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Process-induced deformation in composite laminates usually occurs during the curing process and is strongly dependent on the cure parameters. To study the influence of the cure parameters on the warpage deformation, an experimental study was carried out with different heating rates and first dwell times. The full-field process-induced deformation was captured by contactless 3 D scanning system to validate the numerical method. Second, a multi-scale thermo-viscoelastic modeling technique was employed to predict the process-induced deformation. The results showed a good agreement between the simulation and experimental deformation, which demonstrates the effectiveness of the numerical model. The influence of six different cure parameters of the vacuum bagging process on the warping deformation and residual stress were considered in the numerical study. Knee points were found on the curves depicting the influence of cure parameters on the maximum residual deformation. Within the manufacturer’s specified range, a slower heating rate of 1.5°C/min reduced the process-induced deformation by 7%, while the other parameters affected the process-induced deformation by less than 1.4%. Meanwhile, the residual stress was also reduced with a slower heating rate. Consequently, a lower heating rate is suggested to reduce both the process-induced deformation and the residual stress for thin parts or the zero-bleeding process.

The ballistic impact behavior of hybrid 3-dimentional woven composites (3DWC) is predicted through a novel numerical modeling technique and validated the outcome through experimental data. Continuum shell elements with Hashin failure criterion and cohesive surface contact algorithm with traction separation law are used to predict the damage behavior and failure mechanisms during the impact process at interply and intraply levels. Z-yarns are represented by the connector elements with uniaxial behavior having stress-based failure criterion. The proposed methodology predicted the different damage mechanisms during the impact process in comparison with the experimental data and estimated the residual velocities with acceptable accuracy. Different failure mechanisms incurred due to the hybrid nature of the material are also captured by the numerical simulation and the effect of z-yarns are truly depicted by the use of simple 1D elements over the different phases of perforation process. Overall, the finite element (FE) methodology by using simplest form of the elements, their constitutive and damage behavior gives promising results by sufficiently reducing the computational cost.

In this paper, a broadband double-layer metamaterial absorber (MMA) based on carbon fibers (CFs) is proposed, which consists of three basic elements. Firstly, the reflectivity of CFs double-layer MMA in normal incidence is analyzed, and compared to the copper double-layer MMA with same size, the simulated results show that it obtains better absorbing effect. With the increase of incident angle, the multi-frequency resonance is activated, leading to larger effective bandwidth. Moreover, when incident angle is between 55 and 70 degrees, the continuous absorption band above 85% can totally cover X and Ku microwave band. Meanwhile, when the EM frequency is at 6-7.5GHz, the change of incident angle hardly affects the absorbing performances of CFs double-layer MMA. A specimen of the CFs double-layer MMA was fabricated by the vacuum bag molding process to verify its effectiveness and the measured results fitted well with the simulated results. In order to discuss the mechanism of EM absorption, the distribution of E-fields and power loss density were monitored. The results indicate that incident angle effects the distribution of E-fields and the way of absorption to change the absorbing performances. The proposed MMA has potential application prospects in the structural and functional integration design of CFs.

Aluminum honeycombs are widely applied in weight sensitive applications, increasing their specific strength and energy absorption capacity are rather important. Thin-walled metallic tube was used to enhance the mechanical properties of aluminum honeycomb and formed a novel tube-reinforced honeycomb structure. Its compressive and three-point bending performances were studied experimentally and numerically. Due to tube filling, the specific compressive strength, elastic modulus and energy absorption have been increased by 16%, 26% and 73%, and the specific bending load and stiffness increased by 42% and 62% respectively. The strengthen mechanism study indicated that the sum of tube and honeycomb caused the increase of compressive properties, and aluminum tube filling changed the stress distribution and expanded the stress concentration region which led to a transformation of bending failure mode. The present reinforcement method will make honeycomb more competitive in light-weight structure applications.

