Hybrid truss concepts for carbon fiber composite pyramidal lattice structures

A novel hybrid truss construction concept is presented that incorporates a second-phase core material into trusses of carbon fiber (CF) composite pyramidal lattice (CPL) structures. Embedded core materials of wood and silicone rubber are selected for trial structures. Hybrid CPL structures were fabricated using a hot press molding approach and out-of-plane compression tests were performed to investigate properties. For prototype wood–core truss CPL, structural efficiencies increased comparing with solid truss CPL, while energy absorption capability was enhanced for rubber–core truss CPL. Employing the hybrid truss construction, the density-specific performance space of CF composite lattice structures can be expanded, and desired functional potential can be realized by judicious selection of core materials while simultaneously retaining structural properties.     

Mechanical Response of All-composite Pyramidal Lattice Truss Core Sandwich Structures

The mechanical performance of an all-composite pyramidal lattice truss core sandwich structure was investigated both theoretically and experimentally.Sandwich structures were fabricated with a hot compression molding method using carbon fiber reinforced composite T700/3234.The out-of-plane compression and shear tests were conducted.Experimental results showed that the all-composite pyramidal lattice truss core sandwich structures were more weight efficient than other metallic lattice truss core sandwich structures.Failure modes revealed that node rupture dominated the mechanical behavior of sandwich structures.     

Compressive efficiency of stretch–stretch-hybrid hierarchical composite lattice cores

Composite lattice cores featuring structural hierarchy were developed to achieve greater buckling resistance. The stretch–stretch-hybrid hierarchical lattice cores were fabricated with a two-step approach by assembling composite pyramidal lattice (CPL) sandwiches into macroscopic truss configurations. Analysis and experiments were performed to determine the out-of-plane compressive strength. Hierarchical CPL cores were evaluated based on their failure mechanism maps, and the structural efficiency was affected by the ratio of strut length at different scales (e.g. L/l₁). With the specific limited L/l₁, the optimized hierarchical CPL core was almost 5 times stronger than lower-order CPL cores with rectangular trusses (at relative density 0.01). The fully optimized hierarchical CPL cores can be as efficient as optimized CPL cores with hollow trusses. Effects of topologies at two different length scales on the performance of hierarchical structures were also assessed.     

Hybrid carbon fiber composite lattice truss structures

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from linear carbon fiber braids and Divinycell H250 polymer foam trapezoids. These have been stitched to 3D woven carbon fiber face sheets and infused with an epoxy resin using a vacuum assisted resin transfer molding process. Sandwich panels with carbon fiber composite truss volumes of 1.5–17.5% of the core volume have been fabricated, and the through-thickness compressive strength and modulus measured, and compared with micromechanical models that establish the relationships between the mechanical properties of the core, its topology and the mechanical properties of the truss and foam. The through thickness modulus and strength of the hybrid cores is found to increase with increasing truss core volume fraction. However, the lattice strength saturates at high CFRP truss volume fraction as the proportion of the truss material contained in the nodes increases. The use of linear carbon fiber braids is shown to facilitate the simpler fabrication of hybrid CFRP structures compared to previously described approaches. Their specific strength, moduli and energy absorption is found to be comparable to those made by alternative approaches.     

Designing and compression behaviors of ductile hierarchical pyramidal lattice composites

To improve the ductility of lightweight cellular material, hierarchical pyramidal lattice truss composites were designed and manufactured. Rib of the hierarchical pyramidal lattice truss composite is made of glass fiber reinforced woven textile sandwich structure and designed weft-loaded. Flat-wise compression experiments were carried out to explore the strength and deformation mode of the hierarchical pyramidal lattice truss composite. Progressive crushing of the sandwich rib enables the hierarchical lattice composite to have a long stable deformation plateau. Stress of the deformation plateau of the hierarchical lattice composite is rather close to its strength, indicating that the hierarchical lattice composite would have excellent specific energy absorption, even better than aluminum lattice structures. The experiments reveal that the hierarchical structure makes the fiber reinforced lattice composite much more ductile and weight efficient in energy absorption.     

Fabrication and crushing behavior of low density carbon fiber composite pyramidal truss structures

A new method for fabricating carbon fiber composite pyramidal truss cores was developed based on the molding hot-press technique. In this method, all the continuous fibers of composite are aligned in the direction of struts and thus, the truss structure can fully exploit the intrinsic strength of the fiber reinforced composite. The microstructure and organizations of fibers of fabricated composite structures were examined using scanning electron microscope. The crushing response of the truss cores was also investigated and the corresponding failure modes were studied and complemented with an analytic model of the core crushing response. Our results show that the fabricated low-density truss cores have superior compressive strength and thus, could be used in development of novel lightweight multifunctional structures.     

