Research ArticleMATERIALS SCIENCE

Lightweight, tough, and sustainable cellulose nanofiber-derived bulk structural materials with low thermal expansion coefficient

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Science Advances  01 May 2020:
Vol. 6, no. 18, eaaz1114
DOI: 10.1126/sciadv.aaz1114

Abstract

Sustainable structural materials with light weight, great thermal dimensional stability, and superb mechanical properties are vitally important for engineering application, but the intrinsic conflict among some material properties (e.g., strength and toughness) makes it challenging to realize these performance indexes at the same time under wide service conditions. Here, we report a robust and feasible strategy to process cellulose nanofiber (CNF) into a high-performance sustainable bulk structural material with low density, excellent strength and toughness, and great thermal dimensional stability. The obtained cellulose nanofiber plate (CNFP) has high specific strength [~198 MPa/(Mg m−3)], high specific impact toughness [~67 kJ m−2/(Mg m−3)], and low thermal expansion coefficient (<5 × 10−6 K−1), which shows distinct and superior properties to typical polymers, metals, and ceramics, making it a low-cost, high-performance, and environmental-friendly alternative for engineering requirement, especially for aerospace applications.

INTRODUCTION

Since the dawn of human existence, materials have been fundamental to the development of society. In all kinds of materials, structural materials, such as metals, ceramics, and polymers, are the most widely used (1). In recent years, designing structural materials with high performance on mutually exclusive properties (e.g., strength and toughness) at the same time, especially based on nanoscale building blocks, attracted more and more interest (28). When these nanoscale building blocks are assembled into macroscale materials, many extraordinary nanoscale properties can be scaled to macroscopic level and new macroscopic properties that are ascribable to the assembly of individual units emerge. In particular, it is of great importance and remains a huge challenge to construct high-performance, all-green bulk structural materials from renewable and sustainable nanoscale building blocks (915).

Cellulose nanofiber (CNF), which can be derived from plant or produced by bacteria, is one of the most abundant all-green resources on Earth (16, 17). Many attractive properties of CNF including low density, low thermal expansion coefficient, high strength, high stiffness, and easily modifiable surface make CNF an ideal nanoscale building block for constructing macroscopic high-performance materials. Although a variety of efforts have been made to scale those extraordinary nanoscale properties of CNFs to macroscopic level, only macro fibers and films can be prepared by different strategies up until now. For example, macro fiber was obtained from wood CNF by flow-assisted organization, with a Young’s modulus of 86 GPa and a tensile strength of 1.57 GPa, exceeding any known natural or synthetic biopolymers (18). Macro fiber with high performance was also prepared by a wet-drawing and wet-twisting process of ultralong bacterial CNF (19). In addition, films consisting of CNF with high strength, high transparency, and low thermal expansion coefficient have been designed and used in many fields such as electron devices and flexible display (2024). However, challenges remain in scaling those extraordinary nanoscale properties of CNFs to three-dimensional bulk structural materials. If sustainable high-performance bulk structural materials can be constructed, it will certainly promote the development of CNF, expand its application fields, and provide more materials selection in engineering design.

Here, we report a robust and feasible strategy to process CNFs into a high-performance bulk structural material with low density, outstanding strength and toughness, and great thermal dimensional stability. The obtained CNF plate (CNFP) has high specific strength [~198 MPa/(Mg m−3)], high specific impact toughness [~67 kJ m−2/(Mg m−3)], and low thermal expansion coefficient (<5 × 10−6 K−1), which shows distinct and superior properties to typical polymers, metals, and ceramics, making it a low-cost, high-performance, and environmental-friendly alternative for engineering requirement, especially as spacecraft materials. Moreover, CNFP also shows good serviceability under extreme temperature or rapid thermal shock and high energy absorption properties, and its mass production can be achieved in large scale at low cost.

