随着现代科技的迅猛发展，人们对工程材料/结构的力学性能提出了更高的要求，以适用于恶劣、复杂的环境。在众多性能中，抗冲击性能备受关注，由冲击引起的失效往往会造成巨大的经济损失和人员伤亡。传统的防护器具结构简单，吸能不高，大多采用金属制成，过于笨重且成本高，轻质高效吸能的材料/结构的设计依然是防护领域中的难题。自然界中的生物经过亿万年的自然选择和进化，其结构和性能得到了优化和提高，其优异结构为人造材料的优化设计提供了有益启示。本文寻找自然界中耐冲击的生物材料，掌握其微结构特征及作用机理，设计了几种高效吸能的仿生结构，结合3D打印、摆锤冲击实验和有限元模拟对比了仿生结构和传统结构的抗冲击性能，探讨了仿生结构性能提升机理，为抗冲击复合结构的设计提供了参考。论文的主要工作和结论如下：

    （1）以强韧化著称的贝壳遭遇螳螂虾，却能被其掠足轻松击穿，这归因于掠足表面和内部的弹性模量分别呈梯度和周期性变化。本文仿照螳螂虾掠足，分别设计了弹性模量在表面层梯度变化和沿整个厚度方向周期变化的两类仿生靶板。采用有限元模拟了落锤冲击靶板过程。结果表明，在保持靶板等效弹性模量相同时，采用表面梯度和周期设计，不仅大幅度提升靶板的临界冲击能，分别是均匀靶板的2.35倍和2.91倍，同时大幅度降低靶板在冲击过程中承受的最大冲击力，分别是均匀靶板的0.49倍和0.34倍。深入分析了两种仿生靶板抗冲击性能优于均匀靶板的机理，发现均匀靶板呈现块状脱落的单一失效模式，靶板表面中心是其主要的应变储能区，而表面梯度靶板和周期靶板多处出现损伤、失效，从而改善了应力分布，提高了变形协调能力，使得应变储能区扩大，从而提高了吸能。最后讨论了表面梯度层厚度和弹性模量比值对靶板抗冲击性能的影响，结果表明，降低表面梯度靶板的梯度层厚度，或增大周期靶板的弹性模量比值，均可提高靶板的临界冲击能，并降低锤头的最大冲击力。

    （2）珍珠母具有典型的砖-泥结构，由95%硬脆的矿物质和5%柔软的有机质组成，能够完美解决软硬界面物性的不匹配，具有比单一矿物质高几倍的韧性和抗冲击性能。另外，微结构尺寸梯度设计具有比传统均匀设计更高的吸能，已广泛应用在多孔结构、金属材料中。本文将微结构尺寸梯度引入到砖-泥结构中，设计了一种胞元尺寸呈梯度变化的新型砖-泥结构，并利用3D打印制备了均匀和梯度砖-泥结构。通过摆锤冲击实验研究了胞元尺寸梯度对砖-泥结构抗冲击性能的影响，结果表明梯度砖-泥结构的吸能高于均匀砖-泥结构，且随胞元尺寸梯度增大，其吸能越高，最高可达均匀砖-泥结构的4.0倍。利用有限元模拟研究了梯度砖-泥结构吸能提高的机理，相比均匀砖-泥结构，梯度砖-泥结构极大地改善了应力分布，沿试样长度方向应力曲线光滑且出现平台段，沿试样宽度方向应力变均匀，有效避免了高应力区域的产生，使得整体结构在发生断裂前可承受更大的弯曲变形，从而提高了应变储能。

    （3）呈重叠分布的外部鱼鳞片可有效分散外部载荷、降低应力集中，呈螺旋分布的内部胶原纤维允许这些鳞片发生相对运动，使得整体结构具有足够高的灵活性。本文模仿鱼鳞，设计了一种螺旋-重叠的砖-泥结构，并进行摆锤冲击实验，与吸能较高的仿海螺壳砖-泥结构进行对比。结果表明仿鱼鳞砖-泥结构可以大幅度改善抗冲击性能，其吸能是仿海螺壳砖-泥结构的1.4倍。探讨了线性和非线性层旋转角度、层内软材料倾斜角度和鳞片尺寸缩放系数对仿生螺旋-重叠砖-泥结构抗冲击性能的影响，发现模仿天然生物材料的螺旋结构特征，取层旋转角度为18.95°时，其吸能最高，除该角度外，随层旋转角度增大，仿生螺旋-重叠砖-泥结构的吸能呈现先降低后升高的趋势，当取90°时，吸能最低；随层内软材料倾斜角度的增大，仿生螺旋-重叠砖-泥结构的吸能逐渐降低，当软材料倾斜角度取90°时，吸能最低，当软材料倾斜角度取15°时，吸能最高；随鳞片尺寸缩放系数增大，仿生螺旋-重叠砖-泥结构的吸能逐渐降低，当鳞片尺寸缩放系数取2.0时，吸能最低，当鳞片尺寸缩放系数取0.5时，吸能最高。

