Compressive and flexural properties of biomimetic integrated

Materials and Design 64 (2014) 214–220

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Materials and Design journal homepage: www.elsevier.com/locate/matdes

Compressive and flexural properties of biomimetic integrated honeycomb plates Jinxiang Chen a,⇑, Chenglin He a,⇑, Chenglong Gu a, Jianxun Liu a, Changwen Mi a, Shijie Guo b a b

School of Civil Engineering & International Institute for Urban Systems Engineering, Southeast University, Nanjing 210096, China RIKEN-TRI Collaboration Center, RIKEN, Nagoya 463-0003, Japan

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 19 April 2014 Accepted 11 July 2014 Available online 30 July 2014

The characteristics of honeycomb plates composed of an upper and lower lamination are employed to create a novel single-sided bonded honeycomb plate (SBHP) design, and the compressive and flexural properties of these biomimetic integrated honeycomb plates are investigated. The results demonstrate that even during the fracturing of the honeycomb plates (honeycomb core), no abrupt compression paralysis occurs (which would cause the load to decrease rapidly); furthermore, our honeycomb plates exhibit superior compressive properties compared to biomimetic sandwich plates manufactured using Zhang’s needle-injection method. The interfacial bonding surface and bonding quality have no significant effect on the flexural stiffness but do affect the failure modes and flexural failure strength of the honeycomb plates. The ultimate failure of the biomimetic integrated honeycomb without a bonding layer between the panel–core layers is determined by the material strength itself; therefore, the honeycomb possesses good mechanical properties. This experimental study confirms, for the first time, the effectiveness of the biomimetic integrated honeycomb structure manufacturing method. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Biomimetics Fibers Structural composites Mechanical properties Sandwich structures

1. Introduction Honeycomb structures are a common form of lightweight, high-strength composite [1–3]. Studies and applications related to honeycomb structures are numerous [4–8], ranging from nanomaterials [9,10] to large structures such as those used in Boeing 787 and Airbus 380 airplanes [11]. Many studies have been conducted on the development of honeycomb structures and on the mechanical properties of honeycomb structures, yielding positive results using both experimental and theoretical methods [12–19]. Commercially available honeycomb sandwich plates are currently manufactured by adhesively joining the plate and core components, which are produced separately using different processes [20,21]. However, in sandwich plates produced using the aforementioned process, the side plates and the core are easily separated, and this separation is a factor that limits both strength and side sealing. Furthermore, the use of adhesive is not only environmentally harmful but also expensive [22,23]. Learning from living creatures is a productive approach to the development of lightweight composite materials [24,25]. Motivated by the fact that the forewings of beetles are of high strength ⇑ Corresponding authors. Tel.: +86 25 83793831. E-mail addresses: [email protected], [email protected] (C. He).

[email protected]

http://dx.doi.org/10.1016/j.matdes.2014.07.021 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.

(J.

Chen),

and minimal weight, characteristics that are required for defense and flight, respectively, we have been studying their architectures since 1997 [26]. We have also investigated the three-dimensional (3D) structures and mechanical properties of these forewings [27,28]. These investigations have led to the discovery of a new type of lightweight biomimetic composite that consists of a completely integrated honeycomb structure with fiber-reinforced trabeculae at the corners of the honeycomb cores. Presently, commercial honeycomb plates are manufactured by adhesively joining the plate and core components, which are fabricated separately using different processes [29]. The side plates and core of these sandwich plates are easily separated, limiting both the strength and side sealing of such structures. We developed a manufacturing method that integrates the fabrication and joining of the plate and core components, and this integrated manufacturing method overcomes the weaknesses of the traditional manufacturing method [30,31]. However, the mechanical properties of integrated honeycomb plates have primarily been discussed only from a theoretical perspective based on biomechanical laboratory findings and finite element analysis from the literature [30]. In this study, the characteristic features of honeycomb plates with upper and lower lamination (Fig. 1) are used to design single-sided bonded honeycomb plates (SBHPs, Fig. 1(b)). The lower lamination (Fig. 1(b), left) and core lamination (Fig. 1(b), right) are assembled into an integrated whole, whereas the upper lamination and core

J. Chen et al. / Materials and Design 64 (2014) 214–220

(a)

(b)

215

(c)

Fig. 1. Schematics of the manufacturing methods for honeycomb sandwich plates, which are manufactured by adhesively joining: (a) three parts or (b) two parts (SBHP), as well as using (c) integrated honeycomb technology.

