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Anisotropic abrasive wear behaviour of bamboo (Dentrocalamus strictus)
Wear 262 (2007) 1031–1037
Anisotropic abrasive wear behaviour of bamboo (Dentrocalamus strictus) Navin Chand a,∗ , U.K. Dwivedi a , S.K. Acharya b a
Regional Research Laboratory (CSIR), Hoshangabad Road, Habibganj Naka, Bhopal 462026, India b NIT Rourkela, Rourkela, India Received 18 April 2006; received in revised form 18 September 2006; accepted 17 October 2006 Available online 20 November 2006
Abstract In this paper, an experimental study has been conducted to determine the abrasive wear behaviour of bamboo, which is a cellulosic fibre reinforced composite in different directions namely LL, LT and TT by using a Suga Abrasion Wear Tester. Three different types of abrasive wear behaviours have been observed in bamboo in three orientations and follow the following trends. WTT < WLT < WLL Unidirectional vascular fibre bundles present in bamboo provide unique directional abrasive wear properties. Wear anisotropy magnitude of bamboo is a function of load and abrasive grit size. Worn surfaces and debris after wear test were observed by using a SEM. It has been found that the debris generated were of fibrous type in longitudinal direction, which came out after ploughing and microcutting of vascular fibres, while in normal fibre direction, debris were of particulate type. This unique difference in the nature of worn debris explains the difference in abrasive wear behaviour in three different directions of bamboo. © 2006 Elsevier B.V. All rights reserved. Keywords: Bamboo; High-stress abrasive wear; SEM; Anisotropy
1. Introduction Natural fibres and their composites are gaining importance due to their non-carcinogenic and bio-degradable nature [1–4]. Bamboo (Dentrocalamus strictus) is a natural composite and is used as a constructional material in the tropical countries. Bamboo is an organic lignocellulosic material, in which cellulosic fibres are embedded in a lignin matrix. Two types of cells exist in bamboo and are named as: matrix tissue cells and sclerenchyma cells. Matrix tissue cells are leptodermous. Other type of cells are Sclerenchyma cells, which are enveloped in the matrix tissue. Vascular bundles made up of to Sclerenchyma cells act as reinforcement in bamboo. Vascular bundle is made of several phloem fibres. A phloem fibre consists of several layers of pillar fibres. Micro-fibres in each layer of the pillar fibres are spirally arranged at a fixed spiral angle, which varies for
∗
Corresponding author. E-mail address: [email protected] (N. Chand).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.10.006
different layers of pillar fibres. A vascular bundle is composed of several right-handed spiral phloem fibres [1,3]. The main chemical constituents of bamboo are cellulose, hemicellulose and lignin. Hemicellulose and cellulose are present in the form of holocellulose in bamboo, which contribute to more than 50% of the total chemical constituents present in bamboo. Another important chemical constituent present in bamboo is lignin. Lignin acts as a binder for the cellulose fibres and also behave as an energy storage system [4]. Mechanical properties of bamboo as a composite material have been reported in the past [5]. It has excellent mechanical properties with specific strength and modulus which are comparable to unidirectional glass-reinforced plastic. Tong et al [3] reported the three-body abrasive wear (low stress) results of bamboo (Phyllostachys pubescens) against a free abrasive consisting of quartz sand and bentonite in the past on a rotary-disk type abrasive wear tester. Fibres due to the molecular or chain alignment in axial direction exhibit anisotropy in their physical and electrical properties [6]. Model of Tandon and Weng the self-consistent scheme [7] for unidirectional orientation
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N. Chand et al. / Wear 262 (2007) 1031–1037
Table 1 Some chemical and physical properties of bamboo in different directions Direction
Density (g/cc)
Moisture content (%)
Microfibril angle (deg)
Cellulose (%)
Lignin content (%)
Elongation (%)
Tensile strength (MPa)
In the direction of fibres Perpendicular the fibre direction
0.66 0.66
11.2 –
2–10 –
60–62 –
38–40 –
11.3 0.14
167 19
composites and the Halpin-Tsai [8] equations for modulus are used. The arbitrary fibre orientation modeling has been done by Mlekusch [9]. This anisotropy occurs due to the difference in orientation of the fibres. Different properties of a composite are obtained for each fibre orientation. An overview of the methodology and the different possibilities for micro mechanical modeling of fibre-reinforced systems is proposed by Hashin [10]. Critical comparison of the results for continuous-fibrereinforced composites from different theories is also reported in the past [11]. Chand and Fahim in their earlier paper [12] investigated abrasive wear for glass fibre reinforced composites. In another paper Chand et al. [13] reported abrasive wear mechanisms for FRP composites. A new macro mechanism theory for slurry abrasion of short glass fibre reinforced polyester composites was given by Chand and Fahim [14]. Chand et al. [15] found difference in ac conductivity values in two directions; parallel and perpendicular fibre axis of bamboo. Minimum wear in carbon fibre polyester composites was found by Lancaster [16], when fibres were perpendicular to the sliding surface. However, maximum wear was observed in the composite having fibres parallel to the sliding direction. Relative wear rates between longitudinal and transverse fibre cases depended on the type of carbon fibres [17]. Tsukizoe and Ohmea [17] reported that high strength carbon fibre epoxy composite shows high wear in transverse orientation than in longitudinal orientation, but in case of high modulus fibre composite the reverse trend was found. But there is no report in the literature related to anisotropic high stress abrasive wear behaviour of bamboo, which is an important aspect in development, applications of bamboo. In fibre reinforced composite, structure, dimension and orientation of fibres are important factors affecting their tribological properties. Wear data on natural composite structure of bamboo can provide ideas for the design of composites for tribological
