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Mode I interlaminar fracture toughness behavior and mechanisms of bamboo
Materials and Design 183 (2019) 108132
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Mode I interlaminar fracture toughness behavior and mechanisms of bamboo Qi Chen a,b,c, Chunping Dai c, Changhua Fang a,b, Meiling Chen a,b,c, Shuqin Zhang a,b, Rong Liu a,b, Xianmiao Liu a,b,⁎, Benhua Fei a,b,⁎ a b c
Department of Biomaterials, International Centre for Bamboo and Rattan, Beijing 100102, China SFA and Beijing Co-built Key Laboratory of Bamboo and Rattan Science and Technology, State Forestry Administration, Beijing 100102, China The University of British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Initiation energy and crack-growth energy of bamboo are lowest in the region with highest fiber content. • The highest growth-energy occurs in the middle region with a balanced content of fiber and parenchyma cells. • Initiation energy is related to the intrinsic toughening mechanism through the plastic zoning and crack kinking. • Crack-growth energy is associated with the extrinsic toughening mechanism via fiber bridging.
a r t i c l e
i n f o
Article history: Received 24 April 2019 Received in revised form 15 July 2019 Accepted 17 August 2019 Available online 18 August 2019 Keywords: Bamboo Fracture toughness Intrinsic mechanisms Extrinsic mechanisms Fiber bridging
a b s t r a c t Bamboo, with its fast growth rate and outstanding mechanical properties, has received increasing attention as a green material for engineering applications. Compared to other mechanical properties, little is known about fracture toughness, especially the toughening mechanism of bamboo. In this study, the Mode I interlaminar fracture toughness of bamboo with different proportions of fiber cells (FCs) and parenchyma cells (PCs) was tested, using in situ SEM to investigate the intrinsic mechanisms and extrinsic mechanisms at the cellular level. The results showed that both initiation and crack growth energies of high fiber density region were the lowest, and the crack growth energy of middle fiber density region was the highest. The intrinsic toughening in bamboo is associated with plastic zone size and crack kinking which are governed by PCs. The extrinsic toughening is related to fiber bridging which is governed by FCs. Having the highest fracture toughness, the middle region of bamboo may be the best choice of natural fiber for manufacturing high performance green composite materials. This work fills the gaps in the knowledge of fracture toughness and mechanisms of bamboo, which is vital to the utilization of bamboo in the structural applications. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
1. Introduction ⁎ Corresponding authors at: Department of Biomaterials, International Centre for Bamboo and Rattan, Beijing 100102, China. E-mail addresses: [email protected] (X. Liu), [email protected] (B. Fei).
Bamboo is a natural composite material which has been widely used in building houses, bridges and long span roofs among many other
https://doi.org/10.1016/j.matdes.2019.108132 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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engineering applications [1]. Due to its rare combination of both high strength and toughness, bamboo is an ideal and safe structural material for uses in construction applications [1,2]. Nonetheless, when the bamboo culm is subjected to bending or compression, as is common in many raw bamboo constructions, failures tend to occur in longitudinal fiber splitting [3,4] due to weak interlaminar bond strength [5]. Interlaminar fracture toughness therefore plays a critical role in determining mechanical performance of bamboo when cracks appear in the longitudinal direction. There are some studies focused on the interlaminar fracture toughness of bamboo. Shao et al. [6–8] has studied the Mode I, II and III interlaminar fracture toughness and conducted in-depth studies of factors influencing Mode I fractures, which is considered to be the most common and dangerous category of fractures. The results showed that Mode I interlaminar fracture toughness of bamboo with a tangentiallongitudinal (TL) crack, which is the main cracking mode after bamboo drying [9], was independent of density and height, but the bamboo nodes had a remarkable toughening effect [10,11]. Specifically, fracture toughness increased with the wall thickness of bamboo culm [12]. The other crack opening mode is radial- longitudinal (RL) which is commonly seen in bamboo slivers constituents for manufacturing multidimensional bamboo based composites, such as filament wound tubes and train carriages [13]. Mannan et al. [14] found that the Mode I interlaminar fracture toughness of bamboo in the RL configuration decreased from the inner to the outer portion in radial direction. Although the Mode I interlaminar fracture toughness and some effect parameters of bamboo were investigated in the previous studies, toughening mechanisms have not yet been analyzed. Toughening mechanisms which are classified into intrinsic mechanisms and extrinsic mechanisms have been widely used to analyze the toughening behavior of such natural materials as alloy, shell and bone [15]. Intrinsic mechanisms operate ahead of the crack tip and govern the crack-initiation toughness, while extrinsic mechanisms operate in the wake of the crack tip and govern the crack-growth toughness. The extrinsic mechanism is dependent on the crack size and this consequence is crack-resistance curve (R-curve) behavior [15,16]. Extrinsic toughening occurs primarily in brittle materials, while intrinsic toughening mainly takes place in ductile materials. Many natural materials rely on both intrinsic and extrinsic toughness [15]. In bamboo, no rising R-curve was obtained in the previous studies, therefore the toughness of bamboo was considered to be exclusively governed by the intrinsic mechanisms [4,6,10]. However, in wood, a significant rising R-curve was obtained when cracks opened in the TL direction [17,18], and fiber bridging toughening was thought to be the primary extrinsic mechanism [17–21]. Fiber bridging is the main extrinsic mechanism of fiber reinforced composites [22,23]. Bamboo is essentially a natural fiber reinforced composite consisting of unidirectional fiber cells (FCs) as reinforcement and parenchyma cells (PCs) as matrix. However, there was a lack of evidence that fiber bridging occurs in bamboo when subjected to interlaminar fracture loading. As well the role of PCs in resisting fracture growth has not been analyzed either. To further understand the toughening mechanism of FCs and PCs, bamboo strips with different proportions of FCs and PCs were used in this study to test the Mode I interlaminar fracture toughness. Intrinsic and extrinsic mechanisms were discussed at the cellular level. 2. Materials and methods 2.1. Samples preparation Testing specimens were prepared from mature (5 years old) moso bamboo (Phyllostachys edulis) obtained from Zhejiang province, China. They were collected from the internode sections located at a height between 1.5 m and 2.5 m of the bamboo culms (Fig. 1) and were kept in a conditioning chamber at 20 °C and 65% relative humidity until reaching
equilibrium moisture content (about 11%). This moisture content represents a common product service condition and is consistent with literature addressing similar mechanical properties [6,13,24]. The volume fraction of fiber (Vf) increases radially, from the inner to the outer wall [25,26]. To evaluate the effects of different proportions of FCs and PCs on Mode I interlaminar fracture toughness behavior and mechanism, bamboo strips were cut from the stalks and divided into three categories along the radial direction. As shown in Fig. 1(a), the outer part consisting of the highest amount of FCs was defined as region I. As the fiber volume fraction decreases, the area between region I and the inner part was defined as region II, and the most inner part was region III. The Vf of specimens were measured by Image J (National Institutes of Health, USA). The size of the specimens was 2.0 × 5.0 × 40 mm (h × b × W) and five specimens were prepared for each set.