Fig. 2. Structural schematic of a trabecula–honeycomb: (a) integrated honeycomb plate and (b) schematic illustration of the reinforcing fibers in the cross section of a forewing. The thin and thick arrows indicate the honeycomb wall and the trabeculae, respectively.

lamination (Fig. 1(b), right) are formed using an integrated molding [30]. The mechanical properties of the integrated honeycomb plates (Fig. 1(c)) and those of commercial honeycomb plates (Fig. 1(a)) are compared and analyzed using SBHP tests. The compressive and flexural properties of the biomimetic integrated plates are discussed, and an effective method for determining the mechanical properties of biomimetic integrated honeycomb plates is provided. 2. Experimental details 2.1. Background Approximately 10 years ago, we sought to create a 3D structural model of biomimetic integrated honeycomb plates with a honeycomb–trabecular structure based on the forewing structure of the beetle Allomyrina dichotoma (Fig. 2(a)) [32,33]. This research revealed that the fibers in the small trabeculae and in the upper and lower laminations formed a continuous structure (Fig. 2(b)) [27]. Due to the complicated biological structure of A. dichotoma, establishing a method to imitate this structure was a formidable task. Two distinct integrated molding methods, which are illustrated in Fig. 3, had been previously reported [32,34,35]; however, these methods only include the sandwich plates with the trabeculae, not the honeycomb structure. After years of effort, we recently developed a set of biomimetic integrated molding tools that include both the trabeculae and honeycombs. Furthermore, we have successfully developed a prototype of the integrated honeycomb product. The matrix material is epoxy resin (ER), and the reinforcing material is basalt fiber (BF, black arrows in Fig. 4). The shapes of the honeycomb and trabeculae can be clearly observed in Fig. 4: short fibers are distributed throughout the entire specimen, and long fibers are distributed in the upper and lower laminations. However, there is still a significant difference between the biomimetic specimen and the beetle forewing structures, including the former’s low BF ratio and uneven fiber distribution. In addition, the mechanical properties of the biomimetic specimen were initially only discussed from a theoretical perspective by comparison with structures reported in the literature [26–31,36]; that is, the properties

were not confirmed experimentally. Therefore, this study focuses on experimentally determining the mechanical properties of biomimetic integrated honeycomb plates. 2.2. Experiment To save time and improve efficiency, an experimental scheme that features both the assembled and integrated molding in one honeycomb plate was designed, combining the characteristics of the honeycomb sandwich structure with two plates. The lower plate and core are an integrated molding (referred to as the partially integrated honeycomb plate), whereas the upper plate and core are bonded together (forming the SBHP). The mechanical properties of the materials produced using the two different preparation methods could then be compared through mechanical property tests. 2.2.1. Preparation of SBHPs Fig. 5 illustrates the molds used to prepare the SBHPs. Fig. 6 presents images of the SBHP samples with trabeculae. The samples in Fig. 6(c) were produced by adhesively joining the two parts shown in Fig. 4(a) and (b). The mold is primarily composed of two parts: the partially integrated mold (Fig. 4(a)) and the mold of the upper plate (Fig. 4(b)). A round thimble apparatus was designed to facilitate demolding (Fig. 4(a), red1 coils). In accordance with the GB/T 1456-2005 standard [37], the height of one side of the adhesively integrated honeycomb plate (Fig. 6(a)) and that of the upper plate (Fig. 6(b)) were designed to be 8.7 and 2.3 mm, respectively; the total height was 11 mm (Fig. 6(c)). The thickness of the cellular wall was 2.0 mm, and the trabecular diameter was 6.9 mm. On average, each cell contained two thirds of a trabecula (Fig. 6(a)). The experimental materials and preparation of the SBHPs are as follows: (1) Reinforced fiber: basalt fiber. (2) Preparation of adhesive solution: thoroughly mix the epoxy resin, curing agent, and diluent at a ratio of 10:3:1 [38,39]. 1 For interpretation of color in Figs. 4 and 8, the reader is referred to the web version of this article.

