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Character-level Chinese dependency parsing has been a concern of several studies that naturally handle word segmentation, POS (Part of Speech) tagging and dependency parsing jointly in an end-to-end way. Previous work mostly concentrates on a transition-based framework for this task because of its easy adaption, which is extremely important when feature representation relies heavily on the decoding strategy, particularly under the traditional statistical setting. Recently, on the one hand, sophisticated deep neural networks and deep contextualized word representations have greatly weakened the dependence between feature representation and decoding. On the other hand, (first-order) graph-based models, especially the biaffine parsers, are straightforward for dependency parsing, and meanwhile they can yield competitive parsing performance. In this paper, we make a comprehensive investigation of the deep graph-based character-level dependency parsing for Chinese. We start from an extension of a standard graph-based biaffine parser, and then exploit Chinese BERT as well as our improved encoders based on transformers to enhance the character-level dependency parsing model. We conduct a series of experiments on the Chinese benchmark datasets, showing the performances of various graph-based character-level models and analyzing the advantages of the character-level dependency parsing under the deep neural setting.

A new theory developed from extended high-order sandwich panel theory (EHSAPT) is set up to assess the static response of sandwich panels by considering the geometrical and material nonlinearities simultaneously. The geometrical nonlinearity is considered by adopting the Green–Lagrange-type strain for the face sheets and core. The material nonlinearity is included as a piecewise function matched to the experimental stress–strain curve using a polynomial fitting technique. A Ritz technique is applied to solve the governing equations. The results show that the stress stiffening feature is well captured in the geometric nonlinear analysis. The effect of the geometric nonlinearity in the face sheets on the displacement response is more significant when the stiffness ratio of the face sheets to the core is large. The geometric nonlinearity decreases the shear stress and increases the normal stress in the sandwich core. By comparison with open literature and finite element simulations, the present nonlinear EHSAPT is shown to be sufficiently precise for estimating the nonlinear static response of sandwich beams by considering the geometric and material nonlinearities simultaneously.

In this study, a novel multistable mechanical metamaterial, composed of multiple magnets systems, was presented. Several configurational models of magnets system were proposed. Using theoretical and experimental methods, the energy storage performance of the metamaterial was optimized, the parameters affecting the energy storage performance were discussed and the optimum solution was given. The energy trapping mechanism of this metamaterial was introduced. The specimen of the material was fabricated by combination of Fused Deposition Modeling (FDM) technology and interlock method. The quasi-static uniaxial compression was used to study the energy trapping ability of the metamaterial. The impact experiments were also conducted to test the cushion performance of the metamaterials. Experimental results show that the presented metamaterial exhibits good property, like thresholding the acceleration and completely reusable, in the application of impact protection. Moreover, one special behavior, earlier transformation, was firstly found in this study, and the mechanism was introduced.

Sandwich core materials that offer superior mechanical properties at minimum weight are essential in designing high-performance sandwich structures. Hierarchical materials are ideal templates for this purpose. In this paper, we investigate the mechanical performance of a pyramidal–pyramidal hierarchical lattice material to highlight its potential as the core material for sandwich structures. Three-dimensional failure mechanism maps for the pyramidal–pyramidal hierarchical lattice material are developed under different loading conditions and the results are compared to finite element simulations. Next, we study the mechanical response and failure modes of a sandwich panel with self-similar pyramidal lattice core construction subjected to in-plane compression and three-point bending. The current study indicates that the pyramidal–pyramidal hierarchical configuration can improve the load bearing capacity and core buckling resistance of the sandwich structures at low density. The study provides insights into the role of structural hierarchy in tuning the mechanical response of the lattice materials and expands the application envelope of lightweight sandwich structures by effectively increasing the structural buckling resistance.

[EN] By studying platonic crystals based on lattices of cavities containing N-beam resonators, we conclude that crystals made of 1-beam resonators easily produce low-frequency omnidirectional bandgaps. Based on this favorable property, hardly obtained for resonant cavities containing a higher number of beams N >= 2, we have designed single-phase metamaterial plates for the suppression of low frequency flexural waves in a broad range of frequencies. These metamaterials are obtained by using resonant cavities containing a multiple number M of identical 1-beam resonators uniformly distributed in the cavity. Square lattices of this type of resonators have been studied by using the impedance matrix approach and the multiple scattering method. This semi-analytical method has been employed to show the existence of complete bandgaps whose width can be optimized by increasing M. For the case M = 4, the largest number of resonators studied here, three complete bandgaps separated by two narrow passbands appear in the band structure. The formation of these complete bandgaps originates from the dynamic interaction between different local resonators as well as their interaction with the propagating waves in the host plate. By using composite structures consisting of platonic crystal slabs with complementary bandgaps, these separated bandgaps easily merge into a broadband wave attenuation region. The normalized width, defined as the percentage of the bandwidth to its central frequency, reaches 95.3%, representing an enhancement of about one order of magnitude compared with the absolute bandwidth obtained for the case of a single 1-beam resonator in the cavity. It is shown that the gaps can be easily tuned to lower frequencies by changing the geometrical parameters, such as the length of the beam, the radius and thickness of the smaller circular plate. Since the metamaterial is made of a single-phase material without attaching heavy masses, the work reported here provides a simple approach to construct low-cost structures with potential applications in aeronautic and astronautic industries for broadband vibration suppression at low frequencies. (C) 2018 Published by Elsevier Ltd., This work was supported by the Ministerio de Economia y Competitividad of the Spanish government and the European Union Fondo Europeo de Desarrollo Regional (FEDER) [Grant No. TEC2014-53088-C3-1-R], and the National Natural Science Foundation of China [Grant Nos. 11432004 and 11421091]. Penglin Gao acknowledges a scholarship provided by China Scholarship Council [Grant No. 201606120070].

