Experimental Investigations on the Mechanical Performances of Auxetic Metal-Ceramic Hybrid Lattice under Quasi-Static Compression and Dynamic Ballistic Loading

Abstract

In recent years, there have been increasing research interests in investigating the compression and ballistic responses of metal-ceramic hybrid structures, mainly making use of the synergistic effects of conventional metal honeycomb structures and infilled ceramic matrix materials. In this paper, a novel hybrid auxetic re-entrant metal-ceramic lattice is designed and manufactured to overcome the intrinsic conflicts between the strength and toughness of architected mechanical metamaterials, synergistic effects of auxetic re-entrant metal honeycombs and infilled ceramic materials are experimentally and numerically studied, and auxetic deformation features and failure modes are characterized with the digital image correlation (DIC) technique as well. It was found that (1) the infilled ceramic matrix of conventional honeycomb frames only endure longitudinal compression or impact loading along the external loading direction, while auxetic metal re-entrant honeycomb components endure both longitudinal and transverse loading due to the negative Poisson′s ratio effect and (2) the collaborative effects of infilled auxetics and the constraint frames’ hybrid structure dramatically moderate the stress concentration state and improve the impact resistance of single-phase ceramic materials. Our results indicate that the auxetic hybrid design exhibits promising industrial application potentials for blast protection engineering.

1. Introduction

Composite structures consisting of tough metals and brittle ceramics with rational spatial distribution configurations can be manufactured to generate inaccessible properties that single-phase constituent material cannot provide. Wilkins first proposed metal-ceramic composite armor and conducted a series of ballistic studies [1]. Afterward, various types of novel metal-ceramic composites were proposed for advanced ballistic protection structure applications, including sandwich structure [2,3], functionally graded structure [4,5], and lateral restraint structure [6]. These novel composites show better ballistic performances than single metal or ceramic constituent materials separately.

Additive manufacturing enables the fabrication of industrial components with complex, intricate geometrical structures. With the development of novel mechanical design schemes and the progress of advanced manufacturing techniques, more and more periodically, mode arranged porous lattices filled with ceramics have been studied [7,8]. Wadley et al. [9] studied the synergistic enhancing effects of hybrid structures consisting of ductile metals and infilled brittle ceramics under dynamic projectile penetration loading conditions. It was found that infilled ceramics in the metal lattice can attenuate shear band failure mode and proposed a novel design of hybrid lattice structure consisting of triangular metal lattice and built-in ceramics [10]. The compressive strength and energy absorption [11], bending stiffness and strength [12], and impact response [13] of this novel hybrid metal-ceramic lattice were studied systematically. Dong et al. [14] designed a novel hybrid lattice consisting of a quadrilateral metal lattice and infilled ceramics and analyzed the relation between energy absorption performance indicators and spatial distributions of infilled ceramic constituent materials. Similarly, the penetration resistance of hybrid materials with orthogonal ceramic prisms [15,16] was also studied. Recently, researchers also studied other types of hybrid structures consisting of infilled ceramics and metal lattice frames, such as pyramid lattice [17,18], hexagonal honeycomb lattice [19], and derivative honeycombs [20]. Meanwhile, researchers also performed various types of experimental [21], numerical [22,23], fabrication process [24,25], and theoretical studies [26,27] on the design, and performance evaluation of metal-ceramic lattices for dynamic impact resistance applications.

In this paper, the cooperative design of strength and toughness of a novel ceramic-metal hybrid metal lattice structure based on a structural hybrid of an auxetic metal lattice and the infilled ceramic matrix is performed. The deformation and failure processes are analyzed through finite element analysis, and digital image correlation (DIC) characterization of experimental results is also performed for comparisons. It is found that the ceramic-metal hybrid metal lattice structure based on the auxetic frame can improve the quasi-static mechanical properties of conventional ceramic-metal hybrid honeycomb structures. Moreover, when the ceramic matrix-filled auxetic metal lattice structures are subjected to external dynamic impact loading, the compressive and ballistic resistances of ceramic-filled hybrid lattice structures will be significantly improved.