RESULTS

Material fabrication and characterization

Figure 1 shows a schematic of our bottom-up approach to press multilayered pretreated CNF hydrogels into high-performance CNFP. CNF hydrogels with a robust three-dimensional nanofiber network are produced from glucose by biosynthesis and then treated with a different polymer solution or by surface modification before hot-pressing (Fig. 1, A to C). The CNFPs prepared from untreated, polyvinyl alcohol (PVA)–treated, and silicic acid solution–treated bacterial CNF hydrogels and polyacrylic acid (PAA)–treated surface-oxidized bacterial CNF hydrogels are marked as CNFP-0, CNFP-1, CNFP-2, and CNFP-3, respectively (table S1).

Fig. 1 Fabrication and structure analysis of CNFP.

(A) CNF hydrogel can be produced by biosynthesis. (B) CNF hydrogel and its robust three-dimensional nanofiber network. (C) Numerous layers of treated CNF hydrogels are pressed at 80°C to fabricate CNFP. (D) The diagrammatic drawing of CNFP. (E) The multilayer structure of CNFP. (F) The robust three-dimensional nanofiber network of one layer in CNFP. (G) Cellulose molecular chains are tightly bonded together by hydrogen bonds and expose lots of –OH groups on the surface of CNF to form interfiber hydrogen bonds. (H) Photograph of large-sized CNFP with a volume of 320 mm by 220 mm by 27 mm. (I) Parts with different shapes of CNFP produced by a milling machine. Scale bar, 1 cm (I). (Photo credit: Zi-Meng Han, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.)

Our method can achieve large-scale preparation directly by larger press equipment. A large-sized CNFP was fabricated with a volume of 320 mm by 220 mm by 27 mm and with a weight of 2560 g (Fig. 1H). Since bacterial cellulose hydrogel is a kind of industrial product and its market price is lower than $0.01 per kilogram, CNFP can be prepared at low cost, which is lower than many typical structural materials (tables S2 and S3). Furthermore, CNFP can be processed into desired shape and size; for example, different shapes of parts were obtained by a milling machine, which shows its good processability (Fig. 1I).

The scanning electron microscopy (SEM) images reveal the multilayer structure of CNFP, and each layer is about 20 μm, which is determined by the thickness of CNF hydrogels (Fig. 2A). Inside the layer, there are densified and robust three-dimensional nanofiber networks (Fig. 2, B and C and fig. S1, G to I). Numerous CNFs are intertwined with each other and combined together by strong hydrogen bonds. The interfacial failure of CNFP under external loading shows a large amount of connected CNFs, demonstrating the tightly hydrogen-bonded adjacent layers in CNFP (Fig. 2, D to F, and fig. S1, A to F).

Fig. 2 Structural characterization of CNFP.

(A) SEM image of the marked area in the inset photograph, clearly showing the multilayer structure where each layer is about 20 μm. The inset is a photograph of CNFP. (B) Magnified SEM image of a layer in CNFP, showing the microscopic layered structure of CNFs. (C) SEM image of the marked area in the inset photograph, showing the robust three-dimensional nanofiber network. Numerous CNFs are intertwined with each other and combined together by a strong hydrogen bond. The inset is a photograph of CNFP. (D) Profile of the fractured CNFP-0 showing the sliding between about 20-μm layers. (E) Numerous CNFs are intertwined with each other and combined together between layers [enlarged micrograph of the marked area in (D)]. (F) SEM image of an oblique section of CNFP. Between different layers, a large number of CNFs are pulled out from the layer and intertwine with each other. (Photo credit: Huai-Bin Yang, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.)

