Compression Properties of 3d Printed Honeycomb and Re-Entrant Sandwich Core Materials

In this research, the free vibration analysis is investigated for a sandwich conical shell with two nanocomposite face layers and either a hexagonal honeycomb (HH) or a re-entrant honeycomb (RH) core oriented in an arbitrary direction. Both HH and RH cores are orthotropic structures, but the RH is an auxetic structure and the HH is a non-auxetic one. The nanocomposite face layers are fabricated of a polymeric matrix strengthened with uniformly distributed agglomerated either carbon nanotubes (CNTs) or graphene nanoplatelets (GNPs). The sandwich shell is modeled via Murakami’s zig-zag theory, and the governing equations and boundary conditions are derived through Hamilton’s principle. The influences of various parameters on the natural frequencies are investigated including orientation, wall thickness, and inclined angle of the cells in the honeycomb core, thickness of the honeycomb core, mass fraction and type of the nanofibers, agglomeration intensity, and boundary conditions. It is concluded that in each vibrational mode, there are optimum values for the orientation and wall thickness of the cells and thickness of the honeycomb core which result in the highest natural frequency.

In this paper, the free vibrational response of various non-uniform composite sandwich plates with different hybrid honeycomb core types and varied weight percentages of multi-walled carbon nanotube (MWCNT) is analyzed experimentally and numerically. The governing differential equations of motion of the various honeycomb non-uniform composite sandwich plates are derived using higher order shear deformation theory and numerically solved. The various non-uniform fiber-reinforced composite face plate configurations were fabricated using the vacuum-assisted hand lay-up technique with the ply-dropping off at different domains to achieve different taper configurations. The honeycomb core was fabricated using a corrugated die, and hand lay-up method to yield various configurations. In the corrugated honeycomb core, apart from the standard hexagonal shape, composite laminated strips were longitudinally reinforced between the corrugated shape continuously and intermediately to increase the strength and damping property of the structure. The various weight percentages of R-COOH functionalized MWCNT were distributed homogeneously into epoxy using the titanium probe-assisted ultra-sonication method. Material properties such as Young’s modulus and Poison’s ratio, were evaluated for the face plate using ASTM E1876. An alternative dynamic approach was carried out to obtain the shear moduli along the corrugated direction (Gxz ) and joining direction (Gyz ) of the various core models with and without MWCNT reinforcement. An experimental and numerical investigation was performed on the various prototypes of non-uniform sandwich plates with various hybrid honeycomb core material to obtain natural frequencies and loss factors under various boundary conditions. The effect of MWCNT reinforcement in face sheet and various honeycomb core material, aspect ratio at various boundary conditions on the free vibration and transverse vibrational responses of the structure was also studied. It was found that the MWCNT reinforcement in the various honeycomb core materials increases the shear modulus considerably, thus enhancing the overall dynamic behavior of the various non-uniform honeycomb composite sandwich plates.

The 3D printing technique for sand mold exhibited the good response of precast concrete structure manufacturing process. The designed sandwich core structure, including solid core, diamond core, honeycomb core, pyramid core and round O core were constructed by silica sand with different thickness and different relative density ratios from 0.5 to 0.9 and investigated by mechanical experiment, finite element modeling and digital image correlation. The solid block showed the greatest compressive strength 6.15 MPa and flexural strength 3.85 MPa. The sand core with density ratios of 0.9 performed a significant response with greater 10–50% in strength, compared with specimens with ratios 0.5 and 0.7. The sandwich composites with honeycomb and pyramid core showed a sequence of stability and enhancement with 2–3 times in difference. The experimental, numerical and digital results indicate that the core structure can be optimized to control and apply to 3D printing sand mold and precast concrete structure with a good agreement with under 15% in comparison and verification. These findings offered a good insight into the study of mechanical behavior of sandwich structure in the application and industry.

Composite honeycomb sandwich structure is widely used for aircraft structures such as control surfaces, radomes, engine cowls, and aircraft interior structure because of its lightweight and high strength characteristics. One of the disadvantages of honeycomb sandwich structure is that they are prone to fluid intrusion. The purpose of this study is to determine if the structural properties of honeycomb core are affected by contact with a fluid. The test specimens were manufactured of fiberglass prepreg for the facesheets and Nomex® honeycomb core for the core material in accordance with ASTM C-365/365M. Test specimens were soaked in several different kinds of fluids, such as aircraft fuel, turbine engine oil, hydraulic fluid, and water for a period of 60 days. Some of specimens were tested right after soaking period ended, and some of them were dried before they were tested. A flatwise compressive test was performed, and the test results were analyzed to determine how the contact with aircraft fluids affected the compressive strength of the Nomex® honeycomb core and how the strength was recovered when the specimens were dry. In addition, the investigation of de-bonding between facesheet and core material after soaking was performed to support the study.