Exploring lightweight sandwich structures with excellent load-bearing and vibration damping performances is one of the important topics in structural and functional applications. The aim of the present work is to design, fabricate novel metallic sandwich structures with Hourglass truss cores and investigate their modal characteristics and vibration isolation performances by comparing with the traditional pyramidal sandwich structure experimentally and numerically. It is shown that the natural frequencies of the Hourglass sandwich structures are much higher than that of the pyramidal sandwich structures under free-free boundary condition and equal relative density of the truss cores. The torsional modes and transverse bending modes of the Hourglass sandwich structures separately play a dominant role in lower and higher modes of vibration, which are opposite to the pyramidal sandwich structures. From the results of the acceleration frequency responses and vibration level difference (VLD), it is indicated that the Hourglass sandwich structures exhibit better vibration isolation performance than the pyramidal sandwich structures. Furthermore, experimentally validated finite element analysis (FEA) models are developed to study the influences of truss inclination angles and boundary conditions on the modal characteristics of the present sandwich structures. Some conclusions are summarized, which may be useful for understanding the vibration behavior of such kinds of lattice sandwich structures.

Janus metamaterials, metasurfaces, and monolayers have received intensive attention in nanophotonics and 2D materials. Their core concept is to introduce asymmetry along the wave propagation direction, by stacking different materials or layers of meta-atoms, or breaking out-of-plane mirror asymmetry with external biases. Nevertheless, it has been hitherto elusive to realize a diffusive Janus metadevice, since scalar diffusion systems such as heat conduction normally operate in the absence of polarization control, spin manipulation, or electric-field stimuli, which all are widely used in achieving optical Janus devices. It is even more challenging, if not impossible, for a single diffusive metadevice to exhibit more than two thermal functions. Here a path-dependent thermal metadevice beyond Janus characteristics is proposed, which can exhibit three distinct thermal behaviors (cloaking, concentrating, and transparency) under different directions of heat flow. The rotation transformation mechanism of thermal conductivity provides a robust platform to assign a specific thermal behavior in any direction. The proof-of-concept experiment of anisotropic in-plane conduction successfully validates such a path-dependent trifunction thermal metamaterial device. It is anticipated that this path-dependent strategy can provide a new dimension for multifunctional metamaterial devices in the thermal field, as well as for a more general diffusion process.

A new theoretical model based on the extended high order sandwich panel theory is established to predict the mechanical response of sandwich panels under static loads with the bilinear constitutive stress–strain relation in the core. The constitutive relations of normal stresses related to the longitudinal and vertical normal strains in the bilinear isotropic hardening core are first formulated. The influence of the in-plane rigidity on the elastoplastic response of sandwich structures is analyzed. An in-plane loaded sandwich structure with the bilinear core is first studied based on extended high order sandwich panel theory, and the effect of the bilinear ratio on the mechanical response is evaluated. The governing equations are derived from the principle of minimum potential energy, and a Ritz-based half-analytical method is applied to get the solutions. The plastic response is acquired by an iterative procedure along with the convergence criteria. The results reveal that the local effect can be captured when the axial rigidity of the core is considered. The bilinear characteristic of the core decreases the maximum normal stress with an increase of the average value. The equivalent plastic region extends with the increase of the bilinear ratio when the sandwich structure is loaded in plane. By comparison with open literatures and finite element results, the present theoretical model is proved to be effective and efficient.

Geological disaster recognition on optical image is one of the key techniques in disaster control and disaster relief. Comparing with optical images, remote sensing images contain much higher resolution and more visualized contents. In this paper, we propose a landslide recognition framework which trains a deep auto-encoder network on the compressed domain. ANN or SVM is used as the classifier for decision making. In addition, in order to meet the requirement of some real-time applications, a high performance training network on CUDA-enabled GPUs is designed and implemented. Experiments are conducted on optical images from Google Earth.

In this work, the new type enhanced lattice (defined as hourglass) truss structures with the improved resistance of their core members and attached face sheets have been fabricated by an innovative interlocking method. The shear and bending properties of the produced sandwich structures characterized by three different relative densities have been investigated using various analytical methods and the obtained results were in good agreement with the experimental data. The truss failure under shear loading was dominated by the inelastic buckling of the truss members, while the four possible analytical models describing the bending properties and failure mechanism of the hourglass truss beams included elastic face sheet buckling, face sheet yielding, elastic core member buckling, and core member yielding were studied. The bending specimens, which were designed by constructing failure mechanism maps, were used to examine the yielding behavior of the truss core members as well as the elastic buckling and yielding of the face sheet members. The obtained results revealed that the specific shear strengths and bending failure loads of the studied hourglass truss structures were superior to those of pyramidal truss structures, and that the former could be considered promising candidates for manufacturing lightweight structures with high specific strength.

