasive wear The experimental results of abrasive wear showed that the vascular fiber content had a significant effect on the abrasive wear as shown in Figs. 5–7. Fig. 5 illustrates the abrasive volFig. 6. Effects of the sliding velocity on the abraded volume of bamboo specimens against the abrasive particles of dimension 0.420–0.840 mm at the varied fiber contents.
Fig. 5. Effect of the vascular fiber contents on the abraded volume of bamboo specimens against the abrasive particles of dimension 0.420–0.840 mm at the sliding velocity 2.35 m/s.
Fig. 7. Effects of the abrasive particle size on the abraded volume of bamboo specimens at the sliding velocity 2.35 m/s at varied fiber contents.
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Fig. 8. SEM micrographs of the surfaces of bamboo specimen with vascular fiber content 34.4 vol.% abraded by abrasive particles of 0.104–0.214 mm at the sliding velocity 3.02 m/s (arrows mean the sliding direction of the abrasive particles).
3.2. Abraded surface morphologies Fig. 8 illustrates the morphologies of the abraded surfaces of bamboo specimen with a vascular fiber content of 34.4 vol.% against the abrasive particles of 0.104–0.214 mm at the sliding velocity 3.02 m/s. As seen in Fig. 8a, a certain depth of the matrix tissue was removed to make the vascular bundle fibers protrude on the matrix, which suggested that the vascular fibers had higher abrasive wear resistance than the matrix tissue. The morphology of the abraded surface illustrated in Fig. 8a is a kind not-smooth morphological surface formed during the abrasive wear. The protruding fiber ends on the sliding surfaces can reduce the further wear of the matrix. So, this time, the abrasive wear procedure was controlled by the wear rate of the vascular fiber ends. This not-smooth surface is a better abrasive wear resistant surface because the vascular fibers had higher anti-wear resistance than the matrix tissue. Fig. 8b shows the details of the abraded surface of a protruding fiber end and near it.
4. Discussion 4.1. Mechanical properties of bamboo stem and their effects on abrasive wear Fig. 9 illustrates the tensile strength and elastic modulus of the bamboo stem. Obviously, the tensile strength and elastic modulus of the bamboo stem increased with the vascular fiber content. The tensile strength and the elastic modulus of the bamboo stem were approximately proportional to its vascular fiber content. The tensile strength of the bamboo with the vascular fiber content of 28.9 and 34.4 vol.% was higher by 74.5 and 153.3% than that with the vascular fiber content of 23.1 vol.%, respectively. The elastic modulus of the bamboo with the vascular fiber content of 28.9 and 34.4 vol.% was higher by 24.2% and by 88.7% than that with the vascular fiber content of 23.1 vol.%, respectively. Zeng et al. [17] also demonstrated that an increase in the vascular fiber content
Fig. 9. Relationship of: (a) tensile strength and (b) elastic modulus of the bamboo stem with its vascular fiber content.
can results in increase of the elastic modulus and longitudinal tensile strength of bamboo. Table 1 lists the experimental results of the impact strength. It is shown that the impact strength of the bamboo stem increased with its vascular fiber content, similar as the tensile strength and elastic modulus. The effects of the vascular fiber content on the tensile strength and elastic modulus just corresponded with their effects on the abrasive wear resistance. That was, an increase Table 1 Impact strength of bamboo stem Fiber content (vol.%)
Impact strength (kJ/m2 )
23.1 28.9 34.4
72.8 90.7 117.5
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Fig. 10. The relationship of the abraded volume of bamboo stem with the tensile strength for the wear tests against abrasive particles of dimension 0.420–0.840 mm at the sliding velocity 2.35 m/s.