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Fig. 3. Biomimetic model of the beetle forewing (a and c) and its imitation sample (b and d), as developed by the authors as shown in Fig. 2(a and b) Wang, and (c and d) Zhang et al. [35]. The thick arrows indicate the trabeculae.

(a)

(b)

Fig. 4. Example of a composite produced by integrated molding: (a) top view (lower surface), (b) side view. CBF: continuous basalt fiber; SBF: short basalt fiber. The thick and white arrows indicate some of the trabeculae and honeycomb walls, respectively; the star symbol indicates a positioning hole.

(3) Short fiber laying and dipping: evenly add the short basalt fiber into the mold, followed by the adhesive solution. Cover all of the fibers with the adhesive solution. (4) Vacuum pumping: stir to enable bubbles to escape the adhesive solution, allowing the adhesive to flow more smoothly into the grooves of the mold. (5) Curing and demolding: tightly clamp the mold with the cap using a C-clamp. Next, place the sample in an oven and pre-cure for 1 h at 60 °C and post-cure for 1 h at 120 °C [38]. (6) Finally, demold before and after the mold cools for the upper lamination and the partially integrated honeycomb plate, respectively. (7) Bonding: bond the separately manufactured plate and the core with SY-24B adhesive film (Beijing Institute of Aeronautical Materials).

2.2.2. Compressive and bending tests Compressive tests According to the ASTM:C365/C365M-11a standard, the manufactured SBHPs were cropped to a square with sides 60 mm in length (Figs. 6(c), 7(a) and (b)), and their sizes and weights were measured. In accordance with the aforementioned standard, an SHT4605-W electronic universal testing machine (Fig. 7(a)) was used to test the compressive properties. The pressurized mode was set as displacement loading at a speed of 1 mm/min, and the sample capacity was five. Using the load–displacement curve obtained experimentally, the compressive strength of the SBHPs can be calculated as

r ¼ P=A

ð1Þ

where P is the failure load (N) and A is the surface area of the sample (mm2). Bending tests: according to the GB/T 1456-2005 [37] standard, three-point bending tests of the SBHPs (Fig. 4(c)) were performed using an Instron 3367 electronic universal testing machine (Fig. 7(c) and (d)). The pressurized mode was set as displacement loading at a speed of 2 mm/min, and the effective sample capacity was five. 3. Results and discussion 3.1. Compressive properties of biomimetic integrated honeycomb plates 3.1.1. Results and analysis of the compression tests of the SBHPs Fig. 8 presents a typical loading–displacement curve from a SBHP compression test. The curve is divided into five stages by four critical points (cp0–cp3): stages I and II are linear segments, stages

Fig. 5. Biomimetic honeycomb plate mold with trabeculae (a) for the partially integrated honeycomb plate and (b) for the upper plate.

Fig. 6. SBHP samples: (a) partially integrated honeycomb plate, (b) its panel, and (c) the assembly formed by adhesively joining (a) and (b). The thick and thin arrows indicate a trabecula and a honeycomb wall, respectively. h is the width of the cellular-trabecular structure minimum unit cell.

217


Fig. 7. The compressive and bending tests: (a and b) a photograph and schematic of the compression test equipment, respectively, and (c and d) a photograph and schematic of the bending test equipment, respectively.