2.2. Mode I interlaminar fracture toughness testing The test for fracture toughness was conducted on a universal testing machine (Instron 5582, Instron, Norwood, USA) fitted with 50 N capacity load, at the test rate of 1 mm/min. The test was carried out according to ASTM D5528, as there was currently no relevant standard for calculating fracture toughness in bamboo or wood composites. Mode I interlaminar fracture toughness (GIC) of bamboo was measured using the double cantilever beam (DCB) method. The dimensions of the specimens are shown in Fig. 1(b). Each specimen has two piano hinges bonded to both sides of the specimen at the end with mixed epoxy and hardener resin glue of 50% as illustrated in Fig. 1(b). One edge of each specimen was painted with a thin layer of white lacquer to increase contrast for visual estimation and measurement of crack length and marked with an ultra-fine point marker at 1 mm intervals to provide a reference when measuring the crack length (as Fig. 1(b)). The initial crack was cleaved by a knife along the middle-line of the specimen parallel to the grain in order to simulate a naturally occurring sharp crack [6]. During the test, the crack propagation, load and displacement values were recorded approximately every 0.1 N of load decrease. The mode I strain energy release rate, GIC, was obtained using the Modified Beam Theory (MBT) in accordance with the ASTM D5528 standard:
GIC ¼
3Pδ 2bða þ jΔjÞ
where P is the applied load (N), δ is the load point displacement (mm), b is the specimen width (mm), a is the crack length (mm), and Δ is the intercept of the plot of the cube root of the specimen compliance against the crack length. The tests were conducted at a temperature of 23 °C and a relative humidity of 65%.
2.3. In situ SEM To observe the path of crack growth at the cellular level, an in situ SEM-based testing machine (Quanta2000, FEI, Netherlands) with Micromechanical machining (Microtest 2000, Deben, England) in the sample housing was used to observe the interlaminar fracture process. The size of the specimens was 1.5 × 5.0 × 20 mm (h × b × W), and a0 = 7 mm. Two small bamboo sticks were bonded respectively to both sides of each specimen with adhesive. One edge of each specimen was painted with a layer of platinum by vacuum evaporation before testing. During the fracture process, the Micro-machine was stopped whenever necessary to take images of the crack growth path. The fracture surface of tested specimens was additionally observed by SEM (JSM-6310F, JEOL, Tokyo, Japan).
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Fig. 1. (a) Schematic diagram of specimens prepared from bamboo. Bamboo block were divided into region I (Vf: 42.2% - 46.2%), II (Vf: 23.9% - 26.9%) and III (Vf: 16.1% - 19.3%). (b) A test image and schematic diagram of the specimen, W = 40 mm (longitudinal direction); b = 5.0 mm (tangential direction); h = 2.0 mm (radial direction); a0 = 7 mm.
3. Results and discussion 3.1. Mode I interlaminar fracture toughness behavior Typical load - displacement curves of three specimens respectively from region I, II and III are displayed in Fig. 2(a). The loading decreased rapidly after it reached the maximum value. Subsequently, with an increase in displacement, the descending rate of loading slowed down. The region I specimen was completely split when the displacement reached 8 mm, while the region II and region III specimens were completely split at 18 mm and 21 mm respectively. The maximum loads of region I and region II specimens were similar, both of which were higher than that of region III specimens. The relationships between fracture toughness (GIC) and crack extension (Δa) of region I, II and III specimens are shown in Fig. 2(b). Positive linear relationships between GIC and Δa were found in all regions. The intercepts, which is related to the initial GIC, increased from region I to III; whereas the slopes, which is corresponding to crack-growth GIC,
increased in the order of region I, region III and region II. Variation of Vf in different regions could be mainly responsible for the various initial GIC and crack-growth GIC. As shown in Fig. 2(c), in the initial state (Δa = 2 mm), GIC had the minimum value when the Vf was the highest. It was 429 J/m2 of region I, which was lower than the 614 J/m2 and 567 J/m2 of region II and III. In the crack growth stage (Δa = 15 mm), GIC firstly increased and then decreased with the Vf. The GIC values of region I, II and III were 573 J/m2, 1048 J/m2 and 738 J/m2, respectively. Therefore, crack-growth GIC of these regions were approximately equal to 144 J/m2, 434 J/m2 and 171 J/m2, respectively. These results indicate that both initiation energy and crack growth energy of region I are the lowest, and the crack growth energy of region II is the highest. 3.2. Intrinsic toughening mechanism in bamboo Intrinsic toughening is the primary fracture toughening mechanism for bamboo. It is also the main reason why the Mode I interlaminar fracture toughness varies in different regions across the culm. One
Fig. 2. (a) Typical load - displacement curves of three specimens from different regions. (b) Relationships between fracture toughness (GIC) and crack extension (Δa) of region I, II and III specimens. (c) GIC as a function of the volume fraction of fiber (Vf) when Δa increased from 2 mm to 15 mm. The vertical error bars correspond to the standard deviation of GIC, while the horizontal ones represent the standard deviation of Vf.