Auxiliary metamaterials designed according to the Negative Poisson’s Ratio (NPR) property are exciting structures due to their high impact strength, impact energy absorption abilities, and different damage mechanisms. These good mechanical features are suitable for aviation, automotive, and protective construction applications. These structures, whose most significant disadvantages are production difficulties, have become easier to produce with the development of 3D production technology and have been the subject of many studies in recent years. In this presented study, two conventional core geometries and three different auxetic geometries, commonly used in sandwich structures, were designed and produced with 3D printer technology. The strength and energy absorption capabilities of prototype sandwich structures investigated experimentally under bending loads with static and dynamic compression. Except for the re-entrant (RE) type core, the auxetic core foam sandwich structures demonstrate higher rigidity and load-carrying capacity than classical sinusoidal corrugated (SC) core and honeycomb (HC) core sandwich structures under both quasi-static and impact-loaded compression and three-point bending experiments. Double arrowhead (DAH) and tetrachiral (TC) auxetic cores outperformed honeycomb core in terms of specific quasi-static and impact load-bearing performance under compression by 1.5 ± 0.25 times. In three-point bending experiments under both quasi-static and impact loading conditions, the load-carrying capacity of the double arrowhead and tetrachiral auxetic cores was found to be more than 1,86 ± 0.38 times that of the honeycomb core sandwich panels.

The honeycomb structures, due to minimal density, relative high out-of-plane compression properties, and out-of-plane shear properties, have attracted a lot of attention. Regarding this issue, in the current research, fundamental frequency analysis of the annular plate with two angle-ply multiscale hybrid nanocomposite face sheets and a honeycomb core is investigated. Halpin-Tsai as well as Hamilton’s principle, are presented for obtaining the effective material properties and governing equations of the presented composite system. For modeling the thermal environment, three-kind of thermal loading is presented. Also, the current structure is covered with the Kerr elastic foundation. The generalized differential quadrature method is presented for obtaining the exact fundamental frequency of the annular plate with two angle-ply multiscale hybrid nanocomposite face sheets and a honeycomb core. Finally, the result section is given to illustrate the influences of two angle-ply multiscale hybrid nanocomposite reinforcement, the geometry of honeycomb core, thermal loading, elastic foundation, the weight fraction of nanocomposites, and structural parameters of the annular plate on the fundamental frequency of the annular plate with two angle-ply multiscale hybrid nanocomposite face sheets and a honeycomb core. The outcomes of the current report show that for the clamped edge in the boundary conditions and each increasing is a reason for falling down the frequency of the annular plate with a honeycomb core. Another consequence is that the impact of temperature changes on the frequency of the disk is hardly dependent on the fiber angle.

Laser powder bed fusion is a major additive manufacturing process for manufacturing cellular metallic materials. The main objective of this study was to clarify the influences of microstructure on the compressive deformation behavior and fracture mechanism of selective laser melted (SLM) cellular Ti-6Al-4V alloy with a new cuboctahedron structure using in-situ observation in combination with the digital image correlation technique. The results indicated that the compressive stress–strain curve of the SLM specimen was serrated in the plateau regime due to the brittle struts with α′-martensite. Nevertheless, hot isostatic pressing (HIP) at 1000 °C/150 MPa transformed the microstructure from brittle α′-martensite to ductile α + β dual phases. In the HIP specimen, the struts plastically collapsed layer-by-layer with increasing compressive strain and were then extruded into the surrounding pores, resulting in a smooth stress–strain curve. Furthermore, the HIP treatment also improved the energy absorption at 50 pct strain from 68.1 to 77.4 MJ/m3 and maintained the uniformity in the width of the cellular material. These effects are advantageous to energy absorbing applications and alleviating the risk of biomedical implants.

Carbon fibres used as a honeycomb core material (subject to a proper in-depth analysis of their reinforcement patterns) allows solving the thermo-dimensional stability problem of the units for space systems. Based on the results of numerical simulations with the support of finite element analysis, the paper provides an evaluation of the accuracy of analytical dependencies for the determination of the moduli of elasticity of a carbon fibre honeycomb core in tension/compression and shear. It is shown that a carbon fibre honeycomb reinforcement pattern has a significant impact on the mechanical performance of the carbon fibre honeycomb core. For example, for honeycombs measuring 10 mm in height, the maximum shear modulus values corresponding to the reinforcement pattern of ±45° exceed the minimum values for a reinforcement pattern of 0° and 90° by more than 5 times in the XOZ plane and 4 times for the shear modulus in the YOZ plane. The maximum modulus of the elasticity of the honeycomb core in the transverse tension, corresponding to a reinforcement pattern of ±75°, exceeds the minimum modulus for the reinforcement pattern of ±15° more than 3 times. We observe a decrease in the values of the mechanical performance of the carbon fibre honeycomb core depending on its height. With a honeycomb reinforcement pattern of ±45°, the decrease in the shear modulus is 10% in the XOZ plane and 15% in the YOZ plane. The reduction in the modulus of elasticity in the transverse tension for the reinforcement pattern does not exceed 5%. It is shown that in order to ensure high-level moduli of elasticity with respect to tension/compression and shear at the same time, it is necessary to focus on a reinforcement pattern of ±64°. The paper covers the development of the experimental prototype technology that produces carbon fibre honeycomb cores and structures for aerospace applications. It is shown by experiments that the use of a larger number of thin layers of unidirectional carbon fibres provides more than a 2-time reduction in honeycomb density while maintaining high values of strength and stiffness. Our findings can permit a significant expansion of the area of application relative to this class of honeycomb cores in aerospace engineering.