applications, for examples, anti-friction materials and wearresistant materials. It is expected that the wear rate behaviour of bamboo in different directions such as LL, LT and TT will be different. In this paper, wear behaviour of bamboo in three directions under varying load and abrasive grit size has been determined. 2. Experimental 2.1. Materials Bamboo (Dentrocalamus strictus) was obtained from Sehore, India. Bamboo used in this study was a solid bamboo, in which diameter of vascular fibres bundle is 0.3–0.5 mm and its characteristics are listed in the Table 1 [1]. In this study the samples were prepared from bamboo having external diameter 50 mm and the samples were prepared along the vascular length of fibre and perpendicular to the vascular fibre length. A section between two nodes of bamboo was cut from a solid cylindrical bamboo. Samples were cut along the vascular fibre bundles and perpendicular to the vascular fibre bundles of desired dimension and then well polished by polishing grade emery paper. Two types of bamboo specimens were prepared as shown in Fig. 1a and c, but wear test was carried out in three directions as shown in Fig. 1a–c. The schematic look of vascular fibre bundle orientation and sliding direction for LL, LT and TT samples are shown in Fig. 1. The moisture content MC of bamboo was calculated as the loss in mass, expressed as a percentage of the oven dry mass at 80 ◦ C for 2 h, according to the formula: MC =
m − mo × 100 mo
where m is the mass of the test piece before drying, mo is the same after drying with an accuracy of 0.01 g.
Fig. 1. Schematic diagram of vascular fibre orientation and sliding direction in bamboo sample.
N. Chand et al. / Wear 262 (2007) 1031–1037
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2.2. Tensile testing The tensile samples were prepared as per ASTM D 638 by cutting the bamboo strips. Samples were tested at 10 mm/min cross head speed of testing, by using an electronic universal testing machine model UT-10 Scientific Testing (India). Average of three values is reported here. 2.3. Wear testing Two-body abrasive wear tests were performed on bamboo samples by using a SUGA abrasion tester model- NUS1, (Japan), which is described elsewhere [18]. The rectangular specimen of 45 × 35 × 5 mm3 was slid against a rotating wheel, on which abrasive papers of 120, 180, 320 and 400 grit sizes (John Oakey & Mohan Ltd, India) were mounted by using a double-sided adhesive tape. The embedded hard SiC particles abraded the test sample. The weight-loss measurements were done after each run of 400 cycles, which corresponds to 25.6 m sliding distance. A constant sliding speed of 2.56 m min−1 and four loads 1, 3, 5 and 7 N were applied for testing the samples. The abrasive wear rate (W) was calculated by weight-loss measurements and the following formula was used W=
V , ρD
Where ρ is the density of material, V is the weight loss and D is the sliding distance. Specific wear rate W = W/L where L is the applied load. 2.4. SEM Studies Worn surface and worn out debris of bamboo were observed by using a scanning electron microscope (SEM) model JEOL 35 CF, Japan make. Surfaces of samples were gold coated before observing on SEM. 3. Results and discussions 3.1. Wear study Dependence of wear rate on applied load of bamboo is shown in Fig. 2 for LL, LT and TT samples. The wear rate increased with applied load in all experiments. As the normal load on the abrasive particles increased, the load distributed over all the asperities and each asperity penetrates deeper into the surface on increasing the load. Deeper grooving caused higher wear rate. At higher load, severe plastic deformation of the surface caused heating of the worn surface, which debonded the interface of composites. As a consequence, major portion of the material from the wear surface removed on applying higher load. Wear rate of bamboo in three directions follows the following trend. WTT < WLT < WLL
Fig. 2. Plot between wear rate vs. load for LL,LT and TT samples at sliding distance = 25.6 m.
Wear rate normal to fibre direction (WTT ) is lower than that the transverse fibre direction (WLT ) and even lower than in longitudinal fibre direction (WLL ) at different applied loads. In other words TT sample of bamboo exhibited maximum wear resistance among all the samples. In normal fibre direction TT case, where the long fibres are well embedded deep in the matrix thus offering the greatest resistance to the removal. In case of TT sample, cellulosic cells are oriented normal to the sliding direction, only cross section of vascular bundles come in contact to the hard asperity and could not remove the material easily. The cross sections of vascular bundle fibres created more hindrance in the path of the abrading particles and resisted the movement of abrading particles, which reduced the wear in the TT sample. In LL sample, wear rate is more than TT and LT samples. In this mode ploughing is higher followed by delaminating of fibre. In this mode, the abrasion of fibre is due to removal of complete layer of fibre (cell). Since the vascular fibre bundles are present in parallel direction in LL sample, the possibility of the real contact area with fibres in the sliding direction is more than the TT and LT sample, which led to the highest wear in LL sample. In LT sample, microcutting action is hampered due to phase discontinuity, which is alternatively coming after every vascular fibre bundle. Anisotropy in wear behaviour develops due to the orientation of vascular cellulosic fibres. In case of TT and LT samples, the wear debris and matrix between the fibres are arrested against counterface, resulting in a low wear rate. In the longitudinal LL case, the wear debris of matrix present between the parallel fibres fragmented easily and detached easily by the counterface interaction, resulting in high wear loss. Another mechanism of material removal in LL sample could be the single line motion of hard abrasive asperity i.e. an asperity interaction with fibre or matrix remains in same phase through out the sliding duration and no fibre-matrix interface comes in the path, while in TT and LT samples, hard asperity moves through different interfaces and different layer of matrix and vascular fibre bundle alternatively. Increase in number of interfaces decreases the wear rate. It is due to the reduction in fragmentation of matrix and hence detachment. In LL sample,
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Fig. 3. Plot between specific wear rate vs. load for LL,LT and TT samples at sliding distance = 25.6 m.