2. Synergistic Mechanical Design of Strength and Toughness with Ceramic-Infilled Re-Entrant Lattice

When materials and structures with a negative Poisson′s ratio are compressed, they will shrink laterally. In the following, the classic re-entrant auxetic lattice structure is referred for design and fabrication. As shown in Figure 1a, $l$ is the width of honeycomb cells, h is the height of honeycomb cells, θ is the concave angle, t is the honeycomb cell wall thickness, and in the following examples, the geometrical parameter $l=h$ is specified. The main reason for the formation of a negative Poisson′s ratio within re-entrant honeycomb material is axial compression and hinge rotation, which makes the inclined rib bend inward, resulting in simultaneous contraction of the structure in both transverse and longitudinal directions, as shown in Figure 1b. If the ceramic is filled into re-entrant hexagonal honeycomb frames, the built-in ceramic will be subject to a lateral constraint so that the hybrid re-entrant auxetic lattice with built-in ceramic can be employed for improving both strength and toughness. As shown in Figure 1c,f, design and mechanical tests of the unit cell consisting of different types of hexagonal honeycomb structures are performed first. When the hexagonal unit cell is compressed, internal defects (cracks, voids, etc.) will appear along the horizontal direction, as shown in Figure 1d. Ideally, when the negative Poisson′s ratio unit cell is compressed along vertical direction, cracks will not spread due to metal frame contractions along both the vertical and horizontal directions, as shown in Figure 1g. In order to verify the influence of the external metal frame on the built-in ceramic matrix, this paper verifies the stress state of ceramics within this hybrid material unit cell under compression loading conditions from the perspectives of simulation and experiment comparisons. The internal cell width and internal cell height of the metal honeycomb structure are l = h = 35 mm, the thickness t = 5 mm, and the depth of the structure is 10 mm.

Figure 1i,j illustrate the sideview of quasi-static compression finite element analysis results of single cell hybrid materials, in which only the horizontal stress contour plot of the built-in ceramic is shown. Conventional and re-entrant hexagonal metal ceramic composite structures are subject to vertical compression at a loading speed of 0.004 mm/s. In the numerical calculation of quasi-static compression, the constitutive model of the outer metal frame used in the FEM model is the Johnson–Cook model [28] and the constitutive model of inner ceramic in the FEM model is the Drucker–Prager Cap model [29]. The FEM model of built-in ceramic was simulated with brick (C3D8R) elements with max Warpage = 0, max aspect ratio = 5.44, max Skew = 0, min Jacobian = 0.712, min angle = 60° and max Angle = 60°. Figure 1i,j also shows the contour of horizontal stress within ceramic materials. At the beginning of compression, for the ceramic in the ordinary honeycomb, 73.3% of the volume is transversely compressed and the average horizontal stress is −12.8 MPa, and this value is 85.4% for the transversely compressed re-entrant honeycomb. The average horizontal stress is −108.3 MPa. For the ceramic within the ordinary honeycomb, the horizontal stress of most ceramic elements is between (−100, +27) MPa. However, for the ceramic in the re-entrant honeycomb, the horizontal stress of the ceramic phase is relatively evenly distributed between (−450, +27) MPa. At the moment of compression failure, for the ceramic in the ordinary honeycomb, 60.0% of the volume is transversely compressed and the average horizontal stress is −9.7 MPa, while this value is 90.2% in the re-entrant hexagonal honeycomb, and the average horizontal stress is −141.3 MPa. With the increase in compression loading, the ceramics in the re-entrant honeycomb structure endure larger horizontal compressive stress.