Thermal and mechanical properties

Through this strategy, many remarkable nanoscale properties of CNF can be successfully scaled to macro level on CNFPs, including low thermal expansion coefficient and high strength. Low thermal expansion coefficient is a vitally important feature for materials in many application areas, especially for aerospace applications. From −120° to 150°C, the average thermal expansion coefficients of CNFP are lower than 5 × 10−6 K−1 (parallel to layer) and 7 × 10−6 K−1 (perpendicular to layer), respectively, which is close to those of ceramic materials and much lower than those of typical polymers and metals (Fig. 3A and fig. S2). Meanwhile, the excellent mechanical properties of CNF are also successfully scaled to macro level on CNFPs (Fig. 3B; see also figs. S3 to S6). With a low density of ~1.35 g cm−3, the ultimate flexural strength and the flexural modulus of CNFP can be up to ~269 MPa and ~17 GPa, respectively (Fig. 3B). The organization of CNFs also provides other intriguing macroscopic properties. The Charpy impact test of notched CNFP yields an impact toughness of 87.6 ± 4.3 kJ m−2, which is much higher than those of typical plastics (Fig. 3C). Moreover, it shows no visible change at 200°C, indicating that the thermostability of CNFP is much better than those of other widely used polymer-based materials (Fig. 3, E and F, and fig. S7). After enduring rapid thermal shock for 10 times between two extreme temperature conditions (−196° and 120°C), CNFP still maintains a similar mechanical performance (Fig. 3, G and H). Notably, at 120° and −50°C, the flexural strength of CNFP does not change notably (Fig. 3D), which is vitally important for practical application in extreme environments. In addition, after having been subjected to 95% relative humidity for 60 hours, the thickness and flexural strength of CNFP change only slightly, showing good stability under moisture attack (fig. S8).

Fig. 3 Superb thermal and mechanical properties of CNFP.

(A) Thermal expansion of CNFP (parallel to layer), polyamide (PA), Al alloy (7075 Al), and Al2O3. (B) Comparison of flexural strength and stiffness of different kinds of CNFPs. (C) Comparison of Charpy impact toughness of CNFP-0 with other widely used polymer-based materials. (D) Flexural stress–strain curves of CNFP-0 at different temperatures. (E and F) Comparison of CNFP-0 with other widely used polymer-based materials at (E) 30°C and (F) 200°C. (G) Schematic of rapid thermal shock for 10 times. (H) Flexural stress–strain curves of CNFP-0 before and after rapid thermal shock for 10 times. PMMA, polymethyl methacrylate; PVC, polyvinyl chloride; ABS, acrylonitrile butadiene styrene; PC, polycarbonate; PF, phenolic resin; POM, polyformaldehyde; PP, polypropylene. (Photo credit: Zi-Meng Han, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.)

CNFP has multiple outstanding features in one material, including low thermal expansion coefficient, lightweight, high strength, and high toughness. To demonstrate the overall performance of CNFP, we put its properties into two Ashby maps to compare with different kinds of traditional structural materials (Fig. 4). In Ashby maps, despite the wide range of properties of metals (as an example), the clusters occupy a field that is distinct from that of polymers or that of ceramics. This fact shows that for different kinds of materials, the fields may overlap, but they always have a characteristic place within the whole picture (1). According to this pattern, CNFP appears on the maps as a whole new kind of material, which occupies a distinct field with low thermal expansion coefficient (lower than 5 × 10−6 K−1), high specific strength [up to 198 MPa/(Mg m−3)], and high specific impact toughness [up to 67 kJ m−2/(Mg m−3)]. The specific strength and specific impact toughness of CNFP are higher than those of traditional metals and alloys, making it an all-green and high-performance alternative for engineering design. The position of the fields on Ashby maps can be understood in simple physical terms, as the nature of the fundamental building blocks and how they combine with each other determine the position of the fields that different kinds of materials occupy. Considering CNFP, CNFs are its basic building blocks, while hydrogen bonds are the main interaction that bind them together. The strength (at least 2 GPa) and Young’s modulus (138 GPa) of individual CNFs can be almost as high as those of steel and Kevlar (22), and those superior nanoscale properties can be scaled to macro level (tables S4 and S5), mainly due to the strong interaction between CNFs (2529).

Fig. 4 Comparison of thermal and mechanical properties of CNFP with typical polymers, metals, and ceramics.

(A) Ashby diagram of thermal expansion versus specific strength for CNFP compared with typical polymers, metals, and ceramics (1, 4146). (B) Ashby diagram of thermal expansion versus specific impact toughness for CNFP compared with typical polymers, metals, and ceramics (1).