The Al foams made by space-holder method were physically inserted into carbon fiber reinforced plastic (CFRP) composite thin-wall tubes to obtain the composite structure of Al foam-filled CFRP composite thin-wall tubes. Quasi-static compression tests of CFRP composite tubes, Al foams and Al foam-filled CFRP composite thin-wall tubes were carried out to study their compression properties. Meanwhile, digital image correlation (DIC) was applied to analyze their deformation modes. Furthermore, the compressive properties, energy absorption properties and failure modes of Al foam-filled CFRP composite thin-wall tubes at different temperatures (25-150℃) were studied. The results show that Al foams as fillers change the compression deformation behavior of CFRP composite thin-walled tubes from the scattering flowering failure of a single CFRP composite tube to the fiber layer fracture failure of a foam-filled tube. Comparing to CFRP composite thin-walled tubes, the stress fluctuations of Al foam-filled CFRP composite thin-wall tubes decrease obviously. With environmental temperature increasing, both the compressive properties and energy absorption properties of CFRP composite thin-walled tubes, Al foams and Al foam-filled CFRP composite thin-wall tubes decrease. But the interaction between Al foams and CFRP composite thin-walled tubes is enhanced, the enhancement effect of Al foams on CFRP composite thin-walled tubes is more obvious at high temperature.

In-plane compressive response of composite sandwich panels with local-tight honeycomb cores

The honeycomb (HC) core of sandwich structures undergoes flexural loading and carries the normal compression and shear. The mechanical properties and deformation response of the core need to be established for the design requirements. In this respect, this article describes the development of the smallest possible representative cell (RC) models for quantifying the deformation and failure process of the Nomex polymer-based hexagonal HC core structure under the out-of-plane quasi-static loadings. While the hexagonal single and multi-cell models are suitable for the tension and compression, a six-cell model is the simplest RC model developed for shear in the transverse and ribbon direction. Hashin’s matrix and fiber damage equations are employed in simulating the failure process of the orthotropic cell walls, using the finite element (FE) analysis. The FE-calculated load–displacement curves are validated with the comparable measured responses throughout the loading to failure. The location of the fracture plane of the critical cell wall in the out-of-plane tension case is well predicted. The wrinkling of the cell walls, leading to the structural buckling of the HC core specimen in the compression test, compares well with the observed failure mechanisms. In addition, the observed localized buckling of the cell wall by the induced compressive stress during the out-of-plane shear in both the transverse and ribbon direction is explained. The mesoscale RC models of the polymer hexagonal HC core structure have adequately demonstrated the ability to predict the mechanics of deformation and the mechanisms of failure.

Compressive properties and failure mechanisms of 3D-printed continuous carbon fiber-reinforced auxetic structures

The critical buckling loads for various core densities and materials of honeycomb composite panels are experimentally and numerically investigated in this study. The surface plates of honeycomb composite panels are of polyester/glass fiber composite. Polyester resin-impregnated paper or aluminum is used as the honeycomb core material. Honeycomb panels with different cell sizes, but approximately the same volume, are produced and the effect of the honeycomb core density on the critical buckling load is investigated by compression tests. The critical buckling load of paper core panels is determined to be higher than that of aluminum core panels. It is seen that the buckling strength of the specimens increases by the increase of core density. As the critical buckling load exceeds a certain limit, regional core cell buckling and core crushing are seen in aluminum core panels. In paper core panels, regional cracks are seen, in addition to these failures. The study also calculates the numeric buckling loads of the panels using the ANSYS finite element analysis program. The achieved experimental and numerical results are compared with each other and the results are provided in tables.

A two part research study has been completed on the topic of compression after impact (CAI) of thin facesheet honeycomb core sandwich panels. The research has focused on both experiments and analysis in an effort to establish and validate a new understanding of the damage tolerance of these materials. Part one, the subject of the current paper, is focused on the experimental testing. Of interest are sandwich panels, with aerospace applications, which consist of very thin, woven S2-fiberglass (with MTM45-1 epoxy) facesheets adhered to a Nomex honeycomb core. Two sets of specimens, which were identical with the exception of the density of the honeycomb core, were tested. Static indentation and low velocity impact using a drop tower are used to study damage formation in these materials. A series of highly instrumented CAI tests was then completed. New techniques used to observe CAI response and failure include high speed video photography, as well as digital image correlation (DIC) for full-field deformation measurement. Two CAI failure modes, indentation propagation, and crack propagation, were observed. From the results, it can be concluded that the CAI failure mode of these panels depends solely on the honeycomb core density.

Compression deformation and fracture behaviors of laser powder bed fusion Ti-6Al-4V cellular solid during in situ tests