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mechanism for intrinsic toughening is associated with plastic zone size, which is affected by PCs. PCs are thin-walled and short-grain structures with large lumens compared with FCs (Fig. 3(a) and (c)). The cluster of such PCs ahead of a crack tip will make the material more plastic to deform which help absorb the crack energy. The extent of the strained region ahead of the crack tip is also a function of the yield strength [27]. The yield strength of a fiber reinforced composite generally increased with the decrease of fiber content [28,29]. As shown in Fig. 3(b), in the inner portion, PCs are densely distributed while FCs are sparsely distributed, so the plastic zone size of region III is the biggest. It results in the largest crack opening displacement of region III when the crack extensions are the same (as the testing images depicted in Fig. 3(d)). Another mechanism for intrinsic toughening is crack kinking. At the macro scale, the side view of the fracture surface of region I showed a flat path (Fig. 4(a)) while of region III it was a circuitous path (Fig. 4 (b)). At the cellular level, the crack in FCs propagated along the long axis of fibers and did not change direction at the end of the cell (Fig. 4 (c)). By a close up look at the fracture surface from the side view (Fig. 4(e)), the crack propagation was found often through the middles lamella of FCs. A single FC showed a needle-like shape and the dimension of both ends tapered off (Fig. 4(g)). Thus, when the crack propagated along the boundary of fiber, the path was flat and no obviously direction change. However, in PCs (Fig. 4(d)), crack deflected at the end of the cell and even turned back after encountering FCs because of the hardness of the latter. The crack propagation was also found through the middles lamella of PCs (Fig. 4(f)), and a single PC was short and drum-shaped (Fig. 4(g)). Consequently, when the crack propagated along the boundary of PC, it deflected at the end of the cell and showed a circuitous path. More energy is dissipated if the crack growth path is circuitous. Therefore, intrinsic toughening of bamboo is primarily associated with the plastic zone size and crack kinking which are both governed by PCs. The region with the lowest content of PCs is region I, and the crack initiation energy is consequently the lowest. 3.3. Extrinsic toughening mechanism in bamboo The GIC of all regions increased monotonically with the crack extension (Fig. 2(b)). These increasing curves are rising R-curves. A rising Rcurve indicates the existence of extrinsic mechanism, such as the uncracked ligament bridging in bone [16] and the fiber bridging in wood [17]. As the FCs of bamboo are arranged in parallel to the central axis,
fiber bridging is the primary extrinsic mechanism of toughening for cracking in the longitudinal direction and is responsible for the rising R-curve behavior. As depicted in Fig. 5(a), when the crack tip grew along the fiber, fiber bridging would bond both two fracture surfaces without pulling out or breaking. The FCs in bamboo are long, narrow and needle-like (Fig. 4 (g)), thus it is easy to form fiber bridging. Fig. 5(c) depicts fiber bridging of the FC during crack growth. When the crack tip reached the FCs, the latter bonded the upper and lower fracture surface simultaneously. As the crack grew further, the fiber was detached and the bridging effect disappeared. In certain cases, the fiber would continue to bond both fracture surfaces even after the sample were completely split (Fig. 5(b)). The region II and region III demonstrated faster rising R-curves than region I (Fig. 2(b)), which indicate that the low and middle fiber density regions has strong fiber bridging effect than the high fiber density region. The effectiveness of bridging is dictated by the size, the area fraction, and load-bearing capacity of the bridging [16]. The region I has the highest content of FCs and therefore the highest area fraction of the bridges. However, the size of the bridging in region I (Fig. 6(a)) was much shorter than in region II (Fig. 6(b)). This might be accounted for by the smaller opening displacement of region I (Fig. 3(d)). Besides, the structures of vascular bundles in region I and region III are different [30,31]. It is a gradient change from the semi-open structure in region I to the open structure in region III. With the bigger opening, it was more likely to observe fiber bundle bridging (Fig. 6(c)) in region II and III. Theoretically speaking, fiber bundles have higher load-bearing capacity than single fiber. Therefore, the larger size and higher load-bearing capacity of bridging led to the faster rising R-curves in region II and III. Compared with region II, region III has fewer FCs which results in the weaker fiber bridging effect. 3.4. Primary toughening mechanism in bamboo Previous research found no example of fiber bridging in bamboo for cracking in the longitudinal direction [6,10,14] and assumed that the GIC was primarily governed by the PCs through the intrinsic toughening mechanism [14]. In this study, however, the fiber bridging effect was detected through the rising R-curve, which indicates that the GIC of bamboo is also governed by the extrinsic toughening mechanism. Such discrepancy may be caused by the disparity in the specimen thickness. The thickness used in this study is 2 mm which is small compared to
Fig. 3. SEM images of (a) FCs and (c) PCs. (b) Optical micrograph of bamboo showing that PCs are densely distributed in outer side and sparsely distributed in inner side. (d) The respective testing images of the three kinds of specimens.