Mechanism analysis of thermal and mechanical properties

According to previous researches (16, 22, 30, 31), since the CNFs are aggregates of semi-crystalline extended cellulose chains, its thermal expansion coefficient is as small as 1 × 10−7 K−1, which is lower than that of silica glass. When those CNFs are bound by strong interfiber hydrogen bonds, a low thermal expansion coefficient is achieved.

The ultrafine nanofiber network structure in CNFP results in more extensive hydrogen bonding, the high in-plane orientation, and “three-way branching points” of the microfibril networks (Fig. 2F) (17). Those structural features allow the CNFP to withstand high stress without breaking and also disperse stress and suppress crack formation and crack propagation. According to a photograph of a broken sample (fig. S9A), the main fracture mode of CNFP is interlaminar debonding, which means that the interlaminar shear strength between layers is the limiting factor of the strength of CNFP. Thus, we tune the interlaminar shear strength by treating the CNF hydrogel surface with different polymer solutions. For untreated CNFP-0, the interaction between layers is hydrogen bond, while PVA-treated CNFP-1 has weaker interlaminar shear strength, since PVA forms weaker hydrogen bonds between layers than cellulose itself. For CNFP-2 treated by silicic acid solution, silicic acid dehydrates and provides covalent strong cross-links during the hot-press process (32), which effectively improves the interlayer interaction compared to CNFP-0 and CNFP-1. For CNFP-3, the oxidation process introduces an amount of carboxyl groups on the surface of CNF. Those carboxyl groups of CNFs are further cross-linked by the calcium ions and polyacrylic acid. The calcium ions and carboxyl groups form strong ionic bond and further improve the interaction between layers. The nuclear magnetic resonance (NMR)–13C and Fourier transform infrared spectroscopy (FTIR) results verify the mechanism above, while the x-ray diffraction (XRD) result shows that the crystallinity of CNFs has no visible change after pretreatment (figs. S10, S11, and S12) (33, 34). By tuning the interaction between layers, it can be seen that when the interaction is enhanced, the interlaminar shear strength and flexural strength increase simultaneously (Fig. 3B; see also fig. S9B).

A long-standing challenge in engineering material design is the conflict between strength and toughness, because these properties are, in general, mutually exclusive (25, 3537). The high ultimate flexural strength of CNFP is not accompanied by low impact toughness (Fig. 3, B and C). This unique toughening mechanism can be understood at multiscale. At microscale, bending force initiated the sliding between ~20-μm layers (Figs. 1E and 2D; see also fig. S1, A to C), and the sliding dispersed the stress, avoiding stress concentration. At nanoscale, tightly hydrogen-bonded CNFs at interface are pulled out from the opened layer during the interlaminar sliding (Figs. 1F and 2, E and F; see also fig. S1, D to F), which further disperse the stress and prevent the generation and propagation of crack. At molecular scale, owing to the rich hydroxyl groups in cellulose molecular chains, when CNFPs are subjected to deformation, relative sliding among CNFs involves an enormous number of hydrogen-bond formation, breaking, and reformation (Fig. 1G) (25, 38). Owing to the above mechanisms, deformation of the CNF network can absorb a large amount of energy. To demonstrate the role of interfacial properties on toughness, we further investigated the failure behavior of CNFP under single-edge notched bend (SENB) simulation. As a comparison, the monolithic bulk shows the stress concentration around the crack tip and soon displays the brittle damage through the fracture section (Fig. 5B). With the decrease of interfacial strength, it is clear that laminar bulk illustrates brittle-to-tough transition from the force-displacement curves (Fig. 5C). For an appropriate interfacial strength, the detailed failure behavior is given in Fig. 5A, where the deformation energy is dissipated by interfacial sliding and opening instead of brittle damage of laminae. For CNFP, therefore, suitable interfacial modification can enormously enhance the toughness and retain considerable strength at the same time.

Fig. 5 Superb toughness mechanism and impact resistance properties of CNFP.