Lead-free dielectric ceramics with both a high recoverable energy storage density (Wrec) and excellent mechanical performance are highly desirable for practical applications in next-generation advanced pulsed power capacitors (APPCs). However, lead-free dielectric ceramics exhibit low Wrec owing to small breakdown strength (Eb) and poor mechanical properties because of their large pore size and low relative density, which restrict devices miniaturization and operation in severe environments. Here, we propose a new strategy, namely, grain size engineering, to develop K0.5Na0.5NbO3 (KNN)-based ceramics with both an extremely high Wrec and large mechanical properties. Interestingly, a large Wrec of 2 J cm−3 was achieved in 0.9K0.5Na0.5NbO3-0.1BiFeO3 (0.9KNN-0.1BF) ceramics at 206 kV cm−1, which is superior to other lead-free dielectric ceramics under moderate electric fields (

Nomex honeycombs are widely used in lightweight structures due to their unique porous structure. By adding an absorbent agent to Nomex honeycomb wall and filling carbon fiber reinforced polymer (CFRP) tube, a novel tube-reinforced absorbent honeycomb sandwich structure was proposed. Its compressive properties and energy absorption characteristics were studied experimentally and numerically. Compared with empty absorbent honeycomb, the normalized elastic modulus E, peak stress and energy absorption of tube-reinforced absorbent honeycomb can be increased by 55.74%, 621.69% and 327.86% respectively. Moreover, the energy absorption of the tube-reinforced absorbent honeycomb was increased by 30.34%, compared with the sum of empty absorbent honeycomb and CFRP tube tested separately. The interaction effects and enhancement mechanism between absorbent honeycomb and CFRP tube were also discussed. The excellent electromagnetic absorption and mechanical properties make this novel honeycomb sandwich structure much more competitive not only in electromagnetic wave stealth application but also in load-carry structures.

Polylactic acid (PLA) hexagonal honeycomb structures were fabricated by using 3D-printing technology. By filling with absorbent polymethacrylimide (PMI) foam, a novel absorbent-foam-filled 3D-printed honeycomb was obtained. The in-plane (L- and W-direction) and out-of-plane (T-direction) compressive performances were studied experimentally and numerically. Due to absorbent PMI foam filling, the elastic modulus, compressive strength, energy absorption per unit volume, and energy absorption per unit mass of absorbent-foam-filled honeycomb under L-direction were increased by 296.34%, 168.75%, 505.57%, and 244.22%, respectively. Moreover, the elastic modulus, compressive strength, energy absorption per unit volume, and energy absorption per unit mass, under W-direction, also have increments of 211.65%, 179.85, 799.45%, and 413.02%, respectively. However, for out-of-plane compression, the compressive strength and energy absorption per unit volume were enhanced, but the density has also been increased, thus, it is not competitive in energy absorption per unit mass. Failure mechanism and dimension effects of absorbent-foam-filled honeycomb were also considered. The approach of absorbent foam filling made the 3D-printed honeycomb structure more competitive in electromagnetic wave stealth applications, while acting simultaneously as load-carrying structures.

Foam sandwich structures reinforced with the fiber column arrays are proposed to achieve broadband absorption at microwave frequencies as well as good mechanical properties. To achieve the microwave absorption, two layers of resistive films are embedded into the sandwich structures. The results show that the proposed sandwich structure with the whole thickness of 9.73 mm can achieve broadband absorption with the reflection less than −10 dB in the frequency range of 2.6–21 GHz. The absorption mechanism has been discussed by using the equivalent circuit model. It is further indicated that introduction of the fiber column array in the sandwich structure has little influence on the absorption performance, but can remarkably enhance the mechanical properties. This has been well demonstrated experimentally by three-point bending experiment. Compared with the ordinary sandwich absorbing structures, enhanced mechanical properties make the designed structure suitable in some loading applications. It is expected that the proposed multifunctional structure may find great potentials in the applications of stealth technology, electromagnetic interferences and so on. Keywords: Sandwich structure, Broadband absorption, Mechanical properties, Equivalent circuit model

A light-weight sandwich structure is designed in this paper to achieve the broadband reflection reduction based on scattering cancellation. The multifunctional sandwich structure is composed of checkerboard metasurface (MS), foam core and carbon fabric/epoxy composite reflector. The checkerboard MS can redirect the incident waves away from the specular direction to achieve the reflection reduction. In addition, the carbon fabric/epoxy composite has the dual role as the reflection layer of incident waves and load bearing face material of whole structure. A series of simulation and optimization results indicate that the whole structure obtains reflection reduction with the reflectivity less than −10 dB in the frequency of 6.7–19.8 GHz under normal incidence, and the reflectivity less than −15 dB in 9.8–14.0 GHz. The proposed structure is lightweight and the density is 0.58 g/cm3. By fabricating the sample, electromagnetic property has been well demonstrated experimentally.

Aluminum foam was limited when applied as load carrying structures for its lower strength. Therefore, an effective enhancement method of aluminum foam was reported in the present study by filling of 304 stainless steel tube into a pre-perforated hole and fixed by epoxy glue. The experimental results indicate that the novel tube enhanced foam (with equivalent density of foam) can doubled the specific compressive strength and energy absorption of that of aluminum foam, and even larger than that of the sum of tube and foam which were tested separately. The coupling strengthening mechanisms are suggested to be the instability limitation of steel tube due to the lateral supports (external and internal) supplied by aluminum foam.