Compression and impact testing of two-layer composite pyramidal-core sandwich panels

Quasi-static uniform compression tests and low-velocity concentrated impact tests were conducted to reveal the failure mechanisms and energy absorption capacity of two-layer carbon fiber composite sandwich panels with pyramidal truss cores. Three different volume-fraction cores (i.e., with different relative densities) were fabricated: 1.25%, 1.81%, and 2.27%. Two-layer sandwich panels with identical volume-fraction cores (either 1.25% or 2.27%), and also stepwise graded panels consisting of one light and one heavy core, were investigated under uniform quasi-static compression. Under quasi-static compression, load peaks were identified with complete failure of individual truss layers due to strut buckling or strut crushing, and specific energy absorption was estimated for different core configurations. In the impact test, the damage resulting from low-velocity concentrated impact was investigated. Our results show that compared with glass fiber woven textile truss cores, two-layer carbon fiber composite pyramidal truss cores have comparable specific energy absorptions, and thus could be used in the development of novel light-weight multifunctional structures.     

陶瓷棒填充点阵金属夹层结构的制备及抗侵彻实验

为提高轻量化复合装甲的抗侵彻能力,提出了内部填充陶瓷棒并由混杂短切玻璃纤维的环氧树脂封装的点阵金属夹层防护结构。首先,通过弹道冲击实验研究了陶瓷棒填充点阵金属夹层防护结构的抗弹丸侵彻能力;然后,结合失效模式和吸能效率,综合分析了该夹层防护结构的抗侵彻机制。结果表明:陶瓷棒填充点阵金属夹层防护结构的主要失效模式包括点阵金属结构和混杂填充材料的拉伸断裂、陶瓷棒的破裂、面板和背板的局部剪切破坏以及背板的总体弯曲变形。在球形弹丸侵彻过程中,由于点阵金属结构的塑性大变形和剪切扩孔、陶瓷棒和环氧树脂的断裂破坏以及面板的宏观弯曲变形,防护结构的抗侵彻能力得到大幅提高。研究结果可为新型轻质复合装甲的防护设计提供一定参考。      

Ballistic resistance of hybrid-cored sandwich plates: Numerical and experimental assessment

The impact responses and ballistic resistance of sandwich plates having three different types of hybrid cores are investigated. The hybrid cores include metallic pyramidal lattice trusses, metallic pyramidal lattice trusses with ceramic prism insertions, and metallic pyramidal lattice trusses with ceramic prism insertions and void-filling epoxy resin. Three-dimensional (3D) finite element (FE) simulations are carried out for each sandwich type impacted by a hemispherical projectile. Upon validating the FE simulation results with experimental measurements, the ballistic limit velocity, energy absorption and failure mechanisms for each type of the sandwich as well as the influence of key material, structural and topological parameters are investigated systematically. Sandwich plates having metallic pyramidal lattice core with ceramic insertions and epoxy resin filling void spaces are found to outperform the other two sandwich types. It is also demonstrated that the back face-sheet plays a more significant role than the front face-sheet in resisting ballistic impacts.     

Mechanical behavior of carbon fiber reinforced polymer composite sandwich panels with 2-D lattice truss cores

Composite sandwich structures with lattice truss cores are attracting more and more attention due to their superior specific strength/stiffness and multi-functional applications. In the present study, the carbon fiber reinforced polymer (CFRP) composite sandwich panels with 2-D lattice truss core are manufactured based on the hot-pressing method using unidirectional carbon/epoxy prepregs. The facesheets are interconnected with lattice truss members by means of that both ends of the lattice truss members are embedded into the facesheets, without the bonding procedure commonly adopted by sandwich panels. The mechanical properties of the 2-D lattice truss sandwich panels are investigated under out-of-plane compression, shear and three-point bending tests. Delamination of the facesheets is observed in shear and bending tests while node failure mode does not occur. The tests demonstrate that delamination of the facesheet is the primary failure mode of this sandwich structure other than the debonding between the facesheets and core for conventional sandwiches.     