This paper aims to propose a new theory on the elastoplastic transient response of sandwich beams under large displacement with moderate rotation, including an analysis strategy for multi-dimensional elastoplasticity and the corresponding numerical solving technique. The material behaviors follow the bilinear proportional hardening rule. According to the classifications of stress statuses, the updated structural stiffness of the sandwich beam in time history is acquired and a systematic procedure for the multi-dimensional elastoplastic dynamic analysis is subsequently developed. The dynamic governing equations involving geometric and material nonlinearities are solved by a modified Ritz-based approach, in association with the Newmark-beta technique. The closed linear solution and the finite element simulation validate the accuracy of the proposed nonlinear theory. Case study reveals that the strain hardening has significant effects on the dynamic response of sandwich beams. The application to the blast-resisting study of sandwich beams as sacrificial coatings on ship hulls further proves the effectiveness of the current theory.

A theoretical model has been developed to predict the bending properties and failure mechanism of the human skull. The structure of the skull bone, which is naturally a porous material, was obtained via non-destructive 3D micro-computed tomography. By considering the combined influence of curvature and porosity of the skull, the mid-span deflection of the skull bone loaded in three-point bending was calculated using theoretical prediction. The cranial bending peak load corresponding to several possible failure modes was also analyzed, and the relevant failure mechanism map was generated. The models also showed that the curvature is a critical consideration for accurate precisions to be achieved. To verify the reliability of the theoretical results, as well as to gain a good understanding of the intrinsic connections between internal geometric features and mechanical properties of the cranium, 3D finite element (FE) models, based on posteriorly reconstructed by 2D cranial scanning images, were developed. The bending properties of various reconstructed sections were studied in detail using the FE simulation models, leading to an understanding of the bending failure mechanism and crack propagation path. The numerical results of deflection and peak loads agree well with the theoretical results. In addition, the failure modes of the numerical models have been verified using published experimental results. This work has overall demonstrated that the theoretical and numerical models would provide a reliable basis for predicting cranial bending performance.

The impedance matrix method is applied to study the scattering of flexural waves propagating in an infinite thin plate containing an $N$-beam resonator. The resonator consists of a circular hole containing a smaller plate connected to the background plate by a number $N$ of rectangular beams. After representing the boundary conditions in a modal multipole expansion form, a compact expression is obtained for the T-matrix, which relates the incident and the scattered transverse (out-of-plane) waves. The analysis of the scattering cross-section reveals interesting scattering features, like resonances and anisotropy, associated with this type of resonators. Numerical experiments performed within the framework of the finite element method support the accuracy of the model here developed., 16 pages, 8 figures

The composite pyramidal lattice truss core sandwich panel, in which the lattice core is strengthened by end frames between various nodes and the struts are reinforced with unidirectional fibers, is fabricated by the hot-press molding technique and interlocking method. Because all of the continuous fibers are aligned in the rod direction, the fiber’s intrinsic strength can be fully exploited; thus, the lattice truss core has the maximum load-carrying capacity. Furthermore, the composite lattice truss core (strengthened by the top and bottom aluminum connectors) has the ability to collaboratively deform under compressive and shear loading. Compressive and shear experiments were conducted to investigate the mechanical properties and failure mechanism of this new composite pyramidal lattice structure. The experimental results show that the present composite pyramidal lattice structure has some significant advantages compared with other existing composite pyramidal lattice structures.

The low through-thickness thermal conductivity limits the thermal management application of carbon fiber reinforced polymer-matrix (CFRP) composites. A simple method of constructing a 3D thermal conductive pathway is developed to improve the through-thickness thermal conductivity of CFRP laminated composites. Then, a z-filler laminated composite panel is fabricated by filling the copper spheres and coating the aluminum foils, and its effective thermal conductivity is investigated, both experimentally and theoretically. Analytical predictions agree well with measured values. The through-thickness thermal conductivity of z-filler laminated composites has been significantly improved by increasing the z-filler volume fraction but by decreasing the interface thermal resistance. A maximum improvement in the through-thickness thermal conductivity is up to approximately 12 times compared with the unmodified composite materials. Moreover, with high through-thickness thermal conductivity, z-filler laminated composites possess both low coefficient of thermal expansion (CTE) and adequate mechanical properties for structural applications. Thus, z-filler laminated composites can be considered as promising candidates for multifunctional material in the high-end heat dissipation fields.