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Fig. 4. The crack paths in region I (a, c and e) and III (b, d and f) from the macro to micro scales and the optical micrograph images of a single FC and PC (g). (a) and (b), optical micrograph of crack path; (c) and (d), a side view of In situ SEM image of crack growth path; (e) and (f), a close up look at the fracture surface from the side view.
previous studies where specimen thickness is 10–20 mm [6,14]. The mechanical properties of bamboo depended on its wall thickness [32,33]. The effect of fiber bridging in bamboo was not noticeable probably because of weak interlaminar bond strength [5]. If the thickness of the specimen is high, the fiber bridging effect would be overshadowed;
only when the sample is thin enough can the fiber bridging effect function observably. Another factor concerning the primary toughening mechanism in bamboo is crack direction. In this study, the direction of the crack was radial-longitudinal (RL), and there was a fast rising R-curve for region II. Shao et al. [6,10] have studied fracture toughness of bamboo in case
Fig. 5. (a) Schematic diagrams of fiber bridging mechanism. (b) Image of a split specimen with the fiber still connecting both fracture surfaces. (c) In situ SEM images of the entire process of fiber bridging during crack growth.
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Fig. 6. Schematic images, SEMs of side view and SEMs of fracture surfaces of (a) fiber bridging phenomenon in region I specimen; (b) fiber bridging phenomenon in region II specimen; (c) fiber bundle bridging phenomenon in region II specimen. Scales in all SEM images are 100 μm.
of tangential-longitudinal (TL) crack and found that the R-curve of bamboo was a flat R-curve (as shown in Fig. 7). The crack directional effect has also been found in wood [17,18], but in reverse order. Fracture toughness of wood has a flat R-curve when the crack direction was RL, while a rising R-curve when the direction was TL [17] (as shown in Fig. 7).
The crack directional effect may be related to the distribution of the fiber bridging zone. Fiber bridging in wood is caused by non-fractured latewood zone distributed in concentric rings (as shown in Fig. 7). A TL crack propagates through a path with the uniform distribution of early wood and late wood, whereas a RL crack grows through a path mostly with early wood [17]. Similarly, fiber bridging in bamboo is
Fig. 7. Fracture toughness of bamboo and wood with RL and TL crack.
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caused by FCs which are densely distributed in the outer side and sparsely in the inner side at scattered points (as shown in Fig. 7). Therefore, a RL crack in the region II propagates through a path with the uniform distribution of the FCs and PCs, while a TL crack grows through a path with the non-uniform distribution of FCs. 4. Conclusions In this paper, the effect of FCs and PCs of bamboo on Mode I interlaminar fracture toughness was investigated. Intrinsic and extrinsic mechanisms were discussed at the cellular level. The results indicated that both initiation and crack growth energies of outer portion (region I) were the lowest, and crack growth energy of middle portion (region II) was the highest. The intrinsic toughening mechanism of bamboo is associated with the plastic zone size and crack kinking which are governed by the content of PC. The region with the lowest content of PC is in region I, hence its crack initiation energy is the lowest. The extrinsic toughening mechanism in bamboo is related to fiber bridging which is affected by the fiber density and structure of vascular bundles. Generally speaking, the intrinsic toughening is the dominant toughening mechanism for bamboo when subjected to Mode I interlaminar fracturing. However, when the bamboo strips are thin and subjected to a radial- longitudinal cracking, both intrinsic mechanisms and extrinsic mechanisms play important roles in governing the material toughness. CRediT authorship contribution statement Qi Chen: Data curation, Formal analysis, Investigation, Writing original draft. Chunping Dai: Methodology, Writing - review & editing. Changhua Fang: Writing - review & editing. Meiling Chen: Supervision. Shuqin Zhang: Resources. Rong Liu: Resources. Xianmiao Liu: Conceptualization. Benhua Fei: Conceptualization, Funding acquisition, Methodology. Acknowledgment We would like to thank the 13th Five-Year the National Key Research and Development of China projects (2016YFD0600906) and the Fundamental Research Funds for the International Centre for Bamboo and Rattan (1632017001) for their financial support for this research. References [1] C. Fang, Z. Jiang, Z. Sun, H. Liu, X. Zhang, R. Zhang, B. Fei, An overview on bamboo culm flattening, Constr. Build. Mater. 171 (2018) 65–74. [2] K.F. Chung, W.K. Yu, Mechanical properties of structural bamboo for bamboo scaffoldings, Eng. Struct. 24 (4) (2002) 429–442. [3] J.J. Janssen, Designing and Building With Bamboo, International Network for Bamboo and Rattan Netherlands, 2000. [4] Z. Shao, F. Wang, Fracture Mechanics of Plant Materials: Wood and Bamboo, Springer, 2019. [5] M.K. Habibi, Y. Lu, Crack propagation in bamboo's hierarchical cellular structure, Sci. Rep. 4 (4) (2014) 5598. [6] Z. Shao, C. Fang, G. Tian, Mode I interlaminar fracture property of moso bamboo (Phyllostachys pubescens), Wood Sci. Technol. 43 (5–6) (2009) 527–536.
