Design and assembly
Natural hard materials with the hierarchical heterogeneous structure can inspire new paths to construct some advanced materials with outstanding mechanical properties27,28. We designed a protective NISC composite material that could embody a unique macroscopic lamellar architecture and a higher impact resistance capability or more excellent anti-penetration performance. As shown in Fig. 1B, the special steel-bar framework, which consisted of ordered quadrate patterns on the plane and an insertion of out-of-plane steel bar array with about 30 mm height, was first constructed by the welding process. These soldering points in the steel bar structural frame were sufficiently strong to withstand considerable impact without damage. Next, the steel bar framework was placed into a regular shuttering constructed with concrete placing and curing to form steel bar framework-reinforced concrete platelet as an ideal building block (Length × Width × Height (Dimension) = 300 mm × 300 mm × 50 mm), mimicking nacreous platelet. The dimension was chosen to create platelets with an aspect ratio of 6, which is slightly lower than the mineral platelets (10) in natural nacre8,16. Furthermore, a steel framework-reinforced concrete platelet was assembled/glued on the top surface of staggered nacre-like arrangement by a ~5 mm silicone polymeric interlayer, in order to form the polymer-coated steel-concrete structural unit. An important step was to select the polymers with mechanical attributes similar to the interfaces in natural mollusk shell. Several polymers we tested in shear were too brittle (leading to poor energy absorption) and/or too strong (resulting in unwanted fracture of the glued layer). Silicone polymer material was eventually selected as the cross-linked interface material because of its relatively low strength, very high deformability in shear, strain hardening, and high energy absorption. Several framework-reinforced concrete platelets were laminated with ~5 mm thick polymeric interlayers. During the assembly, the framework-reinforced concrete platelets were carefully aligned so that the tablets formed a staggered nacre-like arrangement analogous to natural nacre13. Finally, the NISC target material with two different sizes (Dimension = 300 mm × 300 mm × 100 mm and 300 mm × 300 mm × 300 mm), could be obtained by the bottom-up assembly technique, named by NISC-100 and NISC-300, respectively. Clearly, the effect of macroscopic nacre-like architectural design on the mechanical properties will be reflected in superior rigidity of the NISC composite compared with conventional SC composite at the same overall geometric density, similar with the construction of the impact-resistant nacre-like super glass composite27.
The whole preparation process of the NISC composite material is shown in Fig. 1C. We could observe that (from left to right): I) the unique steel bar framework was finely welded; II) the concrete was poured into the framework to form the steel framework-reinforced concrete platelet; III) the 5mm-thickness silicone polymer material was uniformly coated on the surface of the steel-concrete structural unit; IV) the NISC composite material was finally constructed to be applied as test target material, as shown in Fig. 1C. For the convenience of subsequent performance testing, these different types of target materials were packaged by home-made galvanized iron molds (see detailed in Supplementary Fig. 1).
Static mechanical performance
To prove the obvious validity of this nacre-inspired design strategy to construct the steel bar reinforced concrete material, the static mechanical responses of these concrete target materials with special sizes were studied first, as shown in Fig. 2 and Supplementary Table 1. Firstly, we can observe that the special specimen of the bulk concrete was deformed/broken by compressive tests (Fig. 2A). Obviously, the NISC target materials displays a better compressive mechanical performance compared with pure concrete. The compression stress of the NISC concrete material is about 68.5 MPa, which is about 32.0% higher than that of pure concrete (51.9 MPa). The value of compress modulus and Mohs hardness for the NISC composite material is ~42 GPa and ~5.5 GPa, respectively, clearly superior to that of the pure concrete (Fig. 2C). Bending tests further revealed that the NISC composite material achieved a better bending resistance capability in comparison with pure concrete. Compared to pure concrete, the maximum Young’s modulus and shear modulus of the NISC composite material is clearly enhanced, up to ~41.5 GPa and ~18.4 GPa, respectively, superior to those of the pure concrete (36.6 ± 4.5 GPa and 15.6 ± 2.0 GPa).
Penetration of projectiles
The anti-penetration capabilities of the NISC composite targets were then systematically investigated by the hypervelocity impact tests to compare with that of the pure concrete under different striking velocities (v = 1 km/s and 2 km), as shown in Figs. 3-5, Supplementary Figs. 2-6 and Movies 1-3. Several shots of pointy ovoid-head steel projectiles (30CrMnSiA) penetrating into pure concrete and NISC targets are conducted. Firstly, the hypervelocity impact impressions on these target materials are observed with optical photographs and Movies (Figs. 3 and 4, and Supplementary Movies 1-3). Before the shots, the intact structural state of the target in the test equipment is shown in Fig. 3A-C. After the shots under a striking velocity of 1 km/s, there is a 5-mm bullet hole in the pure concrete. And a crack propagation along the bullet hole (the long crack cut through the whole target) to generate and the surficial concrete around the bullet hole also isn’t destroyed, as displayed in Fig. 3D. Obviously, the localized damages occur on the impact surfaces of the concrete target, confirming that the dimensions of the target are large enough to neglect the boundary influences29. To capture the shape and dimensions of impact craters more precisely, the silicone casting method was used to reconstruct the 3D crater and the projectile trajectory within the concrete, as shown in Fig. 3E. The damage of the pure concrete target, dimensions of crater, and a j-shaped crater followed by a cylindrical tunnel can be clearly observed. And then, the typical concrete targets were carefully cut along the penetration boreholes, the sectional views of the typical concrete targets are exhibited in Fig. 3F,G. We can observe that the projectile penetration forms a frustum-shaped crater followed by a cylindrical tunnel. As we all know, the diameter of tunneling is consistent to that of projectile when the striking velocities are relatively low where the projectiles can be considered as rigid bodies30. However, when the striking velocity was enhanced up to 2 km/s, the bullet hole in the pure concrete was obviously broaden, up to about 15 mm. Clearly, the tunneling diameter under is larger than that of the projectile, which further confirms the assumptions made by several researcher’s groups29,30. Several large cracks propagate around the bullet hole (these large cracks pass through the whole target), a large area of the surficial concrete around the bullet hole is seriously destroyed. And a heavy damaged state can be observed inside the concrete target. These results reveal that it isn’t unaffordable for the common concrete against hypervelocity impacts.
