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Materials Science & Engineering A

journal homepage: www.elsevier.com/locate/msea

Mechanical properties and deformation behavior under compressive loadingof selective laser melting processed bio-inspired sandwich structures

Kaiming Hua,b, Kaijie Lina,b, Dongdong Gua,b,*, Jiankai Yanga,b, Haoran Wanga,b, Luhao Yuana,b

a College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing, 210016, Jiangsu Province, PR Chinab Jiangsu Provincial Engineering Laboratory for Laser Additive Manufacturing of High-Performance Metallic Components, Nanjing University of Aeronautics andAstronautics, Yudao Street 29, Nanjing, 210016, Jiangsu Province, PR China

A R T I C L E I N F O

Keywords:Bio-inspiredSandwich structureSelective laser meltingDeformation behaviorFinite element analysis

A B S T R A C T

Sandwich structures are widely used in aviation and aerospace applications because of their outstandingcharacteristics of light-weight, excellent energy absorption and continuous compression behaviors. Nature hasprovided us with extraordinary resources to meet the demanding for modern industry. In this study, inspired bythe microstructures of the Norway spruce stem, four light-weight sandwich structures were designed andmanufactured by selective laser melting (SLM). The structures had good formability using an optimized laserprocessing parameter setting. The uniaxial compression tests of SLM-processed bio-inspired sandwich structureswere conducted to evaluate their specific compressive strength and energy absorption performance. Finiteelement analysis was employed to study stress distributions in structures during compression tests and analyzefracture mode combining with the observation of fracture surface morphology. The experimental and numericalresults indicated that the gradient structure, with tube size gradually decreasing from top and bottom platetowards the center, exhibited the highest specific absorption energy, ultimate strength and specific strength,which was 5.73 J/g, 214.8MPa and 98.99 MPa/(g/cm3), respectively. The FEA results revealed that the ar-rangement of tubes significantly affected the stress distribution and fracture locations of structures. The gradientstructure, with the highest specific absorption energy and ultimate strength, had the most uniform stress dis-tributions, contributing to its excellent compressive performance.

1. Introduction

The sandwich structures have high bending and buckling resistance[1,2], excellent energy absorption capability [3,4], and thermal in-sulation property [5–7] and are widely used in aviation and aerospacefields. For sandwich structures, many factors [8] can affect their me-chanical properties and the design of core structures is one of the keyfactors. During the past few decades, many literature focus on the in-fluence of core design on the mechanical properties of sandwichstructures and demonstrate that the parameters of core structures, suchas arrangement and size of cells [9], relative density of cores [10] andwall thickness of cells [11], greatly influence the compressive behaviorof sandwich structures. With the maturation of manufacturing tech-nologies, increasing amount of sandwich structures with complex corestructures are designed, including corrugated cores [12–16], honey-comb cores [17–19], lattice/truss cores [20–22] and metal foam cores[23–25].

After billions of years of evolution, the structures of living organ-isms have developed outstanding properties and provided inexhaustibleprototypes for technical improvements and innovations [26,27]. Inorder to meet the demand for light-weight sandwich structures in theaerospace field, many bio-inspired complex structures with superiorperformance have been designed and evaluated in recent years[28–30]. However, due to the complexity of bio-inspired structures,numerical simulation is commonly applied to study the mechanicalperformance of structures [31–36]. Peng et al. [37] designed bio-in-spired honeycomb column thin-walled structures and investigated thecrushing behavior and energy absorption characteristics using numer-ical simulation. Liu et al. [38] analyzed the crash responses of bio-in-spired aluminum honeycomb sandwich structures and found that thespecific energy absorption increased obviously under high impact ve-locity and the crashworthiness characteristics were more sensitive tocore length compared with the core height.

The emerging of the high-performance bio-inspired structures drives

https://doi.org/10.1016/j.msea.2019.138089Received 30 April 2019; Received in revised form 26 June 2019; Accepted 28 June 2019

* Corresponding author. College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing, 210016,Jiangsu Province, PR China.

E-mail address: dongdonggu@nuaa.edu.cn (D. Gu).

Materials Science & Engineering A 762 (2019) 138089

Available online 29 June 20190921-5093/ © 2019 Elsevier B.V. All rights reserved.