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Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Mode I interlaminar fracture toughness behavior and mechanisms of bamboo Qi Chen a,b,c, Chunping Dai c, Changhua Fang a,b, Meiling Chen a,b,c, Shuqin Zhang a,b, Rong Liu a,b, Xianmiao Liu a,b,⁎, Benhua Fei a,b,⁎ a b c
Department of Biomaterials, International Centre for Bamboo and Rattan, Beijing 100102, China SFA and Beijing Co-built Key Laboratory of Bamboo and Rattan Science and Technology, State Forestry Administration, Beijing 100102, China The University of British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Initiation energy and crack-growth energy of bamboo are lowest in the region with highest fiber content. • The highest growth-energy occurs in the middle region with a balanced content of fiber and parenchyma cells. • Initiation energy is related to the intrinsic toughening mechanism through the plastic zoning and crack kinking. • Crack-growth energy is associated with the extrinsic toughening mechanism via fiber bridging.
a r t i c l e
i n f o
Article history: Received 24 April 2019 Received in revised form 15 July 2019 Accepted 17 August 2019 Available online 18 August 2019 Keywords: Bamboo Fracture toughness Intrinsic mechanisms Extrinsic mechanisms Fiber bridging
a b s t r a c t Bamboo, with its fast growth rate and outstanding mechanical properties, has received increasing attention as a green material for engineering applications. Compared to other mechanical properties, little is known about fracture toughness, especially the toughening mechanism of bamboo. In this study, the Mode I interlaminar fracture toughness of bamboo with different proportions of fiber cells (FCs) and parenchyma cells (PCs) was tested, using in situ SEM to investigate the intrinsic mechanisms and extrinsic mechanisms at the cellular level. The results showed that both initiation and crack growth energies of high fiber density region were the lowest, and the crack growth energy of middle fiber density region was the highest. The intrinsic toughening in bamboo is associated with plastic zone size and crack kinking which are governed by PCs. The extrinsic toughening is related to fiber bridging which is governed by FCs. Having the highest fracture toughness, the middle region of bamboo may be the best choice of natural fiber for manufacturing high performance green composite materials. This work fills the gaps in the knowledge of fracture toughness and mechanisms of bamboo, which is vital to the utilization of bamboo in the structural applications. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
1. Introduction ⁎ Corresponding authors at: Department of Biomaterials, International Centre for Bamboo and Rattan, Beijing 100102, China. E-mail addresses: [email protected] (X. Liu), [email protected] (B. Fei).
Bamboo is a natural composite material which has been widely used in building houses, bridges and long span roofs among many other
https://doi.org/10.1016/j.matdes.2019.108132 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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engineering applications [1]. Due to its rare combination of both high strength and toughness, bamboo is an ideal and safe structural material for uses in construction applications [1,2]. Nonetheless, when the bamboo culm is subjected to bending or compression, as is common in many raw bamboo constructions, failures tend to occur in longitudinal fiber splitting [3,4] due to weak interlaminar bond strength [5]. Interlaminar fracture toughness therefore plays a critical role in determining mechanical performance of bamboo when cracks appear in the longitudinal direction. There are some studies focused on the interlaminar fracture toughness of bamboo. Shao et al. [6–8] has studied the Mode I, II and III interlaminar fracture toughness and conducted in-depth studies of factors influencing Mode I fractures, which is considered to be the most common and dangerous category of fractures. The results showed that Mode I interlaminar fracture toughness of bamboo with a tangentiallongitudinal (TL) crack, which is the main cracking mode after bamboo drying [9], was independent of density and height, but the bamboo nodes had a remarkable toughening effect [10,11]. Specifically, fracture toughness increased with the wall thickness of bamboo culm [12]. The other crack opening mode is radial- longitudinal (RL) which is commonly seen in bamboo slivers constituents for manufacturing multidimensional bamboo based composites, such as filament wound tubes and train carriages [13]. Mannan et al. [14] found that the Mode I interlaminar fracture toughness of bamboo in the RL configuration decreased from the inner to the outer portion in radial direction. Although the Mode I interlaminar fracture toughness and some effect parameters of bamboo were investigated in the previous studies, toughening mechanisms have not yet been analyzed. Toughening mechanisms which are classified into intrinsic mechanisms and extrinsic mechanisms have been widely used to analyze the toughening behavior of such natural materials as alloy, shell and bone [15]. Intrinsic mechanisms operate ahead of the crack tip and govern the crack-initiation toughness, while extrinsic mechanisms operate in the wake of the crack tip and govern the crack-growth toughness. The extrinsic mechanism is dependent on the crack size and this consequence is crack-resistance curve (R-curve) behavior [15,16]. Extrinsic toughening occurs primarily in brittle materials, while intrinsic toughening mainly takes place in ductile materials. Many natural materials rely on both intrinsic and extrinsic toughness [15]. In bamboo, no rising R-curve was obtained in the previous studies, therefore the toughness of bamboo was considered to be exclusively governed by the intrinsic mechanisms [4,6,10]. However, in wood, a significant rising R-curve was obtained when cracks opened in the TL direction [17,18], and fiber bridging toughening was thought to be the primary extrinsic mechanism [17–21]. Fiber bridging is the main extrinsic mechanism of fiber reinforced composites [22,23]. Bamboo is essentially a natural fiber reinforced composite consisting of unidirectional fiber cells (FCs) as reinforcement and parenchyma cells (PCs) as matrix. However, there was a lack of evidence that fiber bridging occurs in bamboo when subjected to interlaminar fracture loading. As well the role of PCs in resisting fracture growth has not been analyzed either. To further understand the toughening mechanism of FCs and PCs, bamboo strips with different proportions of FCs and PCs were used in this study to test the Mode I interlaminar fracture toughness. Intrinsic and extrinsic mechanisms were discussed at the cellular level. 2. Materials and methods 2.1. Samples preparation Testing specimens were prepared from mature (5 years old) moso bamboo (Phyllostachys edulis) obtained from Zhejiang province, China. They were collected from the internode sections located at a height between 1.5 m and 2.5 m of the bamboo culms (Fig. 1) and were kept in a conditioning chamber at 20 °C and 65% relative humidity until reaching
equilibrium moisture content (about 11%). This moisture content represents a common product service condition and is consistent with literature addressing similar mechanical properties [6,13,24]. The volume fraction of fiber (Vf) increases radially, from the inner to the outer wall [25,26]. To evaluate the effects of different proportions of FCs and PCs on Mode I interlaminar fracture toughness behavior and mechanism, bamboo strips were cut from the stalks and divided into three categories along the radial direction. As shown in Fig. 1(a), the outer part consisting of the highest amount of FCs was defined as region I. As the fiber volume fraction decreases, the area between region I and the inner part was defined as region II, and the most inner part was region III. The Vf of specimens were measured by Image J (National Institutes of Health, USA). The size of the specimens was 2.0 × 5.0 × 40 mm (h × b × W) and five specimens were prepared for each set.