III and IV are concave plateaus, and stage V is an exponential curve. The load increases slowly in stage I, primarily because the tests are not preloaded. Stage II is the elastic stage, in which the load increases rapidly. After the second critical point, the loading slightly decreases (III), followed by the plastic stage, which exhibits a large displacement (IV). The final stage is the compaction stage (V). The average compressive strength of the second critical point of the SBHPs calculated using Eq. (1) is 29.2 ± 1.3 (MPa). The failure modes of the honeycomb plate are shown in Fig. 8. All of the fractures occurred in the core layer, as shown in Fig. 8(b) and (c); however, the lower and upper plates (including the bonding layer itself) displayed no obvious damage (Fig. 8(b)–(d)) because the core area only occupies one fourth of the entire plate area. The core stress was far greater than the plate stress under in-plane compressive loading. Therefore, fracture primarily occurred in the core layer. The typical failure mode was tilt damage (i.e., damage resulting from a tilting of the cores), which can be attributed to cracks due to dislocations (arrow in Fig. 8(c)) within the core layer. Therefore, the lower and upper plates may exhibit slight horizontal shifting. Furthermore, tilting and shifting of the core layer due to dislocation expose many short fibers to the exterior of the honeycomb wall and trabeculae, forming hair-like structures (Fig. 8, slightly yellow region marked by an asterisk). As is well known, there have been many related studies on the compressive properties of honeycomb plates [40–43]. For example, the load–displacement curve of a plate with an aluminum alloy honeycomb core is typically divided into four stages: elastic, elastoplastic, plastic, and compaction. Fig. 8(a) illustrates that the biomimetic honeycomb plate also exhibited four stages. However, the difference is that in the elastoplastic stage (III), the compressive curve of the honeycomb plates with an aluminum core exhibits a rapid decline and sharp fall and can decrease by more than half of the maximum load [41]. However, the biomimetic sample decreased by less than 10% (Fig. 8), after which the sample underwent a large plastic deformation despite the load being essentially

140

(a)

120

cp 2

stable. Next, the load continued to increase, even exceeding the initial yield load, because the biomimetic honeycomb plates consist of composite material; not only is the thickness of the cellular wall of the honeycomb core (approximately 2 mm) [41,42,44–46] nearly 10 times larger than that of the aluminum honeycomb core and paper honeycomb core, but the trabeculae also act as reinforcement, and the failure mode of the composite material reinforced by short fibers is almost a tilt (drift) fracture (arrow in Fig. 8(a)). During the fracture, some of the short fibers between the two sides of the fracture have not been pulled out or cut off, as shown in Fig. 8. Although there are a large number of hair-like skin cells in the core layer, they are not separated from the cellular wall; therefore, the core layer (cellular wall of the trabeculae) can still withstand an additional compressive load. Second, after the tilt axis near the upper and lower plates of the honeycomb structure moves, one of the sides will be able to touch the panel, allowing a certain amount of additional pressure to be withstood. Therefore, even when the curve crosses the second critical point, the load does not rapidly decline under a compressive load. In other words, although the honeycomb core of the honeycomb plate is broken, the abrupt pressure paralysis phenomenon does not occur from the viewpoint of the macroscopic compressive properties. The mechanical properties possess excellent practical value for the mechanical stability of maintenance structures and production, which is why evolution has provided the beetle forewing with such excellent mechanical properties.

3.1.2. Mass-specific compressive strength of the biomimetic honeycomb plates and analysis Because lightweight materials are judged by their mass-specific strength [43,47,48], the equivalent density of the biomimetic honeycomb plates and the mass-specific compressive strength were calculated first. The rationality of the biomimetic honeycomb plates and imitation technologies can then be further discussed

(b)

cp 3

(d)

100 80 5mm

cp 1

60

cp0

40 20 0

0

1

2

3

4

5

5mm

4mm

(c)

Fig. 8. Results and failure modes of the compression test: (a) load–displacement curve, (b and c) side view, and (d) top view.