Hybrid composite pyramidal truss sandwich panels combined with multiple damping configurations are fabricated in this work. Modal and quasi-static compressive tests are carried out to investigate the damping and stiffness efficiency of the candidate structures. Experimental results show that such structures combined with damping materials would significantly improve the damping loss efficiency but decrease simultaneously the stiffness efficiency in varying degrees compared with the bare hybrid sandwich panels. In order to evaluate the compatible effect of total damping and stiffness efficiency of the present sandwich structures, a synthetic evaluation criterion is developed, which shows that bare sandwich panels filled with hard polyurethane foam (B-II-HPF) and soft polyurethane foam (B-II-SPF) can yield the best performance up to 2–4 times higher than the base hybrid sandwich panels. It is also shown that multiple patch damping treatments based on the finite element-modal strain energy (FE-MSE) approach are suitable and effective to further improve the total damping efficiency.

Carbon fiber reinforced three dimensional egg and pyramidal honeycomb grids cores with interconnected void spaces were fabricated using an interlocking method. The out-of-plane compression properties, failure modes, and energy absorption capacity of all-composite sandwich panels made of the new 3D grid cores were investigated. The analytical models for predicting the compressive stiffness and strength of both egg and pyramidal honeycomb grids cores were derived. The results showed that the fabricated sandwich panels have higher specific energy absorbing abilities compared to lightweight square honeycombs of same density (10–100 Kg/m3). The new core design promises novel applications for lightweight multifunctional structures due to increased flow in the inner spaces of the core construction, embedding of electrical lines, enhanced heat transfer, fuel storage and higher energy absorption compared to traditional cores.

This paper focuses on improving the strength of truss lattice sandwich structures by optimizing the topology structure. A new Hourglass truss lattice sandwich structure is then designed and fabricated by a snap-fit method. The out-of-plane and in-plane compressive properties of the Hourglass lattice sandwich structure are investigated theoretically and experimentally, and the measured results are compared with the analytical predictions. Results show that the low relative density Hourglass lattices have higher peak out-of-plane compressive strengths than the pyramidal lattices. Moreover, the peak in-plane compressive loads of Hourglass lattice sandwich structures are superior to the pyramidal sandwich structures. Therefore, the Hourglass lattice sandwich structure can be considered as a promising candidate in high specific strength lightweight structures.

The novel composite pyramidal truss core sandwich panels with reinforced joints are manufactured by the water jet cut and interlocking assembly method. Here, the relative density [Formula: see text] of the carbon fiber composite pyramidal truss cores varies in the range 0.88–4.4%. The out-of-plane compression and shear performance of sandwich structures are investigated. Analytical estimates for sandwich panel strength are presented for possible competing failure modes. In general, the measured failure loads show good agreement with the analytical predictions. The measured quasi-static uniaxial compressive strength increases from 0.53 to 8.9 MPa with increasing [Formula: see text] over the relative density range investigated here. The shear strength of lattice cores is improved by eliminating core-to-face sheet bond failures. The measured quasi-static shear strength varies between 0.78 and 1.75 MPa. Compared with node non-reinforced composite pyramidal truss sandwich panels, under the relative density of 1.8%, the shear strength of node reinforced sandwich panels increases by 120%, and its shear modulus increases by 26.5%.

This paper focuses on the effect of temperature on the out-of-plane compressive properties and failure mechanism of carbon fiber/epoxy composite pyramidal truss cores sandwich panels (CF/CPTSP). CF/CPTSP with novel reinforced frames are manufactured by the water jet cutting and interlocking assembly method in this paper. The theoretical analysis is presented to predict the out-of-plane compressive stiffness and strength of CF/CPTSP at different ambient temperatures. The tests of composite sandwich panels are performed throughout the temperature range from − 9 0 ∘ C to 18 0 ∘ C . Good agreement is found between theoretical predictions and experimental measurements. Experimental results indicate that the low temperature increases the compressive stiffness and strength of CF/CPTSP. However, the high temperature causes the degradation of the compressive stiffness and strength. Meanwhile, the effects of temperature on the failure mode of composite sandwich panels are also observed.

A large amplitude vibration analysis is presented for a shear deformable doubly curved panel made of functionally graded materials (FGMs) resting on elastic foundations in thermal environments. The effective material properties are evaluated using the Mori–Tanaka micromechanics model. The formulations are based on a higher order shear deformation theory and von Karman strain–displacement relationships. The panel–foundation interaction and thermal effects are also included. The temperature-dependent material properties of FGMs are assumed to be graded in the thickness direction according to a simple power law distribution. The motion equations are solved by a two-step perturbation approach to determine the nonlinear frequencies of the FGM doubly curved panel. The numerical illustrations cover small- and large-amplitude vibration characteristics of FGM doubly curved panels resting on elastic foundations of Pasternak-type. The results obtained from the Mori–Tanaka model are compared with those obtained from the Voigt model. The results confirm that in most cases Voigt model and Mori–Tanaka model have the same accuracy for predicting the vibration characteristics of FGM doubly curved panels. The effects of volume fraction index, temperature variation, foundation stiffness and panel curvature ratio on the nonlinear free vibration behaviors of FGM doubly curved panels are also discussed in detail.