In this study, a closed-cell aluminum foam was filled into the interspaces of a sandwich panel with corrugated cores to form a composite structure. The novel structure is expected to have enhanced foam-filled cores with high specific strength and energy absorption capacity. An out-of-plane compressive load under low-velocity impact was experimentally and numerically carried out on both the empty and foam-filled sandwich panels as well as on the aluminum foam. It is found that the empty corrugated sandwich panel has poor energy absorption capacity due to the core member buckling compared to that of the aluminum foam. However, by the filling of the aluminum foam, the impact load resistance of the corrugated panel was increased dramatically. The loading-time response of the foam-filled panel performs a plateau region like the aluminum foam, which has been proved to be an excellent energy absorption material. Numerical results demonstrated that the aluminum foam filling can decrease the corrugated core member defects sensitivity and increase its stability dramatically. The plastic energy dissipation of the core member for the foam-filled panel is much higher than that of the empty one due to the reduced buckling wavelength caused by the aluminum foam filling.

Aimed to enhance electromagnetic (EM) wave transmission and compressive properties of sandwich structures with FR-4 face sheets and the polymethacrylimide (PMI) foam core, a kind of spoof surface plasmon polaritons (SSPPs) structure realized by gradient metal wire arrays was proposed and inserted into the PMI foam core to form a novel antireflection sandwich structure. The transmission performances under normal and oblique incidence were studied by simulation and experimental methods. The results showed that this sandwich structure can achieve a broadband enhanced transmission in X and Ku bands. The enhanced transmission mechanism was suggested to be the effect of the inserted SSPPs structure which changed the transmission path of EM waves in a local area, increased the interface refraction angle and greatly suppressed reflection of each interface. In addition, compressive properties of the sandwich structures were studied experimentally, and the results showed that compared with normal PMI foam sandwich structure, the compressive strength and the energy absorption per unit volume of the antireflection sandwich structure were increased by 72% and 60%. The proposed antireflection sandwich structure is more effective and competitive in load bearing and EM wave transmission applications than traditional structures.

The TiO 2 /SiO 2 monoliths with macro- and mesopore structure have been synthsized by sol-gel method. The double pore structure was developed by adding polyethylene glycol and urea in the precurser solution. The morphology, crystallinity, surface area and porosity of the TiO 2 /SiO 2 monoliths were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), N 2 adsorption/desorption and mercury intrusion techniques. The photocatalytic activity and applicablity of the monoliths were investigated by degradation of methylene blue and purificaiton of actual dyeing and finishing wastewater. The result showed the prepared TiO 2 /SiO 2 monolith to have enhanced photocatalytic activity, reusability and easy recovery, so it may be considered a promising material for practical application in wastewater treatment.

To improve the load carrying capacity, structural stiffness, and impact resistance of a honeycomb sandwich structure, the honeycomb holes are filled with metallic tubes. Experimentally and numerically studies were carried out on the drop weight impact response of the tube-reinforced honeycomb sandwich structure. The results show that the stiffness and peak load of the honeycomb sandwich structure were increased by the metallic tube filler. The addition of tube filler made the Mises stress and deformation distribution of the front and back face-sheets more uniform. In addition, the tube-reinforced structure absorbed the impact energy more quickly than the empty honeycomb sandwich structure and the front face-sheet deformation was markedly reduced by tube reinforcement. The peak load and contact energy (the energy absorbed by the local deformation of sandwich structures) were predicted theoretically for both empty honeycomb sandwich structure and tube-reinforced honeycomb sandwich structure. The finite element parametric study shown that, compared with the empty honeycomb sandwich structure, the maximum deflections of the front and back face-sheets of the globally filled tube-reinforced honeycomb sandwich structure were reduced by 18.6% and 36.4% respectively. The tube-reinforced honeycomb sandwich structure is a promising structure for weight sensitive applications owing to its improved load carrying capacity and impact resistance.