3. Fabrication and Experiment Schedule

Hybrid lattice sample fabrication and mechanical experiments are divided into two parts: the single unit cell lattice experiment (Figure 2a) and the multi-cell lattice experiment (Figure 2b). Two different types of hybrid structures consisting of ceramic and metal frame mass are shown in Figure 2a1, where the metal frames are conventional and re-entrant honeycomb shape, and the samples are made of a single unit cell, respectively. Afterwards, the quasi-static compression experiment (Figure 2a2) and the Hopkinson dynamic compression bar experiment (Figure 2a3) are performed, respectively, to study the mechanical properties of these two types of hybrid honeycomb lattices. Meanwhile, two different types of hybrid structures architected with multi-unit cells are designed and fabricated to understand the effects of layer numbers and unit cell numbers within each layer (Figure 2b1,b3), where the metal frames within the ceramic and metal frame hybrid structures are conventional and re-entrant unit cells, respectively. For these multi-unit cell lattice samples, only the Hopkinson compression bar experiment is performed (Figure 2b2). In the unit cell quasi-static compression test, the cell internal width and internal height of metal honeycomb frame structure is l = h = 35 mm, the thickness t = 5 mm and the depth of the structure is 10 mm. For the sample used in the Hopkinson experiment, the unit cell width and height of the metal honeycomb structure are l = h = 5 mm, the thickness t = 1.2 mm and the depth of the structure is 9 mm.

As to the fabrication details of the 316L stainless steel external frame, powder of 316L stainless steel (Zhongmai Metal Co. Ltd., Nanjing, China), certified with the diameter of 45 μm and the chemical composition of raw powder materials before the laser powder bed fusion process is shown in

Table 1. Chemical composition of the 316L stainless steel.

Fe	Cr	Ni	Mo	Mn	Si	P	C	S
Balance	16–18	10–14	2–3	≤2	≤1	≤0.045	≤0.03	≤0.03

, where energy dispersive X-ray spectroscopy (EDS) in the SEM was used to characterize the chemical composition of the as-manufactured samples. We have chosen LPBF as the manufacturing method for exterior metal because LPBF is a timesaving, energy-saving, and clean manufacturing method. LPBF is a very mature way of manufacturing stainless steel 316. The 316L stainless steel external frames were fabricated on FS271M (Farsoon^® Technology Co. Ltd., Shanghai, China), where the building direction of the re-entrant and ordinary honeycomb frames is along out-of-plane z direction, which is perpendicular to the x-y in-plane of 2D honeycomb. To obtain as-fabricated specimens with maximal density and minimal porosity, the process parameters were set as: laser beam power of 200 W, laser beam diameter of 0.1 mm, laser scanning speed of 2000 mm/s, layer thickness of 0.05 mm. In addition, the argon gas shielding technique is employed throughout the 3D printing process to avoid the risk of oxidation and contamination of alloy powder raw materials. The isotactic pressing sintering alumina ceramics were manufactured by ShenZhen Dachuan Ceramic Co. LTD, China. Afterwards, metal honeycomb frames and ceramic matrices are integrated through the snap-fit mechanical process for harvesting the final hybrid lattice (Figure 2a1,b1). In total, three specimens were tested for each experiment. As shown in Figure 2d, the samples are fabricated along the out-of-plane of the 2D honeycomb, built layer by layer with 316L powder with a bi-directional, double pass of laser beam, and 90$°$ rotation scan vector between layers, thus the in-plane anisotropic mechanical properties can be minimized. As to the mechanical properties of constituent materials, 316L stainless steel is a good elastic-plastic material with a yield strength of 310 MPa, an ultimate strength of 690 MPa, and an elongation limit of 40%. The ceramic matrix exhibits linear brittle mechanical properties with a modulus of 350 GPa, and an ultimate strength of 3500 MPa.

The Instron-5985 electronic universal testing machine is used to perform a compression test of an as-fabricated hybrid structure, and the digital image correlation (DIC) technique is employed for deformation field characterization, and the compression loading rate is 0.004 mm/s. The dynamic mechanical tests are performed on the ZDSHPB-40-100 Hopkinson compression rod system (manufactured by Shandong ZongDe Electromechanical Equipment Co. Ltd., Jinan, China) for the single cell Hopkinson test and the ZDSHPB-4080 compression rod system for the multi-cell lattice sample experiment. Afterwards, the incident wave and transmission wave are identified and separated, and the corresponding parameters such as strain rate, stress, and strain energy are calculated according to the experimental parameters.