Composite thin-walled curved structures are widely used in aerospace, marine, automotive and building engineering application. In this paper we design and fabricate a series of carbon fibre composite axial and circular corrugated sandwich cylindrical panels (ACSCPs and CCSCPs) by an in-house hot press moulding method. Low velocity impact tests are carried out to evaluate the impact resistance and failure mechanisms of such structures. Furthermore, validated finite element analysis (FEA) models based on the Hashin failure criteria are adopted to study the effects of the relative density, impact energy and impact position on their impact responses. It is observed that generally the peak forces and absorbed energies of the specimens ascend with the increase of the relative density. The impact responses, especially the ultimate loads of the present structures are particularly dependent on the impact position, but insensitive to the increase of the impact energy. By comparison, the ACSCPs generally have a more excellent impact resistance and energy absorption properties than that of CCSCPs. In addition, the corresponding energy contribution of the components for the ACSCPs and CCSCPs under different impact energy is also revealed, which could be useful for the multifunctional design of such kinds of composite curved sandwich structures.

[EN] The Poisson-like effect, describing the redirection of waves by 90 degrees, is shown to be feasible for flexural waves propagating in perforated thin plates. It is demonstrated that the lowest order symmetric leaky guided mode (S0 mode) is responsible for the splitting of wave motion in two orthogonal directions. The S0 mode shows a feature of stationary waves containing standing wave modes in one and two orthogonal directions for smaller and larger holes, respectively. The former case is well understood thanks to the phenomenon of Wood's anomaly, which was first observed in optical gratings supposed to be transparent. On the contrary, the strong scattering caused by the larger holes leads to a mixed mode occurring when the incident wave is totally transmitted. The mixed mode easily couples with the incoming waves and, therefore, the Poisson-like effect activated under this mechanism is much stronger. Using the Poisson-like effect, a device is proposed in which about 82% of the incident mechanical energy is redirected to the perpendicular direction. Results obtained with arrays of free holes also apply to inclusions with parameters properly chosen. The findings may provide applications in beam splitting and waveguiding. (C) 2018 Acoustical Society of America., Work supported by the Ministerio de Economia y Competitividad of Spain and the EU Fondo Europeo de Desarrollo Regional under Project No. TEC2014-53088-C3-1-R, and the National Natural Science Foundation of China under Grants Nos. 11432004 and 11421091. P.G. acknowledges a scholarship with No. 201606120070 provided by China Scholarship Council.

This paper presents a novel tri-directional negative stiffness (NS) mechanical metamaterial, consisting of disk structure elements arranged in three-dimensions. The NS behavior was achieved through the elastic instability of the disk structure. Through the combination of numerical simulations and experiments, the NS property, the deformation mechanisms, and the basic mechanical performance of the disk structure and the novel NS mechanical metamaterial were systematically investigated. The influence of the geometric parameters on the snap-through behaviors of the NS metamaterial was investigated through the experimentally verified numerical method, and further designs were conducted on the disk structure to tailor its mechanical responses. Also based on the instability of the disk structure, several novel multidirectional NS structures, which can achieve NS behavior along five and seven loading directions, were firstly proposed. This innovation improves the homogeneity of the NS metamaterials, and is expected to expand the application scope of such metamaterial, considering that the previous reported NS metamaterial can exhibit NS behavior along three-dimensions at most. Moreover, the mechanical performance of the proposed NS metamaterial is tunable, benefited from the special structure form of itself. To our knowledge, this work is first to design NS metamaterial using the disk structures, and report the multidirectional (more than tri-directions) NS metamaterials.

Topological management of materials at a Micro-scale is one of the fundamental building principles of nature. This combination of material and structural properties results in marked changes in the properties of solids. Nowadays physicists, chemists, materials scientists and engineers explore those effects by synthesizing, characterizing, and modeling Micro-lattice materials from all material classes. Applications have been identified in the fields of ultra-lightweight structures, thermal equipment, electrochemical devices, high absorption capacity and bio-repair materials. This article aims to review recent progress in the development of such advanced Micro-lattice materials.

The vibration and damping characteristics of free–free composite sandwich cylindrical shell with pyramidal truss-like cores have been conducted using the Rayleigh–Ritz model and finite element method. The predictions for the modal properties of composite sandwich cylindrical shell with pyramidal truss-like cores showed good agreement with the experimental tests. The influences of fiber ply angles on the natural frequency and damping loss factor were investigated. Three types of such composite sandwich cylindrical shells were manufactured using a hot press molding method and the relevant modal characteristics of various sandwich cylindrical shells could be obtained by modal tests. It can be found that modal strain energy approach was effective to estimate the structural damping loss factors and the variation of damping for each vibration modes could be explained suitably through the contributions of each strain energy components. Results showed that the structural damping loss factors not only depended on inherent material damping, but also relied on the vibration modes. The natural frequencies of composite sandwich cylindrical shell increased with the increasing of the ply angle of the inner and outer curve face sheets, whereas the damping loss factors of present shells did not increase monotonically.