matrix fragmentation and fibre delamination, tilting and microcutting are the responsible mechanisms of wear process. In LT sample, matrix fragmentation followed by fibre micro-cutting is the responsible mechanisms of wear process. In TT sample, matrix fragmentation and fibre cross section micro-cutting are the responsible mechanisms of wear process. Fig. 3 illustrates the graph between specific wear rate and applied load for the LL, LT and TT samples. Directional wise trends of specific wear rate were same as found in Fig. 2 at different applied loads. In TT sample, on applying 1N load, bamboo fibres may have no cutting action by hard abrasive (due to insufficient load) only binder (lignin) content ploughed out from bamboo surface, which causes minimum weight loss is observed at 1N. Otherwise specific wear rate (which is defined in Section 2.3) decreased on increasing applied load. This is because of depth of penetration by hard asperities do not follow linear relationship with increased applied load. Real stress reduces with increased applied load because real contact area of hard asperity increased. Fig. 4 shows that increase in abrasive grit size from 400 to 120 grit increase the weight loss of the bamboo. The wear rate is
Fig. 4. Plot between wear rate vs. grit size for LL and TT samples for applied load = 5 N, Sliding distance = 25.6 m.
Fig. 5. Plot between anisotropy coefficient vs. applied load for bamboo.
primarily depending on the depth and width of the groove made by the abrasives. At coarser abrasives, the depth of penetration of the abrasive particle is so high during delaminating mechanism a large portion of material is removed from the specimen surface leaving behind large cavities in the worn surface. The depth of cut is increased significantly with coarser grit size hence large debris are removed from the surface. If the applied load is fixed, then the effective stress on individual abrasives increases with coarser abrasive particles, as the load is shared by less number of abrasives. When the abrasive particles are finer in size, they make only elastic contact with the test specimen surface, as the effective stress in individual abrasive is less. As a result, these abrasive particles only support the applied load without contributing sufficient material removal. However, at higher load regime, the effective stress on each individual abrasive particles reach to a level where the abrasives make plastic contact with the specimen surface and causing more surface damage even at finer abrasive size. The radius of abrasive tip is varying with increase size. But the width of groove increases substantially with increase in abrasive size. This may be attributed to the fact, the effective stresses on each individual abrasive increase
Fig. 6. Plot between anisotropy coefficient vs. grit size for bamboo.
N. Chand et al. / Wear 262 (2007) 1031–1037
substantially when become coarser in size, and make it more effective to penetrate deeper into the surface. An attempt to introduce anisotropy coefficient has been made in this study. Anisotropy coefficient is defined as the ratio of the wear loss value in perpendicular to parallel fibre direction in unidirectional fibre reinforced composites. Physical significance of anisotropy coefficient is to show the anisotropy magnitude of material property in the composites. Anisotropy coefficient can be written
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Anisotropy coefficient (n) WTT = if property W is less in TT case than LL case WLL WTT or n = if property W is less in LL case than TT case WLL n = 1 for isotropic composites; n = 0, for ideal anisotropic composites (or Infinite anisotropic composites); 0 < n < 1 for anisotropic composites.
Fig. 7. Scanning electron micrograph of (a) worn surface of LL sample, (b) worn surface of LT sample, (c) worn surface of TT sample, (d) wear debris of LL sample and (e) wear debris of TT sample.
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Generally the value of anisotropy coefficient will lie between 0 and 1. Here magnitude of wear rate is minimum in the case of TT than LL, so n = WTT /WLL . The dependency of wear anisotropy coefficient for different loads and different abrasive grit size for bamboo has also been determined in this study. Fig. 5 shows the variation of wear anisotropy coefficient with applied loads ranging from 1 to 7 N for 25.6 m sliding distance. The observed trend of wear anisotropy coefficient is a function of load and approaching to maximum value or near to one (which is the indication to become nearly isotropic behaviour). Following dependency between wear anisotropy coefficient and applied load for bamboo is proposed, which may be further tested for all unidirectional fibre reinforced composites
c. This debonding would have occurred due to the heat generation during sliding. This causes volume mismatching due to difference in thermal expansion coefficient of fibres and matrix [13]. Debris observation of LL and TT samples showed the different type of debris, which supports the mechanism, which has discussed in earlier paragraph. In LL sample, microfibres/cell type debris, which had got abraded on the emery paper was observed as shown in Fig. 7d. In TT case, fine divided powder type debris, which had adhered on the emery paper was observed as shown in Fig. 7e. Debris obtained were supports microcutting, microploughing action in TT-sample and complete layer of fibres/cells removal in LL sample.
n = f (L)
4. Conclusions
Similarly Fig. 6 shows the variation of wear anisotropy coefficient with different abrasive grit size. On increasing the grit size more material removal occurred due to the deep ploughing action. But this experimental study exhibited the decreasing trend of wear anisotropy coefficient with increasing abrasive grit size (GS). This behaviour of wear anisotropy coefficient is represented as a function of GS for bamboo.