2.2. Mode I interlaminar fracture toughness testing The test for fracture toughness was conducted on a universal testing machine (Instron 5582, Instron, Norwood, USA) fitted with 50 N capacity load, at the test rate of 1 mm/min. The test was carried out according to ASTM D5528, as there was currently no relevant standard for calculating fracture toughness in bamboo or wood composites. Mode I interlaminar fracture toughness (GIC) of bamboo was measured using the double cantilever beam (DCB) method. The dimensions of the specimens are shown in Fig. 1(b). Each specimen has two piano hinges bonded to both sides of the specimen at the end with mixed epoxy and hardener resin glue of 50% as illustrated in Fig. 1(b). One edge of each specimen was painted with a thin layer of white lacquer to increase contrast for visual estimation and measurement of crack length and marked with an ultra-fine point marker at 1 mm intervals to provide a reference when measuring the crack length (as Fig. 1(b)). The initial crack was cleaved by a knife along the middle-line of the specimen parallel to the grain in order to simulate a naturally occurring sharp crack [6]. During the test, the crack propagation, load and displacement values were recorded approximately every 0.1 N of load decrease. The mode I strain energy release rate, GIC, was obtained using the Modified Beam Theory (MBT) in accordance with the ASTM D5528 standard:
GIC ¼
3Pδ 2bða þ jΔjÞ
where P is the applied load (N), δ is the load point displacement (mm), b is the specimen width (mm), a is the crack length (mm), and Δ is the intercept of the plot of the cube root of the specimen compliance against the crack length. The tests were conducted at a temperature of 23 °C and a relative humidity of 65%.
2.3. In situ SEM To observe the path of crack growth at the cellular level, an in situ SEM-based testing machine (Quanta2000, FEI, Netherlands) with Micromechanical machining (Microtest 2000, Deben, England) in the sample housing was used to observe the interlaminar fracture process. The size of the specimens was 1.5 × 5.0 × 20 mm (h × b × W), and a0 = 7 mm. Two small bamboo sticks were bonded respectively to both sides of each specimen with adhesive. One edge of each specimen was painted with a layer of platinum by vacuum evaporation before testing. During the fracture process, the Micro-machine was stopped whenever necessary to take images of the crack growth path. The fracture surface of tested specimens was additionally observed by SEM (JSM-6310F, JEOL, Tokyo, Japan).
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Fig. 1. (a) Schematic diagram of specimens prepared from bamboo. Bamboo block were divided into region I (Vf: 42.2% - 46.2%), II (Vf: 23.9% - 26.9%) and III (Vf: 16.1% - 19.3%). (b) A test image and schematic diagram of the specimen, W = 40 mm (longitudinal direction); b = 5.0 mm (tangential direction); h = 2.0 mm (radial direction); a0 = 7 mm.
3. Results and discussion 3.1. Mode I interlaminar fracture toughness behavior Typical load - displacement curves of three specimens respectively from region I, II and III are displayed in Fig. 2(a). The loading decreased rapidly after it reached the maximum value. Subsequently, with an increase in displacement, the descending rate of loading slowed down. The region I specimen was completely split when the displacement reached 8 mm, while the region II and region III specimens were completely split at 18 mm and 21 mm respectively. The maximum loads of region I and region II specimens were similar, both of which were higher than that of region III specimens. The relationships between fracture toughness (GIC) and crack extension (Δa) of region I, II and III specimens are shown in Fig. 2(b). Positive linear relationships between GIC and Δa were found in all regions. The intercepts, which is related to the initial GIC, increased from region I to III; whereas the slopes, which is corresponding to crack-growth GIC,
increased in the order of region I, region III and region II. Variation of Vf in different regions could be mainly responsible for the various initial GIC and crack-growth GIC. As shown in Fig. 2(c), in the initial state (Δa = 2 mm), GIC had the minimum value when the Vf was the highest. It was 429 J/m2 of region I, which was lower than the 614 J/m2 and 567 J/m2 of region II and III. In the crack growth stage (Δa = 15 mm), GIC firstly increased and then decreased with the Vf. The GIC values of region I, II and III were 573 J/m2, 1048 J/m2 and 738 J/m2, respectively. Therefore, crack-growth GIC of these regions were approximately equal to 144 J/m2, 434 J/m2 and 171 J/m2, respectively. These results indicate that both initiation energy and crack growth energy of region I are the lowest, and the crack growth energy of region II is the highest. 3.2. Intrinsic toughening mechanism in bamboo Intrinsic toughening is the primary fracture toughening mechanism for bamboo. It is also the main reason why the Mode I interlaminar fracture toughness varies in different regions across the culm. One
Fig. 2. (a) Typical load - displacement curves of three specimens from different regions. (b) Relationships between fracture toughness (GIC) and crack extension (Δa) of region I, II and III specimens. (c) GIC as a function of the volume fraction of fiber (Vf) when Δa increased from 2 mm to 15 mm. The vertical error bars correspond to the standard deviation of GIC, while the horizontal ones represent the standard deviation of Vf.