A study on the bending response of a composite curved panel with pyramidal metallic truss cores suitable for functional applications is presented using a combination of analytical modeling, three-point bending experiments and finite element (FE) based simulations. The aluminum pyramidal cores were constructed using an interlocking method before curing with composite face sheets to fabricate the final structure. A theoretical model was developed to analyze the experiments and develop failure criteria. Three failure modes: (i) face wrinkling, (ii) face crushing, and (iii) debonding between face sheet and truss cores, were considered and theoretical relationships for predicting the collapse load associated with each mode were developed. The experiments were carried out on two sets of specimens with differing face sheet thickness which clearly indicated the important role played by core debonding in determining the peak load of the structure. Localized buckling instabilities were also reported for samples with thinner face sheets. The role of debonding in determining strength was further highlighted by a comparison with FE simulations with suppressed debonding.

The effects of high temperature exposure on the low velocity impact behaviors and damage mechanisms of all-composite pyramidal truss core sandwich panels were investigated by experiment in this paper. The composite sandwich panels were manufactured from unidirectional carbon/epoxy prepreg and exposed to different temperatures for 6 h. The impact tests of exposed specimens were performed at three different energy levels, and the effects of exposure temperature and impact energy level on the damage mechanism, absorbed energy and maximum impact force were analyzed. The impact-damaged specimens were subsequently subjected to in-plane compressive tests in order to investigate the effect of exposure temperature and impact energy level on the compressive failure load. The results have shown that the high temperature exposure has a significant effect on impact properties and damage mechanisms of specimens. The fiber fracture, node failure, delamination and buckling were observed during low velocity impact tests and the extent of damage area was significantly affected by exposure temperature. In addition, the absorbed energy increased with increasing exposure temperature, while the maximum impact force and compressive failure load after impact decreased with increasing exposure temperature due to the degradation of the matrix properties and fiber–matrix interface properties at higher temperatures.

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.

Mechanical properties and failure modes of carbon fiber composite egg and pyramidal honeycombs cores under in plane compression were studied in the present paper. An interlocking method was developed for both kinds of three-dimensional honeycombs. Euler or core shear macro-buckling, face wrinkling, face inter-cell buckling, core member crushing and face sheet crushing were considered and theoretical relationships for predicting the failure load associated with each mode were presented. Failure mechanism maps were constructed to predict the failure of these composite sandwich panels subjected to in-plane compression. The response of the sandwich panels under axial compression was measured up to failure. The measured peak loads obtained in the experiments showed a good agreement with the analytical predictions. The finite element method was used to investigate the Euler buckling of sandwich beams made with two different honeycomb cores and the comparisons between two kinds of honeycomb cores were conducted. Fil: Xiong, Jian. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Zhang, Meng. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Stocchi, Ariel Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación En Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingeniería; Argentina Fil: Hong, Hu. The Hong Kong Polytechnic University; China Fil: Ma, Li. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Wu, Linzhi. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Zhang, Zhong. Beijing Institute of Structure and Environment Engineering; China

An experimental study was performed to investigate the effect of high temperature exposure on mechanical properties of carbon fiber composite sandwich panel with pyramidal truss core. For this purpose, sandwich panels were exposed to different temperatures for different times. Then sandwich panels were tested under out-of-plane compression till failure after thermal exposure. Our results indicated that both the thermal exposure temperature and time were the important factors affecting the failure of sandwich panels. Severe reductions in residual compressive modulus and strength were observed when sandwich panels were exposed to 300 °C for 6 h. The effect of high temperature exposure on failure mode of sandwich panel was revealed as well. Delamination and low fiber to matrix adhesion caused by the degradation of the matrix properties were found for the specimens exposed to 300 °C. The modulus and strength of sandwich panels at different thermal exposure temperatures and times were predicted with proposed method and compared with measured results. Experimental results showed that the predicted values were close to experimental values.

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.

Bending properties and failure modes of carbon fiber composite egg and pyramidal honeycomb beams were studied and presented in this paper. Three point bending responses of both sandwich beams were tested. Face wrinkling, face crushing, core member crushing and debonding were considered, and theoretical relationships for predicting the failure load associated with each mode were presented under three point bending load. Failure mechanism maps were constructed to predict the failure of composite sandwich beams with pyramidal and egg honeycomb cores subjected to bending. Face wrinkling and core debonding have been investigated under three point bending and the maximum displacement was studied using analytical and experimental methods. The finite element method was employed to determine the ratio (maximum displacement/applied load) of sandwich beam with two different honeycomb cores. Comparisons between two kinds of honeycomb beams were also conducted. Fil: Xiong, Jian. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Ma, Li. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Stocchi, Ariel Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación En Ciencia y Tecnología de Materiales (i); Argentina Fil: Yang, Jinshui. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Wu, Linzhi. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Pan, Shidong. Harbin Institute of Technology; China

Since the concept of invisible cloak was proposed by Pendry and Leonhardt in 2006, many researchers have applied the theory of coordinate transformation to thermodynamics and overcome the complexity of inhomogeneous and anisotropic of material parameters. However, only two-dimensional (2D) thermal illusion devices are researched recently. According to this situation, our study focuses on three-dimensional (3D) thermal illusion devices including shrinker (or invisible cloak), concentrator, amplifier, reshaper, and rotator with arbitrary shape in a general way. In this paper, the corresponding material parameters of thermal illusion devices mentioned above are derived based on the theory of transformation thermodynamics and the simulated results agree well with the theoretical derivations. In addition, the conventional invisible cloak just controls the temperature gradient rather than the temperature value which is more concerned in physical applications. Here, we find that the temperature value of the cloaked object can be controlled by adjusting the location of the original point of the coordinate system.