(A and B) FEM simulations of (A) laminated structure and (B) monolithic bulk for the SENB test. (C) Stress-strain curves of different kinds of interfaces for the SENB test. (D) Force-displacement curve of CNFP-0 for the drop hammer impact test. (E) Schematic of the drop hammer impact tester. (F) Photograph of CNFP-0 after the drop hammer impact test. Scale bar, 1 cm. (G) Schematic of the SHPB. (H) Compressive stress-strain curves of CNFP-0 for the SHPB test under different strain rates. (Photo credit: Huai-Bin Yang, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.)

Impact resistance

To further evaluate the impact resistance of CNFP, dynamic mechanical tests including drop hammer impact and split Hopkinson pressure bar (SHPB) were carried out. The CNFP sample can generate a high resist force during the impact and remarkably decrease the velocity of hammer and absorb its energy (Fig. 5, D and E). The absorption energy in the drop hammer test can be calculated by the force-displacement curve, and for CNFP, it is ~2.7 kJ m–1 (Fig. 5D). CNFP under high-velocity impact can maintain its shape, instead of shattering or deforming. The damage pattern of CNFP demonstrates the stress localization of energy adsorption under high-velocity impact (Fig. 5F). SHPB is a powerful tool to study the behavior of material under high–strain rate deformation. Its structure is shown in Fig. 5G, and the strain gauge on the incident bar and the transmission bar can convert mechanical wave into electronic signal and yield the stress-strain curve. The highest compression stress of CNFP can reach ~1600 MPa during high–strain rate compression (14,000 s−1), and the energy absorption of compression is ~387.5, ~89.3, and ~37.0 MJ m−3 under the strain rates of 14,000, 10,000, and 7500 s−1, respectively (Fig. 5H). The stress-strain curves in the SHPB experiments illustrate anomalous decline after the first peak compared with the common plastic platform area of traditional metal materials and energy absorbing materials. Under initial low strain in the impact, the CNF network and the layered plate as the structural frameworks resist the shock stress. Then, the following large strain would trigger the deformation of the CNF network at microscale and even the shear behavior of cellulose nanocrystals at nanoscale. Recent simulation studies demonstrated that breaking and reformation of interfiber hydrogen bonds as well as the dihedral rotation are non-negligible mechanisms in the origin of outstanding mechanical properties of cellulose nanocrystals (38, 39). At last, the larger compression strain results in the densification of CNFP (CNF network at microscale). While the compression speed increases, adsorption energy increases. The light weight and high energy absorption properties of CNFP indicate that it can be a potential armored material for shock waves from blast (40).

DISCUSSION

In summary, we have developed a robust and feasible strategy to process CNF into an environmentally friendly bulk structural material, CNFP. It has multiple unique features in one material, which are low thermal expansion coefficient, light weight, high strength and toughness, good impact resistance, and easy realization of large-scale production, making it a low-cost and high-performance alternative for engineering design. For example, it can be a strong competitor for the lightweight material used for automobiles and aircraft, and remarkably for aerospace application, such as optical lens bracket for the lunar rover where light weight, high strength, and low thermal expansion coefficient are all vitally essential. From the perspective of environmental life cycle impact assessment (10), CNFP contributes almost nothing to extra greenhouse gas emissions, human health impairment, ecosystem toxicity, or resource depletion, indicating that it is a good example of an all-green structural material. Furthermore, CNFP can be designed at multiscale according to application requirements. Various functional bulk structural nanocomposites can be achieved, and thus, the performance of this emerging sustainable structural material can be further explored.