加载速率对点阵铝力学行为及吸能特性的影响

目的研究基体材料和加载速率对点阵铝力学性能和吸能特性的影响规律。方法针对工业纯铝、6063铝合金为基体的点阵铝在3种不同的加载速率下进行压缩力学试验。结果加载速率从2mm/min增加到250 mm/min时,点阵纯铝的屈服强度增加了2 MPa,点阵6063铝合金的屈服强度增加了7.6 MPa;加载速率从250 mm/min增加到500 mm/min时,点阵纯铝的屈服强度变化不大,而点阵6063铝合金的屈服强度增加了8.2 MPa;当加载速率一定时,点阵6063铝合金的屈服强度要大于点阵纯铝。结论点阵6063铝合金的力学性能和单位体积吸能随着加载速率的增大而增大,并且点阵6063铝合金的力学性能和吸能特性要大于点阵纯铝。     

High Velocity Impact Response of Composite Lattice Core Sandwich Structures

In this research, carbon fiber reinforced polymer (CFRP) composite sandwich structures with pyramidal lattice core subjected to high velocity impact ranging from 180 to 2,000 m/s have been investigated by experimental and numerical methods. Experiments using a two-stage light gas gun are conducted to investigate the impact process and to validate the finite element (FE) model. The energy absorption efficiency (EAE) in carbon fiber composite sandwich panels is compared with that of 304 stainless-steel and aluminum alloy lattice core sandwich structures. In a specific impact energy range, energy absorption efficiency in carbon fiber composite sandwich panels is higher than that of 304 stainless-steel sandwich panels and aluminum alloy sandwich panels owing to the big density of metal materials. Therefore, in addition to the multi-functional applications, carbon fiber composite sandwich panels have a potential advantage to substitute the metal sandwich panels as high velocity impact resistance structures under a specific impact energy range.     

碳纤维增强金字塔点阵夹芯结构的抗压缩性能

提出了一种碳纤维增强复合材料点阵夹芯结构的一体化成型工艺方法。该方法克服了传统夹芯结构面板与芯子之间因需要二次粘接或焊接的方法所带来弱界面的缺点。将纤维杆两端埋入面板内,使面板与芯子成为一体而不存在明显的界面。对用该方法制备的碳纤维增强金字塔点阵夹芯板进行平压试验,研究发现随着载荷的增加,纤维杆发生弹性屈曲并在中间部位出现断裂。理论分析了点阵夹芯结构平压载荷下的弹性模量和纤维杆极限屈曲载荷。通过与传统夹芯材料相比较发现,这种新型复合材料点阵夹芯结构具有密度低、比强度和比刚度高等优点。      

七孔涤纶纤维对CPE-AO2246微观结构和性能的影响

利用氯化聚乙烯(CPE)、 2, 2-亚甲基-双( 4-甲基-6-叔丁基苯酚) (AO2246)和三维卷曲七孔中空涤纶纤维(SHPF)制备了一系列的SHPF/CPE-AO2246三元复合材料。通过DMA、 SEM、 SW230吸声仪以及HD026NE电子织物强力仪等对复合材料的微观结构和性能进行了测试及分析。结果表明, 加入的SHPF纤维充当了结晶诱导作用, 在复合材料中产生了大量AO2246的包覆式结晶, 从而加速了网络结构的形成; 含有20%质量比(以CPE和AO2246总质量为基)纤维的SHPF/CPE-AO2246复合材料的最大储能模量是未加纤维的3倍多, 由于复合材料的储能模量增幅较大而使其损耗因子下降较快, 但材料损耗模量-温度曲线下的面积(LA)随纤维含量增加而增大, 说明材料的阻尼耗能能力并未下降; SHPF纤维的加入使材料的力学性能获得了较大的改善, 中空网络结构的形成赋予三元复合材料吸声性能。     

Effects of local damage on vibration characteristics of composite pyramidal truss core sandwich structure

The effects of local damage on the natural frequencies and the corresponding vibration modes of composite pyramidal truss core sandwich structures are studied in the present paper. Hot press molding method is used to fabricate intact and damaged pyramidal truss core sandwich structures, and modal testing is carried out to obtain their natural frequencies. A FEM model is also constructed to investigate their vibration characteristics numerically. It is found that the calculated natural frequencies are in relatively good agreement with the measured results. By using the experimentally validated FEM model, a series of numerical analyses are conducted to further explore the effects of damage extent, damage location, damage form on the vibration characteristics of composite pyramidal truss core sandwich structures as well as the influence of boundary conditions. The conclusion derived from this study is expected to be useful for analyzing practical problems related to structural health monitoring of composite lattice sandwich structures.     