The vibration and damping performances of hybrid carbon fiber composite pyramidal truss sandwich panels with viscoelastic layers embedded in the face sheets were investigated in this paper. Hybrid carbon fiber composite pyramidal truss sandwich panels containing different thickness of viscoelastic layers were manufactured using a hot press molding method. Analytical models based on modal strain energy approach were developed using ABAQUS software to estimate the damping property of the hybrid sandwich structures. A set of modal tests were carried out to investigate the vibration and damping characteristics of such hybrid sandwich panels with or without viscoelastic layers. The damping loss factors of composite slender beams with different fiber orientations were tested to determine the constitutive damping properties of parent materials for such hybrid sandwich panels. The numerical simulation results showed good agreement with the experimental tests. The damping loss factors of hybrid sandwich panels increased distinctly compared with previous sandwich panels due to the viscoelastic layer embedded in the face sheets.

A combined experimental and numerical study is conducted to assess the effects of impact energy, impact site and core density on the compression-after-impact (CAI) strength of pyramidal truss core sandwich structures. It is found that the severity of impact damage highly depends on the impact site. The CAI tests show that the local buckling occurs for both the un-impact specimens and the specimens impacted under lower energy, while debonding is observed for the specimens impacted under higher energy. In case of the specimens impacted on the middle point of the four adjacent nodes, only a crack appears in the impact-damaged facesheet, which propagates transversely across the facesheet. It is observed that the CAI strength of specimens impacted on the middle point of four adjacent nodes drop much more than that of specimens impacted on the node, and that the specimens with higher density cores have slightly lower normalized CAI strength reduction. Moreover, it is found that the CAI strength of specimens impacted on the node decreases with the impact energy increasing. In addition to experimental tests, the numerical simulation performs well in capturing the failure modes for impact-damaged specimens under compressive load.

We present a novel method for fabricating carbon fiber composite sandwich panels with lattice core construction by means of electrical discharge machining (EDM). First, flat-top corrugated carbon fiber composite cores were fabricated by a hot press molding method. Then, two composite face sheets were bonded to each corrugated core to create precursor sandwich panels. These panels were transformed into sandwich panels with near-pyramidal truss cores by EDM plunge-cutting the corrugated core between the face sheets with a shaped cuprite electrode. The flat top corrugation permits adhesive to be applied consistently, and the selected dimensions leave a substantial bond area after cutting, resulting in a strong core-to-sheet bond. The crushing behavior of this novel construction was investigated in flatwise compression, and the results were compared to analytical expressions for strength and stiffness.

In this paper, response of carbon fiber reinforced polymer (CFRP) laminates subjected to high velocity impact has been investigated by experimental and numerical methods. Experiments using a two-stage light gas gun were conducted to investigate the impact process and to validate the finite element model. The energy absorption efficiency (EAE) of CFRP laminates with different thickness was investigated. According to the results of experiments and numerical calculations, thin CFRP laminates have a good EAE under relative higher velocity impact; by contraries, a superior EAE is displayed in thick laminates under relative lower velocity impact. Subsequently, EAE of CFRP laminates was compared with that of 304 stainless-steel plates. In a specific impact velocity range, EAE of CFRP laminates is higher than that of 304 stainless-steel. Thus, CFRP laminates have a potential advantage to substitute the metal plates to be used in high velocity impact resistance structures under a specific impact velocity range.

An attempt has been made here to evaluate the effect of thermal exposure on the mechanical behavior and failure mechanisms of carbon fiber composite sandwich panel with pyramidal truss core under axial compression. Analytical formulae for the collapse strength of composite sandwich panel after thermal exposure were derived. Axial compression tests of composite laminates and sandwich panels after thermal exposure were conducted at room temperature to assess the degradation caused by the thermal exposure. Experimental results showed that the failure of sandwich panel are not only temperature dependent, but are time dependent as well. The decrease in residual compressive strength is mainly attributed to the degradation of the matrix and the degradation of fiber–matrix interface, as well as the formation of cracks and pores when specimens are exposed to high temperature. The measured failure loads obtained in the experiments showed reasonable agreement with the analytical predictions.

In this paper, the pyramidal truss core sandwich structures consisting of carbon fiber reinforced polymer (CFRP) facesheets and aluminum alloy cores were manufactured based on the slot-fitting method. This hybrid concept is to maximize the specific bending stiffness/strength as well as obtain excellent energy absorption ability. Quasi-static compression tests were conducted to get the stress–strain curves and to evaluate the energy absorption mechanism. Low velocity impact tests were carried out to investigate the damage resistance of such structures. The compressive measurements show that the low density aluminum alloy pyramidal truss cores have superior energy absorption ability compared with other lightweight lattice cores. In the impact tests, the failure of matrix cracking, fiber breakage, delamination of CFRP facesheets and buckling of truss members occurred and the extent of damage was significantly affected by the impact site. In addition to experimental testing, finite element models for compression and impact simulations have been developed using ABAQUS software. The numerical results were validated compared with the experimental tests.