T

the development of manufacturing technologies. Additive manu-facturing (AM) [39,40], a novel manufacturing technology, has creatednew opportunities for the design and fabrication of structures withmulti-scale [41,42], multi-material [43–45] and multi-function [46].AM technology has many advantages, such as less waste, freedom ofdesign and high automation, which has been widely applied in diverseindustries, including aeronautical, automobile and bio-implantationfields [47–49]. In recent years, selective laser melting technology, onekey technology of AM, also finds its application in the fabrication ofcomplex bio-inspired structures. Wang et al. [50] fabricated a light-weight structure inspired by the elytra of beetle using SLM and ana-lyzed its energy absorption and bearing capacity using compression testand finite element simulation. Yang et al. [51] designed a bi-direc-tionally corrugated-core sandwich panel, inspired by the mantisshrimp. Bio-inspired components with high forming precision andquality were successfully fabricated by SLM and the energy absorptionbehavior was investigated through the combination of experimentaltests and numerical simulation.

In this work, a series of novel light-weight sandwich structures in-spired by the microstructure of the Norway spruce stem was designedand fabricated by SLM. Uniaxial compression tests were applied toevaluate the mechanical properties of the bio-inspired structures.Subsequently, the finite element analysis (FEA) was conducted toanalyze the stress distributions of different structures under uniaxialcompression loading. Through the combination of finite element ana-lysis and experimental observation, the deformation behavior and thefracture mechanism of the bio-inspired sandwich structures were re-vealed.

2. Materials and methods

2.1. Structure design and SLM processing

The sandwich gradient structures, proposed in this paper, were in-spired by the Norway spruce stem (shown in Fig. 1). From the cross-sectional SEM images of the Norway spruce stem, it is clear to find thatthe cell size gradually changes toward the center. More specifically,from the edge of the stem to the center, the cell size decreases first andthen increases. In order to investigate the influence of gradient con-figuration on the compression performance, four sandwich structureswith a different arrangement of hollow tubes were designed and pre-sented in Fig. 1 b - e. No gradient structure (NG) with identical tube size

(wall thickness 0.7 mm and external diameter 4mm) was designed as acontrol group and its solid volume was equal to gradient structures(Fig. 1b). All of the three gradient structures were composed of twolayers of big tubes and four layers of small tubes. For the gradientstructure A (GA), two layers of big tubes were arranged in the centerand two layers of small tubes were placed next to the top and bottomplate, respectively (Fig. 1c). In the gradient structure B (GB), the ar-rangement of big tubes and small tubes was opposite to the GA struc-ture, a layer of big tubes was adjacent to the top and bottom plate,respectively, and four layers of small tubes were arranged in the center(Fig. 1d). For the gradient structure C (GC), a layer of small tubes wasadjacent to the top and bottom plate, respectively, and the other twolayers of small tubes were located in the center of the structure, sand-wiched by big tube layers (Fig. 1e). The wall thickness of big tubes andsmall tubes was both 0.5mm. The external diameter of big tubes andsmall tubes was 4mm and 2mm, respectively. In addition, the overalldimensions of four structures were 20mm×20mm×17.2 mm andthe thickness of the top and bottom plate for all structures was 1mm. Inorder to ensure the sound connection between tubes, an overlappingdisplacement between tubes was set as 0.3mm.

The SLM equipment used in this work was self-developed byNanjing University of Aeronautics and Astronautics (Fig. 2a). The SLMequipment consisted of YLR-500-SM ytterbium fiber laser with thehighest power 500W and spot size 70 μm. Based on previous research,an optimized parameter setting was used in this work, which was laserpower 250W, hatch spacing 80 μm, scanning speed 800mm/s and layerthickness 50 μm. Ti6Al4V powder with a spherical morphology wasused in this work (Fig. 2b). In order to improve the quality of SLM-processed components, the building direction was consistent with theaxial direction of the tubes, and the as-fabricated Ti6Al4V bio-inspiredsandwich structures were shown in Fig. 2c. Samples for tensile testswere fabricated along with the bio-inspired sandwich structures usingthe identical parameters. A representative stress-strain curve of theSLM-processed Ti6Al4V was presented in Fig. 1d, and the mechanicalproperties was presented in Table 1.