Anisotropic wear behaviour in bamboo exists due to the vascular fibre’s orientation parallel to the sliding direction or the central axis of the bamboo. Wear rate of bamboo in three directions follows the following trend.
n = f (GS) Where GS is the abrasive grit size. 3.2. SEM study Fig. 7 shows the SEM of worn surfaces of LL, LT and TT samples. In LL sample, whole fibre’s cell got abraded, peel-off and removed during abrasion, which is clearly visible in SEM micrograph (Fig. 7a). In this case, fibres geometry supports the flow of asperities and removal of debris and the abrasion of bamboo is due to removal of complete layer of fibre’s cell. The matrix present between the fibres easily fragmented and swept away by the hard asperity interaction, resulting in high wear. Wear tracks are formed by removing out the debris of fragmented binder and delaminated vascular fibres’s cell. Abrasive particles removed the part of the fibres by delaminating and microploughing mechanism. In LT sample, the abrasive particles have slid perpendicular to the fibre alignment, which has led to the microcutting of fibres and matrix. Fig. 7b clearly shows the wear track and cut fibres on the wear track. Against the transverse motion of fibre direction, fibres were cut and tried to bend in the direction of sliding motion. Wear process mainly due to the microcutting mechanism. In case of TT sample, cells of fibre are oriented normal to the sliding direction. The cross sections of vascular fibre bundles (blackened portion) are clearly visible in Fig. 7c, which shows the flower type geometry and diameter of vascular bundles. Fibres are reserved and deep embedded in the matrix. In this case, fibres geometry resists the flow of asperities and removal of debris. The cross sections of fibres come in contact and due to microcutting and microploughing of the cross sections debris produced alongwith the matrix present between fibres, caused formation of wear track. Matrix fibre debonding is also observed in Fig. 7a and
WTT < WLT < WLL In LL sample, the abrasion of fibre is due to removal of complete layer of fibre and matrix fragmentation and fibre delamination, tilting and micro-cutting are the responsible mechanisms of wear process. In LT sample, matrix fragmentation followed by fibre micro-cutting is the responsible mechanisms of wear process. In TT sample, matrix fragmentation and fibre cross section micro-cutting are the responsible mechanisms of wear process. Anisotropy coefficient is a function of applied load and abrasive grit size. Acknowledgements Authors are thankful to Dr. A.Gupta, Centre Director, Dr. D. Phase and Mr. V. Ahire, IUC, Indore for kind guidance, encouragement and permission for carrying out the above SEM work. One of the authors Mr. U.K Dwivedi is thankful to CSIR for giving fellowship during his research work. References [1] N. Chand, P.K. Rohatgi, Natural Fibres and Their Composites, Publishers, Periodical Experts, Delhi, 1994. [2] N. Chand, U.K. Dwivedi, Effect of coupling agent on high stress abrasive wear of chopped Jute /PP composites, Wear 261 (2006) 1057. [3] J. Tong, L. Ren, J. Li, B. Chen, Abrasive wear behaviour of bamboo, Tribol. Int. 28 (5) (1995) 323–327. [4] S. Jain, R. Kumar, U.C. Jindal, mechanical behaviour of bamboo and bamboo composite, J. Mat. Sci. 27 (1992) 4598–4604. [5] S.C. Lakkad, J.M. Patel, Mechanical properties of bamboo, a natural composite, Fibre Sci. Technol. 14 (1981) 319–322. [6] A.K. Gupta, N. Chand, Polymers 20 (1974) 875. [7] G.P. Tandon, G.J. Weng, The effect of aspect ratio of inclusions on the elastic properties of unidirectionally aligned composites, Poly. Compos. 5 (1984) 4. [8] J.C. Halpin, J.L. Kardos, The Halpin-Tsai equs: a review, Polym. Eng. Sci. 16 (5) (1976) 344–352. [9] B. Mlekusch, Thermoelastic properties of short-fibre-reinforced thermoplastics, Compos. Sci. Technol. 59 (6) (1999) 911–923.
N. Chand et al. / Wear 262 (2007) 1031–1037 [10] Z. Hashin, Analysis of composite materials, J. Appl. Mech. 50 (1983) 481–505. [11] R.M. Christensen, A critical evaluation for a class of micromechanical models, J. Mech. Phys. Solids. 38. (3) (1990). [12] N. Chand, M. Fahim, Tribol. Lett. 1 (1995) 301–307. [13] N. Chand, B. Majumdar, M. Fahim, Ind. J. Eng. Mat. Sci. 1 (1994) 273–278.