4. Results and Discussion

4.1. Mechanical Tests

Figure 3a shows the displacement-load curves of the two hybrid honeycomb samples during the compression process. It can be seen that the load curve of the conventional hex hybrid structure is lower than that of re-entrant hybrid lattice. When the compression displacement of the conventional hexagonal hybrid structure reaches 2.59 mm, the compression load drops drastically from peak 175 kN, and the ceramics materials are damaged and crushed into powder. The pressure value of the re-entrant hybrid lattice also increases with the increase in compression displacement. When the compression displacement is 2.75 mm, the compression load reaches a peak value of 235 kN. However, no sudden drop is observed after the peak value, which is quite different from that of the conventional hex hybrid structure. The compression load decreases slowly because only the upper surface of the re-entrant hybrid lattice is damaged, demonstrating superior compression resistance of the global composite structure. The compression load of the re-entrant hybrid lattice remains at 200 kN, at the compression displacement of 3.33 mm, so the re-entrant hybrid lattice can withstand a higher compression load than the conventional hex hybrid structure, and the re-entrant hybrid honeycomb can also resist a larger compressive displacement. During the compression process of re-entrant hybrid lattice filled with ceramics, the load decreases slowly with the increase of compression displacement, and the infilled ceramics fail accordingly. The conventional hexagonal hybrid structure failed at the compression displacement of 2.59 mm, exhibiting sudden burst failure features, and the compression load decreased instantaneously, as shown in Figure 3b. It shows that stress concentration within the conventional hexagonal hybrid structure will trigger ceramic crushing and bursting and induce global composite structure failure. Through comparison between these two types of hybrid honeycomb structures, it can be seen that the re-entrant hybrid lattice cannot be damaged instantaneously and shows good compression resistance—because concave shrinkage deformation on both sides can produce remarkable constraint, thus increasing the hydrostatic pressure within ceramic materials. It can be seen from Figure 3c that cracks are formed in the upper half of the built-in ceramic within the re-entrant hybrid lattice, while the lower half is intact, and the overall structure exhibits remarkable structural integrity.

Strain field within the built-in ceramic matrix surrounded by a metal structure frame at different displacement steps is detected and recorded with DIC equipment, as illustrated in Figure 4a–f. It can be seen from Figure 4a that when the compression strain of the conventional hex structure along the vertical direction is 0.5%, the strain value is low and evenly distributed within ceramic matrix. The maximum strain of built-in ceramic along the horizontal direction is 0.62%, and the strain of the most built-in ceramic in the horizontal direction is less than 0.06%. When the compress strain along the vertical direction is 2.88%, there is an obvious vertical area with a large horizontal strain within a regular hexagon, as shown in Figure 4b. Afterwards, there is an obvious vertical region with a large horizontal strain. The horizontal strain of most ceramics in this region is greater than 1.80%, and the maximum horizontal strain is 3.10%. Finally, as shown in Figure 4c, when the compression failure strain along the vertical direction is 5.75%, the global horizontal strain of the most ceramic matrix is greater than 5.22%. The maximum horizontal strain is 8.79%, exhibiting high local strain concentration features. Accordingly, the ceramic infilled matrix with an obvious horizontal strain suddenly falls off, as shown in Figure 3a.

Figure 4d,f shows the strain contour of built-in ceramics in the re-entrant hybrid structure at different steps during compression process, and these three compression loading steps correspond to the deformation snapshots of the conventional hex hybrid structure at the same compression loading steps. At the beginning of the compression process, the compress strain along the vertical direction is 0.5%. As shown in Figure 4d, the strain value within the ceramic matrix is low and the strain field is almost evenly distributed. The maximum strain of the built-in ceramic along the horizontal direction is 0.23%, and the strain of most built-in ceramic along the horizontal direction is less than 0.02%. As shown in Figure 4e, the strain value along the vertical direction is 3.70%. Unlike conventional hex, the re-entrant hex does not exhibit a large local horizontal strain concentration band along vertical direction. There are three short stripes with obvious stress concentration near the compression top and bottom boundaries within the re-entrant ceramic matrix. The horizontal strain within these areas is above 0.70%, and the maximum local strain value is 1.97%. At the moment before compression failure, the strain of the compress test along the vertical direction is 7.39%, as shown in Figure 4f. In addition to the original strip areas with an obvious horizontal strain, new short strip areas appear. The strain value within these local strip areas is generally about 1.33%, and the maximum horizontal strain is 2.34%. Due to the limitation of metal materials on horizontal sides, the horizontal strain is also significantly smaller in value than the conventional hexagonal honeycomb. At the same time, due to the influence of the negative Poisson′s ratio effect, the vertical strip area with an obvious horizontal strain will not penetrate the whole ceramic from the top to the bottom.