2.2. Microstructural and mechanical property characterization

The SLM-processed samples for metallurgical examinations werecut, grounded and polished according to standard procedures. Thecross-sectional pore distribution of components was observed by anoptical microscope (OM, OLYMPUS PMG3). The etchant (distilled

Fig. 1. The design of bio-inspired sandwich structures: a, The cross-sectional microstructure of the Norway spruce stem [52]; bio-inspired sandwich gradientstructures: b, no gradient structure (NG); c, gradient structure A (GA); d, gradient structure B (GB); e, gradient structure C (GC).

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water: HNO3: HF=50: 3: 5) was prepared, and the OM was used toobserve the microstructure after etching. Uniaxial compression tests ofthe SLM-processed bio-inspired sandwich structures were conducted atroom temperature using a universal testing machine (CMT5205) andthe displacement rate was set as 1mm/min. The fracture surfacemorphologies of SLM-processed bio-inspired sandwich structures afterthe compression tests were observed by a scanning electron microscope(SEM, JSM-6360LV).

2.3. Crashworthiness indicators

From the load-displacement curves of compression tests, two im-portant indexes, namely energy absorption (EA) and specific energyabsorption (SEA) [53–55], can be obtained to evaluate the bearing andenergy absorption capacity of bio-inspired sandwich structures. Theenergy absorption (EA) indicates the capacity of absorbing energyduring compression, which is determined by the integral area of theload-displacement curve. The equation of energy absorption can beexpressed as follows:

∫=ΕΑ d F x dx( ) ( )0

d

(1)

where F(x) is the instantaneous compression force and d is the totalcompression displacement.

The specific energy absorption (SEA) assesses the absorbed energyper unit mass of a structure, which is an important factor to compare

the energy absorption efficiency of different structures. The specificenergy absorption is determined by the following equation:

=SEA d ΕΑ dm

( ) ( )(2)

where m is the mass of a structure. EA is the energy absorption value ofa structure, which is defined by Eq. (1).

2.4. FEA model and boundary conditions

To better understand the deformation behavior of the bio-inspiredsandwich structures under compression loading, nonlinear explicit fi-nite element code ANSYS LS-DYNA was conducted. The compressionprocess was performed by a dynamic explicit procedure and a numer-ical finite element simulation model was shown in Fig. 3. In order tobalance between simulation accuracy and computational cost, the meshsize of the rigid plate and the gradient structure was determined as1mm and 0.3 mm, respectively. During the simulation process, a rigidplate was placed on the top and bottom of the gradient structure, re-spectively. The contacts between the rigid plates and the structure wereset as a surface to surface contact and the friction coefficient was set as0.2 [56]. Besides, the displacement rate of the top rigid plate was1mm/min and the bottom rigid plate was fixed. The material propertyof the SLM-processed Ti6Al4V, showed in Table 1, was used in the FEmodel.

3. Results and discussion

3.1. Formability and microstructure

In order to avoid the influence of formability on mechanical prop-erties, the optimal processing parameters were applied to manufacturethe bio-inspired sandwich structures. The mass of different SLM-pro-cessed components was closed to each other (showed in Table 2). The

Fig. 2. a, the schematic diagram of selective laser melting machine; b, the morphology of the Ti6Al4V powder; c, the as-fabricated bio-inspired sandwich structures;d, a uniaxial tensile stress-strain curve of the SLM-processed Ti6Al4V.

Table 1The mechanical parameters of Ti6Al4V solid parts fabricated by SLM.

Poisson's ratio(mm/mm)

Tensile strength(MPa)

Young modulus(GPa)

0.32 1021.4 114.6

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Fig. 3. Numerical finite element simulation model: meshing and boundary conditions.

Table 2The mass of SLM-processed Ti6Al4V bio-inspired gradient structures.

No gradient structure Gradient structure A Gradient structure B Gradient structure C

17.2 ± 0.023 g 17.6 ± 0.02 g 17.4 ± 0.025 g 17.1 ± 0.027 g

Fig. 4. The macro-photograph and the cross-sectional OM images of the SLM-processed GA structure: a, macro-photograph of the polished side face of the SLM-processed GA structure; b, OM image of the annular ring position; c, OM image of the intersection position (pores and adhered particles are marked by black arrows).