[14] [15] [16] [17] [18]
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N. Chand, M. Fahim, Ind. J. Eng. Mat. Sci. 1 (1994) 165–168. N. Chand, D. Jain, Compos. A 36 (2005) 594–602. J.K. Lancaster, Br. J. Appl. Phys. 1 (1968) 549. T. Tsukizoe, N. Ohmae, Tribol. Int. 8 (1975) 171. N. Chand, M. Fahim, Tribology of FRP Materials, Allied publishers Ltd. (2000) 47.
Anisotropic abrasive wear behaviour of bamboo (Dentrocalamus strictus) Navin Chand a,∗ , U.K. Dwivedi a , S.K. Acharya b a
Regional Research Laboratory (CSIR), Hoshangabad Road, Habibganj Naka, Bhopal 462026, India b NIT Rourkela, Rourkela, India Received 18 April 2006; received in revised form 18 September 2006; accepted 17 October 2006 Available online 20 November 2006
Abstract In this paper, an experimental study has been conducted to determine the abrasive wear behaviour of bamboo, which is a cellulosic fibre reinforced composite in different directions namely LL, LT and TT by using a Suga Abrasion Wear Tester. Three different types of abrasive wear behaviours have been observed in bamboo in three orientations and follow the following trends. WTT < WLT < WLL Unidirectional vascular fibre bundles present in bamboo provide unique directional abrasive wear properties. Wear anisotropy magnitude of bamboo is a function of load and abrasive grit size. Worn surfaces and debris after wear test were observed by using a SEM. It has been found that the debris generated were of fibrous type in longitudinal direction, which came out after ploughing and microcutting of vascular fibres, while in normal fibre direction, debris were of particulate type. This unique difference in the nature of worn debris explains the difference in abrasive wear behaviour in three different directions of bamboo. © 2006 Elsevier B.V. All rights reserved. Keywords: Bamboo; High-stress abrasive wear; SEM; Anisotropy
1. Introduction Natural fibres and their composites are gaining importance due to their non-carcinogenic and bio-degradable nature [1–4]. Bamboo (Dentrocalamus strictus) is a natural composite and is used as a constructional material in the tropical countries. Bamboo is an organic lignocellulosic material, in which cellulosic fibres are embedded in a lignin matrix. Two types of cells exist in bamboo and are named as: matrix tissue cells and sclerenchyma cells. Matrix tissue cells are leptodermous. Other type of cells are Sclerenchyma cells, which are enveloped in the matrix tissue. Vascular bundles made up of to Sclerenchyma cells act as reinforcement in bamboo. Vascular bundle is made of several phloem fibres. A phloem fibre consists of several layers of pillar fibres. Micro-fibres in each layer of the pillar fibres are spirally arranged at a fixed spiral angle, which varies for
∗
Corresponding author. E-mail address: [email protected] (N. Chand).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.10.006
different layers of pillar fibres. A vascular bundle is composed of several right-handed spiral phloem fibres [1,3]. The main chemical constituents of bamboo are cellulose, hemicellulose and lignin. Hemicellulose and cellulose are present in the form of holocellulose in bamboo, which contribute to more than 50% of the total chemical constituents present in bamboo. Another important chemical constituent present in bamboo is lignin. Lignin acts as a binder for the cellulose fibres and also behave as an energy storage system [4]. Mechanical properties of bamboo as a composite material have been reported in the past [5]. It has excellent mechanical properties with specific strength and modulus which are comparable to unidirectional glass-reinforced plastic. Tong et al [3] reported the three-body abrasive wear (low stress) results of bamboo (Phyllostachys pubescens) against a free abrasive consisting of quartz sand and bentonite in the past on a rotary-disk type abrasive wear tester. Fibres due to the molecular or chain alignment in axial direction exhibit anisotropy in their physical and electrical properties [6]. Model of Tandon and Weng the self-consistent scheme [7] for unidirectional orientation
1032
N. Chand et al. / Wear 262 (2007) 1031–1037
Table 1 Some chemical and physical properties of bamboo in different directions Direction
Density (g/cc)
Moisture content (%)
Microfibril angle (deg)
Cellulose (%)
Lignin content (%)
Elongation (%)
Tensile strength (MPa)
In the direction of fibres Perpendicular the fibre direction
0.66 0.66
11.2 –
2–10 –
60–62 –
38–40 –
11.3 0.14
167 19
composites and the Halpin-Tsai [8] equations for modulus are used. The arbitrary fibre orientation modeling has been done by Mlekusch [9]. This anisotropy occurs due to the difference in orientation of the fibres. Different properties of a composite are obtained for each fibre orientation. An overview of the methodology and the different possibilities for micro mechanical modeling of fibre-reinforced systems is proposed by Hashin [10]. Critical comparison of the results for continuous-fibrereinforced composites from different theories is also reported in the past [11]. Chand and Fahim in their earlier paper [12] investigated abrasive wear for glass fibre reinforced composites. In another paper Chand et al. [13] reported abrasive wear mechanisms for FRP composites. A new macro mechanism theory for