In the ceramic-metal hybrid honeycomb structure design, the synergistic effect for improving the energy absorption during the compression process is expected, where interaction between the metal frame and the infilled ceramic matrix will contribute to the deformation modes evolution, failure features, and improvement of damage limits. The energy consumed during the loading process for the ceramic-metal hybrid honeycomb is larger than the sum of each constituent single phase material under compression separately and can be expressed as 1 + 1 > 2. As shown in Figure 5a,b, the compression displacement-force curve of ceramic-metal hybrid lattice is given. Meanwhile, compression experiments of the single ceramic phase and the single metal phase were also conducted with Instron-5985, and corresponding displacement-force curves are plotted for comparisons.

Table 2. Synergistic effect analysis of quasi static compression test.

Lattice Type	Mechanical Characteristics	Inner	Outer	Hybrid	Arithmetic Sum of Inner and Outer	HAR
		Ceramic	Metal	Structure
Hex	peek load (KN)	95.4	15.3	175	110.7	1.58
	strain (%)	2.3		5.6
	energy absorption (kJ)	1.5	1.3 ([email protected]%)	9.7	2.8	3.46
Rehex	peek load (KN)	256.4	20.1	235	276.5	0.85
	strain (%)	2.9		7.4
	energy absorption (kJ)	4.8	2.4 ([email protected]%)	21.6	7.2	3

shows the details of synergistic effects during the quasi-static compression test, where HAR is the hybrid arithmetic ratio of the arithmetic sum and hybrid structure. Compared with the conventional hexagonal honeycomb structure, the re-entrant hexagonal auxetic structure shows advantages over single ceramic, single metal, and conventional hybrid honeycomb structures. However, in the analysis of the synergistic effect, the phenomenon of the two structures is more complex. It is generally believed that the peak stress and energy absorption capacity of hybrid lattice structures can be greatly improved by filling ceramic built-in structures in metal lattice cells. However, in fact, in the unit cells discussed in this paper, the synergistic effect in terms of peak stress is only reflected in the ordinary hexagonal structure. For the ordinary single hexagonal honeycomb, the peak stress and energy absorption capacity of hybrid honeycomb structures can be greatly improved by filling the metal lattice cells with ceramic built-in structures. The peak stress and energy absorption capacity are due to the arithmetic superposition of separated metal shells and built-in ceramic structures. In terms of peak stress, the combination of the ordinary honeycomb and the built-in ceramic is increased by 1.58 times. This phenomenon is more obvious in the study of energy absorption in the ordinary honeycomb. When the ceramic and metal are combined, the peak load that the whole hybrid structure can bear is significantly greater than that of the single material, and the peak load that the hybrid structure is 1.8 times that of the single ceramic. The maximum strain that withstands is also increased significantly, and the deformation that can be withstood is 2.4 times of that of single ceramics. The energy absorption capacity of the hybrid structure is significantly greater than that of a single ceramic or metal and is also greater than the arithmetic sum of the two. The combination of the ordinary honeycomb and built-in ceramics has increased by 3.46 times.