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gradient structure A (GA) was selected as a representative, and themacro-photograph and the cross-sectional OM images were presentedin Fig. 4. An almost fully dense structure can be clearly seen from theOM images, only a small number of pores were found (Fig. 4 b & c).During the SLM-processing, a molten pool was generated by a high-energy laser beam, the laser melted the metal powder at an extremelyfast speed. During the solidification of liquid metal, gases were trapped

inside the molten pool and formed bubbles [57–59]. Due to the fastcooling rate, the molten liquid cannot fill the bubbles in time and leadto the formation of pores [60–63] Besides of the pores in structures,partially melted particles adhered on the edge of the node can also beseen in Fig. 4c, which increased the surface roughness of the structures.The phenomenon of adhered particles was a common problem for se-lective laser melting. During the laser processing, the residual heat from

Fig. 5. a, The OM micrographs of the SLM-processed GA structure after etching, b, the XRD spectrum of the SLM-processed GA component.

Fig. 6. a, The compressive load-displacement curves of different SLM-processed sandwich structures; b, the peak crush force of different SLM-processed structures.

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the molten pool partially melted powders near the surface of the solidpart and led to the adhesion of particles [64].

Fig. 5a showed the etched cross-sectional microstructure at theannular ring and intersection positions of the GA structure. The OMmicrographs demonstrated that the SLM-processed component mainlyconsisted of fine acicular microstructures. The XRD spectrum of theSLM-processed GA structure was presented in Fig. 5b. It can be clearlyseen that only α′-martensitic phase can be detected from the compo-nent, indicating that the fine acicular microstructures observed inFig. 5a was α′-martensitic phase. The SLM processing had large heatinput and rapid cooling process and led to the forming of acicular α′martensite, which was a typical microstructure of SLM-processedTi6Al4V. Similar results have been reported by some researchers[65–67]. Under the identical SLM-processing parameters, all compo-nents exhibited similar microstructures with the GA structure.

3.2. Mechanical properties

In order to investigate the compressive performance of the SLM-processed bio-inspired sandwich structures, uniaxial compression testswere carried out at room temperature. The load-displacement curves ofdifferent sandwich structures were presented in Fig. 6a. From the load-

displacement curves, NG and GC structures directly fractured after themaximum force. However, for GA and GB structures, after the max-imum force, the force abruptly decreased to a plateau before the finalfracture. Fig. 6b presented the peak crush force of different sandwichstructures. The GB structure exhibited the highest peak crush force of85.9 kN, followed by the NG structure (81.3 kN), the GC structure(77.6 kN) and the GA structure (60.6 kN).

The absorption energy and the specific absorption energy of the bio-inspired sandwich structures were calculated according to Eq. (1) andEq. (2), and the values were charted in Fig. 7a. The energy absorption ofthe GB structure was 99.68 J, which was the highest among all struc-tures. And the energy absorption of NG, GA and GC was 76.18 J, 61.46 Jand 54.47 J, respectively. At the structure design stage, all the fourstructures were designed to have the identical solid volume and overalldimensions, and the mass of SLM-processed components was closed toeach other (Table 2). Therefore, the trend of specific absorption energywas similar to that of energy absorption, which was also presented inFig. 7a. The above results indicated that the GB structure had the bestenergy absorption capacity.

In order to further investigate the bearing performance of the SLM-processed sandwich structures, the concept of specific strength ofstructures was introduced in this work [50]. The specific compressive

Fig. 7. a, The absorption energy and the specific absorption energy of the SLM-processed bio-inspired sandwich structures; b, the ultimate strength and specificstrength of the SLM-processed bio-inspired sandwich structures.

Fig. 8. Deformation of the bio-inspired structures at different displacement during compression.

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strength of a structure is defined as follow:

=σ σρs

u

s (3)

where σs is the specific compressive strength, σu is the ultimate com-pressive strength and ρs is the density of a structure.

For cellular and lattice structures, the density can be calculated bythe following equation [68]:

= ×ρ ρ ρˆ̂S 0 (4)

where ρ0 is the density of the material, ρ‾ is the relative density, whichcan be expressed as follows [69–71]:

=ρ VV‾

s

c (5)

where Vs is the solid volume, Vc is the apparent volume.According to the above equations, the ultimate compressive

strength and specific strength of SLM-processed bio-inspired sandwichstructures were presented in Fig. 7b. The results showed that the GBstructure had the highest ultimate strength (214.8MPa) and specificstrength (98.99 MPa/(g/cm3)). Comparing the specific strength withother SLM-processed structures reported in literature, such as light-weight sandwich structures (54.39 MPa/(g/cm3)) [53] and latticestructures (20.0 MPa/(g/cm3)) [70], the structures, proposed in thiswork, exhibited superior compressive properties.

