Defects can easily appear in composite lattice truss core sandwich structures during the complex preparation process, which may significantly affect the structural response and decrease the load-carrying capability. The purpose of this paper is to investigate the manufacturing defect sensitivity of modal vibration responses of carbon fiber composite pyramidal truss-like core sandwich cylindrical panels by modal experiments and finite element analysis. Defects including debonding between face sheets and truss cores (DFT), truss missing (DTM), face sheet wrinkling (DFW) and gap reinforcing (DGR) are introduced into the present intact specimen artificially and modal testing is conducted to study their dynamic behavior under free-free boundary conditions. Finite element models consistent with the experiments are then developed to further study the effect of defect extents, locations and forms on the modal parameters of the present sandwich cylindrical panels. Results indicate that the degree of sensitivity of natural frequencies of the present sandwich cylindrical panels mainly depends on the vibration modes, defect extents, locations and forms. In addition, damping loss factors are much more sensitive than their corresponding frequencies. Some conclusions and essential mechanisms are summarized, which is helpful to vibration-based non-destructive evaluation (NDE) of such kind of composite lattice sandwich structures.

Sophisticated and efficient technique of sandwich attachment for composite sandwich structure assembly is imperatively required by industries. Certain types of joining inserts are widely used to carry the localized loads, but little is known regarding to the joining method for composite lattice truss core sandwich structures. In this study, a novel hybrid insert fastener, which comprises a plurality of carbon-fiber-reinforced grid cells and a metallic part, is developed for pyramidal truss core sandwich structures. Finite element models are developed to predict the failure modes and the load capabilities of different insert locations. Static pull-out and shear experiments are carried out, and the failure behaviors for each load case are discussed. The results show that the shear performance is significantly improved, and the insert position greatly affects the static pull-out behavior. An optimization of the hybrid joining insert to enhance the pull-out characteristic is addressed and verified by the finite element analysis.

As promising metamaterials, 3D periodic auxetic cellular structures (PACSs) have attracted great interest. However, they usually consist of intricate geometries which make their fabrication a significant challenge. The present paper is focused on introducing the interlocking assembly concept into the fabrication of 3D PACSs. There are distinct advantages of the interlocking assembly method compared with the additive manufacturing methods mainly used before. Based on the interlocking assembly method, the dependences of mechanical properties mainly including the Poisson's ratio and the Young's modulus of the structure on the re-entrant angle were investigated through a combination of uniaxial compression experiments and numerical simulations, excellent qualitative and quantitative agreement was found. Using the experimentally verified numerical model, the effects of the strut thickness and the ratio of the vertical strut length to oblique strut length on the mechanical properties of the structure were investigated. Results show that the compression modulus of the structure will increase with the structure becomes more re-entrant, but there exists an extreme value for Poisson's ratio with the re-entrant angle around 45° which differs from former studies. With the thickening of the struts the compression modulus of the structure monotonously increases and the Poisson's ratio of the structure will gradually changes from negative to positive then gradually approaches to the Poisson's ratio of the parent material. The vertical strut length to oblique strut length ratio plays fewer roles on the mechanical properties compared with the re-entrant angle and the strut thickness.

In this work, an analytical model of a 3D re-entrant auxetic cellular structure has been established based on energy method. In the model the overlapping of the struts as well as axial extension or compression (mostly neglected in former studies) were taken into consideration to make the model applicable when the struts are relative stubby which is common in engineering designs. Analytical solutions for the modulus and Poisson's ratios of the cellular structure in all principal directions were deduced. To validate the analytical model in present study, numerical calculations using brick elements were performed on unit cell models with periodic boundary conditions, and comparisons of the present model with analytical formulae and experimental results available in former literatures were also conducted. The results show that when the struts are slender enough, the bending of the struts play decisive role on the deformation of the structure and other mechanisms can be ignored; while when the struts become relative stubby, all the mechanisms including bending, shearing and axial loading need to be considered; The often-ignored axial extension or compression term may even play decisive role on determining the lateral Poisson's ratio of the structure when the struts are relative stubby.

In recent years, 3D structures with negative Poisson's ratio (auxetic) have attracted great interest. Many polymer and metal 3D auxetic structures have been manufactured using additive manufacturing technology, however composite 3D auxetic structures are rarely reported. Auxetic structures are normally of low stiffness which causes limitations on the structural applications of them. The specific stiffness and strength of auxetic structures can be significantly improved by making them from high-performance fibre reinforced polymer (FRP) composites. Consequently, research of composite 3D auxetic structures made from FRP should be conducted. This paper presents the composite 3D double-arrow-head (DAH) auxetic structure made from carbon fibre reinforced polymer (CFRP) using an assembly method. Experimental, finite element and theoretical methods are adopted to study the mechanical properties of the composite 3D DAH auxetic structures. Results show that the Poisson's ratios and effective compression moduli of the composite 3D DAH auxetic structures vary depending on the compression strain amplitude, and the structures become more auxetic and stiffer with the increase of the compression strain. The specific stiffness of the composite 3D DAH structure is much higher than that of the metal structure. In addition, the dependences of the structure's Poisson's ratio and effective compression modulus on the geometry parameters have also been given. Making auxetic structures from high-performance FRP composites can significantly improve their mechanical properties which will enable them to have a much wider variety of applications.