    （4）刺猬从数十米高度掉落，几乎毫发无损，主要归功于刺猬刺极高的缓冲减震功能。本文模仿刺猬刺的微结构特征，设计了刺猬刺薄壁结构，并在其基础上模仿耐冲击的甲壳虫前翅进行2^nd级设计，引入空心圆作为亚结构，设计了多级刺猬刺薄壁结构。通过有限元模拟了刚性平板冲击薄壁结构过程，对比了单壁圆筒、单级/多级蜘蛛网以及单级/多级刺猬刺薄壁结构的抗冲击性能。结果表明，单壁圆筒和蜘蛛网薄壁结构的应力分布不均匀，无高应变能密度区产生，吸能较低。刺猬刺薄壁结构由于多个肋板的加强作用，改善了与肋板连接内外壁的应力分布，增大了整体结构失效前的弯曲变形，吸能比是单壁圆筒薄壁结构的6.7倍。蜘蛛网薄壁结构经过多级设计后，改善效果不明显，仅中部区域的应变能密度略有增大，吸能比仅提高16%。刺猬刺薄壁结构经过多级设计后，亚圆由圆形变为椭圆，改善了变形协调能力，在失效前具有更大的整体弯曲变形和局部屈曲变形，吸能比提高81%。最后讨论了肋板个数和亚圆半径对多级刺猬刺薄壁结构抗冲击性能的影响，增大肋板数量和亚圆半径，均可改善整体结构沿轴向和周向的应力分布，使其越不容易发生失效，导致结构的整体弯曲变形和局部屈曲变形程度提高，吸能比增大。

    The rapid development of modern science and technology has put forward higher requirements for the performance of engineering materials or structures with minimum weight and cost to adapt to harsh and complex environment. Among many properties, the impact resistance has attracted much attention. Material and structure failure caused by impact can cause huge economic losses and casualties. Traditional protective equipment with simple structure usually has low energy absorption and is mostly made of metal, which is inflexible and costly. The design of light-weight materials and structures with high energy absorption is still a major challenge in the protective equipment field. After hundreds of millions of years of natural selection and evolution, the living beings continuously optimize themselves and improve their structure and performance, where the perfect structures of the biomaterials enlighten the design of artificial materials. The biomaterials with enhanced impact resistance are searched and their microstructure characteristics and impact mechanism are obtained in this dissertation. Several bionic structures with high energy absorption are designed. The impact resistance performance of the bionic and traditional structures is compared by using 3D printing technology, pendulum impact test and finite element simulation. The performance improvement mechanism of bionic structures is discussed, which provides a reference for the design of impact resistant composite structures.

    When a shell with high toughness encounters a mantis shrimp, the former can be easily penetrated by the latter’s stomatopod dactyl club known as a formidable damage-tolerant biological hammer. There is no doubt that the high impact resistance of stomatopod dactyl club is closely related to its microstructure. The stomatopod dactyl club consists of an impact region and a periodic region from outside to inside. While the elastic modulus gradually decrease from outside to inside in the impact region, they vary periodically in the periodic region. Inspired by this idea, in this dissertation, a surface gradient target and a periodic layered target are designed. The impact process of drop hammer on target is simulated by finite element method. The results show that the critical impact energies of surface gradient target and the periodic layered target are 2.35 and 2.91 times that of the uniform target respectively, and the maximum impact forces are 0.49 and 0.34 times that of the uniform target, respectively. The mechanism that the impact resistance of the two bionic targets is better than that of the uniform target is deeply analyzed. It is found that the uniform target presents a single failure mode of block-fragmentation, and the center of the target surface is its main storage area of strain energy. The surface gradient target and periodic layered target present multiple sites of damage and failures, which optimizes stress distribution and improves compatibility of deformation, thus expanding the storage area of strain energy and improving energy absorption. Moreover, either decreasing the thickness of gradient layer of the surface gradient target or increasing the elastic modulus ratio of the periodic layered target can increase the critical impact energy of the target, and decrease the maximum impact velocity of the bullet.