3.3. Deformation behaviors

According to the trend of force-displacement curves, four SLM-

processed structures can be classified into two categories, namely directfracture (the NG and GC structures) and non-direct fracture (the GA andGB structures). To investigate the deformation progress of structures,the frames, captured from the video of four different structures re-corded during their compression tests, were presented and compared inFig. 8. For the NG structure, obvious deformation can be observed whenthe displacement exceeded 1mm (marked by yellow arrows). Whenfurther increased the displacement to 1.62mm, the structure severelydamaged and the fracture occurred along the diagonal direction, whichindicated that the shear fracture was the dominant fracture mechanismfor the NG structure. Because of the occurrence of shear fracture, theNG structure was completely destroyed, leading to the direct fracture(Fig. 6 a). Similar shear fracture was also witnessed in the GC structurewhen the displacement increased to 1.26mm. For the GB structure,when the displacement reached 1mm, obvious deformation was ob-served which mainly occurred at the top layer of big tubes (marked byyellow arrows). When further increased the displacement to 1.62mm,some big tubes in the top layer damaged resulting in the drop of force inthe force-displacement curve (Fig. 6 a). With the proceeding of thecompression test, the un-damaged tubes in the top layer still providedbearing capacity and led to the raise of force in the force-displacementcurve (Fig. 6 a). When the displacement exceeded 1.85mm (position ofthe second peak), the un-damaged tubes in the top layer graduallydamaged until the final fracture. Similar deformation process was ob-served in the big-tube layers of the GA structure.

The finite element analysis (FEA) was applied to simulate the stressdistributions in structures and analyze the failure mechanism of struc-tures. First of all, in order to verify the accuracy of numerical simulationresults, the simulated force-displacement curves of different structures

Fig. 9. Comparison of experiment and simulation force-displacement curves: a, the NG structure; b, the GA structure; c, the GB structure and d, the GC structure.

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were compared with the experimental curves (Fig. 9). Because the in-fluence of processing accuracy and defects on the mechanical perfor-mance was ignored during the numerical simulation, the simulatedcurves were higher than the experimental curves. However, the trend ofsimulated force-displacement curves was similar to that of the experi-ment curves. In addition, for the GA and GB structures, the second peakappeared in the simulated force-displacement curves. Therefore, it canbe drawn that the FEA model was reliable.

The stress distributions of different gradient structures were calcu-lated by the FEA model and presented in the left column of Fig. 10. Themiddle column of Fig. 10 was the failure mode predicted by FEA modeland the right column was macro-images of the SLM-processed bio-in-spired sandwich structures after compression tests. The damage zoneand deformed zone were marked by red (middle) and white (right)arrows, respectively. For the NG structure, the stress concentrated inthe center position and diagonal area of the structure, which presentedan “X” shape. The highest stress was in the center and reached1100MPa. The simulation results indicated that the fracture occurredalong the principal and secondary diagonal of the structure. The ex-perimental results showed that the principal diagonal position of the

structure was destroyed, but the secondary diagonal position only de-formed. For the GA structure, the stress distribution in the structure wasnon-uniform. Specifically speaking, the stress of small-sized tubes wasmuch lower than that of large-sized tubes, resulting in the fractureoccurred in two layers of large-size tubes. The deformation and fracturelocations predicted by the FEA simulation was consistent with the ex-perimental results. The stress distribution in the GB structure was re-latively uniform compared with other structures. A large area of stressconcentration cannot be found. From the results of FEA simulation andthe compression tests, the first layers of large-size tubes were destroyedunder compression and only a small amount of deformation occurred inother parts of the structure. In the case of the GC structure, the stressconcentrated in the layers of big tubes and reached up to 1102MPa.According to the fracture of the structure predicted by FEA, the layersof big tubes damaged, which was in agreement with the experimentalresults.

3.4. Fractography

In order to investigate the fracture mode, the SLM-processed GA

Fig. 10. The maps of stress distribution and the fracture mode of different structures simulated by FEA, and macro-images of SLM-processed bio-inspired sandwichstructures after compression tests.