However, for the re-entrant hex auxetic structure, the synergistic effect of 1 + 1 > 2 is not found in term of peak load. For the concave structure, the compressive load of the re-entrant hexagonal single-phase ceramic (256.4 kN) is even higher than that of the metal-ceramic honeycomb hybrid structure (235 kN). In terms of energy absorption characteristics, the re-entrant hexagonal auxetic structure also shows obvious synergistic effects. After the combination of ceramic and metal, the peak load of the hybrid structure is slightly lower than that of single ceramic (the peak load that hybrid structure can bear is 0.92 times that of pure ceramic). There is an obvious increase in the highest strain for the hybrid structure (the deformation it can bear is 2.4 times of that of pure ceramics). The combination of the re-entrant honeycomb and built-in ceramics has increased by 3.00 times. It can be seen from the table that even the addition of relatively soft 316L reduces the peak load, but it still improves the energy absorption. In fact, the role of metal frames is not simply to enhance energy absorption.

4.2. Dynamic Mechanical Tests

The dynamic Hopkinson compression bar is used to test the dynamical performances of metal-ceramic hybrid honeycomb structures, and samples with the conventional hex hybrid structure and the re-entrant hex hybrid structure are of the same weight. Figure 4a shows the impact displacement-load curve of the re-entrant hex hybrid and conventional hex hybrid structures at different impact speeds. The re-entrant hex can provide a more significant load with a minor strain than the conventional hexagonal honeycomb at the same strain rate. With the increase in strain rate, this phenomenon becomes more and more prominent at the strain rate of 2.54 × 10³/s. The maximum load of these two structures is 47.5 kN and 60.4 kN, respectively. The re-entrant hexagonal hybrid honeycomb structure can bear a 27.3% higher load than the conventional hex hybrid structure at a strain rate of 2.54 × 10³/s. Compared with the conventional hexagonal honeycomb the re-entrant hex has a minor ultimate strain under the same dynamic impact loading conditions. In the maximum impact involved in the experiment, the strains of conventional and re-entrant hexagonal honeycomb structures are 7.29% and 5.81%, respectively. Figure 5c,d shows the strain-load curves and synergistic effect analysis of hybrid honeycombs under the dynamic Hopkinson compression bar experiments at the strain rate of 2.54 × 10³/s. The compression experiment of the single ceramic and single metal phase was also conducted with the ZDSHPB-40-100 dynamic compression rod system. Similar to the results of quasi-static experiments, the re-entrant hexagonal auxetic structure shows mechanical advantages over the conventional hexagonal structure, in terms of the single ceramic phase, the single metal phase, and hybrid structures, respectively. The peak load produced by the single re-entrant hexagonal ceramic phase under dynamic impact is even higher than that produced by the metal-ceramic composite structure. In the experiment, although the single re-entrant hexagonal ceramic provides a higher peak load, the single re-entrant hexagonal ceramic is more completely crushed than the conventional hex structure after the dynamic impact loading experiment. After the dynamic impact load, conventional hexagonal ceramic was crashed into fragments composed of blocks, and the re-entrant hexagonal ceramic was smashed into powder or particle. After the introducing of metal frame, only cracks appeared in the conventional hexagonal structure, and there are no visible cracks that appeared in the concave honeycomb after impact loading. Figure 6a illustrates the protective effect of the metal frame on the built-in ceramics under impact load. With the increase in strain rate, the ultimate load of the conventional hex hybrid structure does not change much. While the strain rate changes from 1.86 × 10³/s to 2.54 × 10³/s, the maximum load only increased by 6.2%. For the re-entrant hex hybrid structures, the ultimate load changes obviously with the strain rate, and the maximum load increased by 16.0% under the exact strain rate change within the strain rate variation range of the experimental study. The metal frame will contract when the hybrid structure is under dynamic loading, and the infilled ceramic is compressed along both the impact direction and the direction perpendicular to the impact direction. With the increase in impact velocity, the phenomenon of simultaneous pressure along two orthogonal directions becomes more obvious, thus the re-entrant hybrid structure can generate elevated ultimate strength at higher speed.