Furthermore, after the penetration test, the pure concrete targets were carefully cut to recover the residual projectiles. Figure 3J presents the photographs of the 1 km/s and 2 km/s recovered projectile (v0 = 1 km/s (top) and v0 = 2 km/s (bottom)). Obviously, compared to the unfired projectile, the length and mass of residual projectile reduced a greater or lesser degree. The residual length and mass of 1 km/s recovered projectile (l1, m1) and 2 km/s recovered projectile (l2, m2) are 25.1 mm/5.48 g and 12.3 mm/3.56 g (see Table 1). The residual length and mass loss of the projectile at a striking speed of 2 km/s are much more serious than that at a striking velocity of 1 km/s. Quantitatively, the residual length and mass loss of the projectile change from 26.4 mm/5.76 g for unfired projectile to 25.1 mm/5.48 g for 1 km/s recovered projectile to 12.3 mm/3.56 g for 2 km/s recovered projectile, respectively. Correspondingly, when the initial velocity of projectiles is 1 km/s, the deformation of projectiles become evident, as shown in Fig. 3J. The blunted length of the recovered projectiles is 4.9%, while the relative mass loss is also 4.9%. This regime indicates that the projectile is deformed but not eroded during the penetration process. When the initial velocity of projectile is 2 km/s, the relative mass loss is exacerbated and reach 38.2%. The mass loss and residual length of projectile increase and decrease with the rising of striking velocity, respectively. This regime is considered as eroding projectile penetration.
In contrast, we found that the impact impressions of the NISC composite target was clearly distinguished from that of pure concrete. After the NISC composite target was impacted under a striking velocity of 1 km/s, a projectile passed through the steel bar on the front NISC-100 target, leaving a small bullet hole in the pure concrete and a small crater; and a small crack propagation around the bullet hole. But these short cracks could not cut through the NISC target, and the surficial concrete around the bullet hole also was slightly damaged (Fig. 4AI). In order to further study the damaged state inside the NISC-100 target at a striking velocity of 1 km/s, we observed the different layers (from the first layer to the third layer), as shown in Fig. 4AII-IV. In first layer, a small area on the surface of the concrete was peeled off. However, the damage gradually reduces from second to third layers. On the back NISC-100 target (Fig. 4BI-IV), we only observed a tiny piece of concrete around the bullet hole. The damage status has no obvious difference among these assembled layers.
However, after shooting at a striking speed of 2 km/s, the bullet hole in the front NISC-100 target became obviously larger than that at a striking velocity of 1 km/s. And several small cracks propagate around the bullet hole (these small cracks could not cut through the NISC target). Also, a 5 cm2 area of the surficial concrete around the bullet hole was destroyed, as shown in Fig. 4CI. With an increasing of the assembled layers, the damaged areas rapidly reduced (Fig. 4CII-IV), clearly lower than that of the pure concrete (Fig. 3). However, on the back of the NISC-100 target (Fig. 4D), a 5 cm2 area of surficial concrete around the bullet hole was peel off, accompanying with some crack propagation around the bullet hole (Fig. 4DI). Although the crack could cut through the NISC target (see the different assembled layers), as shown in Fig. 4DII-IV, the damage degree of the NISC target is notably smaller than that of the pure concrete. Based on the analysis of these damage morphologies, we consider that, our nacre-inspired steel-bar-reinforced concrete lamination can drastically enhance the damage tolerance of concrete materials under hypervelocity impacts.
In addition, to quantitatively estimate the impact resistance capability of these bioinspired composite targets, these important parameters (such as trajectory depth, penetration depth, residual length and mass of projectiles, impact crater area, and impact crater diameter) were summarized and compared in Fig. 5A-D, Table 1 and Supplementary Fig. 6. Generally, the impact resistance ability of the NISC target material was stronger than that of pure concrete material. For example, at a striking speed of 1 km/s, the trajectory depth, penetration depth, residual length and mass of the projectile, impact crater area, and impact crater diameter of the NISC target materials are 94.1 mm (NISC-300), 90.0 mm (NISC-300), 19.13 mm/4.94 g and 20.29 mm/5.01 g (NISC-100 and NISC-300), 1851/2154.99 mm2 (NISC-100 and NISC-300), 48.55/73.08 mm (NISC-100 and NISC-300), respectively, obviously lower than those of the pure concrete (174.1 mm, 169.03 mm, 20.29 mm/5.01 g, 278.56 mm2, and 73.81 mm). However, at a striking velocity of 2 km/s, the trajectory depth, penetration depth, residual length and mass of the projectile, impact crater area, and impact crater diameter of the NISC target materials are 102.3 mm (NISC-300), 97 mm (NISC-300), 10.00/9.11 mm and 2.96/2.72 (NISC-100 and NISC-300), 4018.27/3541.7 mm2 (NISC-100/NISC-300), and 71.53/67.15 mm (NISC-100 and NISC-300), respectively, notably lower than those of the pure concrete (140.3 mm, 140.3 mm, 12.3 mm/3.56 g, 26028.98 mm2, 182.05 mm). Simultaneously, we also found that the impact parameters of NISC-300 were lower than those NISC-100, suggesting that with increasing of the thickness of the bioinspired target, the impact resistance capability could be gradually enhanced.