We investigate the mechanical response of elastic solids perforated with double-U array of parallel fine crack, which leaves tailored mechanical metamaterials containing repeated snapping units with programmed tensile behavior. Our results indicate that under uniaxial tension the metamaterials undergo a large extension caused by buckling snap-through instabilities, and exhibit very small transverse deformation. We find that by largely stretching the pre-cracked specimens, nonlinear mechanical responses including self-recovering snapping and multi-stability enabling snapping behaviors can be generated by tuning the relative stiffness of the curved segments. On this basis, topology analysis is carried out to design three-dimensional (3D) multi-stable configurations for practical applications such as shape-reconfigurable tubes as well as variable stiffness and strength material. This work gives rise to the design, analysis and manufacture of zero Poisson's ratio and shape-reconfigurable materials.

The mechanical responses of two novel kinds of two-dimensional (2D) mechanical metamaterials containing opposite or parallel snapping curved (U-shaped) segments with elastic snap-through instability mechanism are systematically investigated. Under uniaxial loading, the metamaterials undergo a large deformation caused by stiffness mismatch between snapping (buckling instabilities) and supporting (relative stiffer/thicker) components, exhibiting very small transverse deformation after every snapping. Based on the multi-stable mechanism, phase transformation/shape-reconfiguration and zero Poisson’s ratio are achieved up to large morphological change. Nonlinear mechanical responses including self-recovering snapping and multi-stability enabling snapping behaviors can be generated by tuning the geometric parameters (the relative thickness of the snapping and supporting segments as well as the amplitude of the snapping curved segments). Then topology analysis is carried out to develop the 2D structures to a series of 3D hierarchical configurations from which can be chosen for various engineering conditions with enhanced snapping mechanism. Specifically, multi-stable/shape-reconfigurable tubes and cylinders are designed using the 3D configurations. Besides, one of the 3D metamaterials is developed for functional applications as shock absorber and damper, i.e., the process from fully stretched state to fully compacted state is used to absorb energy and reduce incoming pressure with small stiffness and strength; then the fully compacted metamaterials are used to carry load and attenuate vibration with relative bigger stiffness and strength. This work gives advance to the design, analysis and manufacture of functionally reconfigurable mechanical metamaterials.

Auxetic materials or structures, especially 3D cellular lattice architectures with negative Poisson's ratio (NPR) have attracted great attention due to their unprecedented mechanical behaviors and promising applications in recent years. Many 3D auxetic architected cellular materials have been manufactured and characterized with polymers and metals using additive manufacturing technology as well as some fibre reinforced polymers (FRP) composites via assembly method, however metal 3D printed auxetic structures with high-quality are rarely reported and FRP composites using assembly method contain a variety of defects which result in a knock-down effect on mechanical properties. Auxetic structures made from polymers are normally of low stiffness and strength and FRP composites are of low toughness and impact resistance, which causes limitations on their structural and functional applications. If rationally designed structures can be made from high-performance metal materials the specific stiffness and strength of which are much higher than polymers, their specific stiffness and strength can be significantly increased which will make them more suitable for various potential applications. This paper presents novel 3D double-U hierarchical structures (DUHs) based on double-V hierarchical structures (DVHs) with tunable Poisson's ratio and Young's modulus by tailoring the geometry parameters. Experimental, numerical and theoretical results show that DUHs have smooth geometry that can reduce manufacturing damage and stress concentration under elastic loading. Another advantage of DUHs is that the curved configurations have enhanced auxetic behavior and higher static collapse stress than DVHs under crushing. The advanced 3D printing technology and those excellent mechanical properties of 3D DUHs meet the requirements of structural and functional applications.

In this paper, the sound transmission loss (STL) through sandwich structure with pyramidal truss cores immersed in the surrounding acoustic fluids is investigated. The periodic model is established in two dimensions. Trusses are replaced by translational and rotational springs and the stiffnesses are calculated by the theory of Euler beam. Fluid-structure coupling is considered by imposing velocity continuity condition at fluid-structure interfaces. The dynamic equations are derived by using space harmonic expansions and the principle of virtual work. The sound transmission characteristics of structure are studied by using the numerical calculation. Then, the effects of incident wave, geometry of truss and material properties are discussed for thorough understanding and system design.

This paper presents a theoretical analysis of sound transmission through a sandwich structure in the presence of external mean flow. The structure is comprised of two panels connected by two-layered pyramidal cores and filled with fibrous material. The fibrous absorptive material in the core is characterized by an equivalent fluid model. The interaction between the structure and the surrounding fluid is taken into account by imposing a velocity continuity condition at the interfaces. The inclined beams in the core exert forces and moments on the panels. The space-harmonic approach and virtual work principle are applied to derive the governing equations. A validation is carried out by comparison with finite element simulations and existing experimental measurement. Numerical calculations are used subsequently to explore the influence of the material, mean flow and structure geometry. The results demonstrate that the absorptive material influences the sound transmission loss (STL) at low frequencies and that the effectiveness is affected by the geometry of the structure.