Figure 6b,c show the samples before and after dynamic impact loading of conventional hex hybrid structures consisting of a one unit cell. Figure 6f illustrates the multi-layer periodic architected sample clamped by the incident rod and the transmission rod before dynamic impact loading of the incident rod. Figure 4g illustrates the multi-layer periodic architected sample after impact, and it can be seen that a crack is triggered within the internal ceramic matrix. The external metal cells are also impacted and deformed, which is similar to the compression deformation of the conventional hex structure. Figure 6d,e show the test sample of the re-entrant hybrid structure sample consisting of a one unit cell before and after impact. It can be seen that there is no obvious change before and after the dynamic Hopkinson compression bar experiment. There is no obvious crack in the internal ceramics, and it is more closely combined with the external metal. Due to the impact force and negative Poisson′s ratio effect of the re-entrant hex structure, the internal ceramics are wrapped more tightly, and the ceramics are well protected from fragmentation. Metal-ceramic composite structures can generate the full advantages of high strength and compression resistance of ceramics when enduring high-speed impact loading. At the same time, structural advantages of external metals can protect ceramics from crushing failure. In order to focus on the protective effect of different metal frames on the built-in ceramics, in the subsequent periodic lattice Hopkinson experiment, the mass loss of the built-in ceramics is employed for evaluating the protecting performances.

In order to better understand the protection effects of metal frames on built-in ceramics, the method of statistical ceramic volume loss after impact is employed for evaluation. Figure 6f–i illustrates the experimental results comparisons between the traditional conventional hex lattice and the re-entrant hex auxetic lattice. Figure 4f,g demonstrate the typical results of two different lattices after impact (the strain rate is 8.8 × 10²/s). Figure 6h,i show the corresponding scanning 3D imaging results, and the 3D scanner used in this study is Zeiss Atos Q with a 170 mm focal length lens and 8M pixels. The internal ceramic materials of the two hybrid lattices are damaged into cracks after high-speed impact. Under the strain rate of 8.8 × 10²/s, the average volume of the ceramic loss of the traditional honeycomb lattice is 65 mm³. Under the same train rate, the volume loss of the re-entrant hex auxetic lattice is 17.8 mm³ (Figure 6j). This experimental result is similar to previous theoretical analysis and experimental results of the single unit cell sample, which further shows the mechanical benefits of strength and toughness.

5. Conclusions and Discussions

Novel hybrid metastructures consisting of an auxetic metal lattice honeycomb and ceramic matrices are designed and fabricated for improving the specific energy absorption and dynamic protection performances of brittle ceramic materials. Lateral constrained compression experiments are performed for verification. Experimental and numerical results show that when the external re-entrant auxetic honeycomb is subjected to compression, uniaxial compression loading along the vertical direction can be converted into both vertical and horizontal compression loading within the infilled ceramic matrix, thus can greatly increase the compressive and impact mechanical performances of single phase infilled ceramic materials. The proposed metal-ceramic hybrid metastructures can provide mechanical advantages of synergistic effects between the ceramics matrix and the metal honeycombs through the phase boundary interaction so that ceramics and metals have better deformation compatibility.

Based on DIC deformation field analysis and thde in-situ quasi-static compression experiment, it was found that infilled ceramic matrix within the re-entrant hexagonal metal honeycomb structure can effectively reduce stress concentration. A benefit from the negative Poisson′s ratio effect and phase boundary metal-ceramic interaction, the outer metal honeycomb can impede crack propagation within infilled ceramic during the compression process. Afterwards, dynamic Hopkinson impact experiments were performed on the single-phase unit cell and the metal-ceramic hybrid honeycomb structure, and it was shown that under the same impact loading conditions, the impact protection performances of the ceramic filled auxetic re-entrant honeycomb was better than that of the ceramic filled ordinary honeycomb structure. Such a protection effect is particularly more remarkable at a higher impact speed. It can also be seen from the experimental results that the unconstrained interface of ceramics is quite vulnerable to impact. Therefore, the hybrid design method of strength and toughness will be more advantageous if 3D auxetic structures are employed as the metal lattice in the hybrid material. Recent studies have shown that the impact resistance of lattice structures is related to impact speed [30]. Our study found that the impact resistances are related to both loading speed and unit cell lattice geometrical configurations. Single ceramic material is superior to the metal-ceramic hybrid structure in terms of dynamic impact energy absorption in Hopkinson tests, respectively. Such dynamic experimental results are quite different from static compression loading results. Such abnormal phenomenon and underlying physical mechanisms will be further studied in the future.