Al2O3‐Assisted Confinement Synthesis of Oxide/Carbon Hollow Composite Nanofibers and Application in Metal‐Ion Capacitors

Hollow micro‐/nanostructures are widely explored for energy applications due to their unique structural advantages. The synthesis of hollow structures generally involves a “top‐down” casting process based on hard or soft templates. Herein, a new and generic confinement strategy is developed to fabricate composite hollow fibers. A thin and homogeneous atomic‐layer‐deposition (ALD) Al₂O₃ layer is employed to confine the pyrolysis of precursor fibers, which transform into metal (or metal oxide)–carbon composite hollow fibers after removal of Al₂O₃. Because of the uniform coating by ALD, the resultant composite hollow fibers exhibit a hollow interior from heads to ends even if they are millimeter long. V, Fe, Co, and Ni‐based hollow nanofibers, demonstrating the versatility of this synthesis method, are successfully synthesized. Because of the carbon constituent, these composite fibers are particularly useful for energy applications. Herein, the as‐obtained hollow V₂O₃–C fiber membrane is employed as a freestanding and flexible electrode for lithium‐ion capacitor. The device shows an impressive energy density and a high power density.     

倒三角截面空间桁架拱平面外弹塑性稳定设计方法研究

该文研究了倒三角截面两铰桁架拱和固支桁架拱的平面外稳定性能:首先介绍了倒三角形桁架拱的平面外弹性屈曲荷载,此后采用大挠度弹塑性有限元方法,通过引入纯压正则化长细比和纯弯正则化长细比,分别建立了纯压和纯弯桁架拱的平面外弹塑性稳定设计方法。在此基础上,研究了杆件的变形对桁架拱平面外整体稳定性能的影响,并通过两个相关系数将该影响体现在稳定承载力设计方程中。进一步,依据有限元数值计算结果,提出了全跨水平均布荷载或半跨水平均布荷载作用下桁架拱的轴力和弯矩稳定承载力设计方程,供倒三角截面桁架拱平面外稳定性设计。     

Mechanical behavior of the sandwich structures with carbon fiber-reinforced pyramidal lattice truss core

Sandwich panel construction with carbon fiber-reinforced pyramidal lattice truss is attracting more and more attention due to its superior mechanical properties and multi-functional applications. Pyramidal lattice truss sandwich panels made from carbon fiber reinforced composites materials are manufactured by hot-pressing. The facesheets are interconnected with truss cores, the facesheets and truss cores are manufactured in one manufacturing process without bonding. The buckling and splitting of truss member is observed in the compressive and shear tests and no nodal failure is observed. The predicted results show that the mechanical behavior of the pyramidal lattice truss core sandwich panels depends on the relative density of core and the material properties of truss members.     

Excellent Mechanical Properties and Energy Absorption of Hexagonal-Body-Centered Lattice Structure Fabricated by Selective Laser Melting

Inspired by the crystal lattice characteristics of hexagonal-close-packed and body-centered-cubic metals, a novel hexagonal-body-centered (HBC) lattice structure is constructed for energy absorption. HBC lattice structures with three different c/a ratios are prepared by selective laser melting (SLM) using 316 L stainless steel powder. The geometric features and energy absorption performance of the fabricated HBC lattice structures with different c/a ratios are studied by scanning electron microscopy and quasistatic compression tests, respectively. The results show that the HBC lattice structure prepared by SLM not only exhibits good formability, but also demonstrates excellent mechanical properties and energy absorption capacity. The c/a ratio significantly affects the mechanical properties and energy absorption performance of HBC lattice structures. The Young's modulus, yield strength, and energy absorption increase as the c/a ratio decreases. Compared with other lattice structures, the HBC lattice structure exhibits better energy absorption at the same relative density, thus indicating the usefulness of the HBC lattice structure as a lightweight energy-absorbing structure.     

The compressive responses of glass fiber composite pyramidal truss cores sandwich panel at different temperatures

A new method for fabricating glass fiber composite sandwich panel with pyramidal truss cores was developed based on the vacuum assisted resin transfer molding technology. The microstructure and organizations of fabricated sandwich panels were examined by the scanning electron microscope. The out-of-plane compressive tests of composite sandwich panels were performed throughout the temperature range from −60 °C to 125 °C. Then the effects of temperature on the compressive strength, compressive modulus and failure mechanism were investigated and analyzed. Our results indicated that cryogenic temperature resulted in the increasing of the compressive modulus and strength, while high temperature caused the degradation of the compressive modulus and strength. The effect of temperature on failure mode of composite sandwich panel was also observed. Analytical expressions were presented to predict the compressive modulus and strength of composite sandwich panels at different temperatures.