MATERIALS AND METHODS

Fabrication of CNFPs

All reagents and raw materials were commercially available. Bacterial cellulose hydrogels were produced by Gluconacetobacter xylinus 1.1812 (China General Microbiological Culture Collection Center) on the liquid nutrient media at 30°C (24). One liter of liquid nutrient media consisted of glucose (50 g liter−1), yeast extract (5 g liter−1), citric acid (2 g liter−1), Na2HPO4·12H2O (4 g liter−1), and KH2PO4 (2 g liter−1). A part of bacterial cellulose hydrogels were surface-oxidized by the 2,2,6,6-Tetramethylpiperidine-1-oxyl-oxidized method. They were then immersed into CaCl2 solution (0.1 M) for 24 hours and rinsed three times with deionized water. The bacterial cellulose hydrogels (untreated/surface oxidation) were cut into sheets, and they were immersed in the surface treatment solution for 6 hours. Then, a certain number of bacterial cellulose sheets were stacked by hydrogel layer-by-layer (HLBL) and were pressed with a pressure of ~1 MPa. Last, a hot-pressing step with a pressure of 100 MPa was applied at 80°C until CNFP was completely dry. The treatment method of bacterial cellulose hydrogels, surface treatment solution, and the corresponding numbers are shown in table S1. For a typical CNFP-0 with a thickness of 6 mm, it was pressed with a pressure of ~1 MPa for about 3 hours and then was hot-pressed with a pressure of ~100 MPa at 80°C for about 1 hour.

Characterization

SEM images were taken with a Carl Zeiss Supra 40 field emission scanning electron microscope (5 kV), and all samples were gold-sputtered for 30 s at a constant current of 30 mA before observation. XRD data were measured by a PANalytical X’pert PRO MRD X-ray diffractometer equipped with Cu Kα radiation (λ = 1.54056 Å), and the samples were prepared by cutting down a ~0.5-mm layer of CNFPs. Thermogravimetric analysis data were measured in air atmosphere with a TA Instruments SDT Q600 thermogravimetric analyzer, and the samples were prepared by grinding CNFPs into powders. FTIR spectra were acquired by a Bruker Vector-22 FTIR spectrometer in attenuated total reflectance mode, and the samples were prepared by cutting down a ~0.5-mm layer of CNFPs. 13C-NMR cross-polarization magic angle spinning spectra were recorded at 100.62 MHz on a Bruker Avance III 400 WB spectrometer using a 90° pulse of 4 μs, an acquisition time of 33.9 ms, a contact time of 3 ms, a recycle delay of 5 s, and a spin rate of 14 kHz.

Thermal and mechanical testing

Three-point bending test was performed on an Instron 5565A universal testing machine according to ASTM D790-15e1. The specimens were carefully cut with a size of about 25 mm by 2 mm by 2 mm. The test was carried out at a loading rate of 1.0 mm min−1 with a support span of 12.5 mm. For all the mechanical tests, the applied loading direction was perpendicular to the layers, and each kind of specimen was tested at least five times, unless noted otherwise. Tensile test and compression test were performed on an MTS 809 material testing machine. The dimensions for tensile specimens (dog-bone-shaped specimens) were approximately 100 mm long, 10 mm wide, and 2 mm thick. The specimens were clamped at both ends and stretched along the specimen length direction with a constant test speed of 1.0 mm min−1 at room temperature. The dimensions for compression specimens were about 9 mm by 9 mm by 4.5 mm, and the specimens were compressed along the thickness direction (4.5 mm) at room temperature with a loading rate of 1 mm min−1. The Charpy impact test of CNFPs was performed on a Chengde Bao Hui XJJY-5 pendulum impact tester, and the dimensions for Charpy V-notch specimens were about 50 mm long, 10 mm wide, and 2 mm thick with a notch depth of 1 mm. The drop hammer impact test of CNFPs was performed on an Instron CEAST 9340 drop hammer tester. The hammer with a weight of 15 kg (total mass) was falling freely from a height of 30 cm, and the dimensions of the tested specimens were about 50 mm by 50 mm by 2 mm. For the SHPB test, the dimensions of the tested specimens were about 6 mm by 6 mm by 2 mm.