of the mechanical properties of the bamboo stem resulted in an improvement of its abrasive wear property, as shown. Fig. 10 illustrates the relationship of the abraded volume of bamboo stem with the tensile strength. It can be found that the tensile strength of the bamboo stem was approximately proportional to its vascular fiber content as shown in Fig. 9a and the abraded volume of bamboo stem was approximately inversely proportional to its tensile strength as shown in Fig. 10. Lo et al. [7] demonstrated that the compressive strength of bamboo Bambusa pervariabilis and Phyllostachys pubescens increased with their vascular fiber content. The increase of both tensile strength and compressive strength is in favor of the abrasive wear resistance of materials. Amada and Untao [8] found that the fracture toughness K1C of the bamboo stem is proportional to the volume fraction of vascular fibers. Yakou and Sakamoto [15] measured the hardness of Phyllostachys pubescens and considered that reason for the higher abrasive wear resistance of vascular fibers than the matrix tissue is the higher hardness of the former than that of the latter. A better mechanical properties of the bamboo with higher vascular content will result in a higher abrasive wear resistance. 4.2. Effects of not-smooth morphology The abrasive particles are in freely flowing state in the sliding contact system of a solid surface with the free abrasive material in this work. The motion of one abrasive particle at the sliding contact is restricted by other particles nearby and the contacting solid surface. The abrasive wear action of the abrasive particles to the solid surface is dependent upon the motion state of the abrasive particles, including the motion of the bulk abrasive material and the motion of each particle. As far as a solid surface is concerned, there are two factors influencing its abrasive wear, the property of solid material and the geometrical morphology of the solid surface. The material factor mainly includes the mechanical properties, such as tensile strength, impact strength, hardness, and/or fracture
toughness. The effects of the geometrical morphology means, in this work, mainly the effects of the not-smooth geometrical morphology of the abraded surfaces of the bamboo stem on the abrasive wear. The friction force acting on a smooth solid surface by the abrasive particles can be considered uniform in distribution and the abrasive wear of the solid surface is also uniform in distribution. When a solid surface is a not-smooth surface with an arrangement of not-smooth geometrical construction units, the streaming direction and contacting state of the free abrasive material are changed near the construction units, not like smooth surface. The total friction force acting on this not-smooth surface consists of two parts: the friction force acting on the flat parts of the solid surface and on the geometrical not-smooth construction units. The abraded surface of bamboo stem is a not-smooth one. The fiber ends protruding on the matrix surface are the not-smooth construction units. The not-smooth units would guide abrasive material stream and would make more abrasive particles roll near the protruding fiber ends to reduce the damaging action on the vascular fibers and matrix tissue. The size of the fiber spacing along the sliding direction was 0.5–1.0 mm or more. The size of the abrasive particles of 0.104–0.214 mm and 0.214–0.420 mm was less than that of the fiber spacing. The size of the abrasive particles of 0.420–0.840 mm was near to that of the fiber spacing. So, it was found that the not-smooth surface morphology appeared for all abrasive particles in this work. It can be estimated that the not-smooth surface morphology can be easily formed if the spacing of the construction units is more than that of the abrasive particles. 4.3. Effects of experimental conditions on the abrasive wear The not-smooth morphology appearing on the abraded surface against the bigger size was not obvious as compared to that on the abraded surface under smaller size. The abraded surfaces against the larger abrasive particles possessed a morphology displaying serious damage, comparing the morphology shown in Fig. 11 with that shown in Fig. 8a. The force acting on the sliding surface of a unit area by the abrasive particles was mainly resulted from the centrifugal force of the abrasive particles for the tests running on the rotary disk type of abrasive wear testing machine. The centrifugal force was related to the density and the motion velocity of the abrasive material. The centrifugal force was increased as the sliding velocity because density of the abrasive material was a constant during tests in this work. When the sliding velocity was a constant, the force acting on the abrading surface of a unit area can be considered a constant. The stress acting on the abrading surface consisted of the two components: shearing stress and compressive stress. The average shearing stress on an abrading surface of unit apparent contact area was the sum of the shearing force generated by all contacting abrasive particles on this unit area. Likewise, the average compressive stress on an abrading surface of unit apparent
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impact strength. A higher mechanical property results in an increase of the abrasive wear resistance.
Acknowledgements This project was supported by National Science Fund for Distinguished Young Scholars of China (grant no. 50025516) and by National Natural Science Foundation of China (grant no. 50275037).
References
Fig. 11. SEM micrograph of the surface of bamboo specimen with vascular fiber content: 28.89 vol.% abraded by abrasive particles of 0.420–0.840 mm at the sliding velocity 1.68 m/s (arrow means the sliding direction of the abrasive particles).
contact area was the sum of the normal force generated by all contacting abrasive particles on this unit area. There were more abrasive particles contacting the abrading surface on a unit area for the smaller size of abrasive particles than those for the larger size of abrasive particles. Therefore, the real shearing stress and real normal load acting on the abrading surface by a larger abrasive particle was higher than those by a smaller abrasive particle. As a result, the larger abrasive particles tended to quicken the rupture procedure of the abrading surface layer under an identical sliding velocity due to the higher real shearing stress and normal stress. So, the larger abrasive particles were resulted in a larger abraded volume. The higher centrifugal force of the abrasive material appeared when the sliding velocity was higher. This would be resulted in a higher abraded volume of the bamboo stem at a higher sliding velocity.
5. Conclusions For the bamboo stem specimens with a normal orientation of the vascular fibers with respect to the sliding surface, the vascular fiber of bamboo content had a significant effect on the abrasive wear. The abrasive wear resistance increased with the fiber content. An anti-wear not-smooth morphological surface was produced during the abrasive wear procedure. The effects of the vascular fiber content on the abrasive wear resistance corresponded right to the fiber content effects on some mechanical properties, such as tensile strength and
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