The design of lightweight sandwich structures with high specific strength and energy absorption capability is valuable for weight sensitive applications. A novel all-metallic foam-filled Y-shape cored sandwich panel was designed and fabricated by using aluminum foam as filling material to prevent core member buckling. Experimental and numerical investigation of out-of-plane compressive loading was carried out on aluminum foam-filled Y-shape sandwich panels to study their compressive properties as well as on empty panels for comparison. The results show that due to aluminum foam filling, the specific structural stiffness, strength, and energy absorption of the Y-shape cored sandwich panel increased noticeably. For the foam-filled panel, aluminum foam can supply sufficient lateral support to the corrugated core and vertical leg of the Y-shaped core and causes a much more complicated deformation mode, which cannot occur in the empty panel. The complicated deformation mode leads to an obvious coupling effect, with the stress&ndash, strain curve of the foam-filled panel much higher than those of the empty panel and aluminum foam, which were tested separately. Metallic foam filling is an effective method to increase the specific strength and energy absorption of sandwich structures with lattice cores, making it competitive in load carrying and energy absorption applications.

The mechanical and failure behavior under high velocity impact of asymmetric hybrid three-dimensional orthogonal woven composite (3DWC) were investigated through experiments and finite element modeling. Two types of non-hybrid and one asymmetric hybrid 3DWCs panels were manufactured with the combination of carbon and Kevlar fiber tows and tested under different impact energies. The hybrid panels were impacted on both faces separately to analyze the directional behavior under impact loading. The energy absorption capability of the hybrid material is sufficiently increased when impacted on carbon face due to diversified failure behavior of carbon and Kevlar layers. Damage in Kevlar layers was due to fiber rupture and pull-out while in carbon layers, damage was induced by matrix cracking and brittle fiber failure. The impact response is simulated through continuum shell based finite element model using connector element technique. The proposed methodology gives a fair comparison with the experimental data in terms of failure behavior of the hybrid material during the impact process.

In this paper, the idea of spoof surface plasmon polaritons (SSPPs) realized by unidirectional carbon fibers (CFs) arrays are proposed and investigated. A structure/function integrated sandwich structure (SISS) with excellent microwave absorption and mechanical properties was designed. It is composed of two corrugation layers with gradient CFs arrays between them. According to our research, the absorption performance can be modulated by adjusting structure parameters. As the corrugation angle increases, the absorption performance becomes better whether in normal or oblique incidence. In addition, compression tests were also performed and the results indicate that this proposal can realize a higher compressive strength at a lower sandwich core density. The compressive strength is more than 3 times comparing with 304 stainless steel (SS) corrugation when the core density is 0.25 g/cm3. This structure/function integrated design is more competitive in practical applications comparing with other metamaterials topologies.

Lightweight structures with multi-functions such as electromagnetic wave absorption and excellent mechanical properties are required in spacecraft. A three-dimensional metamaterial absorber consisting of honeycomb and resistive films was proposed and fabricated through 3D printing and silk-screen printing technology. According to simulation and experiment results, the present three-dimensional metamaterial absorber can realize an absorptivity of more than 90% in a wide band of 3.53–24.00 GHz, and improve absorbing efficiency for transverse magnetic (TM) waves of oblique incidence angle from 0° to 70°. The compression test results reveal that compressive strength of the 3D printed honeycomb can reach 10.7 MPa with density of only 254.91 kg/m3, and the energy absorption per volume W v and per unit mass W m are 4.37 × 103 KJ/m3 and 17.14 KJ/Kg, respectively. The peak compressive strength and energy absorption per mass are at least 2.2 and 3 times comparing to metallic lattice cores with the same density. Outstanding electromagnetic wave absorption and mechanical performance make the present three-dimensional metamaterial absorber more competitive in engineering applications.

Sandwich panels having metallic corrugated cores had distinctly different attributes from those having metal foam cores, the former with high specific stiffness/strength and the latter with superior specific energy absorption capacity. To explore the attribute diversity, all-metallic hybrid-cored sandwich constructions with aluminum foam blocks inserted into the interstices of steel corrugated plates were fabricated and tested under three-point bending. Analytical predictions of the bending stiffness, initial failure load, peak load, and failure modes were obtained and compared with those measured. Good agreement between analysis and experiment was achieved. Failure maps were also constructed to reveal the mechanisms of initial failure. Foam insertions altered not only the failure mode of the corrugated sandwich but also increased dramatically its bending resistance. All-metallic sandwich constructions with foam-filled corrugated cores hold great potential as novel lightweight structural materials for a wide range of structural and crushing/impulsive loading applications.