    The nacre, known as stiff and tough, has a typical brick-mud microstructure, which is composed of 95% stiff and brittle mineral substances and 5% soft organic matters. Since the mismatching of physical properties of soft and stiff interface is perfectly solved, its impact resistance is several times higher than base mineral. In addition, the size gradient of microstructure has higher energy absorption than the traditional uniform structure, which has been widely used in the porous structures and metal materials. A novel brick-mud structure is proposed by introducing cell size gradient design into the nacre-like brick-mud microstructure. The uniform and gradient brick-mud samples are successfully prepared with the 3D printing technology. It is found through a pendulum impact test that the energy absorption of the gradient structure is higher than that of the uniform structure. With the gradient increase in the cell size, the energy absorption increases and can reach up to 4.0 times that of the uniform structure. A finite element simulation method is used to explore the energy absorption improvement mechanism of the gradient structure. The results show that compared with the uniform structure, the gradient structure can achieve a much better stress distribution, which leads to greater bending deformation before fracture, thus improving strain energy storage.

    The external scales with overlapping distribution can effectively disperse the external load and reduce the stress concentration. The spiral distribution of internal collagen fibers allows the scales to move relatively, which makes the whole structure flexible enough. In this dissertation, a kind of spiral overlapping brick-mud structure is designed by imitating the fish scale. The pendulum impact tests are carried out to compare with the conch shell brick-mud structure with high energy absorption. The results show that the impact resistance of the fish scale brick-mud structure can be greatly improved, and its energy absorption is 1.4 times that of the bionic conch shell brick-mud structure. The effects of linear and nonlinear rotation angle of layer, inclination angle of soft material in layer and scaling factor of scale size on the impact resistance of bionic spiral overlapping brick-mud structure are discussed. It is found that the maximum energy absorption is obtained when the rotation angle of the layer is 18.95° which imitates the spiral structure of natural biomaterials. Except this angle, with the increase of the rotation angle of layer, the energy absorption of bionic spiral overlapping brick-mud structure first decreases and then increases, lowest energy absorption under 90°. With the increase of the inclination angle of soft material in layer, the energy absorption of the bionic spiral overlapping brick mud structure decreases gradually, also lowest energy absorption under 90° and highest energy absorption under 15°. Moreover, with the increase of scaling factor of scale size, the energy absorption of the bionic spiral overlapping brick mud structure decreases gradually, lowest energy absorption for 2.0 and highest energy absorption for 0.5.

    The hedgehog is almost intact when it drops from tens of meters, which is mainly due to the extremely high shock absorption behavior of hedgehog spine. A bionic hedgehog spine thin-walled structure is designed by imitating the microstructure characteristics of hedgehog in this dissertation. On the basis of it, the 2^nd level design is taken and the hierarchical hedgehog spine thin-walled structure with sub-circles is proposed according to the beetle forewing. A finite element method is used to simulate the impact process of rigid plate on the thin-walled structure. The impact resistance of single walled cylinder, spider web, multiple spider web, hedgehog spine and multiple hedgehog spine thin-walled structures are compared. The results show that the stress distribution of single wall cylinder and spider web thin-walled structures without high strain energy density area are both uniform, and their absorbed energies are low. Due to the strengthening effect of ribs, the stress distribution of the connections between the inner/outer walls and the ribs are improved and the bending deformation of whole structure before failure is increased. The specific energy absorption of the hedgehog spine thin-walled structure is 6.7 times that of the single walled cylinder thin-walled structure. After hierarchical design, the improvement of specific energy absorption is not obvious only 16% with the strain energy density in the middle region increasing slightly. By comparison, the sub-circles of the multiple hedgehog spine thin-walled structure change to ellipses, which is conducive to improve the deformation coordination ability. The overall bending deformation and local buckling deformation of the multiple hedgehog spine thin-walled structure are increased and the improvement of specific energy absorption is 81%. Finally, the effects of rib number and sub-circle radius on the impact resistance of the multiple hedgehog spine thin-walled structure are discussed. Increasing the rib number or the of sub-circle radius can improve the stress distribution along the axial and circumferential direction of whole structure, making it less prone to failure and increasing the overall bending deformation, local buckling deformation and the specific energy absorption.