slurry abrasion of short glass fibre reinforced polyester composites was given by Chand and Fahim [14]. Chand et al. [15] found difference in ac conductivity values in two directions; parallel and perpendicular fibre axis of bamboo. Minimum wear in carbon fibre polyester composites was found by Lancaster [16], when fibres were perpendicular to the sliding surface. However, maximum wear was observed in the composite having fibres parallel to the sliding direction. Relative wear rates between longitudinal and transverse fibre cases depended on the type of carbon fibres [17]. Tsukizoe and Ohmea [17] reported that high strength carbon fibre epoxy composite shows high wear in transverse orientation than in longitudinal orientation, but in case of high modulus fibre composite the reverse trend was found. But there is no report in the literature related to anisotropic high stress abrasive wear behaviour of bamboo, which is an important aspect in development, applications of bamboo. In fibre reinforced composite, structure, dimension and orientation of fibres are important factors affecting their tribological properties. Wear data on natural composite structure of bamboo can provide ideas for the design of composites for tribological
applications, for examples, anti-friction materials and wearresistant materials. It is expected that the wear rate behaviour of bamboo in different directions such as LL, LT and TT will be different. In this paper, wear behaviour of bamboo in three directions under varying load and abrasive grit size has been determined. 2. Experimental 2.1. Materials Bamboo (Dentrocalamus strictus) was obtained from Sehore, India. Bamboo used in this study was a solid bamboo, in which diameter of vascular fibres bundle is 0.3–0.5 mm and its characteristics are listed in the Table 1 [1]. In this study the samples were prepared from bamboo having external diameter 50 mm and the samples were prepared along the vascular length of fibre and perpendicular to the vascular fibre length. A section between two nodes of bamboo was cut from a solid cylindrical bamboo. Samples were cut along the vascular fibre bundles and perpendicular to the vascular fibre bundles of desired dimension and then well polished by polishing grade emery paper. Two types of bamboo specimens were prepared as shown in Fig. 1a and c, but wear test was carried out in three directions as shown in Fig. 1a–c. The schematic look of vascular fibre bundle orientation and sliding direction for LL, LT and TT samples are shown in Fig. 1. The moisture content MC of bamboo was calculated as the loss in mass, expressed as a percentage of the oven dry mass at 80 ◦ C for 2 h, according to the formula: MC =
m − mo × 100 mo
where m is the mass of the test piece before drying, mo is the same after drying with an accuracy of 0.01 g.
Fig. 1. Schematic diagram of vascular fibre orientation and sliding direction in bamboo sample.
N. Chand et al. / Wear 262 (2007) 1031–1037
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2.2. Tensile testing The tensile samples were prepared as per ASTM D 638 by cutting the bamboo strips. Samples were tested at 10 mm/min cross head speed of testing, by using an electronic universal testing machine model UT-10 Scientific Testing (India). Average of three values is reported here. 2.3. Wear testing Two-body abrasive wear tests were performed on bamboo samples by using a SUGA abrasion tester model- NUS1, (Japan), which is described elsewhere [18]. The rectangular specimen of 45 × 35 × 5 mm3 was slid against a rotating wheel, on which abrasive papers of 120, 180, 320 and 400 grit sizes (John Oakey & Mohan Ltd, India) were mounted by using a double-sided adhesive tape. The embedded hard SiC particles abraded the test sample. The weight-loss measurements were done after each run of 400 cycles, which corresponds to 25.6 m sliding distance. A constant sliding speed of 2.56 m min−1 and four loads 1, 3, 5 and 7 N were applied for testing the samples. The abrasive wear rate (W) was calculated by weight-loss measurements and the following formula was used W=
V , ρD
Where ρ is the density of material, V is the weight loss and D is the sliding distance. Specific wear rate W = W/L where L is the applied load. 2.4. SEM Studies Worn surface and worn out debris of bamboo were observed by using a scanning electron microscope (SEM) model JEOL 35 CF, Japan make. Surfaces of samples were gold coated before observing on SEM. 3. Results and discussions 3.1. Wear study Dependence of wear rate on applied load of bamboo is shown in Fig. 2 for LL, LT and TT samples. The wear rate increased with applied load in all experiments. As the normal load on the abrasive particles increased, the load distributed over all the asperities and each asperity penetrates deeper into the surface on increasing the load. Deeper grooving caused higher wear rate. At higher load, severe plastic deformation of the surface caused heating of the worn surface, which debonded the interface of composites. As a consequence, major portion of the material from the wear surface removed on applying higher load. Wear rate of bamboo in three directions follows the following trend. WTT < WLT < WLL
Fig. 2. Plot between wear rate vs. load for LL,LT and TT samples at sliding distance = 25.6 m.