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Table 1 Comparison of the SBHPs and Zhang’s biomimetic plates [34,35]. Item

Specific density (kg/m3)

Compressive strength (MPa)

Specific strength (m/s)2*104

SBHPs Zhang’s biomimetic platesa

779 528 545

29.3 8.1 10.3

3.76 1.53 1.88

a The panel thickness of the sandwich structure trabeculae is 1.3 mm, and it includes a composite material and a core layer made of a foam composite material. The panel reinforcement material is woven roving glass fiber, and the transition layer is made from short glass fibers. The trabeculae are a mixture of the short glass fibers and felt; the matrix material is unsaturated polyester resin.

by comparing and analyzing the test results with the biomimetic experimental results obtained by Zhang et al. [35]. According to Wang’s report, the equivalent density of the honeycomb core qc (kg/m3) [49] can be calculated as

d a

qc ¼ 1:54 c q0

ð2Þ

where dc is the thickness of the cellular wall, q0 is the density of the honeycomb core, and a is the side length of the cellular wall. The equivalent density of the assembled honeycomb plate can be calculated as

q¼

2t f qf þ 2tr qr þ hc qc 2t f þ 2t r þ hr

mimetic model, and comparative results can be confirmed using qualitative analysis by observing the manufactured plates: the thicknesses of the cellular wall and trabeculae of the SBHPs (Fig. 8(c), Fig. 6(a) and (c)) are well-distributed, whereas Zhang’s biomimetic sandwich plates [35] (Fig. 3(d)) only possess trabeculae, almost half of which are uneven (Fig. 3(d)). Therefore, the former structure (Fig. 6), which is a complete honeycomb–trabecula structure, is more similar to the actual biological structure than the latter, which only features trabeculae (Fig. 3(d)). According to the principle of biomimetics, the mechanical properties of the former structure are superior [50].

ð3:1Þ

3.2. Bending properties of the biomimetic integrated honeycomb plates

where tf, tr and qf, qr are the thicknesses and densities of the panel and bonding layers, respectively, and hc is the thickness of the core layer. The equivalent density is calculated by translating the volume of the trabeculae into the same volume of the honeycomb core without the trabeculae using the equivalence principle. The bonding layer is rather thin compared to the core layer, allowing 2trqr to be omitted. Furthermore, the same materials were adopted for the panel and core layer in the experiment; thus, qf = q0. Therefore, formula (3.1) can be simplified as

3.2.1. Results of the SBHP bending tests Fig. 9 presents the load–displacement curves and failure modes of the SBHPs. Although the failure loads of the SBHPs differ considerably, the slopes of the linear segments at the beginning of the curves are nearly identical. The failure modes of the curves are divided into two types: type A, where the load reaches a certain point and then falls sharply, and type B, where a buffer process occurs after the load reaches a certain point. The failure modes can be divided into two types: skin-core debonding (Fig. 9(b)) and lower skin fracture (Fig. 9(c)). The former is a result of the skin-core debondings caused by the interface between the core and facing plate at the defective or weak side, whereas the latter is a result of the extremely strong bond between the core and facing plate that hinders the fracture phenomenon from occurring in the bonding layer, which only occurred in one case in this experiment.

q¼

2t f þ 1:54 dac h qf 2t f þ h

ð3:2Þ

The equivalent density and specific strength of the SBHPs can be calculated using Eq. (3.2), as shown in Table 1. The test results of the biomimetic integrated plates with a composite material trabecula/foam sandwich structure (Fig. 2(d), called Zhang’s biomimetic plates) manufactured using the needle-injection method [34,35] are also listed in Table 1. Table 1 demonstrates that the specific strength of the SBHPs is twice that of Zhang’s biomimetic plates [35]. The SBHPs and Zhang’s biomimetic plates [35] differ in terms of structure, material, manufacturing method, and molding technology, limiting the ability to compare the two. However, they possess an identical bio-

3.2.2. Failure modes and mechanical property analysis of the SBHPs To analyze the characteristics of the mechanical properties of the assembled honeycomb plates under a bending load, it was assumed that no bonding layer existed between the upper plate and the partially integrated honeycomb plate, as shown in Fig. 10. Under the bending load, the aforementioned two parts turn

Fig. 9. Results and failure modes for the SBHPs under the three-point bending test: (a) load–displacement curves, (b) upper plate-core debonding (side view), and (c) lower plate fracture (top view).