Thermal expansion coefficient of the CNFP was measured by NETZSCH TMA 402F3. The specimens were carefully cut with a size of about 20 mm by 4 mm by 2 mm, and the test was carried out from −130° to 150°C. Thermal expansion coefficient (α) was calculated by the equation α = ΔL/(L ΔT) K−1. Density (ρ) of the CNFP was calculated by first treating the material into a cuboid and then using the equation ρ = mass/volume. The specific values of the impact fracture toughness and the ultimate flexural strength were calculated by dividing the density. The short-beam shear strength can be calculated by three-point bending test according to ASTM D2344/D2344M-16. Short-beam shear strength (Fsbs) was calculated byFsbs=0.75 Pmbh(1)where Pm is the maximum load in three-point bending test, b is the measured specimen width, and h is the measured specimen thickness.

Computational methods

The SENB simulation for CNFPs is implemented by finite element method (FEM) with the software ABAQUS. The geometric model consists of three components: a plate specimen, an indenter, and two symmetrical fixtures. The plate specimen is pre-cracked with a notch in the middle of the specimen. The thickness of the notch is half that of the specimen. The indenter and the fixtures are set as the rigid body in the simulation. The contact constraints are defined between the component surface pairs.

The failure behavior of monolithic bulk without lamination is simulated first. The property of CNFP elements is defined by the user-defined field subroutine. The elements are linear elastic at the initial stage. When the element stress is greater than the material strength σmax, the element Young’s modulus is degraded to 2 × 10−9 of the initial value as an approximate simulation of fracture damage. In the simulation, the indenter pushes down with a uniform speed. The reaction force and falling displacement of indenter are compared and adopted to plot the stress-strain curve (Fig. 5C). It can be seen that the curve grows linearly at first and then decreases rapidly after reaching the peak value. The overall failure of the specimen is due to the stress concentration at the pre-crack tip. The crack expands upward rapidly along the initial crack path, causing the specimen to break (Fig. 5B).

For multilayered CNFPs, the specimen is divided into isopachous multiple layers to consider the effect of interface on the bending property. Contact constraints with cohesive behavior are defined between the layers. The maximum shear stress τmax and fracture energy Gτ of cohesive elements are adjusted with different values to observe the change of failure mode. When the values of τmax and Gτ are big enough, the failure mode is almost unchanged, and the specimen is still broken from the middle. The flexural modulus is reduced due to interlayer slippage. With the decrease of interfacial strength, we can see that the failure mode varies, in which the interface reaches the critical strength earlier than the elements during the indenter falling process. The cracks appear at the middle layer first and expand to both ends gradually (Fig. 5A). Then, more and more interlaminar cracks appear inside the specimen, whereas the prefabricated cracks cannot expand upward because energy is dispersed through layers. It is found that the toughness of specimen is much higher according to the stress-strain curve. As the interface strength continues to decrease, the failure mode remains the same, while the total strength and toughness are gradually reduced.

In summary, the interface property has a significant impact on the SENB test of CNFPs. The toughness of CNFPs can be improved by adjusting the interface strength.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/18/eaaz1114/DC1

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: Funding: This work was supported by the National Natural Science Foundation of China (Grant 51732011), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant QYZDJ-SSW-SLH036), the National Basic Research Program of China (Grant 2014CB931800), the Fundamental Research Funds for the Central Universities (WK2090050043), the Users with Excellence and Scientific Research Grant of Hefei Science Center of Chinese Academy of Sciences (2015HSC-UE007), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB22040402), the National Natural Science Foundation of China (11525211), and the USTC Research Funds of the Double First-Class Initiative (YD2480002002). Author contributions: S.-H.Y. and Q.-F.G. conceived the idea and designed the experiments. S.-H.Y. supervised the project. Q.-F.G., H.-B.Y., Z.-M.H., and Z.-C.L. carried out the synthetic experiment and analysis. H.-A.W., L.-C.Z., and Y.-B.Z. contributed to both mechanical simulations and analysis. H.-B.J. and P.-F.W. contributed to SHPB test and analysis. T.M. contributed to the 3D illustrations. Q.-F.G., H.-B.Y., Z.-M.H., and S.-H.Y. wrote the paper, and all authors discussed the results and commented on the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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