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mechanism for intrinsic toughening is associated with plastic zone size, which is affected by PCs. PCs are thin-walled and short-grain structures with large lumens compared with FCs (Fig. 3(a) and (c)). The cluster of such PCs ahead of a crack tip will make the material more plastic to deform which help absorb the crack energy. The extent of the strained region ahead of the crack tip is also a function of the yield strength [27]. The yield strength of a fiber reinforced composite generally increased with the decrease of fiber content [28,29]. As shown in Fig. 3(b), in the inner portion, PCs are densely distributed while FCs are sparsely distributed, so the plastic zone size of region III is the biggest. It results in the largest crack opening displacement of region III when the crack extensions are the same (as the testing images depicted in Fig. 3(d)). Another mechanism for intrinsic toughening is crack kinking. At the macro scale, the side view of the fracture surface of region I showed a flat path (Fig. 4(a)) while of region III it was a circuitous path (Fig. 4 (b)). At the cellular level, the crack in FCs propagated along the long axis of fibers and did not change direction at the end of the cell (Fig. 4 (c)). By a close up look at the fracture surface from the side view (Fig. 4(e)), the crack propagation was found often through the middles lamella of FCs. A single FC showed a needle-like shape and the dimension of both ends tapered off (Fig. 4(g)). Thus, when the crack propagated along the boundary of fiber, the path was flat and no obviously direction change. However, in PCs (Fig. 4(d)), crack deflected at the end of the cell and even turned back after encountering FCs because of the hardness of the latter. The crack propagation was also found through the middles lamella of PCs (Fig. 4(f)), and a single PC was short and drum-shaped (Fig. 4(g)). Consequently, when the crack propagated along the boundary of PC, it deflected at the end of the cell and showed a circuitous path. More energy is dissipated if the crack growth path is circuitous. Therefore, intrinsic toughening of bamboo is primarily associated with the plastic zone size and crack kinking which are both governed by PCs. The region with the lowest content of PCs is region I, and the crack initiation energy is consequently the lowest. 3.3. Extrinsic toughening mechanism in bamboo The GIC of all regions increased monotonically with the crack extension (Fig. 2(b)). These increasing curves are rising R-curves. A rising Rcurve indicates the existence of extrinsic mechanism, such as the uncracked ligament bridging in bone [16] and the fiber bridging in wood [17]. As the FCs of bamboo are arranged in parallel to the central axis,
fiber bridging is the primary extrinsic mechanism of toughening for cracking in the longitudinal direction and is responsible for the rising R-curve behavior. As depicted in Fig. 5(a), when the crack tip grew along the fiber, fiber bridging would bond both two fracture surfaces without pulling out or breaking. The FCs in bamboo are long, narrow and needle-like (Fig. 4 (g)), thus it is easy to form fiber bridging. Fig. 5(c) depicts fiber bridging of the FC during crack growth. When the crack tip reached the FCs, the latter bonded the upper and lower fracture surface simultaneously. As the crack grew further, the fiber was detached and the bridging effect disappeared. In certain cases, the fiber would continue to bond both fracture surfaces even after the sample were completely split (Fig. 5(b)). The region II and region III demonstrated faster rising R-curves than region I (Fig. 2(b)), which indicate that the low and middle fiber density regions has strong fiber bridging effect than the high fiber density region. The effectiveness of bridging is dictated by the size, the area fraction, and load-bearing capacity of the bridging [16]. The region I has the highest content of FCs and therefore the highest area fraction of the bridges. However, the size of the bridging in region I (Fig. 6(a)) was much shorter than in region II (Fig. 6(b)). This might be accounted for by the smaller opening displacement of region I (Fig. 3(d)). Besides, the structures of vascular bundles in region I and region III are different [30,31]. It is a gradient change from the semi-open structure in region I to the open structure in region III. With the bigger opening, it was more likely to observe fiber bundle bridging (Fig. 6(c)) in region II and III. Theoretically speaking, fiber bundles have higher load-bearing capacity than single fiber. Therefore, the larger size and higher load-bearing capacity of bridging led to the faster rising R-curves in region II and III. Compared with region II, region III has fewer FCs which results in the weaker fiber bridging effect. 3.4. Primary toughening mechanism in bamboo Previous research found no example of fiber bridging in bamboo for cracking in the longitudinal direction [6,10,14] and assumed that the GIC was primarily governed by the PCs through the intrinsic toughening mechanism [14]. In this study, however, the fiber bridging effect was detected through the rising R-curve, which indicates that the GIC of bamboo is also governed by the extrinsic toughening mechanism. Such discrepancy may be caused by the disparity in the specimen thickness. The thickness used in this study is 2 mm which is small compared to
Fig. 3. SEM images of (a) FCs and (c) PCs. (b) Optical micrograph of bamboo showing that PCs are densely distributed in outer side and sparsely distributed in inner side. (d) The respective testing images of the three kinds of specimens.