A study on the mechanical property degradation of carbon fiber composite sandwich panel with pyramidal truss cores by high temperature exposure is performed. Analytical formulae for the residual bending strength of composite sandwich panel after thermal exposure are presented for possible competing failure modes. The composite sandwich panels were fabricated from unidirectional carbon/epoxy prepreg, and were exposed to different temperatures for different time. The bending properties of the exposed specimens were measured by three-point bending tests. Then the effect of high temperature exposure on the bending properties and damage mechanism were analyzed. The results have shown that the residual bending strength of composite sandwich panels decreased with increasing exposure temperature and time, which was caused by the degradation of the matrix property and fiber-matrix interface property at high temperature. The effect of thermal exposure on failure mode of composite sandwich panel was observed as well. The measured failure loads showed good agreement with the analytical predictions. It is expected that this study can provide useful information on the design and application of carbon fiber composite sandwich panel at high temperature.

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.

A method of manufacturing carbon fibre reinforced polymer (CFRP) tetrahedral lattice truss core sandwich structure by thermal expansion silicon rubber mould was developed. The sandwich structure was manufactured integrally without secondary bonding and the silicon rubber mould can be made mass-production with low cost in this approach. The intrinsic property of the CFRP was fully exploited because of carbon fibres aligned in the axial orientation of the truss member. The mechanical properties of CFRP tetrahedral lattice truss core sandwich structures were investigated by flatwise compression and shear test. The experimental results indicate that CFRP tetrahedral lattice truss core sandwich structures have higher weight-specific compressive strength than some metal truss cores, and are competitive with conventional honeycombs.

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.

A combined theoretical, experimental and numerical investigation of carbon fiber composite pyramidal core sandwich plates subjected to torsion loading is conducted. Pyramidal core sandwich plates are made from carbon fiber composite material by a hot compression molding method. Based on the Classical Laminate Plate Theory and Shear Deformation Theory, the equivalent mechanical properties of laminated face-sheet are obtained; based on a homogenization concept combined with a mechanical of materials approach, the equivalent in-plane and out-of-plane shear moduli of pyramidal core are obtained. A torsion solution is derived with Prandtl stress function and can be used in the sandwich plate with orthotropic face-sheets and orthotropic core. The influences of material properties and geometrical parameters on the equivalent torsional stiffness are explored. In order to verify the accuracy of the analytical torsion solution, experimental tests of sandwich plate samples with different face-sheet thicknesses are conducted and two types of finite element models are developed. Good agreements among analytical predictions, finite element simulations and experimental evaluations are achieved, which prove the validity of the present derivation and simulation. The proposed method could also be applied in design applications and optimization of the pyramidal core sandwich structures.

With the equivalent mechanical properties of composite materials, analytical formulae of critical load for an all-composite sandwich column with pyramidal truss core are derived. Four failure modes are considered: macro Euler buckling, macro shear buckling, face-sheet wrinkling and face-sheet crushing. Failure mechanism maps are constructed with the four competing failure modes, and the relationship between the failure mechanism maps and material mechanical properties is discussed. Selected experiments validate the analytical predictions, and reasonable agreement is obtained. Macro shear buckling is the main failure mode for the sandwich column specimens, which is attributed to the low stiffness of core. The final failure loads is related to the strength of the nodes between face-sheets and truss core, so the node strength is the key of improving the failure load. Given by numerical simulations, the failure loads and failure modes agree well with analytical predictions.

In this paper, low-velocity impact characteristics and residual tensile strength of carbon fiber composite lattice core sandwich structures are investigated by experimentally and numerically. Low-velocity impact tests and residual tensile strength tests are performed using an instrumented drop-weight machine (Instron 9250HV) and static test machine (Instron 5569), respectively. The FE (finite element) software, ABAQUS/Explicit is employed to simulate low-velocity impact characteristics and predict residual tensile strength of carbon fiber composite lattice core sandwich structures. These numerical investigations create a user-defined material subroutine (VUMAT) to enhance the damage simulation which includes Hashin and Yeh failure criteria. The impact contact force and the tensile strength are accurately estimated using the present method. From results of this paper, the degradation of residual tensile strength can be divided to three stages for different impact energies, and amplitudes of degradation are affected by stacking sequences.

A series of analytical and experimental investigations is presented to study the response and failure of pyramidal truss core sandwich panels made of carbon fiber composite under axial compression. In the analytical part of the study, three failure modes: (i) Euler or core shear macro-buckling, (ii) face wrinkling, and (iii) face sheet crushing, were considered and theoretical relationships for predicting the failure load associated with each mode were presented. In the experimental part of the study, three different sets of specimens were manufactured to probe the three failure modes mentioned above. Pyramidal truss cores were fabricated using a hot-press molding technique and were bonded to composite face sheets made of fiber reinforced composite. The response of the sandwich panels under axial compression was measured up to failure. The measured peak loads obtained in the experiments showed good agreement with the analytical predictions. The experiments also provide insight into the post-failure response of the sandwich panels.