Wear rate normal to fibre direction (WTT ) is lower than that the transverse fibre direction (WLT ) and even lower than in longitudinal fibre direction (WLL ) at different applied loads. In other words TT sample of bamboo exhibited maximum wear resistance among all the samples. In normal fibre direction TT case, where the long fibres are well embedded deep in the matrix thus offering the greatest resistance to the removal. In case of TT sample, cellulosic cells are oriented normal to the sliding direction, only cross section of vascular bundles come in contact to the hard asperity and could not remove the material easily. The cross sections of vascular bundle fibres created more hindrance in the path of the abrading particles and resisted the movement of abrading particles, which reduced the wear in the TT sample. In LL sample, wear rate is more than TT and LT samples. In this mode ploughing is higher followed by delaminating of fibre. In this mode, the abrasion of fibre is due to removal of complete layer of fibre (cell). Since the vascular fibre bundles are present in parallel direction in LL sample, the possibility of the real contact area with fibres in the sliding direction is more than the TT and LT sample, which led to the highest wear in LL sample. In LT sample, microcutting action is hampered due to phase discontinuity, which is alternatively coming after every vascular fibre bundle. Anisotropy in wear behaviour develops due to the orientation of vascular cellulosic fibres. In case of TT and LT samples, the wear debris and matrix between the fibres are arrested against counterface, resulting in a low wear rate. In the longitudinal LL case, the wear debris of matrix present between the parallel fibres fragmented easily and detached easily by the counterface interaction, resulting in high wear loss. Another mechanism of material removal in LL sample could be the single line motion of hard abrasive asperity i.e. an asperity interaction with fibre or matrix remains in same phase through out the sliding duration and no fibre-matrix interface comes in the path, while in TT and LT samples, hard asperity moves through different interfaces and different layer of matrix and vascular fibre bundle alternatively. Increase in number of interfaces decreases the wear rate. It is due to the reduction in fragmentation of matrix and hence detachment. In LL sample,
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Fig. 3. Plot between specific wear rate vs. load for LL,LT and TT samples at sliding distance = 25.6 m.
matrix fragmentation and fibre delamination, tilting and microcutting are the responsible mechanisms of wear process. In LT sample, matrix fragmentation followed by fibre micro-cutting is the responsible mechanisms of wear process. In TT sample, matrix fragmentation and fibre cross section micro-cutting are the responsible mechanisms of wear process. Fig. 3 illustrates the graph between specific wear rate and applied load for the LL, LT and TT samples. Directional wise trends of specific wear rate were same as found in Fig. 2 at different applied loads. In TT sample, on applying 1N load, bamboo fibres may have no cutting action by hard abrasive (due to insufficient load) only binder (lignin) content ploughed out from bamboo surface, which causes minimum weight loss is observed at 1N. Otherwise specific wear rate (which is defined in Section 2.3) decreased on increasing applied load. This is because of depth of penetration by hard asperities do not follow linear relationship with increased applied load. Real stress reduces with increased applied load because real contact area of hard asperity increased. Fig. 4 shows that increase in abrasive grit size from 400 to 120 grit increase the weight loss of the bamboo. The wear rate is
Fig. 4. Plot between wear rate vs. grit size for LL and TT samples for applied load = 5 N, Sliding distance = 25.6 m.
Fig. 5. Plot between anisotropy coefficient vs. applied load for bamboo.
primarily depending on the depth and width of the groove made by the abrasives. At coarser abrasives, the depth of penetration of the abrasive particle is so high during delaminating mechanism a large portion of material is removed from the specimen surface leaving behind large cavities in the worn surface. The depth of cut is increased significantly with coarser grit size hence large debris are removed from the surface. If the applied load is fixed, then the effective stress on individual abrasives increases with coarser abrasive particles, as the load is shared by less number of abrasives. When the abrasive particles are finer in size, they make only elastic contact with the test specimen surface, as the effective stress in individual abrasive is less. As a result, these abrasive particles only support the applied load without contributing sufficient material removal. However, at higher load regime, the effective stress on each individual abrasive particles reach to a level where the abrasives make plastic contact with the specimen surface and causing more surface damage even at finer abrasive size. The radius of abrasive tip is varying with increase size. But the width of groove increases substantially with increase in abrasive size. This may be attributed to the fact, the effective stresses on each individual abrasive increase
Fig. 6. Plot between anisotropy coefficient vs. grit size for bamboo.
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substantially when become coarser in size, and make it more effective to penetrate deeper into the surface. An attempt to introduce anisotropy coefficient has been made in this study. Anisotropy coefficient is defined as the ratio of the wear loss value in perpendicular to parallel fibre direction in unidirectional fibre reinforced composites. Physical significance of anisotropy coefficient is to show the anisotropy magnitude of material property in the composites. Anisotropy coefficient can be written
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Anisotropy coefficient (n) WTT = if property W is less in TT case than LL case WLL WTT or n = if property W is less in LL case than TT case WLL n = 1 for isotropic composites; n = 0, for ideal anisotropic composites (or Infinite anisotropic composites); 0 < n < 1 for anisotropic composites.
Fig. 7. Scanning electron micrograph of (a) worn surface of LL sample, (b) worn surface of LT sample, (c) worn surface of TT sample, (d) wear debris of LL sample and (e) wear debris of TT sample.
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Generally the value of anisotropy coefficient will lie between 0 and 1. Here magnitude of wear rate is minimum in the case of TT than LL, so n = WTT /WLL . The dependency of wear anisotropy coefficient for different loads and different abrasive grit size for bamboo has also been determined in this study. Fig. 5 shows the variation of wear anisotropy coefficient with applied loads ranging from 1 to 7 N for 25.6 m sliding distance. The observed trend of wear anisotropy coefficient is a function of load and approaching to maximum value or near to one (which is the indication to become nearly isotropic behaviour). Following dependency between wear anisotropy coefficient and applied load for bamboo is proposed, which may be further tested for all unidirectional fibre reinforced composites
c. This debonding would have occurred due to the heat generation during sliding. This causes volume mismatching due to difference in thermal expansion coefficient of fibres and matrix [13]. Debris observation of LL and TT samples showed the different type of debris, which supports the mechanism, which has discussed in earlier paragraph. In LL sample, microfibres/cell type debris, which had got abraded on the emery paper was observed as shown in Fig. 7d. In TT case, fine divided powder type debris, which had adhered on the emery paper was observed as shown in Fig. 7e. Debris obtained were supports microcutting, microploughing action in TT-sample and complete layer of fibres/cells removal in LL sample.
n = f (L)
4. Conclusions
Similarly Fig. 6 shows the variation of wear anisotropy coefficient with different abrasive grit size. On increasing the grit size more material removal occurred due to the deep ploughing action. But this experimental study exhibited the decreasing trend of wear anisotropy coefficient with increasing abrasive grit size (GS). This behaviour of wear anisotropy coefficient is represented as a function of GS for bamboo.