219

Fig. 10. Mechanical analysis of the assembled honeycomb plates: (a) strain distribution and (b) stress distribution.

Fig. 11. Fracture micrographs of the bonding layers of the SBHPs in the three-point tests: (a and c) upper plates and (b and d) core and lower plates.

about their respective neutral axes and produce independent deformations. Therefore, the adhesive layer negatively affects the shear stress. The bonding quality of the bonding layer, which primarily affects the shear strength, directly affects the mechanical properties of the assembled honeycomb plates. When the bonding quality is good, the upper plate and core layer produce the overall deformation, and the final panel failure of the SBHPs results from the tensile stress of the lower plate or the compressive strength of the upper plate. Conversely, when the bonding layer is damaged earlier than the lower plate, the high specific strength and high specific stiffness of the sandwich structure cannot be fully utilized. Thus, the following question arises: why does the same bonding layer fracture lead to two different failure modes, types A and B? This question can be answered by the failure states of the bonding layer (Fig. 11) after the destruction of the honeycomb plate during the bending test. The two micrographs on the left side of Fig. 11 illustrate that the bonding layer either peeled into strips (Fig. 11(a) and (b), white arrow) or remained on the upper plate (Fig. 11(c) and (d), asterisk). The peeling process requires a certain amount of time, which explains the load–displacement curves of the type B failure. Non-adhesion areas can be observed on the right side of the two figures (see Fig. 11(c) and (d), thick arrow). That is, debonding occurs rapidly once peeling begins, resulting in type A failure. Thus, the bonding state may be the important difference between the samples. The fracture micrographs indicate that large differences exist in the bonding of each specimen. This result is consistent with Wang’s result [51] in that interfacial debonding is the weakness of assembled honeycomb plates. To summarize, the assembly method is influenced by many factors [52]. The mechanical properties of adhesively bonded honeycomb plates are extremely discrete, particularly when obtained by manual preparation, as shown in Fig. 9(a). If the quality of the

interfacial bonding surface is good, fracture occurs in the panels, and better mechanical properties are obtained. The integrity and mechanical properties of the biomimetic integrated honeycomb plates are better than those of assembled honeycomb plates. Panel fracture typically occurs under a bending load because the biomimetic integrated honeycomb plates have no bonding surface. In other words, the ultimate failure of biomimetic integrated honeycomb plates that possess desirable material properties is controlled by material failure only. In the next phase of this research, we are planning to investigate these mechanical properties in detail using both numerical simulations and quantitative analysis. 4. Conclusions This paper discusses the mechanical properties of biomimetic composite material honeycomb plates based on mechanical tests of SBHPs. The following conclusions can be drawn based on the results of this research: (1) The compressive process of a biomimetic composite material honeycomb plate is typically divided into four stages: elastic, elastoplastic, plastic, and compacting. However, the load decreases by less than 10% after it reaches the yield point because of the honeycomb plate, which includes thick cellular walls and reinforced trabeculae. Thus, the honeycomb plate possesses mechanical properties that prevent abrupt pressure paralysis, even if the honeycomb core is destroyed. (2) The cellular thickness of the SBHPs and the dimensions of the trabeculae are well-distributed in this study, whereas Zhang’s biomimetic plates, which are manufactured by needle injection, only include the trabeculae, and the dimen-

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sions of the trabeculae are uneven. The experiment confirms that the former has a better specific compressive strength and that the biological structure indeed possesses the biomimetic principle of excellent mechanical properties. (3) The load–displacement curves of the SBHP bending are divided into two types: A and B. The bonding layer and bonding quality do not significantly affect the bending stiffness; however, the failure modes and flexural strength of the honeycomb plates are clearly affected. Why the failure modes are divided into types A and B for the bonding layer fracture is discussed from a microstructure perspective. (4) The bonding layer is easily the weakest component of the assembled honeycomb plate. Furthermore, the biomimetic integrated honeycomb plates lack a bonding surface; therefore, the ultimate failure is determined by the material strength itself. The tests confirmed that the biomimetic integrated honeycomb possess good material properties.

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