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Fig. 4. The crack paths in region I (a, c and e) and III (b, d and f) from the macro to micro scales and the optical micrograph images of a single FC and PC (g). (a) and (b), optical micrograph of crack path; (c) and (d), a side view of In situ SEM image of crack growth path; (e) and (f), a close up look at the fracture surface from the side view.
previous studies where specimen thickness is 10–20 mm [6,14]. The mechanical properties of bamboo depended on its wall thickness [32,33]. The effect of fiber bridging in bamboo was not noticeable probably because of weak interlaminar bond strength [5]. If the thickness of the specimen is high, the fiber bridging effect would be overshadowed;
only when the sample is thin enough can the fiber bridging effect function observably. Another factor concerning the primary toughening mechanism in bamboo is crack direction. In this study, the direction of the crack was radial-longitudinal (RL), and there was a fast rising R-curve for region II. Shao et al. [6,10] have studied fracture toughness of bamboo in case
Fig. 5. (a) Schematic diagrams of fiber bridging mechanism. (b) Image of a split specimen with the fiber still connecting both fracture surfaces. (c) In situ SEM images of the entire process of fiber bridging during crack growth.
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Fig. 6. Schematic images, SEMs of side view and SEMs of fracture surfaces of (a) fiber bridging phenomenon in region I specimen; (b) fiber bridging phenomenon in region II specimen; (c) fiber bundle bridging phenomenon in region II specimen. Scales in all SEM images are 100 μm.
of tangential-longitudinal (TL) crack and found that the R-curve of bamboo was a flat R-curve (as shown in Fig. 7). The crack directional effect has also been found in wood [17,18], but in reverse order. Fracture toughness of wood has a flat R-curve when the crack direction was RL, while a rising R-curve when the direction was TL [17] (as shown in Fig. 7).
The crack directional effect may be related to the distribution of the fiber bridging zone. Fiber bridging in wood is caused by non-fractured latewood zone distributed in concentric rings (as shown in Fig. 7). A TL crack propagates through a path with the uniform distribution of early wood and late wood, whereas a RL crack grows through a path mostly with early wood [17]. Similarly, fiber bridging in bamboo is
Fig. 7. Fracture toughness of bamboo and wood with RL and TL crack.
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caused by FCs which are densely distributed in the outer side and sparsely in the inner side at scattered points (as shown in Fig. 7). Therefore, a RL crack in the region II propagates through a path with the uniform distribution of the FCs and PCs, while a TL crack grows through a path with the non-uniform distribution of FCs. 4. Conclusions In this paper, the effect of FCs and PCs of bamboo on Mode I interlaminar fracture toughness was investigated. Intrinsic and extrinsic mechanisms were discussed at the cellular level. The results indicated that both initiation and crack growth energies of outer portion (region I) were the lowest, and crack growth energy of middle portion (region II) was the highest. The intrinsic toughening mechanism of bamboo is associated with the plastic zone size and crack kinking which are governed by the content of PC. The region with the lowest content of PC is in region I, hence its crack initiation energy is the lowest. The extrinsic toughening mechanism in bamboo is related to fiber bridging which is affected by the fiber density and structure of vascular bundles. Generally speaking, the intrinsic toughening is the dominant toughening mechanism for bamboo when subjected to Mode I interlaminar fracturing. However, when the bamboo strips are thin and subjected to a radial- longitudinal cracking, both intrinsic mechanisms and extrinsic mechanisms play important roles in governing the material toughness. CRediT authorship contribution statement Qi Chen: Data curation, Formal analysis, Investigation, Writing original draft. Chunping Dai: Methodology, Writing - review & editing. Changhua Fang: Writing - review & editing. Meiling Chen: Supervision. Shuqin Zhang: Resources. Rong Liu: Resources. Xianmiao Liu: Conceptualization. Benhua Fei: Conceptualization, Funding acquisition, Methodology. Acknowledgment We would like to thank the 13th Five-Year the National Key Research and Development of China projects (2016YFD0600906) and the Fundamental Research Funds for the International Centre for Bamboo and Rattan (1632017001) for their financial support for this research. References [1] C. Fang, Z. Jiang, Z. Sun, H. Liu, X. Zhang, R. Zhang, B. Fei, An overview on bamboo culm flattening, Constr. Build. Mater. 171 (2018) 65–74. [2] K.F. Chung, W.K. Yu, Mechanical properties of structural bamboo for bamboo scaffoldings, Eng. Struct. 24 (4) (2002) 429–442. [3] J.J. Janssen, Designing and Building With Bamboo, International Network for Bamboo and Rattan Netherlands, 2000. [4] Z. Shao, F. Wang, Fracture Mechanics of Plant Materials: Wood and Bamboo, Springer, 2019. [5] M.K. Habibi, Y. Lu, Crack propagation in bamboo's hierarchical cellular structure, Sci. Rep. 4 (4) (2014) 5598. [6] Z. Shao, C. Fang, G. Tian, Mode I interlaminar fracture property of moso bamboo (Phyllostachys pubescens), Wood Sci. Technol. 43 (5–6) (2009) 527–536.
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