Anisotropic wear behaviour in bamboo exists due to the vascular fibre’s orientation parallel to the sliding direction or the central axis of the bamboo. Wear rate of bamboo in three directions follows the following trend.
n = f (GS) Where GS is the abrasive grit size. 3.2. SEM study Fig. 7 shows the SEM of worn surfaces of LL, LT and TT samples. In LL sample, whole fibre’s cell got abraded, peel-off and removed during abrasion, which is clearly visible in SEM micrograph (Fig. 7a). In this case, fibres geometry supports the flow of asperities and removal of debris and the abrasion of bamboo is due to removal of complete layer of fibre’s cell. The matrix present between the fibres easily fragmented and swept away by the hard asperity interaction, resulting in high wear. Wear tracks are formed by removing out the debris of fragmented binder and delaminated vascular fibres’s cell. Abrasive particles removed the part of the fibres by delaminating and microploughing mechanism. In LT sample, the abrasive particles have slid perpendicular to the fibre alignment, which has led to the microcutting of fibres and matrix. Fig. 7b clearly shows the wear track and cut fibres on the wear track. Against the transverse motion of fibre direction, fibres were cut and tried to bend in the direction of sliding motion. Wear process mainly due to the microcutting mechanism. In case of TT sample, cells of fibre are oriented normal to the sliding direction. The cross sections of vascular fibre bundles (blackened portion) are clearly visible in Fig. 7c, which shows the flower type geometry and diameter of vascular bundles. Fibres are reserved and deep embedded in the matrix. In this case, fibres geometry resists the flow of asperities and removal of debris. The cross sections of fibres come in contact and due to microcutting and microploughing of the cross sections debris produced alongwith the matrix present between fibres, caused formation of wear track. Matrix fibre debonding is also observed in Fig. 7a and
WTT < WLT < WLL In LL sample, the abrasion of fibre is due to removal of complete layer of fibre and matrix fragmentation and fibre delamination, tilting and micro-cutting are the responsible mechanisms of wear process. In LT sample, matrix fragmentation followed by fibre micro-cutting is the responsible mechanisms of wear process. In TT sample, matrix fragmentation and fibre cross section micro-cutting are the responsible mechanisms of wear process. Anisotropy coefficient is a function of applied load and abrasive grit size. Acknowledgements Authors are thankful to Dr. A.Gupta, Centre Director, Dr. D. Phase and Mr. V. Ahire, IUC, Indore for kind guidance, encouragement and permission for carrying out the above SEM work. One of the authors Mr. U.K Dwivedi is thankful to CSIR for giving fellowship during his research work. References [1] N. Chand, P.K. Rohatgi, Natural Fibres and Their Composites, Publishers, Periodical Experts, Delhi, 1994. [2] N. Chand, U.K. Dwivedi, Effect of coupling agent on high stress abrasive wear of chopped Jute /PP composites, Wear 261 (2006) 1057. [3] J. Tong, L. Ren, J. Li, B. Chen, Abrasive wear behaviour of bamboo, Tribol. Int. 28 (5) (1995) 323–327. [4] S. Jain, R. Kumar, U.C. Jindal, mechanical behaviour of bamboo and bamboo composite, J. Mat. Sci. 27 (1992) 4598–4604. [5] S.C. Lakkad, J.M. Patel, Mechanical properties of bamboo, a natural composite, Fibre Sci. Technol. 14 (1981) 319–322. [6] A.K. Gupta, N. Chand, Polymers 20 (1974) 875. [7] G.P. Tandon, G.J. Weng, The effect of aspect ratio of inclusions on the elastic properties of unidirectionally aligned composites, Poly. Compos. 5 (1984) 4. [8] J.C. Halpin, J.L. Kardos, The Halpin-Tsai equs: a review, Polym. Eng. Sci. 16 (5) (1976) 344–352. [9] B. Mlekusch, Thermoelastic properties of short-fibre-reinforced thermoplastics, Compos. Sci. Technol. 59 (6) (1999) 911–923.
N. Chand et al. / Wear 262 (2007) 1031–1037 [10] Z. Hashin, Analysis of composite materials, J. Appl. Mech. 50 (1983) 481–505. [11] R.M. Christensen, A critical evaluation for a class of micromechanical models, J. Mech. Phys. Solids. 38. (3) (1990). [12] N. Chand, M. Fahim, Tribol. Lett. 1 (1995) 301–307. [13] N. Chand, B. Majumdar, M. Fahim, Ind. J. Eng. Mat. Sci. 1 (1994) 273–278.
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