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structure was selected as an illustrative example and the fracture sur-faces were observed by SEM (Fig. 11). As shown in Fig. 11 a, thefracture occurred at the tube wall and the crack was parallel with theaxis of the tubes and perpendicular to the direction of compression.Fig. 11 b & c showed the fracture morphology of the SLM-processed GAstructure. From low magnification SEM images, the fracture surfacesmainly consisted of dimple area and cleavage facet area, and the dimplearea was dominated. Under the high magnification SEM image ofdimple area (Fig. 11 d), the dimple exhibited small size (3–18 μm) andshallow morphology, which indicated high tensile strength, low tensileductility [64,72–74]and low elongation [75–78]. Some researchersbelieved that the reason for this phenomenon can be attributed to alarge amount of supersaturated solid solution of Al in Ti matrix due tothe extremely high cooling speed of SLM-processing [79]. Under thehigh magnification SEM image of cleavage facet area (Fig. 11 e), theshell patterns can be found, which was a typical indication of brittlerupture. Apart from the fracture morphology, the compression curvescan also illustrate the fracture modes of structures. According to theloading-displacement curves of Ti6Al4V bio-inspired sandwich struc-tures shown in Fig. 6 a, the curves displayed a suddenly dropping afterreaching the peak value, which indicated the brittle fracture feature[80]. Therefore, the SLM-processed bio-inspired Ti6Al4V sandwichstructure exhibited a mixed mode of ductile and brittle fracture undercompression loading.

It is generally known that the microstructure of the TiAl6V4 alloydepends on the cooling rate during solidification. The extremely fast

cooling rate during the SLM process results in a fine acicular martensiticphase, which is a main factor affecting the mechanical properties of theSLM-processed Ti6Al4V samples. According to the literature [81], thepresence of the metastable α′ martensite contributed to the enhance-ment of yield strength and ultimate tensile strength, but resulted in thelow ductility. The preheating of powder bed during the SLM processingcan effectively reduce the cooling rate [82], adjust the microstructure[83,84] and reduce residual stress in the structure [84], which couldimprove the ductility of SLM-processed Ti6Al4V samples. In addition,post-treatments, such as HIP treatment, could improve ductility ofsamples by adjusting the microstructure [81,85–87] and eliminatingmetallurgical defects inside components [88–91].

4. Conclusions

In this study, SLM technology was successfully applied to fabricategradient sandwich structures inspired by the Norway spruce stem. Themechanical properties and the energy absorption behavior of thestructures were evaluated by uniaxial compression experiments. Inorder to investigate the fracture mechanism of the bio-inspired gradientstructures, the LS-DYNA finite element analysis software was applied toanalyze the stress distribution and predict fracture behavior. The fol-lowing conclusions can be drawn from this study:

(1) The Ti6Al4V bio-inspired gradient structures, manufactured by theoptimized SLM parameters, exhibited a high relative density and

Fig. 11. The fracture surface of the SLM-processed GA structure after compression tests. a, the macro-graph of the gradient structure A after compression tests; b andc, low magnification SEM images of the fracture surfaces; d and e, high magnification SEM images of the fracture surfaces.

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similar microstructures among structures.(2) The mechanical properties of gradient structures under compres-

sion loading closely related to the arrangement of large and smalltubes. The gradient structure, with a layer of big tubes adjacent tothe top and bottom plate and four layers of small tubes arranged inthe center, possessed the highest SEA (5.73 J/g) and specific com-pressive strength (98.99 MPa/(g/cm3)) among all structures.

(3) The results of FEA simulation revealed that the arrangement of bigand small tubes significantly affected the stress distributions in bio-inspired sandwich structures. The gradient structure, with thehighest SEA and specific compressive strength, exhibited the mostuniform stress distribution, contributing to its excellent compres-sive performance. In addition, the fracture location predicted byFEA simulation was in agreement with that of the experiment.

(4) The fracture morphologies of SLM-processed bio-inspired sandwichstructures consisted of shallow dimples and cleavage facets, in-dicating a mixed mode of ductile and brittle fracture under com-pression loading.

Acknowledgments

The authors gratefully acknowledge the financial support from theNational Natural Science Foundation of China (No. 51735005), the fi-nancial support from Natural Science Foundation of Jiangsu for Youths(No. BK20170787), the Graduate Innovation Base (Laboratory) OpenFund of Nanjing University of Aeronautics and Astronautics (No.kfjj20180620), and the Priority Academic Program Development ofJiangsu Higher Education Institutions.

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