A microwave absorption/transmission integrated sandwich structure (MATSS) with excellent mechanical properties was proposed and investigated in this paper. This structure is composed of composite corrugation panels with foams filled in its channels. The absorption/transmission property is produced based on the mechanism of spatial dispersion engineering and electromagnetic resonance. Different from previous designs, this proposal can realize a broadband absorption (5.74–7.84 GHz) below transmission band (8.96–12.45 GHz) with the absorptivity and transmissibility more than 80%. Moreover, compressive experiment results show that filling foams in channels can obviously improve the mechanical properties of this structure. The peak compressive strength of the foam-filled MATSS can reach to 25.21 MPa with a core density 0.32 g/cm3, which are more competitive comparing with many metallic sandwich core topologies.

By the addition of a carbon-based electromagnetic absorbing agent during the foaming process, a novel electromagnetic absorbent polymethacrylimide (PMI) foam was obtained. The proposed foam exhibits excellent electromagnetic wave-absorbing properties, with absorptivity exceeding 85% at a large frequency range of 4.9&ndash, 18 GHz. However, its poor mechanical properties would limit its application in load-carrying structures. In the present study, a novel enhancement approach is proposed by inserting metallic tubes into pre-perforated holes of PMI foam blocks. The mechanical properties of the tube-enhanced PMI foams were studied experimentally under compressive loading conditions. The elastic modulus, compressive strength, energy absorption per unit volume, and energy absorption per unit mass were increased by 127.9%, 133.8%, 54.2%, and 46.4%, respectively, by the metallic tube filling, and the density increased only by 5.3%. The failure mechanism of the foams was also explored. We found that the weaker interfaces between the foam and the electromagnetic absorbing agent induced crack initiation and subsequent collapses, which destroyed the structural integrity. The excellent mechanical and electromagnetic absorbing properties make the novel structure much more competitive in electromagnetic wave stealth applications, while acting simultaneously as load-carrying structures.

All-metallic corrugate core sandwich panels as primary loading structures may rapidly soften under compressive loading due mainly to core member buckling once the peak compressive stress is reached, resulting in reduced load-carrying capability. Inserting close-celled aluminum foam into the corrugate core has been envisioned as a feasible way to enhance the load capacity. The enhancement due to foam filling were firstly explored experimentally under quasi-static out-of-plane compression and the underlying mechanisms subsequently numerically studied using finite element simulations. The foam filled corrugated panel was found to have strength and energy absorption much greater than the sum of those of an empty corrugated sandwich panel and the aluminum foam alone. It was demonstrated that the core members in the foam-filled panel were considerably stabilized by the filling foam against lateral deflection. In particular, the elastoplastic buckling wavelength of the core members was significantly reduced and the transition from axial deformation to bending of the core member was much delayed, both of which contributing to the enhanced strength and energy adsorption capability of the foam filled panel.

Novel multilayer structured TiBN coatings were deposited on Si (100) substrate using TiBN complex cathode plasma immersion ion implantation and deposition technique (PIIID). The coatings were characterized by X-ray diffraction (XRD), high-resolution transmission electron microcopy (HRTEM), energy-dispersive spectrometer (EDS) and ball-on-disk test. XRD results reveal that both samples of TiBN coatings have the main diffraction peak of TiN (200) and (220). Cross-section TEM images reveal that these coatings have the character of self-forming multilayer and consists of face-centered cubic TiN and hexagonal BN nanocrystalline embedded in amorphous matrix. Because of the existence of hexagonal BN, the friction coefficient of the new TiBN coating in room temperature is obviously lower than that of the monolithic TiN nanocrystalline coating.

We demonstrate a simple cross-property correlation between effective conductivity (both thermal and electrical) and effective elastic modulus for high porosity cellular metallic foams with non-spherical closed cells. A particular solution to the equation of Laplace heat conduction for modelling foam thermal conductivity is extended to account for non-spherical gaseous pores; and this model is further associated with the estimation of effective elastic modulus. The simple cross-property correlation thus obtained shows that both the effective conductivity and modulus significantly decrease with increasing porosity. Non-spherical pores contribute to further reduce the effective conductivity and modulus, due mainly to increased tortuosity and stress concentration caused by enlarged surface area and sharp corners as spherical pores are replaced by non-spherical ones.

An origami-based building block of mechanical metamaterials is proposed and explained by introducing a mechanism model based on its geometry. According to our model, this origami mechanism supports response to uniaxial tension that depends on structure parameters. Hence, its mechanical properties can be tunable by adjusting the structure parameters. Experiments for poly lactic acid (PLA) samples were carried out, and the results are in good agreement with those of finite element analysis (FEA). This work may be useful for designing building blocks of mechanical metamaterials or other complex mechanical structures.